WALTER AND ANDRÉE DE NOTTBECK FOUNDATION SCIENTIFIC REPORTS No. 30

Antipredator behaviour of Baltic planktivores

EVELIINA LINDÉN

Academic dissertation in Hydrobiology, to be presented, with the permission of the Faculty of Biosciences of the University of Helsinki, for public examination in the Auditorium Aura, Dynamicum, Erik Palménin aukio 1, Helsinki, on August 4th, 2006, at 12 noon.

HELSINKI 2006 This thesis is based on the following articles, which are referred to by their Roman numerals:

I Lindén, E., Lehtiniemi, M. & Viitasalo, M. 2003: Predator avoidance behaviour of Baltic littoral mysids Neomysis integer and Praunus fl exuosus. – Marine Biology 143: 845–850.

II Lehtiniemi, M. & Lindén, E. 2006: Cercopagis pengoi and Mysis spp. alter their feeding rate and prey selection under predation risk of herring (Clupea harengus membras). – Marine Biology DOI 10.1007/s00227-006-0243-2.

III Lindén, E. 2006: The more the merrier: Swarming as an anti-predator strategy in the mysid Neomysis integer. – Aquatic Ecology (in press).

IV Lindén, E. & Lehtiniemi, M. 2005: The lethal and sublethal effects of the aquatic macrophyte Myrio- phyllum spicatum on Baltic littoral planktivores. – Limnology and Oceanography 50: 405–411.

Articles I–III are reproduced by the kind permission of Springer Science and Business Media and article IV of American society of Limnology and Oceanography, Inc.

Supervised by: Maiju Lehtiniemi Finnish Institute of Marine Research Finland

Markku Viitasalo Finnish Institute of Marine Research Finland

Reviewed by: Heikki Hirvonen University of Helsinki Finland

Sture Hansson Stockholm University Sweden

Examined by: Andrew Sih University of California, Davis USA 3

Contributions

Field Distribution Article I II III IV mortality study experiments EL, ML, Original idea ML, EL EL EL, ML EL, ML TH, MV MV Study design and EL, ML ML, EL EL, ML EL, ML EL, ML, SL TH, MV methods TH, MV, TM, Data collection EL ML, EL EL EL, ML EL, SL SaL, SiL Data analyses EL ML EL EL EL EL Manuscript EL, ML, ML, EL EL EL, ML – – preparation MV

EL = Eveliina Lindén, ML = Maiju Lehtiniemi, MV = Markku Viitasalo, SL = Sally Londesborough, TH = Tomi Hakala, TM = Tuuli Meriläinen, SaL = Sari Lehtinen, SiL = Sirpa Lehtinen 4

Antipredator behaviour of Baltic planktivores

EVELIINA LINDÉN

Lindén, E. 2006: Antipredator behaviour of Baltic planktivores. – W. & A. de Nottbeck Foundation Sci. Rep. 30: 1–58. ISBN 952-99673-1-4 (paperback), ISBN 952-10-3210-3 (PDF).

Predation is an important source of mortality for most aquatic animals. Thus, the ability to avoid being eaten brings substantial fi tness benefi ts to individuals. Detecting predators and modifying behaviour accordingly are of prime importance in escaping predation. Here I contribute to the study of predator-prey interactions by investigating the different behavioural responses of Baltic planktivores to predation risk. Predator detection abilities and antipredator behaviour were examined in various planktivores, i.e. the littoral mysids Neomysis integer and Praunus fl exuosus, three-spined stickleback Gasterosteus aculeatus larvae, pelagic mysids My- sis mixta and M. relicta, and the predatory cladoceran Cercopagis pengoi, with cues from their respective predators European perch Perca fl uviatilis and Baltic herring Clupea harengus membras. The use of differ- ent aquatic macrophytes as predation refuges by the littoral planktivores was also examined. All pelagic planktivores and stickleback larvae were able to detect the presence of their predator by chemical cues alone. Even the nonindigenous C. pengoi, which invaded the Baltic only 14 years ago, re- sponded to chemical cues from herring, suggesting a general avoidance of any fi sh species. In contrast, the littoral mysids N. integer and P. fl exuosus responded only when chemical and visual predator cues were combined. Vision is important for littoral mysids in their well-lit shallow-water habitat, whereas pelagic mysids, performing diel vertical migration, spend most of their time in near darkness, and hence the ability to detect predators by chemical cues alone is essential for their survival. The better predator detection abili- ties of stickleback larvae compared with littoral mysids refl ect the more highly developed sensory systems of vertebrates. In addition, the responses of stickleback larvae were stronger to the combined cues than the chemical cue alone, indicating threat-sensitive behaviour. A common antipredator behaviour in all of the planktivores studied was decreased ingestion rate in response to predator cues. In addition, N. integer and stickleback larvae also decreased their swimming ac- tivity, which reduces encounters with predators and the probability of being detected by predators. Pelagic mysids and C. pengoi also altered their prey selectivity patterns in response to predator cues. Modifi cations in feeding behaviour may refl ect the increased vigilance of individuals under predation threat. The effects of predator cues on the swarming behaviour of N. integer were examined. Swarming has many advantages, including antipredator defences. Swarming also brings clear antipredator advantages to N. integer, since when they feed in a swarm, they do not signifi cantly decrease their feeding rate. This can be attributed to collective vigilance in the swarm. However, the swarming behaviour of N. integer was not affected by predation risk, but was instead a fi xed strategy. Despite the presence or absence of predator cues, N. integer individuals attempted to associate with a swarm and preferred larger to smaller swarms. In studies with aquatic macrophytes, stickleback larvae and P. fl exuosus utilized vegetation as a preda- tion refuge, spending more time within vegetation when under predation threat. There were signifi cant differences between macrophyte species in their suitability to littoral planktivores as a refuge. The two mac- roalgal species studied, bladderwrack Fucus vesiculosus and stonewort Chara tomentosa, were preferred by P. fl exuosus, whereas Eurasian watermilfoil Myriophyllum spicatum was strongly avoided by N. integer and stickleback larvae. In fact, when in dense patches in aquaria, M. spicatum caused acute and high mortal- ity (> 70%) in littoral mysids, but not in sticklebacks, whereas C. tomentosa and northern watermilfoil M. sibiricum did not. The mortality is probably due to polyphenols excreted by M. spicatum. In contrast, only 2-4% mortality in N. integer was observed with intact and broken stems of M. spicatum in fi eld experiments. 5

The distribution of littoral mysids in different vegetations, however, suggests that N. integer avoids areas vegetated by M. spicatum. Each of the Baltic planktivore species studied manifests a unique set of antipredator traits that work in combination to decrease its predation risk. The differences between the pelagic and littoral habitats are re- fl ected in the antipredator behaviours exhibited by the planktivores. Furthermore, past and ongoing changes in the Baltic Sea, e.g. eutrophication-induced changes in the composition of the macrophyte communities to the dominance of unfavourable species, probably play a role in the predator-prey interactions between planktivores and their predators. Behaviour needs to be taken into account in food web studies, since many antipredator behaviours, such as reduced feeding, result in effects on the lower trophic levels similar to those caused by direct predation on the planktivores.

Eveliina Lindén, Finnish Institute of Marine Research, P.O. Box 2, FI-00561 Helsinki, Finland 6

CONTENTS

1. INTRODUCTION ...... 7 1.1 Predation ...... 7 1.2 Predator detection ...... 10 1.3 Antipredator behaviour ...... 11 1.3.1 Decreased activity ...... 12 1.3.2 Swarming ...... 13 1.3.3 Hiding ...... 13 1.3.4 Migration ...... 14 1.3.5 Flight ...... 15 2. STUDY AREA ...... 15 2.1 Baltic Sea ...... 15 2.2 Ekenäs Archipelago ...... 16 3. STUDY OBJECTS ...... 16 3.1 Littoral mysids ...... 16 3.2 Pelagic mysids ...... 18 3.3 Predatory cladoceran Cercopagis pengoi ...... 19 3.4 Three-spined stickleback Gasterosteus aculeatus ...... 21 3.5 Predators ...... 21 3.6 Aquatic macrophytes ...... 22 4. OBJECTIVES OF THE STUDY ...... 23 5. MATERIALS AND METHODS ...... 24 5.1 Laboratory experiments ...... 24 5.1.1 Sampling ...... 24 5.1.1.1 Littoral planktivores ...... 24 5.1.1.2 Pelagic planktivores ...... 24 5.1.1.3 Predators ...... 24 5.1.1.4 Aquatic macrophytes ...... 24 5.1.1.5 Zooplankton and brine shrimp ...... 25 5.1.2 Feeding experiments ...... 25 5.1.3 Video fi lming ...... 25 5.1.4 Swarming experiments ...... 26 5.1.5 Mortality experiments ...... 26 5.2 Field studies ...... 26 5.2.1 Mortality experiments ...... 26 5.2.2 Distribution study ...... 27 5.3 Statistical analyses and equations ...... 28 6. RESULTS AND DISCUSSION ...... 29 6.1 Predator detection ...... 29 6.2 Decreased activity ...... 30 6.3 Prey selection ...... 31 6.4 Swarming ...... 33 6.5 Hiding ...... 34 6.6 Effects of aquatic vegetation ...... 36 6.6.1 Mortality ...... 36 6.6.2 Distribution ...... 37 7. CONCLUSIONS ...... 39 8. ACKNOWLEDGEMENTS ...... 40 9. REFERENCES ...... 41 7

1. INTRODUCTION affected by the inner state of the predator, e.g. hunger (Wootton 1984), density of the 1.1 Predation prey, and the profi tability of the prey, e.g. size (Gerritsen & Strickler 1977, O’Brien Predation, in which an animal consumes 1987, Bailey & Houde 1989, Hirvonen & another animal, is a major process shaping Ranta 1996). Attack probability is affected aquatic ecosystems, and affects individuals, by the skills and strategy of the predator, populations and communities (Aneer 1980, the evasion ability of the prey, density of Kerfoot & Sih 1987, Rudstam et al. 1992, the prey, and the simultaneous presence of Thiel 1996, Abrahams & Kattenfeld 1997). more than one prey animal in the encounter Virtually every animal is potential prey for fi eld of the predator (Gerritsen & Strickler others, at least during certain life stages, 1977, O’Brien 1987, Stemberger & Gilbert and predation is an important source of 1987, Bailey & Houde 1989, Hirvonen & mortality (Bailey & Houde 1989, Sih et al. Ranta 1996). Ingestion probability can be 1992, Brönmark & Hansson 2000, Scharf affected by the chemical and morphologi- et al. 2003). Predators are able to control cal properties of the prey (Wootton 1984, prey populations by altering the relative Scrimshaw & Kerfoot 1987, Stemberger and absolute abundances (Carpenter et al. & Gilbert 1987). Thus, the process of pre- 1985, Uitto et al. 1995, Diehl & Kornijów dation is by no means straightforward and 1997), species composition and population not necessarily always completed, since the structure of their prey (Wooldridge & Webb prey always attempt to stop the cycle at the 1988, Hansson et al. 1990, Benoit et al. earliest possible stage. 2002, Laxson et al. 2003), and can even In addition to direct mortality, preda- drive their prey into extinction (Murdoch tors have important indirect effects on their & Bence 1987, Dorn & Mittelbach 1999). prey, such as changes in morphology (Dill A predation event can be divided into 1987, Larsson & Dodson 1993, Stabell & separate succeeding stages, forming a cy- Lwin 1997, Merilaita 2001, Schoeppner cle: location, pursuit, attack, capture, han- & Relyea 2005), life history (Reznick et dling and ingestion (Gerritsen & Strickler al. 1990, Flinkman et al. 1994, Slusarc- 1977, O’Brien 1987, Ohman 1988, Car- zyk 1999, Chivers et al. 2001a, Sakwinska son & Merchant 2005, Clarke et al. 2005). & Dawidowicz 2005) and behaviour (Sih However, various predators may have dif- 1987, Ohman 1988, Lima 1998a, Brown & ferent skills at each stage, and prey may Cowan 2000, Hölker & Stief 2005). Some likewise differ in their vulnerability at each of these changes are permanent and fi xed, stage (Gerritsen & Strickler 1977, O’Brien i.e. not dependent on ambient predation 1987). Furthermore, many environmental pressure, and some are fl exible and induci- variables and individual properties affect ble (Dill 1987, Havel 1987, Brown & Smith the success of the predator at each stage; 1998, Sakwinska & Dawidowicz 2005, e.g. location probability can be affected Cohen & Forward 2005). Fixed antipreda- by light level, prey density, and the rela- tor traits are common in prey that are not tive sizes and speeds of the predator and likely to survive their fi rst encounter with prey (Gerritsen & Strickler 1977, O’Brien a predator and thus cannot afford to gather 1987, Stemberger & Gilbert 1987, Bailey information on local predation risk to make & Houde 1989). Pursuit probability can be adaptive changes in their traits (Dill 1987, 8

Sih 1987, 1992a). Flexible traits are possi- 1989, Crowl & Covich 1990) and hatching ble when the costs of gathering information (Chivers et al. 2001a, Kusch & Chivers are lower (Havel 1987). Environmental var- 2004). Each of these can be either fi xed or iability also plays a role: fi xed antipredator fl exible traits, depending on the species in traits are favoured in stable environments, question. whereas fl exible traits are benefi cial under An individual may temporarily go hun- variable conditions (Dill 1987, Havel 1987, gry, or fail to fi nd a mate with which to Sih 1987, Brönmark & Pettersson 1994). reproduce within a given time, but these Morphological antipredator adaptations shortcomings may have only minimal in- include changes in body size or shape, such fl uence on the individual’s lifetime fi tness, as spines that make the prey more diffi cult i.e. lifetime production of offspring. Clear- to handle and ingest, e.g. in fi shes (Wootton ly, being eaten reduces fi tness dramatically 1984, Brönmark & Pettersson 1994, Stabell and irrevocably; hence, the need to avoid & Lwin 1997), amphibians (Relyea 2001, predation is often put ahead of all other Schoeppner & Relyea 2005) and cladocer- needs. Often the same behavioural traits ans (Walls & Ketola 1989, Boeing et al. that make an animal effi cient in foraging 2005). Spines and other projections of the simultaneously increase its own risk of be- body may also increase the apparent size of ing eaten (Sih 1992b, Lima 1998a). Even the prey and thus discourage predators with where resources are abundant, the organism a limited gape size from attacking (Wootton may not be able to utilize them due to the 1984, O’Brien 1987, Stemberger & Gilbert confl icting need to avoid predators. There 1987, Makarewicz et al. 2001, Scharf et al. are trade-offs involved in all antipredator 2003). Lateral compression can reduce the traits (Dill 1987, Lima & Dill 1990, Loose vulnerability of zooplankton to gill raker & Dawidowicz 1994, van Duren & Videler retention by planktivorous fi sh (O’Brien 1996, Lima 1998a,b, Chivers et al. 2001b, 1987). Morphological antipredator struc- Viherluoto & Viitasalo 2001b, Boyero et al. tures can also be obtained through behav- 2006). Prey with defences often have lower ioural processes in caddisfl y larvae (Boyero metabolic, feeding, growth, developmental et al. 2006). Prey may also deter predators and/or reproductive rates than prey without by being distasteful or producing toxic sub- defences, e.g. those living in permanently stances (Scrimshaw & Kerfoot 1987). predator-free environments (Stemberger & Life history adaptations include a high Gilbert 1987, Walls & Ketola 1989, Loose birth rate that offsets the death rate (Stem- & Dawidowicz 1994, Jachner 1997, Tsuda berger & Gilbert 1987, Reznick et al. 1990), et al. 1998, van Buskirk 2000). synchronized reproduction and emergence The benefi t of fl exible defences is that of the young (Mordukhai-Boltovskoi & they only need to be produced or manifest- Rivier 1971, Sih 1987, Bailey & Houde ed when the risk of predation is real. The 1989, Johnston & Ritz 2001), small matu- time required to prepare a certain defence ration size (Reznick et al. 1990, Burks & is dependent on its nature: behavioural de- Lodge 2002, Sakwinska 2002, Sakwinska fences can be manifested immediately and & Dawidowicz 2005), production of diges- affect the fi tness of the individual, but mor- tion-resistant eggs (Flinkman et al. 1994), phological and life history defences oper- diapause (Hairston 1987, Slusarczyk 1999), ate on an intergenerational time scale and and timing of reproduction (Bailey & Houde thus affect the fi tness of the population. 9

The intensity of these defences can change, vulnerability of the alternative prey, and on depending on the hunger level, mating op- the possibility of the predator to respond portunities and patterns in current versus numerically to increased prey availability future reproductive success and potential of (Cooper & Goldman 1980, Abrams 1987, the prey individual (van Duren & Videler Bailey & Houde 1989). Predation may fa- 1996, Lima 1998b, Brown & Cowan 2000, cilitate invasions by exotic species if the Hartman & Abrahams 2000, Skajaa et al. native species is a more vulnerable or pre- 2004). In certain situations individuals are ferred prey than the invader (Dorn & Mit- more willing to take the risk of being eaten telbach 1999). On the other hand, multiple than in others. An individual must balance predators preying on a single species may the immediate benefi ts and long-term costs lead to either enhancement or reduction in of antipredator defences (Lima 1998a,b). its predation risk through nonindependent Predation may also infl uence other eco- predator effects (Diehl & Kornijów 1997, logical interactions of their prey, such as Warfe & Barmuta 2004, Vance-Chalcraft & inter- or intraspecifi c competition for re- Soluk 2005, Van de Meutter et al. 2005b, sources (Coen et al. 1981, Dill 1987, Wie- Griffen & Byers 2006). Omnivory, i.e. derholm 1987, Lima 1998a, Schofi eld 2003). consumption of prey from more than one Through direct lethal effects, predators can trophic level, further complicates the troph- reduce the density of their prey and thus de- ic relationships, because the intraguild crease the intensity of competition. Preda- predators show both competitive and con- tors can also induce changes in prey niches sumer-resource interactions (Johannsson et via nonlethal effects and thus either increase al. 1994, Diehl & Kornijów 1997, Warfe or decrease the intensity of competition, de- & Barmuta 2004, Winkler & Greve 2004, pending on whether the prey use the same Griffen & Byers 2006). or different antipredator strategies (Coen et Predation on one trophic level may in- al. 1981, Dill 1987, Mittelbach & Chesson duce effects on levels lower down, i.e. on 1987, Lima 1998a,b, Schofi eld 2003). Com- levels that the predators do not directly petition increases if predation pressure forces consume. These are referred to as trophic prey to share limited refuges, but decreases cascades or “leapfrog” effects (Carpenter et if prey have similar niches in the absence of al. 1985, Dill 1987, Kerfoot 1987, Romare predators and different when predators are & Hansson 2003, Reisewitz et al. 2006). present. The outcome of competition can Predators may decrease the abundance of also change as an indirect effect of preda- the prey population and thus release the tion (Leibold 1991, Lima 1998a,b, Relyea prey population of their prey from preda- 2000). In addition, there may be “apparent tion pressure (Uitto et al. 1995, Diehl & competition” between prey species that do Kornijów 1997, Laxson et al. 2003, Griffen not directly compete for resources but none- & Byers 2006). Changes in biotic habitats theless have negative effects on each other resulting from trophic cascades may infl u- through their common predator (Holt 1977, ence species utilizing these habitats (Reise- Abrams 1987, Dill 1987, Sih 1987). witz et al. 2006). The effects can also be Alternatively, the predation rates on a mediated through behavioural changes in prey species may decline when another prey the prey population, changing the way they species becomes available to the predator, feed, i.e. what, where and when (Bowers depending on the abundance and predation & Grossnickle 1978, Jeppesen et al. 1997, 10

Lima 1998a,b, Romare & Hansson 2003, likely face a defi cit of accurate information, Reichwaldt & Stibor 2005). In addition and thus the behaviour of prey is not only to top-down effects, trophic cascades can infl uenced by the actual risk of predation, also operate from bottom-up: an increase in but also by the subjectively perceived risk abundance of a prey population may lead to (Sih 1992a, Lima 1998b, Hemmi & Zeil increased foraging activity of its predator, 2005, Wong et al. 2005). Individuals can with a consequent increase in its vulnerabil- also perceive predation threat by monitoring ity to the top predator (Dill 1987). In most the behaviour of their conspecifi cs (Ryer & cases, direct lethal and indirect behavioural Olla 1991, Vilhunen et al. 2005, Wong et al. effects interact to produce the outcome of 2005), or if predators have regular foraging trophic cascades (Lima 1998a,b). patterns, cues related to season or time of day can be used (Havel 1987). The reaction distance is often of central importance in 1.2 Predator detection escape success (Viitasalo et al. 1998, Visser 2001, Scharf et al. 2003, Clarke et al. 2005, The ability to detect the presence of a Gilbert & Buskey 2005). predator gives a prey animal a chance In aquatic environments, predators can to adjust its behaviour to reduce the be detected by the visual, chemical and/or probability of completion of the predation hydromechanical cues that they emit (Blax- cycle, i.e. being detected, attacked, caught ter & Batty 1985, Kiørboe & Visser 1999, and ingested (Rademacher & Kils 1996, Mathis & Vincent 2000, Cohen & Ritz Hartman & Abrahams 2000, Gilbert & 2003, Lehtiniemi 2005). Visual cues include Buskey 2005, Hemmi & Zeil 2005, Hölker the size, shape, colour and movement of the & Stief 2005). The prey can evaluate predator (Batty 1989, Gregory 1993, Ma- predation risk by detection of different this & Vincent 2000, Hemmi & Zeil 2005, cues from predators. Many prey animals Lehtiniemi 2005). However, the underwater have remarkably sophisticated mechanisms visual environment is highly variable com- to distinguish predators from similar pared with most terrestrial environments: il- nonpredators (Mathis & Vincent 2000), lumination and turbidity levels can change different predator species (Relyea 2001), dramatically due to both natural and an- actively foraging predators from inactive thropogenic factors, and this has important (Phillips 1978), predators that have fed consequences, e.g. for predation (Johnsen on different diets (Brönmark & Pettersson 2005). Thus, under certain circumstances, 1994, Stirling 1995, Brown & Cowan 2000, such as darkness, turbidity or in the case of Vilhunen & Hirvonen 2003, Schoeppner sit-and-wait predators, vision is of little or & Relyea 2005), predators with different no use, and hence other cues play a major foraging strategies (Ritz et al. 1997, Boyero role (Jachner 1997, Ejdung 1998, Brown & et al. 2006), predators of different sizes Cowan 2000, Hartman & Abrahams 2000, (Chivers et al. 2001b, Engström-Öst & Mathis & Vincent 2000). Lehtiniemi 2004, Kusch et al. 2004, Carson Chemical predator cues include those & Merchant 2005) and hungry predators cues emitted from the predator per se (i.e. from satiated (Walls & Ketola 1989, kairomones), from the remains of prey in Jachner 1997, Ejdung 1998, Schoeppner & the faeces of the predator and cues released Relyea 2005). However, many prey animals by injured or partly consumed prey in the 11 vicinity of an active predator (i.e. alarm 2003, Browman 2005, Hemmi & Zeil 2005) cues) (Loose et al. 1993, Hamrén & Hans- and chemical cues (Stirling 1995, van Du- son 1999, Quirt & Lasenby 2002, Vilhunen ren & Videler 1996, Ejdung 1998, Cohen & Hirvonen 2003, Cohen & Forward 2005, & Ritz 2003, Åsbjörnsson et al. 2004) in Schoeppner & Relya 2005, Boyero et al. predator detection and they also have hy- 2006). The ability of an animal to inter- dromechanical sensors on their antennae cept and interpret these chemical cues may and body (Haury et al. 1980, Visser 2001, depend on both large-scale environmen- Clarke et al. 2005, Fields & Weissburg tal fl ows and on small-scale movements 2005, Gilbert & Buskey 2005). and currents generated by the animal itself (Phillips 1978, Moore & Grimaldi 2004, Mead 2005). A predator may also be able to 1.3 Antipredator behaviour suppress the release of kairomones to con- ceal its presence when attacking (Cohen & Prey can alter their behaviour both Ritz 2003). before and after their encounter with a All organisms moving in an aquatic en- predator, i.e. when the predator detects vironment create hydromechanical cues and recognizes its prey (Sih 1987, Ohman that can be distinguished, since they can 1988, Brodie et al. 1991, Scharf et al. 2003, vary signifi cantly among species (Blaxter Carson & Merchant 2005). Avoidance & Batty 1985, Viitasalo et al. 1998, Kiør- behaviours act to reduce the probability of boe & Visser 1999, Gilbert & Buskey 2005, encounters (Brown & Smith 1998, Ejdung Mogdans 2005). Detection of a disturbance 1998, Mathis & Vincent 2000, Quirt & in the water is dependent on the strength of Lasenby 2002, Scharf et al. 2003). Escape the disturbance, the distance between the behaviours act to increase the probability disturbance and the animal, the sensitivity of surviving the encounter (Rademacher of the animal’s receptors and the relative & Kils 1996, Viitasalo et al. 1998, Scharf level of noise, such as turbulence (Gerrit- et al. 2003, Clarke et al. 2005, Gilbert & sen & Strickler 1977). Hydromechanical Buskey 2005). Often these behaviours cues may carry the information later than are intermixed and one type of behaviour chemical or visual cues, when the preda- serves both causes. Antipredator behaviour tor is already attacking, and may thus give can be fi xed or fl exible, i.e. a lifestyle or suffi cient warning only for fl ight responses induced by predator detection, respectively (O’Brien 1987, Viitasalo et al. 1998, Clarke (Sih 1987, 1992a). However, fi xed vs. et al. 2005, Gilbert & Buskey 2005). fl exible behaviours are only endpoints To detect predators, fi shes utilize visual along a continuum, and a behaviour that is (Batty 1989, Bishop & Brown 1992, Greg- basically fl exible may appear fi xed due to ory 1993, Mikheev et al. 2002, Lehtiniemi a long time lag in response to a changing 2005) and chemical (Jachner 1997, Brown environment (Sih 1987, 1992a, van Duren & Cowan 2000, Vilhunen & Hirvonen & Videler 1996, Laurel et al. 2004). 2003, Kusch et al. 2004, Lehtiniemi 2005) If behaviour is fl exible, the prey are not sensory modes together with their lateral- only able to express the behaviour when line system (Blaxter & Batty 1985, Mog- needed, but also to vary the intensity of dans 2005). Crustaceans also utilize visual the response according to factors that af- (Rademacher & Kils 1996, Cohen & Ritz fect the predation risk, such as abundance 12 of predators (Loose & Dawidowicz 1994, predators; a change in habitat use or activ- Hölker & Stief 2005), size of the preda- ity to avoid one type of predator may in- tor (Bishop & Brown 1992, Chivers et al. crease vulnerability to some other type (Sih 2001b, Engström-Öst & Lehtiniemi 2004, 1987, Vance-Chalcraft & Soluk 2005, Van Carson & Merchant 2005), distance to the de Meutter et al. 2005b, Laurel & Brown predator (Hemmi & Zeil 2005) or distance 2006). to the refuge (Hartman & Abrahams 2000, Lehtiniemi 2005). In addition, prey that are more susceptible to predation, such 1.3.1 Decreased activity as a size class that is preferred by preda- tors or insect larvae that have undergone Decreased activity reduces the encounter autotomy, show stronger antipredator re- frequency with predators, as well as sponses (Sih 1982, Abrahams & Cartar the probability of being detected and 2000, Mathis & Vincent 2000, Chivers et recognized as prey (Gerritsen & Strickler al. 2001b, Quirt & Lasenby 2002, Gyssels 1977, Sih 1987, Lima & Dill 1990, Lima & Stoks 2006). Adjusting behaviour ac- 1998ab, Weissburg et al. 2002). These cording to the perceived predation risk is activities include swimming and other termed threat-sensitivity (Helfman 1989, types of movement, as well as feeding Bishop & Brown 1992, Mathis & Vincent (Stein & Magnuson 1976, Ejdung 1998, 2000, Chivers et al. 2001b, Hölker & Stief Mathis & Vincent 2000, Hölker & Stief 2005). Usually the behavioural response to 2005, Lehtiniemi 2005). However, the increased predation risk occurs rapidly, but prey swimming speed that best avoids the response to decreased predation risk oc- encounters with predators is dependent on curs much more slowly (Lima & Dill 1990, the predominant type of predator present, Loose 1993, Jachner 1997, Burks & Lodge i.e. cruising or ambush predators (Gerritsen 2002, Dalesman et al. 2006). The recovery & Strickler 1977, Ohman 1988). is also slower with higher predation risk In extreme cases, when the predation (Sih 1987, 1992a, Lima & Dill 1990, Lima risk is perceived as being so high that an es- 1998b, Wong et al. 2005). Behavioural de- cape appears impossible or the prey lack ef- fences, being fl exible and reversible, are fective escape responses, prey may escape useful if the individual has only incomplete passively (e.g. dead-man response in cla- information on predation risk, in contrast to docerans) to reduce the ability of the preda- costly morphological defences (Schoepp- tor to relocate the prey (Sih 1987, Stem- ner & Relyea 2005). berger & Gilbert 1987, Ohman 1988). In However, even strong antipredator re- addition to reducing their swimming speed, sponses can fail to prevent mortality (Sih animals can change the way they swim to 1992b, Lima 1998a,b). Antipredator behav- remain undetected (Ohman 1988, Bailey & iour may be ineffective due to phylogenetic Houde 1989, van Duren & Videler 1996). and developmental constraints or confl ict- The trade-offs of decreased activity include ing demands (Sih 1992b). The responses of lost opportunities to feed or mate, resulting prey to a particular predator can make the in lower growth and/or fecundity (Tiselius prey more susceptible to attacks from other et al. 1993, van Duren & Videler 1996, Tsu- predators. Escaping from one predator can da et al. 1998, Hölker & Stief 2005, Lehti- make the prey more conspicuous to other niemi 2005). 13

1.3.2 Swarming al. 2004). In addition, organic compounds may serve as pheromones that aid in main- Swarming has several antipredator benefi ts taining zooplankton swarms (Burks & (reviewed in Alexander 1974, Ohman Lodge 2002). Mechanoreception may also 1988, Lima & Dill 1990, Magurran 1990, play a role (Clutter 1969, Mauchline 1971c, Ritz 1994). First, it reduces the encounter Zelickman 1974, Buskey 2000), including frequency with predators (Brock & the lateral line in fi shes (Partridge & Pitch- Riffenburgh 1960, Clutter 1969). Second, er 1980, Timmermann et al. 2004). the predator is often capable of consuming Trade-offs of swarming behaviour in- only a fraction of available prey, which clude increased visibility and chemical de- is called the dilution effect (Clutter 1969, tectability of large aggregations (Brock & Major 1978, Foster & Treherne 1981, Riffenburgh 1960, Ohman 1988, Magurran Byholm 1998, Foster et al. 2001). In 1990, Weissburg et al. 2002, Botham et al. combination these two factors effectively 2005), competition for resources between reduce individual predation risk (Turner & group members (Alexander 1974, Eggers Pitcher 1986). The predator may become 1976, Magurran 1990, Ranta et al. 1993, confused in trying to choose a single prey Ritz 1994), and spreading of diseases and individual for attack (Major 1978, Heller & parasites (Alexander 1974, Hamner 1984). Milinski 1979, Magurran 1990, Jakobsen Swarming may be highly disadvantageous et al. 1994, Ritz 1994). The confusion may when predators are many orders of magni- be easier to overcome if the predator can tude larger than their swarming prey, such focus on individuals that differ from the as in the case of euphasiids and baleen others in appearance (Wolf 1985, Landeau whales (Reid et al. 2000, Gill 2002). & Terborgh 1986, Lima & Dill 1990, Reebs & Saulnier 1997, Byholm 1998), which may lead to size-assortive aggregations 1.3.3 Hiding (O’Brien 1988, Carleton & Hamner 1989, Ranta & Lindström 1990, Ribes et al. Hiding decreases the possibility of being 1996, Peuhkuri 1997). Coordinated group detected and recognized by predators avoidance and escape manoeuvres can (Ohman 1988, Lima & Dill 1990, Diehl also be utilized (Mullin & Roman 1986, & Kornijów 1997, Jeppesen et al. 1997, Magurran & Pitcher 1987, O’Brien & Ritz Brönmark & Hansson 2000). Prey can 1988, Magurran 1990, Ritz 1994). utilize physical refuges such as aquatic Swarming behaviour can be triggered vegetation (Gotceitas & Colgan 1987, by predator cues (Pijanowska & Kowalc- Lauridsen et al. 1997, Romare & Hansson zewski 1997, Brown & Smith 1998, Brown 2003, Lehtiniemi 2005, Van de Meutter & Cowan 2000) or changes in light inten- et al. 2005b), benthic habitats (Stein & sity (O’Brien 1988, Modlin 1990, Milne et Magnuson 1976, Eklöv & Persson 1996, al. 2005), the latter refl ecting avoidance of Ejdung 1998, Hossain et al. 2002, Hölker predators that use vision to locate their prey, & Stief 2005) and phytoplankton blooms and also the fact that vision is important to (Engström-Öst et al. 2006). Predators may maintain contact within the aggregation also have diffi culties in capturing prey (Steven 1961, Partridge & Pitcher 1980, in the refugia (Vince et al. 1976, Stoner Ritz 1994, Buskey 2000, Timmermann et 1982, Winfi eld 1986, Diehl 1988, Warfe & 14

Barmuta 2004) and prey may have a better organisms undergo diel vertical migration ability to assess the actual risk of predation, (DVM). Zooplankton and other smaller once they are near their refuge (Hemmi & organisms avoid visual predators, mainly Zeil 2005, Wong et al. 2005). However, the fi sh, by descending at dawn to greater behaviour and size of the predator and prey depths and rising at dusk to surface waters in question determine the effectiveness of to feed (McLaren 1974, Bollens & Frost the refuge (Coen et al. 1981, Savino & 1989, Loose 1993, Stirling 1995, Cohen & Stein 1982, 1989, Ryer 1988, Laurel & Forward 2005, Leising et al. 2005). Some Brown 2006). zooplankton species also use modifi ed As in other antipredator mechanisms, DVM to avoid contact with invertebrate there are trade-offs involved in hiding be- predators that themselves migrate to avoid haviour. While hiding reduces encounter fi sh (Burris 1980, Bowers & Vanderploeg rates with predators, it also tends to reduce 1982, Stemberger & Gilbert 1987, Ohman the encounter rates between prey and their 1988, Lampert 1989). In addition to own resources (Sih 1987, Lima & Dill 1990, antipredator benefi ts, DVM gives the Diehl & Kornijów 1997, Ejdung 1998). Of- phytoplankton community a chance to ten the habitats that are energetically most recover during day from grazing during profi table are also the most dangerous, night, possibly resulting in higher feeding since the distribution of predators tends rates (Lampert 1989, Reichwaldt & Stibor to match their prey’s resource distribution 2005). However, deeper waters usually (Lima 1998a,b). The refugia may be poorly have colder temperatures and less food, adaptable for feeding and other essential ac- and energy must be spent swimming up and tivities, such as reproduction (Lima & Dill down the water column, all of which are 1990, Diehl & Kornijów 1997, Jeppesen et costly to the migrating individuals in terms al. 1997, Warfe & Barmuta 2004, Hölker & of decreased growth rates and fecundity Stief 2005). Predation pressure may be rela- (Lampert 1989, Dodson 1990, Dawidowicz tively high also inside the refugia, by differ- & Loose 1992, Loose & Dawidowicz 1994, ent predator species or size-classes than out- Sakwinska & Dawidowicz 2005). side it (Jeppesen et al. 1997, Burks & Lodge In shallow habitats horizontal migra- 2002, Wojtal et al. 2003, Åsbjörnsson et al. tion is more common. Many zooplankton 2004, Van de Meutter et al. 2005b). Com- species perform diel horizontal migration petition for resources may be intensifi ed especially in lakes, spending the daylight within the limited dimensions of the refuge hours under cover of the littoral macro- (Lima 1998a, Laurel et al. 2004). phyte vegetation and migrating to the open pelagic to feed at night (Lauridsen & Lodge 1996, Lauridsen et al. 1997, Wojtal et al. 1.3.4 Migration 2003). Many macrophytes are repellent to zooplankton, so again there is a trade-off Light level has a crucial effect on the involved (Lauridsen & Lodge 1996, Lau- ability of visual predators to locate their ridsen et al. 1997, Burks & Lodge 2002). prey (O’Brien 1987). A free-swimming In predator-free environments, such as individual can indirectly control its fi sh-free lakes, horizontal migration does ambient light level by migration. In pelagic not occur and the zooplankton reside in the environments with suffi cient depth, many open water (Lauridsen et al. 1997). 15

The proximate cue for migrations (both (Cohen & Ritz 2003, Vilhunen & Hirvonen vertical and horizontal) is light, which also 2003). regulates their amplitude, but cues of pred- The success of a fl ight response is de- ator presence and food availability may pendent on its timing, velocity and orien- modify the pattern (Bohl 1980, Rudstam et tation (reviewed in Neil & Ansell 1995). al. 1989, Loose & Dawidowicz 1994, Lau- Flight success is enhanced by the proxim- ridsen et al. 1997, Gal et al. 1999). In the ity of refuges, so that after a rapid escape case of DVM, common crustacean species response the prey can utilize concealment in temperate areas follow an isolume, mov- (Sih 1987, Lima & Dill 1990). The strength ing vertically to maintain the same subjec- of the fl ight response increases with size tive level of light intensity (Johnsen 2005). and/or developmental stage (Blaxter & Diel and seasonal changes in the buoyancy Batty 1985, Ohman 1988, Bailey & Houde of copepods can also infl uence their verti- 1989, Batty 1989, Fuiman 1993). cal migrations (Thorisson 2006). Migration behaviour is generally more pronounced in the more conspicuous individuals, i.e. 2. STUDY AREA large, pigmented or carrying eggs (Lampert 1989, Lima 1998b). 2.1 Baltic Sea

The Baltic Sea is the largest brackish water 1.3.5 Flight sea in the world, with an area of 415 266 km2 and stretching between 54ºN and 66ºN, Escape responses are the fi nal option to but shallow, with a mean depth of only 55 reduce the chance of being captured when m. It is characterized by a year-round stable the predator is attacking its prey (O’Brien salinity gradient, declining from > 20 psu & Ritz 1988, Ohman 1988, Neil & Ansell in its opening area towards the North Sea to 1995, Brönmark & Hansson 2000). These about 2 psu in the Bothnian Bay (Kullenberg responses include high-speed “leaps” and 1981). The organisms are of both marine swimming bursts, often aided by protean, i.e. and freshwater origin in addition to true unpredictable, movement patterns that aim to brackish water species, their distribution confuse the predator so that it cannot relocate being largely determined by the salinity the prey after an attack (Rademacher & Kils gradient (Haage 1975, Hällfors et al. 1981, 1996, Viitasalo et al. 1998, Cohen & Ritz Snoeijs 1999). The Baltic has relatively 2003, Vilhunen & Hirvonen 2003, Gilbert low species diversity, in part because of its & Buskey 2005). These require abundant young age – it has been in its present form energy, and thus cannot be maintained for for only about 7 500 years. In addition, its longer periods of time (Larsson & Dodson low salinity and temperature and strong 1993, Fields & Weissburg 2005, Gilbert & seasonality with an annual ice cover result Buskey 2005). Flight is most effectively in physiological stress to organisms, many triggered by mechanosensory (Blaxter & of which live near their tolerance limits. Batty 1985, Viitasalo et al. 1998, Green et The Baltic has been and is constantly al. 2003, Gilbert & Buskey 2005) or visual invaded by nonindigenous species that cues (Batty 1989, Bishop & Brown 1992), colonize available niches (Ojaveer et al. but chemical stimuli may also be effective 1999, Leppäkoski et al. 2002a, 2002b). 16

The Baltic has a large drainage basin, 1 and many summer houses are located there. 729 000 km2, with a population of about 85 The anthropogenic impact can be seen as million people living in 14 countries, and local eutrophication, but nutrient loading the impact of human activities is profound. from the open sea of the Gulf of Finland Eutrophication is perhaps the most serious also affects the area (Kangas et al. 1982, and widespread problem, causing commu- Wallström et al. 2000). There is practically nity changes and turbidity (Larsson et al. no tide in the Baltic Sea, which has special 1985, Bonsdorff et al. 1997, Karjalainen implications for littoral ecosystems. The 1999, Laamanen et al. 2004, Weckström hydrolittoral zone of the Baltic is defi ned 2005). The Baltic, except for the Bothnian as the zone that extends above the annual Bay, is strongly stratifi ed, with a seasonal minimum water level to the mean summer- thermocline and a permanent halocline, time level, and the sublittoral is the part that below which the water is irregularly sub- is permanently submerged (Snoeijs 1999). jected to periods of hypoxia or anoxia and The annual wind-induced water level presence of H2S (Larsson et al. 1985, Matt- changes can be substantial, up to 2 m in the häus 1995, Snoeijs 1999). Bothnian Bay.

2.2 Ekenäs Archipelago 3. STUDY OBJECTS

The Ekenäs Archipelago is located in 3.1 Littoral mysids southwest Finland, at the entrance to the Gulf of Finland, where the average salinity is Neomysis integer (Leach 1814) (Mysidacea, about 6 psu. The Archipelago can be divided Crustacea) is the most common and into outer, middle and inner zones that have widespread mysid species in the Baltic different characteristics in exposition and Sea, occurring in all areas except for occurrence of hard and soft bottoms. The the Bothnian Bay (Köhn 1992, Kotta & land is constantly rising about 3–4 mm yr-1 Kotta 1999, Kotta et al. 2004). It also oc- as a result of recovery from the last Ice Age. curs in brackish lakes in Europe (Irvine As a consequence, small semienclosed bays et al. 1993, Aaser et al. 1995, Søndergaard (fl ads) along the coastline are becoming et al. 2000) and estuaries of the North Sea increasingly cut off from the sea. These bays and northwestern Atlantic between 36ºN undergo a succession of changes in their and 63ºN (Apel 1992, Moffat & Jones geology, physical conditions and community 1992, Mees et al. 1994, Hostens & Mees structure, especially macrovegetation 1999, Fockedey 2005). It is a genuine (Munsterhjelm 1997, 2005, Wallström brackish water species, with a salinity et al. 2000). In spring after the ice break- tolerance of 1–38 psu and temperature up, the shallow bays warm up quickly to tolerance of 0–33 ºC (Arndt & Jansen 1986, temperatures higher than in the open sea Köhn 1992, Kotta & Kotta 2001, Fockedey (Munsterhjelm 1997) and act as important 2005). However, sexual maturation is only nursery areas for many fi sh species (Urho possible within a range of 5–15 psu and 2002). 15–25 ºC, and the size-at-maturity increases The Ekenäs Archipelago is widely used with increasing salinity and decreasing for recreation, such as boating activities, temperature (Fockedey 2005). Neomysis 17 integer occurs in large swarms (Mauchline tions (Hansson et al. 1990, Debus et al. 1971c, Arndt & Jansen 1986, Debus et al. 1992, Irvine et al. 1993, Speirs et al. 2002, 1992, Köhn 1992, Byholm 1998), especially Fockedey 2005). Littoral mysids are omniv- in early summer, and these swarms are orous, feeding on bottom detritus, organic often assorted according to size (Köhn material in suspension in the water, vari- 1992, Välipakka 1992, Kauppila 1994, ous phyto- and zooplankton and meiofau- Fockedey 2005). In the northern Baltic Sea, na (Mauchline 1971a,b, Nordström 1997, N. integer breeds from May to September Fockedey 2005, Koho 2005, Gorokhova and usually produces two generations per in press). The relative importance of each year (Rudstam et al. 1986, Köhn 1992, group changes with season, habitat and size Kauppila 1994). of the mysid (Arndt & Jansen 1986, Nord- Praunus fl exuosus (Müller 1776) (My- ström 1997, Fockedey 2005). These two sidacea, Crustacea) is a marine euryhaline mysid species may compete for food, al- species that is common throughout the Bal- though P. fl exuosus utilizes more zooplank- tic Sea, except in the Bothnian Bay (Köhn ton and is more specialized for consuming 1992). It also occurs in estuaries and along certain food items than N. integer, whose the coast of the northern Atlantic (Mauch- diet is broad and contains more detritus line 1971b, Winkler & Greve 2004). Its sa- (Nordström 1997, Winkler & Greve 2004). linity tolerance ranges from 3.5 to 37 psu Neomysis integer also preys on the eggs (McLusky 1979). Praunus fl exuosus is a and larvae of the Baltic herring Clupea phytophilous species: its distribution pat- harengus membras L. 1761 (Lehtiniemi et tern is positively infl uenced by the density al. unpubl.), whereas P. fl exuosus may prey of aquatic vegetation and its entire life cy- on the smaller-sized N. integer (Winkler & cle occurs in the phytobenthic zone (Kotta Greve 2004). In many areas, including the & Kotta 1999). Its most important habitat southern Baltic, N. integer is able to control in the Baltic is the bladderwrack Fucus ve- zooplankton abundance and composition, siculosus L. belt (Kauppila 1994). It breeds but this is unlikely in the northern Baltic, from June to September, usually produc- where Mysis spp. and young-of-the-year ing one but sometimes two generations per clupeids are the dominant planktivores (re- year (Köhn 1992, Kauppila 1994). Praunus viewed in Fockedey 2005). fl exuosus is mainly solitary but may occur Littoral mysids are important prey for in loose shoals (Mauchline 1971c). a number of fi sh species, including Euro- In the Baltic, N. integer and P. fl exuo- pean perch Perca fl uviatilis L. 1758, roach sus occupy mainly shallow coastal waters Rutilus rutilus L. 1758, three-spined stick- (Arndt & Jansen 1986, Välipakka 1992, leback Gasterosteus aculeatus L. 1758 Väinölä & Vainio 1998, Kotta & Kotta and Baltic herring (Thiel 1996, Aarnio & 1999, 2001), but perform seasonal horizon- Bonsdorff 1993, Hostens & Mees 1999, tal migration, leaving the uppermost litto- Granqvist & Mattila 2004, Gorokhova et al. ral in winter, as well as in summer when 2004, Fockedey 2005). Neomysis integer the water temperature reaches about 20 ºC may also compete with larval herring for (Arndt & Jansen 1986, Välipakka 1992, zooplankton food (Fockedey 2005, Koho Kauppila 1994, Nordström 1997, Kotta 2005). In addition to fi sh, macrocrustaceans & Kotta 1999). Neomysis integer also un- and wading birds are important predators of dergoes diel horizontal and vertical migra- N. integer (reviewed in Fockedey 2005). 18

3.2 Pelagic mysids as M. relicta from the Bothnian Sea (II, Ta- ble 1) are most likely M. salemaai, while The pelagic mysid species that has those collected from the Bothnian Bay (II, conventionally been referred to as Mysis Table 1) most likely include both species. relicta sensu lato (Mysidacea, Crustacea) Hybridization between the two species has actually consists of four sibling species with been reported from the Bothnian Bay but different distributions, zoogeographical this is a rare phenomenon (Väinölä & Vainio histories and ecological characteristics 1998). The ecological differences found so (Väinölä 1986, Väinölä et al. 1994, far between M. relicta and M. salemaai re- Väinölä & Vainio 1998, Audzijonyte & late to the timing of breeding (Väinölä 1986, Väinölä 2005). Of these four species, M. Väinölä & Vainio 1998) and to the spectral relicta s. str. (Lovén 1862) and M. salemaai sensitivity of their eyes (Lindström 2000, (Audzijonyte and Väinölä 2005) occur in Audzijonyte et al. 2005), neither of which the Baltic Sea (Väinölä 1986, Väinölä et are likely to be of major importance in the al. 1994, Audzijonyte & Väinölä 2005). present study. Moreover, separation of these Both species are primarily marine taxa, two species in live animals is impossible, but separated from their common marine since it requires dissection followed by the ancestor at different times. Mysis relicta has use of genetic (Väinölä 1986, Väinölä et al. a considerably longer (> 1 Myr) freshwater 1994, Väinölä & Vainio 1998, Audzijonyte history, whereas M. salemaai colonized & Väinölä 2005) and microscopy (Väinölä the continental waters more recently et al. 2002, Audzijonyte & Väinölä 2005) (Audzijonyte 2006). During the last glacial techniques. Hence they are treated as one maximum (20 kyr ago) the main refugia of species and referred to as M. relicta in II M. relicta were along the eastern margins and the following discussion. of the ice sheet, from where it colonized Mysis mixta (Lilljeborg 1852) (Mysida- the Baltic Sea and Northern European lakes cea, Crustacea) is of North Atlantic origin (Väinölä et al. 1994). The main refugium and has adapted to the brackish water of the of M. salemaai appears to have been in the Baltic Sea (Salemaa et al. 1986). It domi- North Sea area, and the species colonized nates mysid communities in the Gulf of the Baltic Sea via the Yoldia Sea connection Finland, the northern Baltic proper and the (Audzijonyte & Väinölä unpubl. data). Gulf of Riga (Rudstam et al. 1986, Salemaa The species referred to as M. relicta et al. 1986, Kotta & Kotta 2001). Mysis (II) most likely includes both M. relicta mixta and M. relicta co-occur in the Both- and M. salemaai. These two species oc- nian Sea, but M. relicta dominates the my- cur sympatrically in the Bothnian Bay, but sid communities in the Bothnian Bay (Sa- only M. salemaai is found in the Bothnian lemaa et al. 1986, 1990, Väinölä & Vainio Sea (Väinölä et al. 1994, Väinölä & Vainio 1998). The northern distribution limit of M. 1998, Audzijonyte & Väinölä 2005). In mixta lies in the Quark area between the other areas of the northern Baltic Sea, M. Bothnian Sea and Bothnian Bay, whereas relicta occurs mostly near the coast and is the southern distribution limit of M. relicta replaced by M. salemaai in deeper offshore lies at 56º N and western at 18º30’ E (Köhn waters (Väinölä 1986, Väinölä et al. 1994, & Gosselck 1989, Salemaa et al. 1990). Väinölä & Vainio 1998, Audzijonyte & Mysis spp. are the most frequently occur- Väinölä 2005). Thus the animals collected ring mysids in Baltic open sea areas (Köhn 19

1992). Most Mysis populations in the leased juveniles at the time of the spring northern Baltic breed in autumn and carry phytoplankton bloom and early summer the embryos in a marsupium over winter (Bowers & Vanderploeg 1982, Rudstam & (Rudstam et al. 1986, Salemaa et al. 1986, Hansson 1990, Viherluoto et al. 2000, Lin- Väinölä & Vainio 1998). The juveniles are dén & Kuosa 2004). In June, zooplanktivo- released in spring, M. relicta earlier than ry is limited due to the high light levels that M. mixta (Salemaa et al. 1986, Rudstam inhibit DVM (Rudstam & Hansson 1990), & Hansson 1990). However, M. salemaai poor availability of zooplankton, and the breeds throughout the year in the Gulf of small size and poor capture ability of the Finland (Väinölä & Vainio 1998). Mysis mysids (Cooper & Goldman 1980, Viher- individuals may breed as one- or two-year- luoto et al. 2000). Later in the season, as olds, but only one generation is produced mysids grow larger and zooplankton more annually (Salemaa et al. 1986, Väinölä & abundant, zooplankton predominate in the Vainio 1998). diet (Bowers & Vanderploeg 1982, Viher- Mysis spp. undergo DVM (Salemaa et luoto et al. 2000, Lehtiniemi et al. 2002, al. 1986, Rudstam et al. 1989, Hansson et Koho 2005). Mysis mixta ingests more de- al. 1990, Rudstam & Hansson 1990). The tritus and other benthic material during the migration is regulated by light intensity: the day when near the bottom, whereas they mysids rise from their daytime near-bottom ingest more zooplankton and also phyto- habitat up the water column to feed at dusk plankton during the night when they have and descend at dawn, avoiding light levels ascended (Rudstam et al. 1989). Mysis mix- above 10-4 lux (Salemaa et al. 1986, Rud- ta is more zooplanktivorous than M. relicta stam et al. 1989, Rudstam & Hansson 1990). (Viherluoto et al. 2000) and may be able Mysis mixta rises higher than M. relicta, but to regulate zooplankton abundance and rarely through the thermocline (Salemaa et species composition in the northern Baltic al. 1986). Part of the M. mixta population proper (Rudstam et al. 1986, 1992, Hans- does not migrate, but also remains near the son et al. 1990, Rudstam & Hansson 1990). bottom during night (Rudstam et al. 1989, Pelagic mysids are important prey for adult Rudstam & Hansson 1990). Baltic herring (Aneer 1980) and European Pelagic mysids are of widespread eco- smelt Osmerus eperlanus L. 1758 (Horp- logical importance in the open sea areas of pila et al. 2003, Ojaveer et al. 2004), but at the Baltic, because they effectively link pri- the same time they compete for the same mary and secondary production to higher zooplankton prey (Hansson et al. 1990, trophic levels as well as benthic to pelagic Rudstam & Hansson 1990, Johannsson et ecosystems (Grossnickle 1982, Viherluoto al. 1994). et al. 2000, Viherluoto 2001). Both Mysis species are opportunistic omnivores, feed- ing on phytoplankton, detritus, copepods, 3.3 Predatory cladoceran cladocerans, rotifers, ciliates and protists Cercopagis pengoi (Rudstam et al. 1989, Rudstam & Hansson 1990, Viherluoto 2001, Albertsson 2004, Cercopagis pengoi (Ostroumov 1884) Koho 2005). Their feeding habits are de- (Cladocera, Crustacea) is native to the pendent on life stage, season and DVM. Ponto-Caspian Basin, occurring in the Herbivory is important for the newly re- Caspian Sea, Sea of Azov, Aral Sea, 20 estuaries of the Black Sea, several rivers al reproduction, i.e. gamogenesis (Mor- and freshwater reservoirs (Mordukhai- dukhai-Boltovskoi & Rivier 1971). The Boltovskoi & Rivier 1971). It was fi rst change from asexual to sexual reproduc- described in the Baltic in 1992 in Pärnu tion in cladocerans is induced by chemi- Bay and the Gulf of Riga (Ojaveer & cal signals from conspecifi cs and possibly Lumberg 1995), and an observation of the also from predators (reviewed in Larsson species was also made in the same year in & Dodson 1993). In the Baltic, partheno- the Gulf of Finland (S. Saesmaa, Finnish genesis is prevalent for most of the summer Institute of Marine Research, pers. comm.). and gamogenic females and males are rare In addition to these areas, it today occurs in (Grigorovich et al. 2000, Gorokhova et al. the Åland Sea, Archipelago Sea, Bothnian 2000, Uitto et al. 1999, Simm & Ojaveer Sea, northern Baltic proper, Curonian and 2006). The major production of gamogenic Vistula lagoons, and the Gulf of Gdansk resting eggs begins in late summer (Krylov (MacIsaac et al. 1999, Bielecka et al. 2000, & Panov 1998, Antsulevich & Välipakka Gorokhova et al. 2000, Leppäkoski et al. 2000, Gorokhova et al. 2004, Simm & Oja- 2002a, Finnish Institute of Marine Research veer 2006). The eggs hatch in May and the unpubl. data). In some years the distribution fi rst generation is morphologically distinct of C. pengoi reaches even the Bothnian from the following parthenogenic genera- Bay in the northernmost Baltic (Finnish tions (Simm & Ojaveer 2006). In the Cas- Institute of Marine Research unpubl. data). pian Sea, cercopagids undergo DVM (Mor- It has also invaded North American lakes: dukhai-Boltovskoi & Rivier 1971), but this it was identifi ed from Lake Ontario in behaviour has not been reported from the 1998 (MacIsaac et al. 1999), from Lake Baltic Sea (Krylov et al. 1999) or Lake Michigan (Charlebois et al. 2001) and the Ontario (Benoit et al. 2002, Laxson et al. Finger Lakes a year later (Makarewicz et al. 2003). The body length of the female is 2001), and from Lake Erie and Muskegon 1.2–2.3 mm and that of the male 1.1–2.1 Lake in 2001 (Therriault et al. 2002). mm (Ojaveer & Lumberg 1995, MacIsaac Cercopagis pengoi is a euryhaline spe- et al. 1999, Bielecka et al. 2000, Grigoro- cies, tolerating salinities from < 1–17 psu vich et al. 2000, Makarewicz et al. 2001). as well as freshwater (Mordukhai-Boltovs- The caudal appendage of C. pengoi can be koi & Rivier 1971, Bielecka et al. 2000, as much as 8–10 times body length, termi- Gorokhova et al. 2000). It is also a ther- nating in a distinctive loop (Mordukhai- mophilous species (Mordukhai-Boltovskoi Boltovskoi & Rivier 1971, MacIsaac et al. & Rivier 1971) and its abundance varies 1999, Grigorovich et al. 2000, Makarewicz yearly according to temperature (Antsu- et al. 2001, Simm & Ojaveer 2006), and levich & Välipakka 2000, Leppäkoski et probably serves as an antipredator defense al. 2002a, but see Ojaveer et al. 2004), (Makarewicz et al. 2001, Laxson et al. reaching a maximum in late summer/early 2003). autumn (Ojaveer & Lumberg 1995, Krylov Cercopagis pengoi is a predatory cla- & Panov 1998, Krylov et al. 1999, Antsu- doceran: it has raptatory thoracopods with levich & Välipakka 2000, Gorokhova et al. no fi ltering exopods (Mordukhai-Boltovs- 2000). koi & Rivier 1971, MacIsaac et al. 1999). It Cercopagis pengoi has two modes of feeds on zooplankton (Laxson et al. 2003, reproduction: parthenogenesis and sexu- Gorokhova et al. 2005) and may control 21 zooplankton abundances during occurrence et al. 1999). During the breeding season, at high densities (Uitto et al. 1999, Benoit stickleback are territorial: the males select et al. 2002, Laxson et al. 2003, Kotta et al. and defend territories in which they build 2004, Ojaveer et al. 2004, Lehtiniemi & nests and to which the females are attracted Gorokhova unpubl. data). Since its invasion to lay eggs (Keenleyside 1955, Wootton of the Baltic, C. pengoi has become impor- 1984). tant prey for Baltic herring, European sprat Although the three-spined stickleback is Sprattus sprattus L. 1758, sticklebacks and of no commercial importance, it is a cen- smelt (Ojaveer & Lumberg 1995, Antsu- tral link in the trophic web of the Baltic Sea levich & Välipakka 2000, Gorokhova et (Lemmetyinen & Mankki 1975). It is a vi- al. 2004, 2005, Ojaveer et al. 2004, Pel- sual predator (Wootton 1984) and feeds on tonen et al. 2004), while pelagic mysids zooplankton, littoral mysids, benthic inver- (Gorokhova & Lehtiniemi unpubl. data) tebrates, and fi sh eggs and larvae (Lemme- and Neomysis integer (Gorokhova in press) tyinen & Mankki 1975, Hangelin & Vuo- also prey on C. pengoi. However, C. pengoi rinen 1988, Thiel 1996, Ojaveer et al. 2004, may be a strong competitor for zooplank- Peltonen et al. 2004). Three-spined stick- ton food with planktivorous fi sh (Benoit leback constitute an important part of the et al. 2002, Laxson et al. 2003, Ojaveer et diet of several waterfowl species and fi sh al. 2004, Peltonen et al. 2004, Gorokhova (Lemmetyinen & Mankki 1975, Wootton et al. 2005) and possibly also with mysid 1984, Reimchen 1994). shrimps (Kotta et al. 2004). Due to its cau- dal appendage, C. pengoi individuals easily 3.5 Predators become entangled, forming dense aggrega- tions, e.g. on fi shing gear, which may cause The European perch is a freshwater species economic losses to fi shermen in the Baltic that is widely distributed throughout Europe, (Ojaveer & Lumberg 1995, Antsulevich & and it is also the most common freshwater Välipakka 2000, Bielecka et al. 2000). species in shallow areas of the Ekenäs Archipelago (Sundell 1994, Lappalainen 3.4 Three-spined stickleback et al. 2001). It is the main target of the Gasterosteus aculeatus L. recreational fi shery in Finnish sea areas (Finnish Game and Fisheries Research The three-spined stickleback is a small, Institute 2004). The perch is a food generalist euryhaline fi sh (adult size 35–80 mm) that undergoes a series of ontogenetic niche that is found in fresh, marine and brackish shifts, whereby its diet changes (Sandström waters throughout the Northern Hemisphere 1999). In the study area, mysids, together (Wootton 1984). It is the most common fi sh with amphipods and fi sh are the main food species in the littoral zones of the northern items for smaller perch (12–20 cm total Baltic Sea (Lemmetyinen & Mankki 1975, length), whereas larger perch (> 20 cm Sundell 1994, Rajasilta et al. 1999). Adult TL) feed mainly on fi sh, including gobies, stickleback migrate from the open sea in herring and three-spined stickleback early spring to spawn in coastal waters, and (Lappalainen et al. 2001). The perch is a the larvae and juveniles are very abundant visual predator that needs suffi cient light in shallow bays during July and August levels for foraging (Diehl 1988, Diehl & (Lemmetyinen & Mankki 1975, Rajasilta Kornijów 1997, Sandström 1999). 22

The Baltic herring is a pelagic schooling dominant (the only one in the study area) species, although it spawns in coastal areas canopy-forming macroalga in the Baltic Sea (Laine 2003). It is a visual predator that (Jansson et al. 1982, Kautsky et al. 1992, consumes mainly zooplankton and mysids Snoeijs 1999). Its northern distribution (Aneer 1980, Hansson et al. 1990, Rudstam limit is at the Bothnian Sea/Bothnian Bay et al. 1992, Arrhenius & Hansson 1993, boundary where the salinity is about 4 Koho 2005), competing for zooplankton psu (Jansson et al. 1982, Kautsky et al. food with sprat (Rönkkönen et al. 2004, 1992, Snoeijs 1999). Fucus vesiculosus Koho 2005). The Baltic herring is the most forms belts between 0.3 and 12 m in depth, important commercial fi sh species in Finn- depending on the water clarity (Haage ish sea areas (Finnish Game and Fisheries 1975, Jansson et al. 1982, Kautsky et al. Research Institute 2005). In recent decades 1992, Snoeijs 1999). It requires clean, hard the weight-at-age of Baltic herring has de- substrates to attach its thallus (Kangas et creased signifi cantly, presumably caused by al. 1982, Eriksson & Johansson 2003) and a salinity-induced change in zooplankton is therefore found on rocky shores (Snoeijs species composition (Flinkman et al. 1998, 1999). It is often covered with epiphytic Cardinale & Arrhenius 2000, Rönkkönen et algae (Kautsky et al. 1992, Snoeijs 1999) al. 2004), and food competition with sprat and supports a diverse community of fi sh (Casini et al. 2006). The mysid populations and invertebrates (Haage 1975, Kangas in the Baltic also have declined, possibly 1978, Jansson et al. 1982, Kautsky et al. due to poor oxygen conditions on the bot- 1992, Hemmi 2003). tom, and these energetically profi table food The stonewort Chara tomentosa L. items occur in the stomachs of large her- (Characeae, Chlorophyta) is a macroalga ring less often than before (Arrhenius & of freshwater origin, but is also found in Hansson 1993, Koho 2005). In addition, brackish waters (Snoeijs 1999). In the Bal- the nonindigenous cladoceran Cercopagis tic Sea it is restricted to sheltered coastal pengoi has altered the pelagic food web bays (Schubert & Yousef 2001). It requires structure, and although several studies in- clear water and soft bottoms, where it is dicate that it is a preferred food item for anchored by rhizoids (Kautsky 1988, Mun- herring (Ojaveer & Lumberg 1995, Ojaveer sterhjelm 1997, 2005, Snoeijs 1999). Cha- et al. 1999, 2004, Antsulevich & Välipakka ra tomentosa is very sensitive to anthropo- 2000, Gorokhova et al. 2004), the full ef- genic changes in the environment, such as fects of this invasion on the predator-prey eutrophication (Koistinen & Munsterhjelm and competitive interactions between Bal- 2001, Munsterhjelm 2005). tic planktivores remain to be resolved. The Eurasian watermilfoil Myriophyl- lum spicatum L. (Haloragaceae) is a fresh- water vascular plant species that is also 3.6 Aquatic macrophytes adapted to living in brackish water (Aiken et al. 1979, Snoeijs 1999). Myriophyllum Aquatic macrophytes were included in spicatum is very abundant in the Ekenäs the study as possible predation refuges for Archipelago (Munsterhjelm 1997, Wall- littoral planktivores. The bladderwrack ström et al. 2000) as well as in other coastal Fucus vesiculosus (Phaeophyceae) is areas in the Baltic Sea, growing at 1–5-m a perennial marine brown alga and the depths (Smith & Barko 1990). It benefi ts 23 from eutrophication and has increased in The experiments with pelagic plankti- recent decades (Munsterhjelm 1997, Eriks- vores, i.e. Mysis mixta, M. relicta, and Cer- son et al. 2004, Roos et al. 2004). It is a copagis pengoi, focused on feeding behav- strong competitor (Grace & Wetzel 1978, iour (II). The objective was to examine how Kautsky 1988, Smith & Barko 1990, Mad- they change their feeding activity and prey sen et al. 1991) and was introduced into selectivity in response to chemical preda- North America by the 1880s, where it has tor cues from herring (II). Other aspects of become a nuisance plant, replacing many antipredator behaviour, i.e. swimming ac- native species (Aiken et al. 1979, Nichols tivity (I, IV), hiding (I, IV) and swarming & Shaw 1986, Smith & Barko 1990, Mad- (III), were also studied in experiments with sen et al. 1991). the littoral planktivores Neomysis integer, The northern watermilfoil Myriophyllum Praunus fl exuosus and stickleback larvae. sibiricum Kom. is a circumpolar freshwater The objectives were to determine how lit- plant species occurring in Eurasia, Green- toral planktivores change their swimming land and North America (Aiken 1981, activity, refuge use and feeding activity in Ceska & Ceska 1986). In North America, it response to predator cues from perch (I, IV) was fi rst described as M. exalbescens Fer- and to examine how the swarming behav- nald, which is now considered a synonym iour of N. integer is used as an antipredator for M. sibiricum (Ceska & Ceska 1986). strategy (III). The species is very similar in appearance In addition to the antipredator behaviour to M. spicatum, but since it requires a more as such, one of the main factors affecting sheltered habitat than M. spicatum, it is not the predation vulnerability of planktivores as widely spread in the study area. in the littoral zone was also examined: aquatic macrophyte vegetation. The rela- tive abundance of the different macrophyte 4. OBJECTIVES OF THE STUDY species changes along with environmental changes, especially eutrophication. The There were two main goals in the studies objectives were to determine the differ- of this thesis, of which the fi rst was to ences between littoral planktivore species reveal how Baltic planktivores, inhabiting in their refuge use, as well as the differ- both littoral and pelagic environments, ences between the macrophyte species detect their predators. The objective was to (I, IV). determine the importance of chemical and Manipulative experiments were car- visual predator cues in predator detection ried out in the laboratory (I–IV). In addi- (I, II, IV). The second main goal was to tion, previously unpublished fi eld data are reveal how planktivores modify their included in the thesis to obtain additional behaviour to avoid predation, once they information on the effects of macrophytes have detected the presence of a predator. All on the survival and distribution of littoral the planktivores studied are important prey mysids. Field mortality experiments were for a number of predators, including perch conducted with N. integer and both intact and Baltic herring. The main objective and broken stems of Myriophyllum spica- was to examine the different antipredator tum to determine the lethality of this mac- strategies and the fl exibility in behaviour of rophyte in a natural setting. In the distri- the various planktivore species (I–IV). bution study, mysids were sampled in four 24 shallow bays with different macrophyte the bottom and then lifted slowly to the communities to investigate the effects of surface. Cercopagis pengoi (mean length macrophyte species composition on the oc- 2.4 mm) were collected during day at a currence and distribution of mysids. station in the Gulf of Finland, from 50 m to the surface with a WP-2 plankton net (200 μm) equipped with a cod end. 5. MATERIALS AND METHODS All studies were conducted at the Tvärminne 5.1.1.3 Predators. Perch (I, III, IV) were Zoological Station (TZS) (University of caught in a fi sh trap from a shallow bay near Helsinki) and its nearby sea areas, located TZS. The trap was set up in the evening in the Ekenäs Archipelago, or aboard the and emptied the following morning. The R/V Aranda (Finnish Institute of Marine fi sh were transported to the laboratory in Research). ambient seawater. Baltic herring (II) were caught in Helsinki with hook and line. The chemical predator cue from perch (I, III, 5.1 Laboratory experiments IV) was prepared by keeping the fi sh for about 6 h before the experiments in a 30- 5.1.1 Sampling 80 l container, where they were fed littoral 5.1.1.1 Littoral planktivores. All sampling mysids or stickleback larvae, according to was performed during the day near TZS in the experiment in which the chemical cue the littoral zone (depth 0–2 m). Neomysis was used. The visual predator cue from integer (I, III, IV) (mean length 16.0–18.2 perch (I, III, IV) was accomplished by mm) were caught with a hand net equipped placing a perch in an aquarium next to the with a long handle, or with a beach seine. experimental aquarium. Since herring were Praunus fl exuosus (I, IV) (mean length not available during the experiments aboard 23.6–24.1 mm) were caught with a hand net the R/V Aranda, the chemical predator cue that was pulled through Fucus vesiculosus from herring (II) was prepared beforehand vegetation. Three-spined stickleback larvae by keeping the fi sh in a 10 l container for (IV) (mean TL 9.1 mm) were caught with about 30 min immediately after catching. a hand net from a shallow lagoon. The The water containing the herring cue was animals were transported to the laboratory frozen, melted just before the experiments in ambient seawater. and sieved through a 100 µm net to remove impurities. 5.1.1.2 Pelagic planktivores. Pelagic mysids and Cercopagis pengoi (II) were 5.1.1.4 Aquatic macrophytes. All collected aboard the R/V Aranda. Mysis macrophytes studied as predation refuges mixta (mean length 12.8 mm) were for littoral planktivores were collected near collected from stations in the Bothnian TZS with a rake. Chara tomentosa (I, IV), Sea, and M. relicta (mean length 15.9 mm) Myriophyllum spicatum and M. sibiricum from stations in the Bothnian Sea and the (IV) were collected from shallow bays and Bothnian Bay. The mysids were caught Fucus vesiculosus (I) from rocky littoral. at night using a plankton net (mesh size The artifi cial vegetation, used as a control, 0.5 mm, diameter 0.8 m and length 3 m) consisted of green plastic strings attached with a cod end, which was lowered near to metal plates. 25

5.1.1.5 Zooplankton and brine shrimp. cause the eyes of Mysis spp. are not used to Zooplankton, which were used as prey in the light. To study the effect of predator pres- experiments in paper II, were collected from ence on prey selection, we used a chemical various stations around the northern Baltic predator cue from herring. The incubations Sea while aboard the R/V Aranda, from 30 were terminated after 2 h (with mysids) or m to the surface with a WP-2 plankton net 12 h (with C. pengoi). (100 μm). For the experiments in papers III In the natural prey experiments (II), in- and IV, zooplankton were collected with a gestion rates and prey selection of mysids plankton net (100 µm) from about 20 m to (M. mixta and M. relicta) in natural zoo- the surface in an open sea area near TZS plankton assemblages was studied with (Storfjärden, 59º57’30’’ N, 23º16’ E, depth and without a chemical predator cue from 33 m). Brine shrimp (Artemia sp.), which herring. The experiments were performed were used as prey in experiments in papers aboard the R/V Aranda, and the experimen- I and III, were hatched from commercially tal procedures were the same as those in the available eggs at 26 ºC in 20-psu water with two-prey experiments. After 2 h of incuba- strong aeration. tion, the mysids were preserved in buffered 4% formalin. The stomach contents of pre- served mysids were examined, compared 5.1.2 Feeding experiments with the zooplankton assemblage offered, Feeding experiments were conducted with and prey selection was estimated by calcu- the littoral mysids Neomysis integer (I, III) lating selection indices. and Praunus fl exuosus (I), with the pelagic The swarming experiments (III) were mysids Mysis mixta and M. relicta (II) and conducted at TZS to compare the effects the cladoceran Cercopagis pengoi (II). In of chemical and visual predator cues from addition, the feeding rate of the stickleback perch on feeding rates of N. integer on brine larvae was determined in the video fi lming shrimp when feeding alone or in a group of experiments (IV). 20 individuals. The experiments with single The ingestion rate experiments (I) were mysids were performed in cubic 1 l aquaria performed to test the effects of visual and (width 10 cm × depth 10 cm × height 10 chemical predator cues from perch on litto- cm) and the experiments with 20 mysids in ral mysid (N. integer and P. fl exuosus) feed- rectangular 20 l aquaria (26 cm × 37 cm × ing rates on brine shrimp. The experiments 20 cm) at ambient water temperature (9–12 were conducted at TZS and performed in 2 l ºC) in normal indoor lighting conditions. aquaria at ambient water temperature (9–16 The experiments lasted about 1 h. ºC) under normal indoor lighting conditions. All feeding rates were calculated as prey Each set of experiments lasted about 3 h. per predator per hour. The two-prey experiments (II) were conducted aboard the R/V Aranda and per- 5.1.3 Video fi lming formed at 13 ºC in 1 l clear glass bottles with two prey types, evasive and noneva- The antipredator behaviour of the littoral sive, offered together to the planktivore, i.e. mysids Neomysis integer and Praunus M. mixta, M. relicta and C. pengoi. Experi- fl exuosus (I, IV) and stickleback larvae (IV) ments with mysids were conducted in the was studied by video fi lming at TZS. The dark and those with C. pengoi in light, be- effects of chemical and visual perch cues 26 and different aquatic macrophyte refuges predation refuges, their survival in different (Chara tomentosa, Fucus vesiculosus, types of macrophyte vegetation was Myriophyllum spicatum and artifi cial investigated in the mortality experiments vegetation) were studied. The experiments (IV). They were conducted at TZS in were conducted under conditions similar rectangular 2.2 l aquaria (width 15.1 to those in the ingestion rate experiments cm × depth 7.2 cm × height 20 cm) with described above, except for the M. spicatum Myriophyllum spicatum (N. integer, P. experiments with mysids (IV) that were fl exuosus, G. aculeatus), M. sibiricum (N. conducted in a larger cubic aquarium (20 integer, P. fl exuosus), Chara tomentosa (N. cm × 20 cm × 20 cm). The behaviour of integer), or without macrophytes (N. integer, the mysids was studied for 15 min and the P. fl exuosus, G. aculeatus), at 16 ºC under behaviour of sticklebacks for 20 min by normal indoor lighting conditions. The video fi lming. The time spent swimming experiments lasted for 3 h, after which the and hiding within the vegetation were survival of the animals was determined. determined from the fi lms, as well as the attack rate of sticklebacks on zooplankton. 5.2 Field studies 5.1.4 Swarming experiments 5.2.1 Field mortality experiments The swarming experiments (III) were undertaken to determine how predator cues The fi eld mortality experiments (un- from perch infl uence the swarming behaviour published) were undertaken to determine of Neomysis integer. The experiments were if Myriophyllum spicatum vegetation conducted at TZS and performed in a 150 l kills Neomysis integer individuals un- rectangular tank (width 100 cm × depth 50 der natural conditions, as it did in the cm × height 30 cm) with 100 l of water at laboratory experiments (IV, see below). The ambient water temperature (8–9 ºC) under experiments were conducted in September normal indoor lighting conditions. The 2004 in a shallow bay (Jovskärsfl adan) tank was divided into three compartments: near TZS. The mysids were collected with the middle part (width 40 cm × depth 50 a hand net from the littoral zone in the cm × height 30 cm) was the experimental evening preceding each experiment and arena, which was separated from both end transported immediately to the laboratory, compartments (each 30 cm × 50 cm × 30 where they were kept in 20 l containers in cm) by a transparent 200 µm net (cf. Fig. ambient seawater with aeration. The mysids 1 in III). The end compartments contained were not fed before the experiments. the stimuli (predator or mysid swarms). The The experiments were conducted in focal mysid was observed for 15 min and rectangular cages (height 100 × width 30 the total time spent within 6 cm of either net × depth 30 cm). The walls of the cages wall was recorded. were plankton nets (mesh size 500 µm) fi t- ted around a metal frame. The fl oors and the roofs of the cages were of cotton fabric 5.1.5 Mortality experiments equipped with opening and closing devices. In addition to studying how the littoral There were two M. spicatum treatments in planktivores use aquatic macrophytes as the fi eld experiments, the fi rst of which was 27 conducted with intact vegetation, because The incubation lasted about 20 h (intact the stems of M. spicatum had to be cut to vegetation 20.02 ± 0.07, broken stems 20.44 fi t the aquaria in the mortality experiments ± 0.12), after which the cages were lifted conducted in the laboratory (IV). With the up. The mysids were immediately counted aid of a self-contained underwater breath- and determined as dead or alive. The water ing apparatus (SCUBA) diver, the cages temperature in the bay ranged from 12.6 to were placed in dense M. spicatum patches 14.3 ºC and oxygen levels from 8.2 to 10.2 about 2 m apart and secured in an upright ppm during the experiments. The water position with the help of weights. Each cage level always reached the upper half of the was carefully placed around approx. eight cages, but at no time were the cages com- intact stems and the fl oor was closed tightly pletely submerged. around them to prevent mysids and preda- tors from exiting and entering the cages. 5.2.2 Distribution study The second M. spicatum treatment was con- ducted with naturally broken stems, which In the distribution study (unpublished), the could be abundantly found from the shore- occurrence and distribution of mysids were line of TZS. The cages, with their fl oors examined in relation to aquatic macrophyte closed, were placed on bare sediment as species composition. The mysids were described above. Eight broken stems were sampled with a beach seine from four added to each cage. In both treatments, 10 different shallow bays in the Ekenäs mysids were added to each cage through the Archipelago: Solbacksfl adan, Älgöfl adan, roof, which was then closed tightly. There Danskogsfl adan, and Åkernäsfl adan (Table were fi ve replicates in each treatment. 1) three times (May, June and July) during

Table 1. Characteristics of the fl ads. There are no data on the area of Danskogsfl adan, but it is approximately the same as in Åkernäsfl adan. The dominant vegetation is listed according to Munsterhjelm (1997) and Wallström et al. (2000).

Solbacks- Älgö- Danskogs- Åkernäs- fl adan fl adan fl adan fl adan N 59º54’01’’ N 59º52’20’’ N 59º53’87’’ N 59º53’39’’ Location E 23º21’25’’ E 23º22’15’’ E 23º23’25’’ E 23º28’36’’ Mean depth (m) 0.8 1.5 App. 1.5 0.6 Max. depth (m) 2.2 > 4.3 2.6 4.0 Area (km2) 0.13 0.6 – 0.16 Chara Myriophyllum Myriophyllum Chara tomentosa, spicatum, Dominant spicatum, tomentosa, C. baltica, Potamogeton vegetation Potamogeton Potamogeton Najas pectinatus spp. pectinatus marina Vaucheria May 8th May 8th May 8th May 8th Sampling dates June 11th June 12th June 11th June 12th July 9th July 11th July 9th July 11th 28 the summer of 2001. Five seine samples 5.3 Statistical analyses and equations were taken at each bay at each date. All the mysids in the samples were identifi ed to The data were tested for statistical species and counted. The water temperature signifi cance with parametric tests, if the was recorded and the salinity measured appropriate assumptions were fulfi lled, with a salinometer at each location (Fig. i.e. the normal distribution of the data 1a-b). The macrophyte species composition and homogeneity of variances. In some at each sample site was taken from the cases transformations (log(x+1) or arcsin) literature (Munsterhjelm 1997, Wallström were used to fulfi l these assumptions. The et al. 2000). parametric tests used were the t-test (I– III), one-way (III) and two-way (I, III, IV) analysis of variance (ANOVA), and three- way analysis of covariance (ANCOVA) (II). Otherwise nonparametric tests were used, i.e. the Mann-Whitney U-test (II–IV), Wilcoxon signed ranks test (III), Kruskal-Wallis one- way ANOVA (IV) and Fisher’s exact test 25 (II, IV, fi eld mortality experiments). Tukey a) HSD multiple comparisons was used as the 20 posthoc test (IV). The chi-squared-based selectivity index

15 C (II) was calculated using Yates’ correc- tion for continuity (Pearre 1982). The C

10 index was chosen because it is zero-val- temperature ºC ued for no selection, is symmetrical (rang- Älgö 5 ing between –1 and +1), is not sensitive to Danskog Åkernäs rare prey species and is statistically testable Solbacks 0 (Pearre 1982). 6 b)

5 2 1/2 C = ± (χ y/n) or 2 1/2 C = ± [(|adbe-bdae| – n/2) / abde] 4 (1) where 3

salinity psu salinity Species 2 A Others Total

1 Diet ad bd ad + bd = d

0 Environ- a b a + b = e May June July e e e e ment Figure 1. Water temperature (ºC) and salinity (psu) Total a + a = a b + b = b a + a + b + b = n in each bay during the fi eld sampling period. d e d e d e d e 29

6. RESULTS AND DISCUSSION ators: chemical cues are relatively more important for pelagic than littoral mysids, 6.1 Predator detection and the ability of pelagic mysids to detect predators by chemical cues alone is impor- All the planktivores studied detected tant for their survival. predators by the emitted cues. Changes Although the sticklebacks used in these in feeding rate were used as a measure of experiments were immature, their predator predator detection (Magurran 1990, Jachner detection abilities (IV) were more precise 1997, Ejdung 1998). Hamrén & Hansson than that of adult littoral mysids (I), which (1999) found that Mysis mixta decreases occupy the same habitat. This difference its feeding rate in response to the odour of is derived most likely from the better-de- Baltic herring, indicating that the mysid veloped sensory system of vertebrates can detect its predator by chemical cues. compared with invertebrates. The sensory The experiments reported in this thesis systems of larval fi sh are not all functional confi rmed this fi nding for M. mixta (II), and throughout posthatching development (Bai- it also held for M. relicta (II), the cladoceran ley & Houde 1989), and larger larvae are Cercopagis pengoi (II), and three-spined better able to judge the magnitude of threats stickleback larvae (IV). In contrast, the (Fuiman 1993). Thus, the predator detec- littoral mysids Neomysis integer and tion abilities of stickleback larvae continue Praunus fl exuosus did not respond to either to develop as they grow and interindividual chemical or visual predator cues alone, but variation in this development plays a ma- when combined these predator cues resulted jor role in their survival. Growth rate alone in a reduction in their feeding rate (I). does not explain the differences in early life The differences in predator detection mortality, but it is coupled with the devel- abilities of pelagic and littoral mysids may opment of sensory and locomotory perfor- be explained by the different habitats they mance (Fuiman et al. 2005). There may be live in. Mysids have well-developed com- signifi cant differences in fi sh larvae of the pound eyes and are known to use vision in same species and size in their predator rec- various situations (Fulton 1982, Lindström ognition and avoidance abilities (Vilhunen 2000). However, pelagic mysids live in near & Hirvonen 2003, Fuiman et al. 2005). darkness, remaining near the bottom during The response of stickleback larvae was day and rising up only at night (Salemaa also stronger to combined chemical and et al. 1986, Rudstam et al. 1989, Hansson visual cues than to the chemical cue alone et al. 1990, Rudstam & Hansson 1990), and (IV). Similar results were obtained previ- probably have little use for vision. Littoral ously with both stickleback and northern mysids, in contrast, can utilize vision in the pike Esox lucius L. 1758 larvae (Lehtiniemi well-lit shallow water they inhabit (Lind- 2005, Lehtiniemi et al. 2005). Stickleback ström 1992). Both N. integer and P. fl exuo- larvae are clearly capable of threat-sensi- sus are well adapted to the light regime in tive behaviour (Bishop & Brown 1992), i.e. aquatic vegetation, where the chlorophyll they interpret two simultaneous predator of macroalgae and vascular plants absorbs cues as a more convincing threat of preda- longer wavelengths (Lindström 2000). tion. For larval newts Notophthalmus viri- Thus, the differences in habitats apparently descens (Rafi nesque 1820), visual predator affect the way in which mysids detect pred- cues alone carry only general information 30 on danger, and chemical cues are needed to Cercopagis pengoi has a large compound fully evaluate the threat (Mathis & Vincent eye (Grigorovich et al. 2000), but its visual 2000). For fi shes, the situation is reversed capabilities, e.g. for image forming, are and visual cues are more important than unknown. It is likely that chemical and hy- chemical (Hartman & Abrahams 2000, dromechanical cues are the most important Chivers et al. 2001b). Vision is also the pre- for C. pengoi in detection of both prey and dominant sensory modality of sticklebacks predators. A general fi sh avoidance strategy and they have good colour and form vi- as well as acquiring species-specifi c preda- sion (Wootton 1984). Thus, visual predator tor detection would both favour the inva- cues combined with chemical cues result in sion and establishing of viable populations stronger antipredator behaviour (IV). to new areas and thus be a major advantage Cercopagis pengoi invaded the Baltic for C. pengoi. Sea only 14 years ago (Ojaveer & Lumberg 1995); before that time it was not exposed to Baltic herring. Therefore, its ability to 6.2. Decreased activity detect herring by chemical cues is quite re- markable and most probably a general strat- The responses of the planktivores studied to egy to avoid any fi sh predator, because C. predator cues were numerous and variable pengoi is prey for many fi sh species in the (Table 2). However, a feature common to Baltic and elsewhere (Antsulevich & Väli- all of the species was that they decreased pakka 2000, Charlebois et al. 2001, Bush- their feeding rate in response to predation noe et al. 2003, Gorokhova et al. 2004, threat (I–IV), i.e. exhibited an inducible Ojaveer et al. 2004, Peltonen et al. 2004). antipredator defence (Sih 1987). Decreased

Table 2. Antipredator behaviours of Baltic planktivores (I–IV).

Neomysis Praunus Mysis Mysis Cercopagis Gasterosteus Study object integer fl exuosus mixta relicta pengoi aculeatus Chemical Chemical Cues needed for Chemical Chemical Chemical Chemical, + visual + visual predator detection (II) (II) (II) visual (IV) (I, IV) (I, IV) Feeding Decreased Decreased Decreased Decreased Decreased Decreased rate (I, III) (I) (II) (II) (II) (IV) Prey Decreased Decreased Increased –– – selection (II) (II) (II) Responses Swimming Decreased No change Decreased to predator –– – activity (I, IV) (I, IV) (IV) cues No change Swarming –––– – (III) No change Increased Increased Hiding –– – (I, IV) (I, IV) (IV) 31 feeding activity may refl ect the increased 1999, but see Torgersen 2003). Prey may vigilance of the individuals, which remain also limit their activity to places or times at alert for possible attacks by the predator which predators are inactive, i.e. use spatial detected, and hence focusing less on locating or temporal refuges (Lima 1998b, Tsuda and capturing food items. Reduced feeding et al. 1998, Torgersen 2003, Takahashi et as an antipredator behaviour involves a al. 2004). For example, planktonic animals clear trade-off, i.e. reduced energy intake, with full guts are visually more conspicu- which can have a fundamental impact on ous, and hence they may limit their feeding individual fi tness (Tiselius et al. 1993, van activity to nighttime (Ohman 1988, Cieri & Duren & Videler 1996, Tsuda et al. 1998, Stearns 1999). Hölker & Stief 2005, Lehtiniemi 2005). In addition to antipredator benefi ts, Continuous, effi cient feeding is especially swarming may increase feeding (Foster important to larval fi sh, which need to grow 1985, Clark & Mangel 1986, Ryer & Olla rapidly to be able to survive the following 1992, Ritz 1994, Milne et al. 2005) and winter and to outgrow predators (Adams mating effi ciency (Clutter 1969, O’Brien & DeAngelis 1987, Bailey & Houde 1989, 1988, Ritz 1994, Ribes et al. 1996) and Pedersen 1997). bring advantages in energy use (Hough & Neomysis integer (I, IV) and stickleback Naylor 1992, Ritz 2000). When N. integer larvae (IV) also decreased their swimming were feeding in a swarm, their feeding rate activity in response to predation threat, was not signifi cantly decreased in the pres- whereas Praunus fl exuosus did not (I, IV). ence of a predator, as when the mysids were Praunus fl exuosus is closely associated feeding alone (III). This may be attributed to with aquatic vegetation and relies primar- collective vigilance, i.e. the individual that ily on hiding in its predator avoidance be- has detected the predator informs the oth- haviour (I, IV). Hiding may decrease the ers in the group, which enables individuals need for the prey to reduce its activity in to spend less time being alert for predators the presence of predators, because hid- and allocate more time to feeding (Clutter ing decreases the abilities of predators to 1969, Clark & Mangel 1986, Lima & Dill detect prey (Diehl & Kornijów 1997). On 1990, Magurran 1990, Ritz 1994). Group the other hand, reduction in activity may in size, position of the individual within the some cases result from the increased use of group and group composition are impor- a refuge (Lima & Dill 1990, Lima 1998a,b, tant factors in vigilance (Lima & Dill 1990, Laurel & Brown 2006), which may have Peuhkuri 1997, Lima 1998b). been the case for stickleback larvae (IV). Decreased swimming may be, as an alterna- tive to hiding, a way to avoid being detected 6.3 Prey selection by the approaching predator (Lima 1998b), especially when the predator locates prey Reduced feeding activity can involve prey primarily through vision or the lateral line. selection: the prey switches its own diet Decreased activity is especially useful in towards food items that are easier to detect, environments without physical structure, catch and ingest, which reduces the effort where movement is easily perceived visu- required in swimming activity and enables ally (O’Brien 1987, Tiselius et al. 1993, alertness (Lima & Dill 1990, Lima 1998b, Kiørboe & Visser 1999, Kiørboe et al. Mikheev et al. 2002, Torgersen 2003), or 32 towards food items that, once ingested, do contains more energy. However, the strong not make them more detectable by predators fl ight reaction of copepodites (Browman et (Dill 1987). In addition to decreased feeding al. 1989, Viitasalo & Rautio 1998, Viitasalo rate, the pelagic mysids Mysis mixta and M. et al. 1998) may explain why M. mixta se- relicta as well as the cladoceran Cercopagis lected the nonevasive prey. In the two-prey pengoi altered their prey selection patterns experiments, M. relicta did not exhibit sig- in response to predation risk (II). nifi cant prey selection in either predator In the absence of predator cues, C. pen- treatment, although it consumed the more goi did not select for either prey type, eva- evasive prey, copepodites, in the absence of sive or nonevasive, and consumed them predator cues (II). at similar rates, whereas when predator In contrast, both mysid species selected cues were present, it consumed more non- cladocerans in the absence of predator cues evasive than evasive prey, signifi cantly se- when they fed on a natural zooplankton as- lecting for the nonevasive prey items (II). semblage, but exhibited no signifi cant se- The evasive prey, i.e. copepod nauplii, are lection in their presence (II). Despite the energetically more profi table than the non- ability of mysids to switch between rap- evasive prey, i.e. rotifers (Flinkman et al. torial and suspension feeding modes in 1998, Koski et al. 1999, Viherluoto & Vii- response to prey availability (Viitasalo & tasalo 2001a, Pertola et al. 2002). Thus C. Rautio 1998), M. mixta and M. relicta ex- pengoi switched to prey items that were hibited no switching behaviour in response easier to catch when under predation risk, to predation risk. However, they abandoned although it would have gained more energy their selective feeding when predator cues from the evasive prey. This gives the plank- where present, indicating that they were tivore an opportunity to continue feeding being more vigilant towards predators and and still maintain vigilance for predators, focusing less on feeding, as was C. pengoi. while remaining relatively inconspicuous Both increases, as in C. pengoi, and de- (Lima & Dill 1990, Lima 1998b, Mikheev creases, as in Mysis spp., in food selectiv- et al. 2002, Torgersen 2003), but includes a ity in response to predation risk have been trade-off in terms of energy gain. reported in vertebrates (Lima & Dill 1990, The behaviour of the mysids differed Lima 1998b). somewhat between the two types of ex- In addition to the abundance of different periment (II). In the experiments in which prey items and predation pressure, the diet the mysids were offered two types of prey, of an organism in the fi eld is partly deter- evasive and nonevasive, M. mixta selected mined by its ability to detect an item from the nonevasive prey regardless of preda- a distance (Gerritsen & Strickler 1977, Vi- tor presence or absence (II). This is some- herluoto & Viitasalo 2001a, Weissburg et what surprising, since the nonevasive prey, al. 2002). Mysids utilize mainly mecha- the rotifers, contain much less energy than noreception to locate their prey (Cooper & the evasive prey, Eurytemora affi nis Poppe Goldman 1980, Viitasalo et al. 1998, Viher- (1880) copepodites (Flinkman et al. 1998, luoto & Viitasalo 2001b), whereas the prey Koski et al. 1999, Viherluoto & Viitasalo detection modes of C. pengoi are unknown. 2001a, Pertola et al. 2002). Thus, in the Another predatory cladoceran, Leptodora absence of predator cues, M. mixta would kindti (Focke 1884) (Haplopoda), initiates have been expected to select the prey that an attack only upon direct contact with po- 33 tential prey (Browman et al. 1989). More the initial size of the aggregation increases, knowledge of the basic biology of C. pen- relatively fewer individuals will become goi is needed to further examine its feeding separated from the main group. The avoid- behaviour. ance of perch by N. integer individuals was stronger when the swarm was present (III), which indicates that stragglers, i.e. 6.4 Swarming individuals separated from a swarm, are at greatest risk of being preyed upon (Parrish The aggregative behaviour of mysids has 1989, Lima & Dill 1990, Magurran 1990, been studied extensively throughout the Ritz 1994, Byholm 1998). Solitary N. in- world (e.g. Steven 1961, Clutter 1969, teger individuals utilize decreased activity Mauchline 1971c, Zelickman 1974, (I) and fl ight (Rademacher & Kils 1996) to Dadswell 1975, O’Brien 1988, Carleton avoid predation. & Hamner 1989, Modlin 1990, Ohtsuka et In addition to protection from predators, al. 1995, Buskey 2000). However, studies swarming also brings energetic (Hough & from the Baltic Sea and of Neomysis integer Naylor 1992, Ritz 2000) and feeding ben- are lacking (but see Byholm 1998). In the efi ts (I) to N. integer. The ingestion rates current study (III), N. integer individuals were consistently higher, despite predator actively tried to join swarms. Swarming presence or absence, when the mysids were tendency was strong both in the presence feeding in a swarm of 20 individuals than and absence of a predator (III), indicating when alone (I). This can result from more that it is a fi xed strategy (Sih 1987) and not effi cient location of patchily distributed an inducible defence. Fixed defences are food or social facilitation (Clark & Mangel favourable when acquiring information on 1986, Ryer & Olla 1992, Ranta et al. 1993, local predation risk is dangerous, or when Ritz 1994). As group size increases, how- local predation risk is consistently high ever, interference and competition between (Sih 1987). This holds for N. integer, which group members become more intense inhabits shallow littoral zones together (Clark & Mangel 1986, Lima & Dill 1990, with a wide array of predators (Aarnio & Ranta et al. 1993, Ritz 1994). Thus, the Bonsdorff 1993, Thiel 1996, Hostens & feeding advantage observed in this study Mees 1999, Granqvist & Mattila 2004). with a relatively small swarm may not be Similarly, N. integer individuals pre- relevant in natural surroundings, where the ferred the larger swarm regardless of densities of N. integer may reach 600 indi- predator presence (III). The predation risk viduals per m2 in the Baltic (Debus et al. of an individual declines as the group size 1992, Thiel 1992) or even higher in brack- increases (Major 1978, Turner & Pitcher ish lakes (Aaser et al. 1995, Søndergaard et 1986, Ritz 1994, Byholm 1998, Foster et al. 2000) and estuaries (Fockedey 2005). al. 2001). Coordinated avoidance or escape The fi xed swarming behaviour of N. in- responses are easier to perform when the teger resembles that of other mysid species swarm is large (O’Brien & Ritz 1988). The (Clutter 1969, Mauchline 1971c, Zelickman predator often aims to break up the cohe- 1974, O’Brien 1988, Buskey 2000). Some sive prey aggregation (Zelickman 1974, other small aquatic animals exhibit more Major 1978, Magurran & Pitcher 1987, fl exible swarming behaviour: grass shrimps Parrish 1989, Foster et al. 2001), and as Palaemonetes pugio Holthuis 1949 and P. 34 vulgaris (Say 1818) increase grouping be- also exhibit variable behavioural defenc- haviour as well as the number of individu- es: in addition to hiding (IV), they utilize als in a group under predation risk (Carson reduced swimming and feeding activity & Merchant 2005), and juvenile Atlantic (Wootton 1984, Bishop & Brown 1992, cod Gadus morhua L. 1758 switch from an Godin & Crossman 1994, Lehtiniemi avoidance and hiding strategy to a school- 2005, IV), schooling, both as young and ing strategy, depending on habitat, predator adults (Keenleyside 1955, Wootton 1984, type and population density (Laurel et al. van Havre & FitzGerald 1988, Peuhkuri 2004, Laurel & Brown 2006). However, N. 1997), predator inspection (Wootton 1984, integer individuals are clearly capable of Bishop & Brown 1992, Godin & Crossman observing their social situation, i.e. solitary 1994) and fl ight (Wootton 1984, Bishop & occurrence vs. swarming, as well as ambi- Brown 1992). Different defences militate ent predation risk, and adapting their be- best against different predators, e.g. the haviour accordingly (III). perch is a superior forager in dense littoral vegetation compared with roach and bream Abramis brama L. 1758 (Diehl 1988). 6.5 Hiding Stickleback larvae probably balance their antipredator behaviours according to the Hiding decreases the possibility of being predator in question, available habitat, and detected and recognized by predators developmental stage, i.e. capabilities of (Ohman 1988, Lima & Dill 1990, Jeppesen each individual, in an adaptive manner. For et al. 1997). Praunus fl exuosus (I, IV) and example, stickleback schools do not hide stickleback larvae (IV) increased their use in vegetation as an antipredator response, of the vegetation refuge in response to the as stickleback do when alone (Keenleyside presence of predator cues, but Neomysis 1955). Swarming is a less effi cient strategy integer did not (I, IV). Instead, N. integer in habitats with aquatic vegetation (Savino utilizes other antipredator behaviours, e.g. & Stein 1982, Flynn & Ritz 1999, Laurel swarming (Mauchline 1971c, Byholm & Brown 2006), under conditions of poor 1998, Fockedey 2005, III), decreased visibility, e.g. night or turbidity (Brock & swimming (I, IV) and feeding (I, III) Riffenburgh 1960, Zelickman 1974, Twin- activity, and fl ight (Rademacher & Kils ing et al. 2000) and against predators that 1996). Praunus fl exuosus spends its entire utilize ambush tactics (Flynn & Ritz 1999, life in contact with aquatic vegetation Laurel & Brown 2006), schooling (Ma- (Kotta & Kotta 1999); thus hiding seems jor 1978, Ritz 1994, Foster et al. 2001) or a natural antipredator strategy for this sensory modalities other than vision to lo- species, although not a fi xed strategy (Sih cate their prey (Magurran 1990, Ritz 1994, 1987), since it is induced by predator cues. Sørnes & Aksnes 2004). Three-spined sticklebacks occupy a There are also qualitative differences wide range of different habitats (Woot- between refugia: aquatic vegetation may ton 1984) and exhibit a similarly wide be structurally and chemically very dif- selection of antipredator defences. They ferent, depending on the macrophyte spe- have morphological defences, e.g. lateral cies that form the community (Coen et al. plates, dorsal spines and cryptic coloration 1981, Ryer 1988, Lauridsen & Lodge 1996, (Wootton 1984, Reimchen 1994). They Warfe & Barmuta 2004, Lehtiniemi 2005) 35 and the density of vegetation also plays a The presence of M. spicatum also de- signifi cant role (Vince et al. 1976, Savino creased the feeding rate of stickleback lar- & Stein 1982, Gotceitas & Colgan 1987, vae (IV). Almost no attacks occurred with- Stansfi eld et al. 1997, Snickars et al. 2004, in M. spicatum vegetation. The complex but see Warfe & Barmuta 2004). The mac- morphology of M. spicatum may lower the rophyte species were utilized as a refuge feeding effi ciency of fi sh (Coull & Wells differently, depending on the planktivore 1983), but since the feeding of stickleback in question (I, IV) (Table 3). Praunus fl ex- larvae is unaffected by similarly structured uosus preferred Fucus vesiculosus vegeta- C. tomentosa (Lehtiniemi 2005), the chem- tion (I), and also sought refuge in Chara ical properties of M. spicatum are likely to tomentosa (I) as well as in Myriophyllum be more important (see below). spicatum vegetation (IV). In contrast, stick- In pelagic environments where hiding leback larvae strongly avoided M. spicatum within physical structures is impossible, in the absence of predator cues and, in the other hiding mechanisms must be used. presence of predator cues, utilized it as a Crypsis, including mimicry of immobile refuge less than artifi cial vegetation (IV). objects and camoufl age coloration, is often Neomysis integer spent signifi cantly less combined with decreased activity (Wootton time within M. spicatum vegetation than in 1984, Sih 1987, Sandström 1999, Merilaita artifi cial vegetation (IV). In addition to the 2001). Many planktonic organisms, e.g. planktivores studied, M. spicatum is repel- crustaceans and larval fi sh, utilize transpar- lent to cladocerans (Pennak 1973, Laurid- ency (Stemberger & Gilbert 1987, Bailey sen & Lodge 1996). & Houde 1989, Langsdale 1993, Thet-

Table 3. Effects of aquatic macrophytes on Baltic littoral planktivores (I, IV, fi eld mortality experiments, and distribution study).

Fucus Chara Myriophyllum Myriophyllum A. Lethality vesiculosus tomentosa spicatum sibiricum Neomysis Laboratory – 0% (IV) 89% (IV) 0% integer Field ––2–4% – Praunus Laboratory ––73% (IV) 0% (IV) fl exuosus Gasterosteus Laboratory ––0% (IV) – aculeatus B. Repellence Neomysis Laboratory No (I) No (I) Yes (IV) – integer Field –NoYes – Praunus Laboratory No (I) No (I) No (IV) – fl exuosus Gasterosteus Laboratory – – Yes (IV) – aculeatus 36 meyer & Kils 1995, Tsuda et al. 1998) and Singleton & Kratzer 1969). However, M. small size (O’Brien 1987, Tiselius et al. sibiricum, albeit belonging to the same 1993, Tsuda et al. 1998, Kiørboe & Visser genus as M. spicatum and having a high 1999, Sakwinska 2002) to avoid detection phenolic acid content (Spencer & Ksand- by predators. DVM, performed by pelagic er 1999), induced no mortality in mysids mysids (Salemaa et al. 1986, Rudstam et (IV). Therefore the mechanism that caused al. 1989, Hansson et al. 1990, Rudstam & the high mortality in mysids remains un- Hansson 1990), can be seen as use of a tem- known. Myriophyllum spicatum is also poral refuge. known to produce alkaloids (Ostrofsky & Zettler 1986) and fatty acids (Nakai et al. 2005) that may play a role. 6.6 Effects of aquatic vegetation Myriophyllum spicatum releases sever- al polyphenols into the surrounding water 6.6.1 Mortality (Gross & Sütfeld 1994), but tissue damage may facilitate the production and excretion Myriophyllum spicatum was acutely lethal of chemicals from aquatic macrophytes to littoral mysids in the laboratory in dense (Levin 1971, Cronin & Hay 1996). In the patches (IV). The mortality of Neomysis mortality experiments conducted in the lab- integer was 89% and that of Praunus oratory (IV), the stems of M. spicatum had fl exuosus 73%. This is the fi rst time a larger to be cut to fi t the aquaria, which probably aquatic invertebrate was shown to be killed accelerated the release of chemicals. How- by submerged macrophytes, although the ever, autofragmentation of M. spicatum negative effects of M. spicatum on the plants typically occurs after fl owering pe- growth and survival of mosquito and midge riods, as does damage by waves and human larvae were reported (Dhillon et al. 1982, activities such as mechanical weed harvest- Johnson & Mulla 1983). The surviving ing and boating traffi c (Grace & Wetzel mysids were also in weak condition, turning 1978, Aiken et al. 1979, Nichols & Shaw from transparent to opaque (IV). In the 1986, Smith & Barko 1990). Damaged M. control (fi ltered seawater), M. sibiricum and spicatum stems commonly occur under Chara tomentosa treatments no mortality natural conditions; therefore we conducted of mysids was observed (IV). Three-spined fi eld mortality experiments with both intact stickleback larvae also experienced no and cut stems of M. spicatum. mortality in the experiments (IV). The mortality of mysids was much low- I suggest that the mortality may have er in the fi eld than in the laboratory, both been caused by polyphenols excreted by in treatments with intact and with broken M. spicatum (Planas et al. 1981, Gross & stems. One out of 50 mysids (2%) was found Sütfeld 1994, Gross et al. 1996, Nakai et dead in the intact vegetation treatment and al. 2000, Gross 2003). Possible mecha- two (4%) in the broken stem treatment. nisms behind the toxicity and repellence The difference in mortality rates between of polyphenols to both invertebrates and the two treatments was not statistically sig- vertebrates are numerous, such as mem- nifi cant (Fisher’s exact test: p = 0.5080). brane-damaging activity or interference A total of fi ve mysids were not found after with catecholamine metabolism and other the experiments; these were probably lost neural control mechanisms (reviewed by in the recovery process. 37

The difference in the results of the labo- may represent unfavourable physical con- ratory and fi eld mortality experiments may ditions for mysids (Köhn 1992, Fockedey be explained by different dilution volumes 2005). present in the aquaria and in the bay. Al- Älgöfl adan is a relatively open bay and though the bay where the fi eld experiment the vegetation is quite variable (Munster- was conducted is shallow, small, sheltered hjelm 1997, Wallström et al 2000). Several and has a low water exchange rate, the con- macrophyte species are found at the site centration of toxins released from M. spi- where the mysids were sampled: M. spi- catum vegetation is still unlikely to rise as catum, M. sibiricum, rigid hornwort Cer- high as in the aquaria where the laboratory atophyllum demersum L., fennel pondweed experiments were conducted. Thus, M. spi- Potamogeton pectinatus L. and fan-leaved catum vegetation may not cause mortality water-crowfoot Ranunculus circinatus in mysids in the fi eld as in the laboratory, Sibth. (Wallström et al. 2000). Neomysis or the mortality may be much lower, as ob- integer individuals were found in Älgöfl a- served in the fi eld experiments. However, dan only in May (Fig. 2). Macrophytes are taking into account the life span of N. in- not yet fully developed by that time and the teger, mortality of even 2% per day can later absence of mysids in the bay may be accumulate to signifi cant mortality on the a sign of avoidance of the predominating population level. M. spicatum vegetation. Älgöfl adan had the lowest temperature and highest salin- ity of the bays studied throughout the study 6.6.2 Distribution period (Fig. 1a-b), and hence these factors should not limit the occurrence of mysids. Under natural conditions mysids are able In addition to N. integer, one Praunus fl ex- to migrate away from toxic or otherwise uosus individual was found in Älgöfl adan unfavourable vegetation. In laboratory in May and one in June. experiments (IV), Neomysis integer avoided Myriophyllum spicatum compared with artifi cial vegetation. The distribution of mysids in areas with different vegetation can give insight into the repellent effects 300 May of M. spicatum under natural conditions, June July although the varying physical conditions 150 in different locations naturally also play a role. The hypothesis was that fewer mysids -1 would be found at sites where M. spicatum 100

dominates the macrophyte community. # ind.sample

The vegetation in Solbacksfl adan is 50 dominated by Chara tomentosa (Wallström et al. 2000). No mysids were found in Sol- backsfl adan at any sampling time. During 0 Älgö Danskog Åkernäs the entire sampling period, the water in Solbacksfl adan was warmer and less saline Figure 2. Number of Neomysis integer individuals than in the other bays (Fig. 1a-b), which (mean + SE) in fi eld samples in each bay. 38

Most mysids were found in Danskogsfl a- Trin. ex Steud. occurs at both sites (Mun- dan and Åkernäsfl adan, especially in June sterhjelm 1997). In June, when the highest (Fig. 2). Most of Danskogfl adan is covered abundance of mysids was recorded, most by Vaucheria cf. dichotoma (L.) Martius and mysids occurred at the Ruppia-Potamogeton M. spicatum occurs only in the opening and site (Fig. 3a), which could indicate avoid- immediately inside it (Munsterhjelm 1997). ance of C. demersum. Three-spined stickle- Mysids were sampled at two locations; one back larvae avoid C. demersum vegetation dominated by C. demersum and one by compared with C. tomentosa, even in the beaked tasselweed Ruppia maritima L. and presence of cues from a predator (Lehtinie- P. pectinatus L. (Munsterhjelm 1997). Some mi 2005). This repellence is probably chem- common reed Phragmites australis (Cav.) ically induced: C. demersum is repellent to a freshwater snail (Sterry et al. 1983) and it has an inhibitory effect on several phyto- plankton species (Nakai et al. 1999, Körner 800 a) & Nicklisch 2002, Gross 2003, Gross et al. 2003) and garden cress Lepidium sativum L.

400 (Kleiven & Szczepanska 1988). It also pro- duces alkaloids (Ostrofsky & Zettler 1986) -1 300 that may serve as chemical deterrents, pos- sibly also to mysids or their food.

# ind. sample 200 In Åkernäsfl adan mysids were sampled at three locations: one dominated by C. to-

100 May mentosa, one by C. demersum and one by June M. spicatum; P. pectinatus also occurs at July 0 each site (Wallström et al. 2000). Again the C. demersum R. maritima, P. pectinatus 400 highest abundance of mysids was recorded b) in June, and most mysids occurred at the Chara-dominated site (Fig. 3b). These re- sults lend support to the hypothesis that 200 -1 mysids do indeed avoid M. spicatum and C. demersum and prefer C. tomentosa veg- etation as a habitat, as found in the labo- # ind. sample# ind. 100 ratory experiments (I, IV) as well as with stickleback larvae (IV, Lehtiniemi 2005). Thus, even if the direct mortality of mysids

0 caused by M. spicatum is quite low in the C. tomentosa C. demersum M. spicatum fi eld, macrophyte species composition can have a strong effect on the distribution of Figure 3. Number of Neomysis integer individuals (mean + SE) in a) Danskogfl adan at two sampling mysids, and most likely on other organ- locations, dominated by Ceratophyllum demersum isms, such as fi sh larvae, as well (Keast and Ruppia maritima/Potamogeton pectinatus, and 1984, Carpenter & Lodge 1986). b) Åkernäsfl adan at three sampling locations, domi- Very high (> 9) and stable pH values nated by Chara tomentosa, Ceratophyllum demer- have been found in M. spicatum vegeta- sum and Myriophyllum spicatum. tion both in the laboratory and in the fi eld 39

(Eicher 1946, Halstead & Tash 1982). This behavioural traits, mysids and Cercopagis probably gives M. spicatum a competitive pengoi hide from predators by being trans- advantage for carbon over phytoplankton, parent, and stickleback larvae by their cryp- since it is able to utilize HCO3- ions at el- tic coloration. Sticklebacks and C. pengoi evated pH (Grace & Wetzel 1978), but it also gain protection by their morphology, also has a major impact on aquatic animals, and Neomysis integer by their swarming since few species, especially very few fi sh behaviour. Regardless of habitat, however, species, have adapted to tolerate high lev- for all of the planktivores studied, reduced els of pH (Trama 1954, McCarraher 1971, feeding activity was a common antipreda- Daye & Garside 1975, Witschi & Ziebell tor defence. The pelagic planktivores fur- 1979, Wootton 1984). Unfortunately there thermore altered their prey selectivity pat- are no data available on the pH of the bays terns in response to predation risk. Each of studied, but pH may have a signifi cant im- the Baltic planktivore species studied thus pact on the distribution of both mysids and manifests a unique set of antipredator traits larval fi sh in vegetated areas. that operate in combination to decrease its predation risk. The actual benefi t of these traits, measured as reduced predation mor- 7. CONCLUSIONS tality, should be studied further. The Baltic Sea is under heavy anthro- The ability to detect predation risk and to pogenic infl uence. This can have major in- modify behaviour accordingly to avoid fl uence on the species composition of the being consumed is critically important to aquatic macrophyte communities (Jeppesen small aquatic animals. The main fi ndings et al. 1997). Chara tomentosa suffers from of my thesis were the variable and fl exible increased turbidity caused by boating activ- behavioural responses of the planktivores ities (Eriksson et al. 2004), eutrophication to predator cues (Table 2), which indicate (Dahlgren & Kautsky 2004) and dredging that they have adapted to different predator and is currently declining in many areas regimes and thus employ different (Munsterhjelm 1997, 2005, Koistinen & antipredator strategies. The differences Munsterhjelm 2001, Schubert & Yousef between the pelagic and littoral habitats 2001). Fucus vesiculosus, the most impor- are profound and are refl ected in the tant habitat for P. fl exuosus, is also declin- predator detection modes and antipredator ing in many areas due to eutrophication behaviours exhibited by the planktivores. (Kangas et al. 1982, Kautsky et al. 1986, Littoral mysids live in well-lit shal- 1992, Vogt & Schramm 1991, Hemmi 2003, low waters and are thus able to use visual Berger et al. 2004, Roos et al. 2004). How- cues in predator detection, whereas pelagic ever, it may benefi t from ferryboat traffi c mysids, which rarely have much ambient due to the mechanical disturbance of the light, must rely on chemical cues of preda- propellers, which cleans the substrate neces- tion risk. In the littoral, Praunus fl exuosus sary for successful recruitment (Eriksson & and three-spined stickleback larvae utilize Johansson 2003, Eriksson et al. 2004, Roos aquatic macrophyte vegetation as a preda- et al. 2004). On the other hand, Myriophyl- tion refuge, whereas pelagic mysids, which lum spicatum and Ceratophyllum demer- lack a structural refuge, utilize a temporal sum, species that are unfavourable habitats one by undergoing DVM. In addition to for littoral mysids and larval fi sh (IV, Lehti- 40 niemi 2005), benefi t from the turbid con- 8. ACKNOWLEDGEMENTS ditions caused by eutrophication, dredging and traffi c (Nichols & Shaw 1986, Smith & There’s one person without whom this thesis would Barko 1990, Munsterhjelm 1997, Dahlgren never have been fi nished: my supervisor Maiju Leh- & Kautsky 2004, Eriksson et al. 2004, Roos tiniemi. From the very beginning, she took me un- et al. 2004). These two species are also less der her wing and guided me through this process. I susceptible to mechanical damage by waves can’t thank Maiju enough for all the help and sup- and boats than Potamogeton spp. (Eriksson port, not only at work but also on other areas of life. et al. 2004, Schutten et al. 2004). The in- During very diffi cult times, she was there, listening, creased dominance of macrophyte species understanding, and encouraging me, never loosing that are unsuitable predation refuges may her faith in me. I enjoyed very much working side substantially increase the predation-related by side with her in Tvärminne and aboard Aranda, not to mention the wonderful sunny day on the para- mortality of littoral planktivores and thus dise island in Spain and other memorable times at have important consequences for the entire conference trips! littoral food web. I also want to thank my second supervisor Turbidity as such has major effects on Markku Viitasalo, who welcomed me in the project the predation vulnerability (Miner & Stein with open arms and has been looking out for me 1996, Utne-Palm 2002, De Robertis et al. ever since. Make also included me in his other 2003, Granqvist & Mattila 2004, Johnsen projects, and thus gave me an opportunity to work 2005) and antipredator behaviour (Hartman with topics other than my own thesis, which was & Abrahams 2000, Snickars et al. 2004, not only educational, but refreshing, and a lot of Johnsen 2005, Lehtiniemi et al. 2005, Van fun. de Meutter et al. 2005a) of aquatic organ- Tvärminne Zoological Station was the scene of isms and may therefore alter trophic inter- most of my experiments, and I wish to thank every- body there that helped to make them happen: Jouko actions directly. Under turbid conditions, Pokki, Raija Myllymäki, Lallu Keynäs, Eva Sand- the use of vision by both predators and berg-Kilpi, Totti Sjölund and Magnus Lindström. A prey is hampered, and thus chemically and special thanks goes to Riggert Munsterhjelm, who hydromechanically mediated predator-prey helped me countless times with all sorts of things interactions become more prevalent. concerning aquatic macrophytes. Soili Saesmaa, The indirect effects of predators on Eila Lahdes and Jan-Erik Bruun at FIMR are ac- planktivores are crucial to the study of knowledged for their help with the Cercopagis data, trophic interactions and implications of en- as are Helena Huttunen and Anne Martikainen in vironmental change. The predation effects the library of FIMR. Asta Audzijonyte clarifi ed the mediated by antipredator behaviour can messy systematics of Mysis spp. for me, and Marja alter consumption rates, prey selectivity Koistinen helped with the systematics of Myrio- and distribution of planktivores. Predators, phyllum sibiricum. Jorma Kuparinen and Johanna Ikävalko helped me to wrap up my studies and get even without causing increased mortality in through all the bureaucracy leading up to this disser- their prey populations, can induce cascad- tation. Maiju, Make, Harri Kuosa and Lasse Paka- ing effects on lower trophic levels through rinen gave valuable comments on the manuscript of decreased feeding rates of their prey. The this thesis, and Sture Hansson and Heikki Hirvonen fl exibility and variability of antipredator pre-examined this thesis, all of which improved it behaviour highlighted in this thesis pose a considerably. Carl-Adam Hæggström was of utmost challenge to forecasting and making gener- importance in making this thesis a real book. This alizations about trophic dynamics. thesis, from the beginning to the end, was fi nanced 41 by Walter and Andrée de Nottbeck Foundation, for 9. REFERENCES which I’m most grateful. Fortunately, these years have not been all work Aarnio, K. & Bonsdorff, E. 1993: Seasonal variation and no fun. For that I give full acknowledgements to in abundance and diet of the sand goby Poma- the so-called EZECO-group: Make, Maiju, Miina, toschistus minutus (Pallas) in a northern Baltic Jonna, SannaS, SannaR, Satu, Roope, Tarja and JP, archipelago. – Ophelia 37: 19–30. and those who used to wear the label: Marja, Tomi, Aaser, H. F., Jeppesen, E. & Søndergaard, M. 1995: Sandra and Samuli. They provided the loud mu- Seasonal dynamics of the mysid Neomysis in- sic, forest-croquet in the dark, Avista-games on the teger and its predation on the copepod Euryte- sunny deck of Aranda, and more laughter than one’s mora affi nis in a shallow hypertrophic brackish stomach can ever put up with, that made me realize lake. – Mar. Ecol. Prog. Ser. 127: 47–56. what a team truly means, and proud of belonging to Abrahams, M. & Kattenfeld, M. 1997: The role of it. I’ve always been able to count on them for any turbidity as a constraint on predator-prey inter- kind of help, from statistics to cuisine tips. I also actions in aquatic environments. – Behav. Ecol. wish to thank all the people of the department of Sociobiol. 40: 169–174. biological oceanography at FIMR, and of the former Abrahams, M. V. & Cartar, R. V. 2000: Within- division of hydrobiology at the university, for com- group variation in the willingness to risk expo- panionship and fun times around the respective cof- sure to a predator: the infl uence of species and fee tables. size. – Oikos 89: 340–344. Warm thanks go to my dear big sister Essi and Abrams, P. 1987: Indirect interactions between spe- her family, and to my wonderful friends Lasse, Mik- cies that share a predator: varieties of indirect ko, Lotta, Olli and all the others, for being there, effects. – In: Kerfoot, W. C. & Sih, A. (eds.), and at least from time to time pretending to under- Predation. Direct and indirect impacts on aquat- stand what it is that I do, and why. Polyteknikkojen ic communities, pp. 38–54. University Press of Orkesteri offered unforgettable musical experiences New England, Hanover. that gave energy to carry on with the science. The Adams, S. M. & DeAngelis, D. L. 1987: Indirect biggest thanks of all goes to the love of my life, Art- effects on early bass-shad interactions on preda- tu, who has made me happier than I ever believed I tor population structure and food web dynamics. could be. – In: Kerfoot, W. C. & Sih, A. (eds.), Predation. Direct and indirect impacts on aquatic commu- nities, pp. 103–117. University Press of New England, Hanover. Aiken, S. G. 1981: A conspectus of Myriophyllum (Haloragaceae) in North America. – Brittonia 33: 57–69. Aiken, S.G., Newroth, P.R. & Wile, I. 1979: The biology of Canadian weeds. 34. Myriophyllum spicatum L. – Can. J. Plant Sci. 59: 201–215. Albertsson, J. 2004: Trophic interactions involv- ing mysid shrimps (Mysidacea) in the near-bot- tom habitat in the Baltic Sea. – Aquat. Ecol. 38: 457–469. Alexander, R. D. 1974: The evolution of social be- havior. – Ann. Rev. Ecol. Syst. 5: 325–383. Aneer, G. 1980: Estimates of feeding pressure on pelagic and benthic organisms by Baltic herring 42

(Clupea harengus v. membras L.). – Ophelia ing the impact of a recent predatory invader: the Suppl. 1: 265–275. population dynamics, vertical distribution, and Antsulevich, A. & Välipakka, P. 2000: Cercopagis potential prey of Cercopagis pengoi in Lake On- pengoi – new important food object of the Baltic tario. – Limnol. Oceanogr. 47: 626–635. herring in the Gulf of Finland. – Internat. Rev. Berger, R., Bergström, L., Graneli, E. & Kautsky, Hydrobiol. 85: 609–619. L. 2004: How does eutrophication affect differ- Apel, M. 1992: Spatial distribution and seasonal oc- ent life stages of Fucus vesiculosus in the Bal- currence of Mysidacea in the Jade estuary (North tic Sea? – a conceptual model. – Hydrobiologia Sea, Germany), with some comments on diurnal 514: 243–248. migrations. – In: Köhn, J., Jones, M. B. & Mof- Bielecka, L., Zmijewska, M. I. & Szymborska, A. fat, A. (eds.), Taxonomy, biology and ecology 2000: A new predatory cladoceran Cercopagis of Baltic mysids (Mysidacea, Crustacea), pp. (Cercopagis) pengoi (Ostroumov 1891) in the 98–108. Rostock University. Gulf of Gdansk. – Oceanologia 42: 371–374. Arndt, E. A. & Jansen, W. 1986: Neomysis integer Bishop, T. D. & Brown, J. A. 1992: Threat-sensitive (Leach) in the chain of boddens south of Darss foraging by larval threespine sticklebacks (Gas- / Zingst (Western Baltic) – ecophysiology and terosteus aculeatus). – Behav. Ecol. Sociobiol. population dynamics. – Ophelia Suppl. 4: 1–15. 31: 133–138. Arrhenius, F. & Hansson, S. 1993: Food consump- Blaxter, J. H. S. & Batty, R. S. 1985: The develop- tion of larval, young and adult herring and sprat ment of startle responses in herring larvae. – J. in the Baltic Sea. – Mar. Ecol. Prog. Ser. 96: Mar. Biol. Ass. U.K. 65: 737–750. 125–137. Boeing, W. J., Wissel, B. & Ramcharan, C. W. 2005: Åsbjörnsson, K., Hansson, L.-A. & Brönmark, C. Costs and benefi ts of Daphnia defense against 2004: Responses of prey from habitats with dif- Chaoborus in nature. – Can. J. Fish. Aquat. Sci. ferent predator regimes: local adaptation and 62: 1286–1294. heritability. – Ecology 85: 1859–1866. Bohl, E. 1980: Diel pattern of pelagic distribution Audzijonyte, A. 2006: Diversity and zoogeography and feeding in planktivorous fi sh. – Oecologia of continental mysid crustaceans. – W. & A. de 44: 368–375. Nottbeck Foundation Sci. Rep. 29: 1–46. Bollens, S. M. & Frost, B. W. 1989: Predator-in- Audzijonyte, A., Pahlberg, J., Väinölä, R. & Lind- duced diel vertical migration in a planktonic ström, M. 2005: Spectral sensitivity differences copepod. – J. Plankton Res. 11: 1047–1065. in two Mysis sibling species (Crustacea, Mysi- Bonsdorff, E., Blomqvist, E. M., Mattila, J. & Nork- da): Adaptation or phylogenetic constraints? – J. ko, A. 1997: Long-term changes and coastal eu- Exp. Mar. Biol. Ecol. 325: 228–239. trophication. Examples from the Åland Islands Audzijonyte, A. & Väinölä, R. 2005: Diversity and and the Archipelago Sea, northern Baltic Sea. distributions of circumpolar fresh- and brackish- – Oceanolog. Acta 20: 319–329. water Mysis (Crustacea: Mysida): descriptions Botham, M. S., Kerfoot, C. J., Louca, V. & Krause, of M. relicta Lovén, 1862, M. salemaai n. sp., J. 2005: Predator choice in the fi eld; grouping M. segerstralei n. sp. and M. diluviana n. sp., guppies, Poecilia reticulata, receive more at- based on molecular and morphological charac- tacks. – Behav. Ecol. Sociobiol. 59: 181–184. ters. – Hydrobiologia 544: 89–141. Bowers, J. A. & Grossnickle, N. E. 1978: The her- Bailey, K. M. & Houde, E. D. 1989: Predation on bivorous habits of Mysis relicta in Lake Michi- eggs and larvae of marine fi shes and the recruit- gan. – Limnol. Oceanogr. 23: 767–776. ment problem. – Adv. Mar. Biol. 25: 1–83. Bowers, J. A. & Vanderploeg, H. A. 1982: In situ Batty, R. S. 1989: Escape responses of herring lar- predatory behavior of Mysis relicta in Lake vae to visual stimuli. – J. Mar. Biol. Ass. U.K. Michigan. – Hydrobiologia 93: 121–131. 69: 647–654. Boyero, L., Rincón, P. A. & Bosch, J. 2006: Case Benoit, H. P., Johansson, O. E., Warner, D. M., selection by a limnephilid caddisfl y [Pota- Sprules, W. G. & Rudstam, L. G. 2002: Assess- mophylax latipennis (Curtis)] in response to dif- 43

ferent predators. – Behav. Ecol. Sociobiol. 59: Byholm, L. 1998: Neomysis integer -halkoisjalkai- 364–372. sen parvikäyttäytymisen vaikutus yksilön saa- Brock, V. E. & Riffenburgh, R. H. 1960: Fish school- liiksi joutumisen riskiin. – M.Sc. thesis, Univer- ing: a possible factor in reducing predation. – J. sity of Helsinki, 43 pp. (In Finnish.) Cons. Int. Explor. Mer 25: 307–317. Cardinale, M. & Arrhenius, F. 2000: Decreas- Brodie, E. D. Jr., Formanowicz, D. R. Jr. & Bro- ing weight-at-age of Atlantic herring (Clupea die, E. D. III 1991: Predator avoidance and anti- harengus) from the Baltic Sea between 1986 and predator mechanisms: distinct pathways to sur- 1996: a statistical analysis. – ICES J. Mar. Sci. vival. – Ethol. Ecol. Evol. 3: 73–77. 57: 882–893. Brönmark, C. & Hansson, L.-A. 2000: Chemical Carleton, J. H. & Hamner, W. M. 1989: Resident communication in aquatic systems: an introduc- mysids: community structure, abundance and tion. – Oikos 88: 103–109. small-scale distributions in a coral reef lagoon. Brönmark, C. & Pettersson, L. B. 1994: Chemical – Mar. Biol. 102: 461–472. cues from piscivores induce a change in mor- Carpenter, S. R., Kitchell, J. F. & Hodgson, J. R. phology in crucian carp. – Oikos 70: 396–402. 1985: Cascading trophic interactions and lake Browman, H. I. 2005: Applications of sensory biol- productivity. – BioScience 35: 634–639. ogy in marine ecology and aquaculture. – Mar. Carpenter, S. R. & Lodge, D. M. 1986: Effects of Ecol. Prog. Ser. 287: 266–269. submersed macrophytes on ecosystem process- Browman, H. I., Kruse, S. & O’Brien, W. J. 1989: es. – Aquat. Bot. 341–370. Foraging behavior of the predaceous cladoceran, Carson, M. L. & Merchant, H. 2005: A laboratory Leptodora kindti, and escape responses of their study of the behavior of two species of grass prey. – J. Plankton Res. 11: 1075–1088. shrimp (Palaemonetes pugio Holthuis and Pa- Brown, G. E. & Cowan, J. 2000: Foraging trade- laemonetes vulgaris Holthuis) and the killifi sh offs and predator inspection in an Ostariophy- (Fundulus heteroclitus Linnaeus). – J. Exp. Mar. san fi sh: switching from chemical to visual cues. Biol. Ecol. 314: 187–201. – Behaviour 137: 181–195. Casini, M., Cardinale, M. & Hjelm, J. 2006: Inter- Brown, G. E. & Smith, R. J. F. 1998: Acquired annual variation in herring, Clupea harengus, predator recognition in juvenile rainbow trout and sprat, Sprattus sprattus, condition in the (Oncorhynchus mykiss): conditioning hatchery- central Baltic Sea: what gives the tune? – Oikos reared fi sh to recognize chemical cues of a pred- 112: 638–650. ator. – Can. J. Fish. Aquat. Sci. 55: 611–617. Ceska, A. & Ceska, O. 1986: Notes on Myriophyl- Burks, R. L. & Lodge, D. M. 2002: Cued in: ad- lum (Haloragaceae) in the Far East: The identity vances and opportunities in freshwater chemical of Myriophyllum sibiricum Komarov. – Taxon ecology. – J. Chem. Ecol. 28: 1901–1917. 35: 95–100. Burris, J. E. 1980: Vertical migration of zooplankton Charlebois, P. M., Raffenberg, M. J. & Dettmers, J. in the Gulf of Finland. – Am. Midland Nat. 103: M. 2001: First occurrence of Cercopagis pen- 316–321. goi in Lake Michigan. – J. Great Lakes Res. 27: Bushnoe, T. M., Warner, D. M., Rudstam, L. G. & 258–261. Mills, E. L. 2003: Cercopagis pengoi as a new Chivers, D. P., Kiesecker, J. M., Marco, A., DeVito, prey item for alewife (Alosa pseudoharengus) J., Anderson, M. T. & Blaustein, A. R. 2001a: and rainbow smelt (Osmerus mordax) in Lake Predator-induced life history changes in amphib- Ontario. – J. Great Lakes Res. 29:205–212. ians: egg predation induces hatching. – Oikos Buskey, E. J. 2000: Role of vision in the aggrega- 92: 135–142. tive behavior of the planktonic mysid Mysidium Chivers, D. P., Mirza, R. S., Bryer, P. J. & Kieseck- columbiae. – Mar. Biol. 137: 257–265. er, J. M. 2001b: Threat-sensitive predator avoid- van Buskirk, J. 2000: The costs of an inducible de- ance by slimy sculpins: understanding the im- fense in an anuran larvae. – Ecology 81: 2813– portance of visual versus chemical information. 2821. – Can. J. Zool. 79: 867–873. 44

Cieri, M. D. & Stearns, D. E. 1999: Reduction of Dalesman, S., Rundle, S. D., Coleman, R. A. & Cot- grazing activity of two estuarine copepods in re- ton, P. A. 2006: Cue association and antipredator sponse to the exudate of a visual predator. – Mar. behaviour in a pulmonate snail, Lymnea stagna- Ecol. Prog. Ser. 177: 157–163. lis. – Anim. Behav. 71: 789–797. Clark, C. W. & Mangel, M. 1986: The evolutionary Dawidowicz, P. & Loose, C. J. 1992: Metabolic advantages of group foraging. – Theor. Popul. costs during predator-induced diel vertical mi- Biol. 30: 45–75. gration of Daphnia. – Limnol. Oceanogr. 37: Clarke, R. D., Buskey, E. J. & Marsden, K. C. 2005: 1589–1595. Effects of water motion and prey behavior on Daye, P. G. & Garside, E. T. 1975: Lethal lev- zooplankton capture by two coral reef fi shes. els of pH for brook trout, Salvelinus fontinalis – Mar. Biol. 146: 1145–1155. (Mitchill). – Can. J. Zool. 53: 639–641. Clutter, R. I. 1969: The microdistribution and social Debus, L., Mehner, T. & Thiel, R. 1992: Spatial and behavior of some pelagic mysid shrimps. – J. diel patterns of migration for Neomysis inte- Exp. Mar. Biol. Ecol. 3: 125–155. ger. – In: Köhn, J., Jones, M. B. & Moffat, A. Coen, L. D., Heck, K. L. Jr. & Abele, L. G. 1981: (eds.), Taxonomy, biology and ecology of Baltic Experiments on competition and predation mysids (Mysidacea, Crustacea), pp. 79–82. Ros- among shrimps of seagrass meadows. – Ecology tock University. 62: 1484–1493. De Robertis, A., Ryer, C. H., Veloza, A. & Brodeur, Cohen, J. H. & Forward, R. B. Jr. 2005: Photobehav- R. D. 2003: Differential effects of turbidity on iour as an inducible defense in the marine cope- prey consumption of piscivorous and planktivo- pod Calanopia americana. – Limnol. Oceanogr. rous fi sh. – Can. J. Fish. Aquat. Sci. 60: 1517– 50: 1269–1277. 1526. Cohen, P. J. & Ritz, D. A. 2003: Role of kairomones Dhillon, M. S., Mulla, M. S. & Hwang, Y.-S. 1982: in feeding interactions between seahorses and Allelochemics produced by the hydrophyte mysids. – J. Mar. Biol. Ass. U.K. 83: 633–638. Myriophyllum spicatum affecting mosquitoes Cooper, S. D. & Goldman, C. R. 1980: Opossum and midges. – J. Chem. Ecol. 8: 517–526. shrimp (Mysis relicta) predation on zooplank- Diehl, S. 1988: Foraging effi ciency of three fresh- ton. – Can. J. Fish. Aquat. Sci. 37: 909–919. water fi shes: effects of structural complexity and Coull, B. C. & Wells, J. B. J. 1983: Refuges from light. – Oikos 53: 207–214. fi sh predation: experiments with phytal meio- Diehl, S. & Kornijów, R. 1997: Infl uence of sub- fauna from the New Zealand rocky intertidal. merged macrophytes on trophic interactions – Ecology 64: 1599–1609. among fi sh and macroinvertebrates. – Ecol. Cronin, G. & Hay, M. E. 1996: Induction of sea- Stud. Ser. 131: 24–46. weed chemical defenses by amphipod grazing. Dill, L. M. 1987: Animal decision making and its – Ecology 77: 2287–2301. ecological consequences: the future of aquatic Crowl, T. A. & Covich, A. P. 1990: Predator-induced ecology and behaviour. – Can. J. Zool. 65: 803– life-history shifts in a freshwater snail. – Science 811. 247: 949–951. Dodson, S. 1990: Predicting diel vertical migration Dadswell, M. J. 1975: Some notes on shoaling be- of zooplankton. – Limnol. Oceanogr. 35: 1195– havior and growth of Mysis gaspensis (Mysida- 1200. cea) in a small Newfoundland estuary. – Can. J. Dorn, N. J. & Mittelbach, G. G. 1999: More than Zool. 53: 374–377. predator and prey: a review of interactions be- Dahlgren, S. & Kautsky, L. 2004: Can different veg- tween fi sh and crayfi sh. – Vie et Milieu 49: etative states in shallow coastal bays of the Bal- 229–237. tic Sea be linked to internal nutrient levels and van Duren, L. A. & Videler, J. J. 1996: The trade-off external nutrient load? – Hydrobiologia 514: between feeding, mate seeking and predator av- 249–258. 45

oidance in copepods: behavioural responses to Flinkman, J., Vuorinen, I. & Christiansen, M. 1994: chemical cues. – J. Plankton Res. 18: 805–818. Calanoid copepod eggs survive passage through Eggers, D. M. 1976: Theoretical effect of schooling fi sh digestive tracts. – ICES J. Mar. Sci. 51: by planktivorous fi sh predators on rate of prey 127–129. consumption. – J. Fish. Res. Board Can. 33: Flynn, A. J. & Ritz, D. A. 1999: Effect of habitat 1964–1971. complexity and predatory style on the capture Eicher, G. J. Jr. 1946: Lethal alkalinity for trout in success of fi sh feeding on aggregated prey. – J. waters of low salt content. – J. Wildl. Manag. Mar. Biol. Ass. U.K. 79: 487–494. 10: 82–85. Fockedey, N. 2005: Diet and growth of Neomysis Ejdung, G. 1998: Behavioural responses to chemi- integer (Leach, 1814) (Crustacea, Mysidacea). cal cues of predation risk in a three-trophic-level – Ph.D. thesis, Ghent University, 297 pp. Baltic Sea food chain. – Mar. Ecol. Prog. Ser. Foster, E. G., Ritz, D. A., Osborn, J. E. & Swadling, 165: 137–144. K. M. 2001: Schooling affects the feeding suc- Eklöv, P. & Persson, L. 1996: The response of prey cess of Australian salmon (Arripis trutta) when to the risk of predation: proximate cues for refu- preying on mysid swarms (Paramesopodopsis ging juvenile fi sh. – Anim. Behav. 51: 105–115. rufa). – J. Exp. Mar. Biol. Ecol. 261: 93–106. Engström-Öst, J., Karjalainen, M. & Viitasalo, M. Foster, S. A. 1985: Group foraging by a coral reef 2006: Feeding and refuge use by small fi sh in fi sh: a mechanism for gaining access to defen- the presence of cyanobacteria blooms. – Env. ded resources. – Anim. Behav. 33: 782–792. Biol. Fish. DOI 10.1007/s10641-006-9013-8. Foster, W. A. & Treherne, J. E. 1981: Evidence for Engström-Öst, J. & Lehtiniemi, M. 2004: Threat- the dilution effect in the selfi sh herd from fi sh sensitive predator avoidance by pike larvae. – J. predation on a marine insect. – Nature 293: Fish Biol. 65: 251–261. 466–467. Eriksson, B. K. & Johansson, G. 2003: Sedimenta- Fuiman, L. A. 1993: Development of predator eva- tion reduces recruitment success of Fucus vesi- sion in Atlantic herring, Clupea harengus L. culosus (Phaeophyceae) in the Baltic Sea. – Eur. – Anim. Behav. 45: 1101–1116. J. Phycol. 38: 217–222. Fuiman, L. A., Cowan, J. H. Jr., Smith, M. E. & Eriksson, B. K., Sandström, A., Isæus, M., Schrei- O’Neal, J. P. 2005: Behavior and recruitment ber, H. & Karås, P. 2004: Effects of boating ac- success in fi sh larvae: variation with growth rate tivities on aquatic vegetation in the Stockholm and the batch effect. – Can. J. Fish. Aquat. Sci. archipelago, Baltic Sea. – Est. Coast. Shelf Sci. 62: 1337–1349. 61: 339–349. Fulton, R. S. III 1982: Predatory feeding of two ma- Fields, D. M. & Weissburg, M. J. 2005: Evolutiona- rine mysids. – Mar. Biol. 72: 183-191. ry and ecological signifi cance of mechanosen- Gal, G., Loew, E. R., Rudstam, L. G. & Moham- sor morphology: copepods as a model system. madian, A. M. 1999: Light and diel vertical mi- – Mar. Ecol. Prog. Ser. 287: 269–274. gration: spectral sensitivity and light avoidance Finnish Game and Fisheries Research Institute 2004: by Mysis relicta. – Can. J. Fish. Aquat. Sci. 56: Recreational Fishery 2002. – SVT Agriculture, 311–322. Forestry and Fishery 2004: 51. Gerritsen, J. & Strickler, J. R. 1977: Encounter Finnish Game and Fisheries Research Institute probabilities and community structure in zoo- 2005: Commercial Marine Fishery 2004. – SVT plankton: a mathematical model. – J. Fish. Res. Agriculture, Forestry and Fishery 2005: 57. Board Can. 34: 73–82. Flinkman, J., Aro, E., Vuorinen, I. & Viitasalo, M. Gilbert, O. M. & Buskey, E. J. 2005: Turbulence de- 1998: Changes in northern Baltic zooplankton creases the hydrodynamic predator sensing abil- and herring nutrition from 1980s to 1990s: top- ity of the calanoid copepod Acartia tonsa. – J. down and bottom-up processes at work. – Mar. Plankton Res. 27: 1067–1071. Ecol. Prog. Ser. 165: 127–136. 46

Gill, P. C. 2002: A blue whale (Balaenoptera mus- Griffen, B. D. & Byers, J. E. 2006: Partitioning culus) feeding ground in a southern Australian mechanisms of predator interference in different coastal upwelling zone. – J. Cetacean Res. Man- habitats. – Oecologia 146: 608–614. age. 4: 179–184. Grigorovich, I. A., MacIsaac, H. J., Rivier, I. K., Godin, J.-G. J. & Crossman, S. L. 1994: Hunger- Aladin, N. V. & Panov, V. E. 2000: Comparative dependent predator inspection and foraging be- biology of the predatory cladoceran Cercopagis haviours on the threespine stickleback (Gaster- pengoi from Lake Ontario, Baltic Sea and Cas- osteus aculeatus) under predation risk. – Behav. pian Sea. – Arch. Hydrobiol. 149: 23–50. Ecol. Sociobiol. 34: 359–366. Gross, E. M. 2003: Allelopathy of aquatic auto- Gorokhova, E. (in press): Molecular identifi cation trophs. – Crit. Rev. Plant Sci. 22: 313–339. of the invasive cladoceran Cercopagis pengoi Gross, E. M., Erhard, D. & Iványi, E. 2003: Allelo- (Cladocera: Onychopoda) in stomachs of preda- pathic activity of Ceratophyllum demersum L. tors. – Limnol. Oceanogr. Methods. and Najas marina ssp. intermedia (Wolfgang) Gorokhova, E., Aladin, N. & Dumont, H. J. 2000: Casper. – Hydrobiologia 506-509: 583–589. Further expansion of the genus Cercopagis Gross, E. M., Meyer, H. & Schilling, G. 1996: Re- (Crustacea, Branchiopoda, Onychopoda) in the lease and ecological impact of algicidal hydro- Baltic Sea, with notes on the taxa present and lysable polyphenols in Myriophyllum spicatum. their ecology. – Hydrobiologia 429: 207–218. – Phytochemistry 41: 133–138. Gorokhova, E., Fagerberg, T. & Hansson, S. 2004: Gross, E. M. & Sütfeld, R. 1994: Polyphenols with Predation by herring (Clupea harengus) and algicidal activity in the submerged macrophyte sprat (Sprattus sprattus) on Cercopagis pengoi Myriophyllum spicatum L. – Acta Horticul. 381: in a western Baltic Sea bay. – ICES J. Mar. Sci. 710–716. 61: 959–965. Grossnickle, N. E. 1982: Feeding habits of Mysis Gorokhova, E., Hansson, S., Höglander, H. & An- relicta – an overview. – Hydrobiologia 93: 101– dersen, C. M. 2005: Stable isotopes show food 107. web changes after invasion by the predatory cla- Gyssels, F. & Stoks, R. 2006: Behavioral responses doceran Cercopagis pengoi in a Baltic Sea bay. to fi sh kairomones and autotomy in a damselfl y. – Oecologia 143: 251–259. – J. Ethol. 24: 79–83. Gotceitas, V. & Colgan, P. 1987: Selection be- Haage, P. 1975: Quantitative investigations of the tween densities of artifi cial vegetation by young Baltic Fucus belt macrofauna. 2. Quantitative bluegills avoiding predation. – Trans. Am. Fish. seasonal fl uctuations. – Contr. Askö Lab. 9: Soc. 116: 40–49. 1–88. Grace, J. B. & Wetzel, R. G. 1978: The production Hairston, N. G. Jr. 1987: Diapause as a predator- biology of Eurasian watermilfoil (Myriophyllum avoidance adaptation. – In: Kerfoot, W. C. & spicatum L.): a review. – J. Aquat. Plant Man- Sih, A. (eds.), Predation. Direct and indirect age. 16: 1–11. impacts on aquatic communities, pp. 281–290. Granqvist, M. & Mattila, J. 2004: The effects of tur- University Press of New England, Hanover. bidity and light intensity on the consumption of Hällfors, G., Niemi, Å., Ackefors, H., Lassig, J. & mysids by juvenile perch (Perca fl uviatilis L.). Leppäkoski, E. 1981: Biological oceanography. – Hydrobiologia 514: 93–101. – In: Voipio, A. (ed.), The Baltic Sea, pp. 219– Green, S., Visser, A. W., Titelman, J. & Kiørboe, T. 274. Elsevier Scientifi c Publishing Company, 2003: Escape responses of copepod nauplii in Amsterdam, Netherlands. the fl ow fi eld of the blue mussel, Mytilus edulis. Halstead, B. C. & Tash, J. C. 1982: Unusual diel – Mar. Biol. 142: 727–733. pHs in water as related to aquatic vegetation. Gregory, R. S. 1993: Effect of turbidity on the pred- – Hydrobiologia 96: 217–224. ator avoidance behaviour of juvenile chinook Hamner, W. M. 1984: Aspects of schooling in Eu- salmon (Oncorhynchus tshawytscha). – Can. J. phausia superba. – J. Crustacean Biol. 4 (Spec. Fish. Aquat. Sci. 50: 241–246. No. 1): 67–74. 47

Hamrén, U. & Hansson, S. 1999: A mysid shrimp sponse to chemical stimuli from predators and (Mysis mixta) is able to detect the odour of its resource density. – Behav. Ecol. Sociobiol. 58: predator (Clupea harengus). – Ophelia 51: 187– 256–263. 191. Holt, R. D. 1977: Predation, apparent competition, Hangelin, C. & Vuorinen, I. 1988: Food selection and the structure of prey communities. – Theor. in juvenile three-spined sticklebacks studied in Pop. Biol. 12: 197–229. relation to size, abundance and biomass of prey. Horppila, J., Liljendahl-Nurminen, A., Malinen, – Hydrobiologia 157: 169–177. T., Salonen, M., Tuomaala, A., Uusitalo, L. & Hansson, S., Larsson, U. & Johansson, S. 1990: Vinni, M. 2003: Mysis relicta in a eutrophic Selective predation by herring and mysids, and lake: Consequences of obligatory habitat shifts. zooplankton community structure in a Baltic Sea – Limnol. Oceanogr. 48: 1214–1222. coastal area. – J. Plankton Res. 12: 1099–1116. Hossain, M. A. R., Tanaka, M. & Masuda, R. 2002: Hartman, E. J. & Abrahams, M. V. 2000: Sensory Predator-prey interaction between hatchery- compensation and the detection of predators: the reared Japanese fl ounder juvenile, Paralichthys interaction between chemical and visual infor- olivaceus, and sandy shore crab, Matua lunaris: mation. – Proc. R. Soc. Lond. B 267: 571–575. daily rhythms, anti-predator conditioning and Haury, L. R., Kenyon, D. E. & Brooks, J. R. 1980: starvation. – J. Exp. Mar. Biol. Ecol. 267: 1–14. Experimental evaluation of the avoidance reac- Hostens, K. & Mees, J. 1999: The mysid-feeding tion of Calanus fi nmarchicus. – J. Plankton Res. guild of demersal fi shes in the brackish zone 2: 187–202. of the Westerschelde estuary. – J. Fish Biol. 55: Havel, J. E. 1987: Predator-induced defenses: a re- 704–719. view. – In: Kerfoot, W. C. & Sih, A. (eds.), Pre- Hough, A. R. & Naylor, E. 1992: Distribution and dation. Direct and indirect impacts on aquatic position maintenance behaviour of the estuarine communities, pp. 263–278. University Press of mysid Neomysis integer. – J. Mar. Biol. Ass. New England, Hanover. U.K. 72: 869–876. van Havre, N. & FitzGerald, G. J. 1988: Shoaling Irvine, K., Moss, B., Bales, M. & Snook, D. 1993: and kin recognition in the threespine stickleback The changing ecosystem of a shallow, brackish (Gasterosteus aculeatus L.). – Biol. Behav. 13: lake, Hickling Broad, Norfolk, U.K. I. Trophic 190–201. relationships with special reference to the role Helfman, G. S. 1989: Threat-sensitive predator of Neomysis integer. – Freshwat. Biol. 29: 119– avoidance in damselfi sh-trumpetfi sh interac- 139. tions. – Behav. Ecol. Sociobiol. 24: 47–58. Jachner, A. 1997: The response of bleak to predator Heller, R. & Milinski, M. 1979: Optimal foraging of odour of unfed and recently fed pike. – J. Fish sticklebacks on swarming prey. – Anim. Behav. Biol. 50: 878–886. 27: 1127–1141. Jakobsen, P. J., Birkeland, K. & Johnsen, G. H. Hemmi, A. 2003: Eutrophication and grazing pres- 1994: Swarm location in zooplankton as an anti- sure on the brown alga Fucus vesiculosus by the predator defence mechanism. – Anim. Behav. herbivorous isopod Idotea baltica. – Ph.D. the- 47: 175–178. sis, University of Turku, 28 pp. Jansson, A.-M., Kautsky, N., von Oertzen, J.-A., Hemmi, J. M. & Zeil, J. 2005: Animals as prey: Schramm, W., Sjöstedt, B., von Wachenfeldt, perceptual limitations and behavioural options. T. & Wallentinus, I. 1982: Structural and func- – Mar. Ecol. Prog. Ser. 287: 274–278. tional relationships in a southern Baltic Fucus Hirvonen, H. & Ranta, E. 1996: Prey to predator ecosystem. – Contr. Askö Lab. 28: 1–95. size ratio infl uences foraging effi ciency of lar- Jeppesen, E., Lauridsen, T. L., Kairesalo, T. & Per- val Aeshna juncea dragonfl ies. – Oecologia 106: row, M. R. 1997: Impact of submerged macro- 407–415. phytes on fi sh-zooplankton interactions in lakes. Hölker, F. & Stief, P. 2005: Adaptive behaviour of – Ecol. Stud. Ser. 131: 91–114. chironomid larvae (Chironomus riparius) in re- 48

Johannsson, O. E., Rudstam, L. G. & Lasenby, D. their invertebrate prey. – Can. J. Zool. 62: 1289– C. 1994: Mysis relicta: assessment of metalim- 1303. netic feeding and implications for competition Keenleyside, M. H. A. 1955: Some aspects of the with fi sh in lakes Ontario and Michigan. – Can. schooling behaviour of fi sh. – Behaviour 8: J. Fish. Aquat. Sci. 51: 2591–2602. 183–247. Johnsen, S. 2005: Visual ecology on the high seas. Kerfoot, W. C. 1987: Cascading effects and indirect – Mar. Ecol. Prog. Ser. 287: 281–285. pathways. – In: Kerfoot, W. C. & Sih, A. (eds.), Johnson, G. D. & Mulla, M. S. 1983: An aquatic Predation. Direct and indirect impacts on aquat- macrophyte affecting nuisance chironomid ic communities, pp 57–70. University Press of midges in a warm-water lake. – Environ. Ento- New England, Hanover. mol. 12: 266–269. Kerfoot, W. C. & Sih, A. (eds.) 1987: Predation. Di- Johnston, N. M. & Ritz, D. A. 2001: Synchro- rect and indirect impacts on aquatic communi- nous development and release of broods by the ties. – University Press of New England, Hano- swarming mysids Anisomysis mixta australis, ver. 386 pp. Paramesopodopsis rufa and Tenagomysis tas- Kiørboe, T. & Visser, A. 1999: Predator and prey maniae (Mysidacea: Crustacea). – Mar. Ecol. perception in copepods due to hydromechanical Prog. Ser. 223: 225–233. signals. – Mar. Ecol. Prog. Ser. 179: 81–95. Kangas, P. 1978: On the quantity of meiofauna Kiørboe, T., Saiz, E. & Visser, A. 1999: Hydrody- among the epiphytes of Fucus vesiculosus in the namic signal perception in the copepod Acartia Askö area, northern Baltic Sea. – Contrib. Askö tonsa. – Mar. Ecol. Prog. Ser. 179: 97–111. Lab. Univ. Stockholm 24: 1–32. Kleiven, S. & Szczepańska, W. 1988: The effects Kangas, P., Autio, H., Hällfors, G., Luther, H., Nie- of extracts from Chara tomentosa and two oth- mi, Å. & Salemaa, H. 1982: A general model of er aquatic macrophytes on seed germination. the decline of Fucus vesiculosus at Tvärminne, – Aquat. Bot. 32: 193–198. south coast of Finland in 1977–81. – Acta Bot. Köhn, J. 1992: Mysidacea of the Baltic Sea – state Fennica 118: 1–27. of the art. – In: Köhn, J., Jones, M. B. & Mof- Karjalainen, M. 1999: Effect of nutrient loading on fat, A. (eds.), Taxonomy, biology and ecology of the development of the state of the Baltic Sea Baltic mysids (Mysidacea, Crustacea). Rostock – an overview. – W. & A. de Nottbeck Found. University. Sci. Rep. 17: 1–35. Köhn, J. & Gosselck, F. 1989: The recent distribu- Kauppila, M. 1994: Rantavyöhykkeen halkoisjal- tion of glacial relict Malacostraca in the western kaäyriäisten (Crustacea; Mysidacea) lisäänty- and southern Baltic. – Zool. Anz. 222: 57–74. minen ja energetiikka Tvärminnen saaristossa. Koho, M. J. 2005: Silakan (Clupea harengus mem- – M.Sc. thesis, University of Helsinki, 53 pp. bras L.), kilohailin (Sprattus sprattus L.) ja (In Finnish.) mysidiäyriäisten (Mysidacea) ravinnonkäyttö Kautsky, H., Kautsky, L., Kautsky, N., Kautsky, U. Suomenlahdella. – M.Sc. thesis, University of & Lindblad, C. 1992: Studies on the Fucus ve- Helsinki, 64 pp. (In Finnish.) siculosus community in the Baltic Sea. – Acta Koistinen, M. & Munsterhjelm, R. 2001: Charo- Phytogeogr. Suec. 78: 33–48. phytes of the Finnish coastal waters. – Schrift- Kautsky, L. 1988: Life strategies of aquatic soft bot- enr. Landschaftspfl ege Naturschutz 72: 27–29. tom macrophytes. – Oikos 53: 126–135. Körner, S. & Nicklisch, A. 2002: Allelopathic Kautsky, N., Kautsky, H., Kautsky, U. & Waern, M. growth inhibition of selected phytoplankton 1986: Decreased depth penetration of Fucus ve- species by submerged macrophytes. – J. Phycol. siculosus (L.) since the 1940’s indicates eutro- 38: 862–871. phication of the Baltic Sea. – Mar. Ecol. Prog. Koski, M., Viitasalo, M. & Kuosa, H. 1999: Season- Ser. 28: 1–8. al development of mesozooplankton biomass Keast, A. 1984: The introduced aquatic macrophyte, and production on the SW coast of Finland. Myriophyllum spicatum, as habitat for fi sh and – Ophelia 50: 69–91. 49

Kotta, I. & Kotta, J. 1999: Distribution and migra- Landeau, L. & Terborgh, J. 1986: Oddity and the tion of mysids in the Gulf of Riga (northern Bal- “confusion effect” in predation. – Anim. Behav. tic). – Proc. Estonian Acad. Sci. Biol. Ecol. 48: 34: 1372–1380. 284–295. Langsdale, J. R. M. 1993: Developmental changes in Kotta, I. & Kotta, J. 2001: Distribution of mysids on the opacity of larval herring, Clupea harengus, bank slopes in the Gulf of Riga. – Proc. Estonian and their implications for vulnerability to preda- Acad. Sci. Biol. Ecol. 50: 14–21. tion. – J. Mar. Biol. Ass. U.K. 73: 225–232. Kotta, J., Simm, M., Kotta, I., Kanosina, I., Kallaste, Lappalainen, A., Rask, M., Koponen, H. & Vesala, K. & Raid, T. 2004: Factors controlling long- S. 2001: Relative abundance, diet and growth of term changes of the eutrophicated ecosystem of perch (Perca fl uviatilis) and roach (Rutilus ruti- Pärnu Bay, Gulf of Riga. – Hydrobiologia 514: lus) at Tvärminne, northern Baltic Sea, in 1975 259–268. and 1997: responses to eutrophication? – Bor. Krylov, P. I., Bychenkov, D. E., Panov, V. E., Rodi- Env. Res. 6: 107–118. onova, N. V. & Telesh, I. V. 1999: Distribution Larsson, P. & Dodson, S. 1993: Invited review. and seasonal dynamics of the Ponto-Caspian in- Chemical communication in planktonic animals. vader Cercopagis pengoi (Crustacea, Cladocera) – Arch. Hydrobiol. 129: 129–155. in the Neva Estuary (Gulf of Finland). – Hydro- Larsson, U., Elmgren, R. & Wulff, F. 1985: Eutro- biologia 393: 227–232. phication and the Baltic Sea: causes and conse- Krylov, P. I. & Panov, V. E. 1998: Resting eggs in quences. – Ambio 14: 9–14. the life cycle of Cercopagis pengoi, a recent in- Laurel, B. J. & Brown, J. A. 2006: Infl uence of vader of the Baltic Sea. – Arch. Hydrobiol. Spec. cruising and ambush predators in 3-dimensional Issues Advanc. Limnol. 52: 383–392. habitat use in age 0 juvenile Atlantic cod Gadus Kullenberg, G. 1981: Physical oceanography. – In: morhua. – J. Exp. Mar. Biol. Ecol. 329: 34–46. Voipio, A. (ed.), The Baltic Sea, pp. 135–181. Laurel, B. J., Gregory, R. S., Brown, J. A., Hancock, Elsevier Scientifi c Publishing Company, Am- J. K. & Schneider, D. C. 2004: Behavioural sterdam, Netherlands. consequences of density-dependent habitat use Kusch, R. C. & Chivers, D. P. 2004: The effects of in juvenile cod Gadus morhua and G. ogac: the crayfi sh predation on phenotypic and life-histo- role of movement and aggregation. – Mar. Ecol. ry variation in fathead minnows. – Can. J. Zool. Prog. Ser. 272: 257–270. 82: 917–921. Lauridsen, T. L., Jeppesen, E., Søndergaard, M. & Kusch, R. C., Mirza, R. S. & Chivers, D. P. 2004: Lodge, D. M. 1997: Horizontal migration of Making sense of predator scents: investigating zooplankton: predator-mediated use of macro- the sophistication of predator assessment abili- phyte habitat. – Ecol. Stud. Ser. 131: 233–239. ties of fathead minnows. – Behav. Ecol. Socio- Lauridsen, T. L. & Lodge, D. M. 1996: Avoidance biol. 55: 551–555. by Daphnia magna of fi sh and macrophytes: Laamanen, M., Fleming, V. & Olsonen, R. 2004: Chemical cues and predator-mediated use of Water transparency in the Baltic Sea between macrophyte habitat. – Limnol. Oceanogr. 41: 1903 and 2004. – http://www.helcom.fi /environ- 794–798. ment2/ifs/archive/ifs2004/_GB/transparency/. Laxson, C. L., McPhedran, K. N., Makarewicz, J. Laine, P. 2003: Regulation of reproductive success C., Telesh, I. V. & MacIsaac, H. J. 2003: Effects in Baltic herring (Clupea harengus membras): of the non-indigenous cladoceran Cercopagis individual effort meets the environment. – Ph.D. pengoi on the lower food web of Lake Ontario. thesis, University of Turku. 32 pp. – Freshwat. Biol. 48: 2094–2106. Lampert, W. 1989: The adaptive signifi cance of Lehtiniemi, M. 2005: Swim or hide – predator cues diel vertical migration of zooplankton. – Funct. cause species specifi c reactions in young fi sh Ecol. 3: 21–27. larvae. – J. Fish Biol. 66: 1285–1299. 50

Lehtiniemi, M., Engström-Öst, J. & Viitasalo, M. – In: Köhn, J., Jones, M.B. & Moffat, A. (eds.), 2005: Turbidity decreases anti-predator behav- Taxonomy, biology and ecology of Baltic mysids iour in pike larvae, Esox lucius. – Env. Biol. (Mysidacea, Crustacea), pp. 120–126. Rostock Fish. 73: 1–8. University. Lehtiniemi, M., Viitasalo, M., Kuosa, H. 2002: Diet Lindström, M. 2000: Eye function of Mysidacea composition infl uences the growth of the pe- (Crustacea) in the northern Baltic Sea. – J. Exp. lagic mysid shrimp, Mysis mixta (Mysidacea). Mar. Biol. Ecol. 246: 85–101. – Boreal Env. Res. 7: 121–128. Loose, C. J. 1993: Daphnia diel vertical migration Leibold, M. A. 1991: Trophic interactions and habi- behavior: Response to vertebrate predator abun- tat segregation between competing Daphnia dance. – Arch. Hydrobiol. Beih. Ergebn. Lim- species. – Oecologia 86: 510–520. nol. 39: 29–36. Leising, A. W., Pierson, J. J., Cary, S. & Frost, B. Loose, C. J. & Dawidowicz, P. 1994: Trade-offs in W. 2005: Copepod foraging and predation risk diel vertical migration by zooplankton: the costs within the surface layer during night-time feed- of predator avoidance. – Ecology 75: 2255– ing forays. – J. Plankton Res. 27: 987–1001. 2263. Lemmetyinen, R. & Mankki, J. 1975: The three- Loose, C. J., von Elert, E. & Dawidowicz, P. 1993: spined stickleback (Gasterosteus aculeatus) in Chemically-induced diel vertical migration in the food chains of the northern Baltic. – Meren- Daphnia: a new bioassay for kairomones exud- tutkimuslait. Julk. 239: 155–161. ed by fi sh. – Arch. Hydrobiol. 126: 329–337. Leppäkoski, E., Gollasch, S., Gruszka, P., Ojaveer, MacIsaac, H. J., Grigorovich, I. A., Hoyle, J. A., H., Olenin, S. & Panov, V. 2002a: The Baltic – a Yan, N. D. & Panov, V. E. 1999: Invasion of sea on invaders. – Can. J. Fish. Aquat. Sci. 59: Lake Ontario by the Ponto-Caspian predatory 1175–1188. cladoceran Cercopagis pengoi. – Can. J. Fish. Leppäkoski, E., Olenin, S. & Gollasch, S. 2002b: Aquat. Sci. 56: 1–5. The Baltic Sea – a fi eld laboratory for inva- Madsen, J. D., Sutherland, J. W., Bloomfi eld, J. A., sion biology. – In: Leppäkoski, E., Gollasch, Eichler, L. W. & Boylen, C. W. 1991: The de- S. & Olenin, S. (eds.), Invasive aquatic species cline of native vegetation under dense Eurasian of Europe – distribution, impacts and manage- watermilfoil canopies. – J. Aquat. Plant Man- ment, pp. 253–259. Kluwer Academic Publish- age. 29: 94–99. ers, Dordrecht, Netherlands. Magurran, A. E. 1990: The adaptive signifi cance Levin, D. A. 1971: Plant phenolics: an ecological of schooling as an anti-predator defence in fi sh. perspective. – Am. Nat. 105: 157–181. – Ann. Zool. Fennici 27: 51–66. Lima, S. L. 1998a: Nonlethal effects in the ecology Magurran, A. E. & Pitcher, T. J. 1987: Provenance, of predator-prey interactions. – Bioscience 48: shoal size and the sociobiology of predator-eva- 25–34. sion behaviour in minnow shoals. – Proc. R. Lima, S. L. 1998b: Stress and decision making un- Soc. Lond. B 229: 439–465. der the risk of predation: recent developments Major, P. F. 1978: Predator-prey interactions in two from behavioral, reproductive, and ecological schooling fi shes, Caranx ignobilis and Stolepho- perspectives. – Adv. Stud. Behav. 27: 215–290. rus purpureus. – Anim. Behav. 26: 760–777. Lima, S.L. & Dill, L.M. 1990: Behavioral decisions Makarewicz, J. C., Grigorovich, I. A., Mills, E., made under the risk of predation: a review and Damaske, E., Cristescu, M. E., Pearsall, W., prospectus. – Can. J. Zool. 68: 619–640. LaVoie, M. J., Keats, R., Rudstam, L., Hebert, Lindén, E. & Kuosa, H. 2004: Effects of grazing and P., Halbritter, H., Kelly, T., Matkovich, C. & excretion by pelagic mysids (Mysis spp.) on the MacIsaac, H. J. 2001: Distribution, fecundity, biomass and size structure of the phytoplankton and genetics of Cercopagis pengoi (Ostroumov) community. – Hydrobiologia 514: 73–78. (Crustacea, Cladocera) in Lake Ontario. – J. Lindström, M. 1992: Spectral sensitivity and light Great Lakes Res. 27: 19–32. tolerance of mysid species in the Baltic area. 51

Mathis, A. & Vincent, F. 2000: Differential use of Milne, S. W., Shuter, B. J. & Sprules, W. G. 2005: visual and chemical cues in predator recognition The schooling and foraging ecology of lake and threat-sensitive predator-avoidance respons- herring (Coregonus artedi) in Lake Opeongo, es by larval newts (Notophthalmus viridescens). Ontario, Canada. – Can. J. Fish. Aquat. Sci. 62: – Can. J. Zool. 78: 1646–1652. 1210–1218. Matthäus, W. 1995: Natural variability and human Miner, J. G. & Stein, R. A. 1996: Detection of pred- impacts refl ected in long-term changes in the ators and habitat choice by small bluegills: ef- Baltic deep water conditions – a brief review. fects of turbidity and alternative prey. – Trans. – Deutsche Hydrographische Zeitschrift 47: Am. Fish. Soc. 125: 97–103. 47–65. Mittelbach, G. G. & Chesson, P. L. 1987: Predation Mauchline, J. 1971a: The biology of Neomysis inte- risk: indirect effects on fi sh populations. – In: ger (Crustacea, Mysidacea). – J. Mar. Biol. Ass. Kerfoot, W. C. & Sih, A. (eds.): Predation. Di- U.K. 51: 347–354. rect and indirect impacts on aquatic communi- Mauchline, J. 1971b: The biology of Praunus fl exu- ties. University Press of New England, Hanover, osus and P. neglectus (Crustacea, Mysidacea). pp. 315–332. – J. Mar. Biol. Ass. U.K. 51: 641–652. Modlin, R. F. 1990: Observations on the aggregative Mauchline, J. 1971c: Seasonal occurrence of mysids behavior of Mysidium columbiae, the mangrove [Crustacea] and evidence of social behaviour. mysid. – Mar. Ecol. 11: 263–275. – J. Mar. Biol. Ass. U.K. 51: 809–825. Moffat, A. M. & Jones, M. B. 1992: Bionomics McCarraher, D. B. 1971: Survival of some fresh- of Mesopodopsis slabberi and Neomysis inte- water fi shes in the alkaline eutrophic waters of ger (Crustacea: Mysidacea) in the Tamar estu- Nebraska. – J. Fish. Res. Bd. Canada 28: 1811– ary. – In: Köhn, J., Jones, M. B. & Moffat, A. 1814. (eds.), Taxonomy, biology and ecology of Baltic McLaren, I. A. 1974: Demographic strategy of ver- mysids (Mysidacea, Crustacea), pp. 109–119. tical migration by a marine copepod. – Am. Nat. Rostock University. 108: 91–102. Mogdans, J. 2005: Adaptations of the fi sh lateral McLusky, D. S. 1979: Some effects of salinity and line for the analysis of hydrodynamic stimuli. temperature on the osmotic and ionic regulation – Mar. Ecol. Prog. Ser. 287: 289–292. of Praunus fl exuosus (Crustacea, Mysidacea) Moore, P. & Grimaldi, J. 2004: Odor landscapes and from Isefjord. – Ophelia 18: 191–203. animal behavior: tracking odor plumes in differ- Mead, K. S. 2005: Reception before perception: ent physical worlds. – J. Mar. Sys. 49: 55–64. how fl uid fl ow affects odor signal encounter by Mordukhai-Boltovskoi, P. D. & Rivier, I. K. 1971: olfactory sensors. – Mar. Ecol. Prog. Ser. 287: A brief survey of the ecology and biology of the 285–289. Caspian Polyphemoidea. – Mar. Biol. 8: 160– Mees, J., Abdulkerim, Z. & Hamerlynck, O. 1994: 169. Life history, growth and reproduction of Neo- Mullin, M. M. & Roman, M. R. 1986: In situ feed- mysis integer in the Westerschelde estuary (SW ing of a schooling mysid, Anisomysis sp., on Netherlands). – Mar. Ecol. Prog. Ser. 109: 43– Davies Reef – MECOR #4. – Bull. Mar. Sci. 39: 57. 623–629. Merilaita, S. 2001: Habitat heterogeneity, predation Munsterhjelm, R. 1997: The aquatic macrophyte and gene fl ow: colour polymorphism in the iso- vegetation of fl ads and gloes, S coast of Finland. pod, Idotea baltica. – Evol. Ecol. 15: 103–116. – Acta Bot. Fenn. 157: 1–68. Mikheev, V. N., Pasternak, A. F. & Wanzenboeck, Munsterhjelm, R. 2005: Natural succession and hu- J. 2002: Feeding response of perch (Perca fl u- man-induced changes in the soft-bottom mac- viatilis) and roach (Rutilus rutilus) fry to the rovegetation of shallow brackish bays on the presence of a predator. – Doklady Biological southern coast of Finland. – W. & A. de Nott- Sciences 383: 155–157. beck Foundation Sci. Rep. 26: 1–53. 52

Murdoch, W. W. & Bence, J. 1987: General preda- Ohtsuka, S., Inagaki, H., Onbe, T., Gushima, K. & tors and unstable prey populations. – In: Ker- Yoon, Y.H. 1995: Direct observations of groups foot, W.C. & Sih, A. (eds.), Predation. Direct of mysids in shallow coastal waters of western and indirect impacts on aquatic communities, Japan and southern Korea. – Mar. Ecol. Prog. pp. 17–30. University Press of New England, Ser. 123: 33–44. Hanover. Ojaveer, H., Lankov, A., Eero, M., Kotta, J., Kotta, Nakai, S., Inoue, Y., Hosomi, M. & Murakami, A. I. & Lumberg, A. 1999: Changes in the ecosys- 1999: Growth inhibition of blue-green algae by tem of the Gulf of Riga from the 1970s to the allelopathic effects of macrophytes. – Wat. Sci. 1990s. – ICES J. Mar. Sci. 56 Suppl.: 33-40. Tech. 39: 47–53. Ojaveer, H. & Lumberg, A. 1995: On the role of Nakai, S., Inoue, Y., Hosomi, M. & Murakami, A. Cercopagis (Cercopagis) pengoi (Ostroumov) 2000: Myriophyllum spicatum -released allelo- in Pärnu Bay and the NE part of the Gulf of Riga pathic polyphenols inhibiting growth of blue- ecosystem. – Proc. Estonian Acad. Sci. Ecol. 5: green algae Microcystis aeruginosa. – Wat. Res. 20–25. 34: 3026–3032. Ojaveer, H., Simm, M. & Lankov, A. 2004: Popula- Nakai, S., Yamada, S. & Hosomi, M. 2005: Anti- tion dynamics and ecological impact of the non- cyanobacterial fatty acids released from Myrio- indigenous Cercopagis pengoi in the Gulf of Riga phyllum spicatum. – Hydrobiologia 543: 71–78. (Baltic Sea). – Hydrobiologia 522: 261–269. Neil, D. M. & Ansell, A. D. 1995: The orientation Ostrofsky, M. L. & Zettler, E. R. 1986: Chemical of tail-fl ip escape swimming in decapod and defences in aquatic plants. – J. Ecology 74: mysid crustaceans. – J. Mar. Biol. Ass. U.K. 75: 279–287. 55–70. Parrish, J. K. 1989: Re-examining the selfi sh herd: Nichols, S. A. & Shaw, B. H. 1986: Ecological life are central fi sh safer? – Anim. Behav. 38: 1048– histories of the three aquatic nuisance plants, 1053. Myriophyllum spicatum, Potamogeton crispus Partridge, B. L. & Pitcher, T. J. 1980: The sensory and Elodea canadensis. – Hydrobiologia 131: basis of fi sh schools: relative roles of lateral line 3–21. and vision. – J. Comp. Physiol. 135: 315–325. Nordström, H. 1997: Rantavyöhykkeen halkoisjal- Pearre, S. Jr. 1982: Estimating prey preference by kaäyriäisten (Mysidacea) ravinnonkäyttö Itäme- predators: uses of various indices, and a propos- ressä. – M.Sc. thesis, University of Helsinki, 57 al of another based on χ2. – Can. J. Fish. Aquat. pp. + app. (In Finnish.) Sci. 39: 914–923. O’Brien, D. P. 1988: Direct observations of cluster- Pedersen, B. H. 1997. The cost of growth in young ing (schooling and swarming) behaviour in my- fi sh larvae, a review of new hypotheses. – Aqua- sids (Crustacea: Mysidacea). – Mar. Ecol. Prog. culture 155: 259–269. Ser. 42: 235–246. Peltonen, H., Vinni, M., Lappalainen, A. & Pönni, J. O’Brien, D. P. & Ritz, D. A. 1988: Escape respons- 2004: Spatial feeding patterns of herring (Clu- es of gregarious mysids (Crustacea: Mysida- pea harengus L.), sprat (Sprattus sprattus L.), cea): towards a general classifi cation of escape and the three-spined stickleback (Gasterosteus responses in aggregated crustaceans. – J. Exp. aculeatus L.) in the Gulf of Finland, Baltic Sea. Mar. Biol. Ecol. 116: 257–272. – ICES J. Mar. Sci. 61: 966–971. O’Brien, W. J. 1987: Planktivory by freshwater fi sh: Pennak, R. W. 1973: Some evidence for aquatic thrust and parry in the pelagia. – In: Kerfoot, W. macrophytes as repellents for a limnetic species C. & Sih, A. (eds.), Predation. Direct and indi- of Daphnia. – Int. Revue ges. Hydrobiol. 58: rect impacts on aquatic communities, pp. 3–16. 569–576. University Press of New England, Hanover. Pertola, S., Koski, M. & Viitasalo, M. 2002: Stoichi- Ohman, M. D. 1988: Behavioral responses of zoo- ometry of mesozooplankton in N- and P-limited plankton to predation. – Bull. Mar. Sci. 43: areas of the Baltic. – Mar. Biol. 140: 425–434. 530–550. 53

Peuhkuri, N. 1997: Size-assortative shoaling in phausia superba at South Georgia. – J. Cetacean three-spined sticklebacks. – PhD thesis, Univer- Res. Manage. 2: 143–149. sity of Helsinki. 18 pp. Reimchen, T. E. 1994: Predator and morphological Phillips, D. W. 1978: Chemical mediation of inver- evolution in threespine stickleback. – In: Bell, tebrate defensive behaviors and the ability to M. A. & Foster, S. A. (eds.), The evolutionary distinguish between foraging and inactive pred- biology of the threespine stickleback, pp. 240– ators. – Mar. Biol. 49: 237–243. 276. Oxford University Press. Pijanowska, J. & Kowalczewski, A. 1997: Predators Reisewitz, S. E., Estes, J. A. & Simenstad, C. A. can induce swarming behaviour and locomo- 2006: Indirect food web interactions: sea otters tory responses in Daphnia. – Freshwat. Biol. 37: and kelp forest fi shes in the Aleutian archipela- 649–656. go. – Oecologia 146: 623–631. Planas, D., Sarhan, F., Dube, L., Godmaire, H. & Relyea, R. A. 2000: Trait-mediated indirect effects Cadieux, C. 1981: Ecological signifi cance of in larval anurans: reversing competition with the phenolic compounds of Myriophyllum spica- threat of predation. – Ecology 81: 2278–2289. tum. – Verh. Internat. Verein. Limnol. 21: 1492– Relyea, R. A. 2001: Morphological and behavioral 1496. plasticity of larval anurans in response to differ- Quirt, J. & Lasenby, D. 2002: Cannibalism and on- ent predators. – Ecology 82: 523–540. togenetic changes in the response of the fresh- Reznick, D. A., Bryga, H. & Endler, J. A. 1990: Ex- water shrimp Mysis relicta to chemical cues perimentally induced life-history evolution in a from conspecifi c predators. – Can. J. Zool. 80: natural population. – Nature 346: 357–359. 1022–1025. Ribes, M., Coma, R., Zabala, M. & Gili, J.-M. 1996: Rademacher, K. & Kils, U. 1996: Predator/prey dy- Small-scale spatial heterogeneity and seasonal namics of fi fteen-spined stickleback (Spinachia variation in a population of a cave-dwelling spinachia) and the mysid (Neomysis integer). Mediterranean mysid. – J. Plankton Res. 18: – Arch. Fish. Mar. Res. 43: 171–181. 659–671. Rajasilta, M., Mankki, J., Ranta-Aho, K. & Vuori- Ritz, D. A. 1994: Social aggregation in pelagic in- nen, I. 1999: Littoral fi sh communities in the Ar- vertebrates. – Adv. Mar. Biol. 30: 155–216. chipelago Sea, SW Finland: a preliminary study Ritz, D. A. 2000: Is social aggregation in aquatic of changes over 20 years. – Hydrobiologia 393: crustaceans a strategy to conserve energy? – 253–260. Can. J. Fish. Aquat. Sci. 57 (Suppl. 3): 59–67. Ranta, E. & Lindström, K. 1990: Assortative school- Ritz, D. A., Osborn, J. E. & Ocken, A. E. J. 1997: ing in three-spined sticklebacks? – Ann. Zool. Infl uence of food and predatory attack on mysid Fennici 27: 67–75. swarm dynamics. – J. Mar. Biol. Ass. U.K. 77: Ranta, E., Rita, H. & Lindström, K. 1993: Competi- 31–42. tion versus cooperation: success of individuals Romare, P. & Hansson, L.-A. 2003: A behavioral foraging alone and in groups. – Am. Nat. 142: cascade: Top-predator induced behavioral shifts 42–58. in planktivorous fi sh and zooplankton. – Lim- Reebs, S. G. & Saulnier, N. 1997: The effect of hun- nol. Oceanogr. 48: 1956–1964. ger on shoal choice in golden shiners (Pisces: Rönkkönen, S., Ojaveer, E., Raid, T. & Viitasalo, Cyprinidae, Notemigonus crysoleucas). – Ethol- M. 2004: Long-term changes in Baltic herring ogy 103: 642–652. (Clupea harengus membras) growth in the Gulf Reichwaldt, E. S. & Stibor, H. 2005: The impact of Finland. – Can. J. Fish. Aquat. Sci. 61: 219– of diel vertical migration of Daphnia on phyto- 229. plankton dynamics. – Oecologia 146: 50–56. Roos, C., Rönnberg, O., Berglund, J. & Alm, A. Reid, K., Brierley, A. S. & Nevitt, G. A. 2000: An 2004: Long-term changes in macroalgal com- initial examination of relationships between the munities along ferry routes in a northern Baltic distribution of whales and Antarctic krill Eu- archipelago. – Nord. J. Bot. 23: 247–259. 54

Rudstam, L. G., Danielsson, K., Hansson, S. & Jo- Savino, J. F. & Stein, R. A. 1982: Predator-prey hansson, S. 1989: Diel vertical migration and interactions between largemouth bass and blue- feeding patterns of Mysis mixta (Crustacea, gills as infl uenced by simulated, submersed veg- Mysidacea) in the Baltic Sea. – Mar. Biol. 101: etation. – Trans. Am. Fish. Soc. 111: 255–266. 43–52. Savino, J. F. & Stein, R. A. 1989: Behavioural in- Rudstam, L. G. & Hansson, S. 1990: On the ecol- teractions between fi sh predators and their prey: ogy of Mysis mixta (Crustacea, Mysidacea) in a effects of plant density. – Anim. Behav. 37: coastal area of the northern Baltic proper. – Ann. 311–321. Zool. Fennici 27: 259–263. Scharf, F. S., Buckel, J. A., McGinn, P. A. & Juanes, Rudstam, L. G., Hansson, S., Johansson, S. & Lars- F. 2003: Vulnerability of marine forage fi shes to son, U. 1992: Dynamics of planktivory in a piscivory: effects of prey behavior on suscepti- coastal area of the northern Baltic Sea. – Mar. bility to attack and capture. – J. Exp. Mar. Biol. Ecol. Prog. Ser. 80: 159–173. Ecol. 294: 41–59. Rudstam, L. G., Hansson, S. & Larsson, U. 1986: Schoeppner, N. M. & Relyea, R. A. 2005: Damage, Abundance, species composition and production digestion, and defence: the roles of alarm cues of mysid shrimps in a coastal area of the north- and kairomones for inducing prey defences. ern Baltic proper. – Ophelia Suppl. 4: 225–238. – Ecol. Let. 8: 505–512. Ryer, C. H. 1988: Pipefi sh foraging: effects of fi sh Schofi eld, P. J. 2003: Habitat selection of two gobies size, prey size and altered habitat complexity. (Microgobius gulosus, Gobiosoma robustum): – Mar. Ecol. Prog. Ser. 48: 37–45. infl uence of structural complexity, competitive Ryer, C. H. & Olla, B. L. 1991: Information transfer interactions, and presence of a predator. – J. and the facilitation and inhibition of feeding in a Exp. Mar. Biol. Ecol. 288: 125–137. schooling fi sh. – Env. Biol. Fish. 30: 317–323. Schubert, H. & Yousef, M. A. M. 2001: Charophytes Ryer, C. H. & Olla, B. L. 1992: Social mechanisms in the Baltic Sea – threats and conservation. facilitating exploitation of spatially variable – Schriftenr. Landschaftspfl ege Naturschutz 72: ephemeral food patches in a pelagic marine fi sh. 7–8. – Anim. Behav. 44: 69–74. Schutten, J., Dainty, J. & Davy, A. J. 2004: Wave- Sakwinska, O. 2002: Response to fi sh kairomone in induced hydraulic forces on submerged aquatic Daphnia galeata life history traits relies on shift plants in shallow lakes. – Ann. Bot. 93: 333– to earlier instar at maturation. – Oecologia 131: 341. 409–417. Scrimshaw, S. & Kerfoot, W. C. 1987: Chemical Sakwinska, O. & Dawidowicz, P. 2005: Life history defenses of freshwater organisms: beetles and strategy and depth selection behavior as alterna- bugs. – In: Kerfoot, W. C. & Sih, A. (eds.), Pre- tive antipredator defenses among natural Daph- dation. Direct and indirect impacts on aquatic nia hyalina populations. – Limnol. Oceanogr. communities, pp. 240–262. University Press of 50: 1284–1289. New England, Hanover. Salemaa, H., Tyystjärvi-Muuronen, K. & Aro, E. Sih, A. 1982: Foraging strategies and the avoidance 1986: Life histories, distribution and abundance of predation by an aquatic insect, Notonecta of Mysis mixta and Mysis relicta in the northern hoffmanni. – Ecology 63: 786–796. Baltic Sea. – Ophelia Suppl. 4: 239–247. Sih, A. 1987: Predators and prey lifestyles: an evo- Salemaa, H., Vuorinen, I. & Välipakka, P. 1990: lutionary and ecological overview. – In: Ker- The distribution and abundance of Mysis popu- foot, W. C. & Sih, A. (eds.), Predation. Direct lations in the Baltic Sea. – Ann. Zool. Fennici and indirect impacts on aquatic communities, 27: 253–257. pp. 203–224. University Press of New England, Sandström, A. 1999: Visual ecology of fi sh – a review Hanover. with special reference to percids. – Fiskeriverket Sih, A. 1992a: Prey uncertainty and the balancing of rapport 2: 45–80. antipredator and feeding needs. – Am. Nat. 139: 1052–1069. 55

Sih, A. 1992b: Integrative approaches to the study Spencer, D. F. & Ksander, G. G. 1999: Phenolic ac- of predation: general thoughts and a case study ids and nutrient content for aquatic macrophytes on sunfi sh and salamander larvae. – Ann. Zool. from Fall River, California. – J. Freshwat. Ecol. Fennici 29: 183–198. 14: 197–209. Sih, A., Kats, L. B. & Moore, R. D. 1992: Effects of Stabell, O. B. & Lwin, M. S. 1997: Predator-induced predatory sunfi sh on the density, drift, and ref- phenotypic changes in crucian carp are caused uge use of stream salamander larvae. – Ecology by chemical signals from conspecifi cs. – Env. 73: 1418–1430. Biol. Fish. 49: 145–149. Simm, M. & Ojaveer, H. 2006: Taxonomic status Stansfi eld, J. H., Perrow, M. R., Tench, L. D., Jowitt, and reproduction dynamics of the non-indig- A. J. D. & Taylor, A. A. L. 1997: Submerged enous Cercopagis in the Gulf of Riga (Baltic macrophytes as refuges for grazing Cladocera Sea). – Hydrobiologia 554: 147–154. against fi sh predation: observations on seasonal Singleton, V. L. & Kratzer, F. H. 1969: Toxicity changes in relation to macrophyte cover and and related physiological activity of phenolic predation pressure. – Hydrobiologia 342/343: substances of plant origin. – J. Agricult. Food 229–240. Chem. 17: 497–512. Stein, R. A. & Magnuson, J. J. 1976: Behavioral re- Skajaa, K., Fernö, A. & Folkvord, A. 2004: Onto- sponse of crayfi sh to a fi sh predator. – Ecology genetic- and condition-related effects of starva- 57: 751–761. tion on responsiveness in herring larvae (Clu- Stemberger, R. S. & Gilbert, J. J. 1987: Defenses of pea harengus L.) during repeated attacks by a planktonic rotifers against predators. – In: Ker- model predator. – J. Exp. Mar. Biol. Ecol. 312: foot, W.C. & Sih, A. (eds.): Predation. Direct 253–269. and indirect impacts on aquatic communities. Slusarczyk, M. 1999: Predator-induced diapause in University Press of New England, Hanover, pp. Daphnia magna may require two chemical cues. 227–239. – Oecologia 119: 159–165. Sterry, P. R., Thomas, J. D. & Patience, R. L. 1983: Smith, C. S. & Barko, J. W. 1990: Ecology of Eur- Behavioural responses of Biomphalaria gla- asian watermilfoil. – J. Aquat. Plant Manage. brata (Say) to chemical factors from aquatic 28: 55–64. macrophytes including decaying Lemna pauci- Snickars, M., Sandström, A. & Mattila, J. 2004: An- costata (Hegelm ex Engelm). – Freshwat. Biol. tipredator behaviour of 0+ year Perca fl uviatilis: 13: 465–476. effect of vegetation density and turbidity. – J. Steven, D. M. 1961: Shoaling behaviour in a mysid. Fish. Biol. 65: 1604–1613. – Nature 21: 280–281. Snoeijs, P. 1999: Marine and brackish waters. – Acta Stirling, G. 1995: Daphnia behaviour as a bioassay Phytogeogr. Suec. 84: 187–212. of fi sh presence or predation. – Funct. Ecol. 9: Søndergaard, M., Jeppesen, E. & Aaser, H. F. 2000: 778–784. Neomysis integer in a shallow hypertrophic Stoner, A. W. 1982: The infl uence of benthic mac- brackish lake: distribution and predation by rophytes on the foraging behavior of pinfi sh, three-spined stickleback (Gasterosteus aculea- Lagodon rhomboides (Linnaeus). – J. Exp. Mar. tus). – Hydrobiologia 428: 151–159. Biol. Ecol. 58: 271–284. Sørnes, T. A. & Aksnes, D. L. 2004: Predation ef- Sundell, J. 1994: Dynamics and composition of lit- fi ciency in visual and tactile zooplanktivores. toral fi sh fauna along the coast of SW-Finland. – Limnol. Oceanogr. 49: 69–75. – Aqua Fennica 24: 37–49. Speirs, D., Lawrie, S., Raffaelli, D., Gurney, W. & Takahashi, K., Hirose, T., Azuma, N. & Kawagu- Emes, C. 2002: Why do shallow-water predators chi, K. 2004: Diel and intraspecifi c variation in migrate? Strategic models and empirical evi- vulnerability of the beach mysid, Archaeomysis dence from an estuarine mysid. – J. Exp. Mar. kokuboi II, 1964, to fi sh predators. – Crusta- Biol. Ecol. 280: 13–31. ceana 77: 717–728. 56

Therriault, T. W., Grigorovich, I. A., Kane, D. D., Twining, B. S., Gilbert, J. J. & Fisher, N. S. 2000: Haas, E. M., Culver, D. A. & MacIsaac, H. J. Evidence of homing behavior in the coral reef 2002: Range expansion of the exotic zooplank- mysid Mysidium gracile. – Limnol. Oceanogr. ter Cercopagis pengoi (Ostroumov) into western 45: 1845–1849. Lake Erie and Muskegon Lake. – J. Great Lakes Uitto, A., Gorokhova, E. & Välipakka, P. 1999: Dis- Res. 28: 698–701. tribution of the non-indigenous Cercopagis pen- Thetmeyer, H. & Kils, U. 1995: To see and not be goi in the coastal waters of the eastern Gulf of seen; the visibility of predator and prey with re- Finland. – ICES J. Mar. Sci. 56 Suppl.: 49–57. spect to feeding behaviour. – Mar. Ecol. Prog. Uitto, A, Kaitala, S., Kuosa, H. & Pajuniemi, R. Ser. 126: 1–8. 1995: Effect of nutrient addition and predation Thiel, R. 1992: Quantitative estimation of mysids of mysid shrimp (Neomysis integer) on a plank- – Neomysis integer (Leach, 1814) – and their ton community in a short-term enclosure experi- production within a typical southern Baltic ment in the northern Baltic. – Aqua Fennica 25: bay. – In: Köhn, J., Jones, M.B. & Moffat, A. 23–31. (eds.), Taxonomy, biology and ecology of Baltic Urho, L. 2002: The importance of larvae and nurs- mysids (Mysidacea, Crustacea), pp. 73–78. Ros- ery areas for fi sh production. – Ph.D. thesis, tock University. University of Helsinki, 118 pp. Thiel, R. 1996: The impact of fi sh predation on the Utne-Palm, A. C. 2002: Visual feeding of fi sh in a zooplankton community in a southern Baltic turbid environment: physical and behavioural bay. – Limnologica 26: 123–137. aspects. – Mar. Fresh. Behav. Physiol. 35: 111– Thorisson, K. 2006: How are the vertical migrations 128. of copepods controlled? – J. Exp. Mar. Biol. Väinölä, R. 1986: Sibling species and phylogenetic Ecol. 329: 86–100. relationships of Mysis relicta (Crustacea: Mysi- Timmermann, M., Schlupp, I. & Plath, M. 2004: dacea). – Ann. Zool. Fennici 23: 207–221. Shoaling behaviour in a surface-dwelling and a Väinölä, R., Audzijonyte, A. & Riddoch, B. J. 2002: cave-dwelling population of a barb Garra bar- Morphometric discrimination among four spe- reimiae (Cyprinidae, Teleostei). – Acta Ethol. 7: cies of the Mysis relicta group. – Arch. Hydro- 59–64. biol. 155: 493–515. Tiselius, P., Jonsson, P. R. & Verity, P. G. 1993: A Väinölä, R., Riddoch, B. J., Ward, R. D. & Jones, model evaluation of the impact of food patchi- R. I. 1994: Genetic zoogeography of the Mysis ness on foraging strategy and predation risk in relicta species group (Crustacea: Mysidacea) in zooplankton. – Bull. Mar. Sci. 53: 247–264. northern Europe and North America. – Can. J. Torgersen, T. 2003: Proximate causes for anti- Fish. Aquat. Sci. 51: 1490–1505. predatory feeding suppression by zooplankton Väinölä, R. & Vainio, J. K. 1998: Distributions, life during the day: reduction of contrast or motion cycles and hybridization of two Mysis relicta – ingestion or clearance? – J. Plankton Res. 25: group species (Crustacea: Mysida) in the north- 565–571. ern Baltic Sea and Lake Båven. – Hydrobiologia Trama, F. B. 1954: The pH tolerance of the common 368: 137–148. bluegill (Lepomis macrochirus Rafi nesque). Välipakka, P. 1992: Distribution of mysid shrimps – Notulae Naturae 256: 1–13. (Mysidacea) in the Bay of Mecklenburg (west- Tsuda, A., Saito, H. & Hirose, T. 1998: Effect of gut ern Baltic Sea). – In: Köhn, J., Jones, M.B. & content on the vulnerability of copepods to vi- Moffat, A. (eds.), Taxonomy, biology and ecol- sual predation. – Limnol. Oceanogr. 43: 1944– ogy of Baltic mysids (Mysidacea, Crustacea), 1947. pp. 61–72. Rostock University. Turner, G. F. & Pitcher, T. J. 1986: Attack abate- Vance-Chalcraft, H. D. & Soluk, D. A. 2005: Es- ment: a model for group protection by com- timating the prevalence and strength of non- bined avoidance and dilution. – Am. Nat. 128: independent predator effects. – Oecologia 146: 228–240. 452–460. 57

Van de Meutter, F., De Meester, L. & Stoks, R. heteroclitus (L.) in relation to prey size and hab- 2005a: Water turbidity affects predator-prey in- itat structure: consequences for prey distribution teractions in a fi sh-damselfl y system. – Oecolo- and abundance. – J. Exp. Mar. Biol. Ecol. 23: gia 144: 327–336. 255–266. Van de Meutter, F., Stoks, R. & De Meester, L. Visser, A.W. 2001: Hydromechanical signals in the 2005b: Spatial avoidance of littoral and pelagic plankton. – Mar. Ecol. Prog. Ser. 222: 1–24. invertebrate predators by Daphnia. – Oecologia Vogt, H. & Schramm, W. 1991: Conspicuous de- 142: 489–499. cline of Fucus in Kiel Bay (Western Baltic): Viherluoto, M. 2001: Food selection and feeding be- what are the causes? – Mar. Ecol. Prog. Ser. 69: haviour of Baltic Sea mysid shrimps. – W. & A. 189–194. de Nottbeck Foundation Sci. Rep. 23: 1–35. Walls, M. & Ketola, M. 1989: Effects of predator- Viherluoto, M., Kuosa, H., Flinkman, J. & Viitasa- induced spines on individual fi tness in Daphnia lo, M. 2000: Food utilisation of pelagic mysids, pulex. – Limnol. Oceanogr. 34: 390–396. Mysis mixta and M. relicta, during their growing Wallström, K., Mattila, J., Sandberg-Kilpi, E., Ap- season in the northern Baltic Sea. – Mar. Biol. pelgren, K., Henricson, C., Liljekvist, J., Mun- 136: 553–559. sterhjelm, R., Odelström, T., Ojala, P., Persson, Viherluoto, M. & Viitasalo, M. 2001a: Temporal J. & Schreiber, H. 2000: Miljötillstånd i grunda variability in functional responses and prey havsvikar. Beskrivning av vikar i regionen Upp- selectivity of the pelagic mysid, Mysis mixta, land-Åland-sydvästra Finland samt utvärdering in natural prey assemblages. – Mar. Biol. 138: av inventeringsmetoder. – Upplandstiftelsen 18: 575–583. 1–114, 6 app. (In Swedish.) Viherluoto, M. & Viitasalo, M. 2001b: Effect of Warfe, D. M. & Barmuta, L. A. 2004: Habitat struc- light on the feeding rates of pelagic and litto- tural complexity mediates the foraging success ral mysid shrimps: a trade-off between feeding of multiple predator species. – Oecologia 141: success and predation avoidance. – J. Exp. Mar. 171–178. Biol. Ecol. 261: 237–244. Weckström, K. 2005: Recent eutrophication of Viitasalo, M., Kiørboe, T., Flinkman, J., Pedersen, coastal waters in southern Finland – a palaeo- L. W. & Visser, A. W. 1998: Predation vulner- limnological assessment. – Ph.D. thesis, Uni- ability of planktonic copepods: consequences versity of Helsinki, 53 pp. + app. of predator foraging strategies and prey sensory Weissburg, M. J., Ferner, M. C., Pisut, D. P. & Smee, abilities. – Mar. Ecol. Prog. Ser. 175: 129–142. D. L. 2002: Ecological consequences of chemi- Viitasalo, M. & Rautio, M. 1998: Zooplanktivory cally mediated prey perception. – J. Chem. Ecol. by Praunus fl exuosus (Crustacea: Mysidacea): 28: 1953–1970. functional responses and prey selection in rela- Wiederholm, A.-M. 1987: Habitat selection and tion to prey escape responses. – Mar. Ecol. Prog. interactions between three marine fi sh species Ser. 174: 77–87. (Gobiidae). – Oikos 48: 28–32. Vilhunen, S. & Hirvonen, H. 2003: Innate anti- Winfi eld, I. J. 1986: The infl uence of simulated predator responses of Arctic charr (Salvelinus aquatic macrophytes on the zooplankton con- alpinus) depend on predator species and their sumption rate of juvenile roach, Rutilus rutilus, diet. – Behav. Ecol. Sociobiol. 55: 1–10. rudd, Scardinius erythrophthalmus, and perch, Vilhunen, S., Hirvonen, H. & Laakkonen, M. V.-M. Perca fl uviatilis. – J. Fish Biol. 29 (Suppl. A): 2005: Less is more: social learning of predator 37–48. recognition requires a low demonstrator to ob- Winkler, G. & Greve, W. 2004: Trophodynamics server ratio in Arctic charr (Salvelinus alpinus). of two interacting species of estuarine mysids, – Behav. Ecol. Sociobiol. 57: 275–282. Praunus fl exuosus and Neomysis integer, and Vince, S., Valiela, I., Backus, N. & Teal, J. M. 1976: their predation on the calanoid copepod Eury- Predation by the salt marsh killifi sh Fundulus temora affi nis. – J. Exp. Mar. Biol. Ecol. 308: 127–146. 58

Witschi, W. A. & Ziebell, C. D. 1979: Evaluation predation threat in fi ddler crabs: trust thy neigh- of pH shock on hatchery-reared rainbow trout. bor as thyself? – Behav. Ecol. Sociobiol. 58: – Prog. Fish-Cult. 41: 3–5. 345–350. Wojtal, A., Frankiewicz, P., Izydorczyk, K. & Zal- Wooldridge, T. H. & Webb, P. 1988: Predator-prey ewski, M. 2003: Horizontal migration of zoo- interactions between two species of estuarine plankton in a littoral zone of the lowland Sule- mysid shrimps. – Mar. Ecol. Prog. Ser. 50: 21– jow Reservoir (Central Poland). – Hydrobiologia 28. 506-509: 339–346. Wootton, R. J. 1984: A functional biology of stickle- Wolf, N. G. 1985: Odd fi sh abandon mixed-species backs. – Croom Helm, London. groups when threatened. – Behav. Ecol. Socio- Zelickman, E. A. 1974: Group orientation in Neo- biol. 17: 47–52. mysis mirabilis (Mysidacea: Crustacea). – Mar. Wong, B. B. M., Bibeau, C., Bishop, K. A. & Biol. 24: 251–258. Rosenthal, G. G. 2005: Response to perceived