Mingand swimming behaviour of Bythotmphes cedemtmemi Schoedler

James Robert Muirhead

A thesis submitted in conformity with the requirernents for the degree of Master of Science Graduate Department of Zoology University of Toronto

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Master of Science, 1999

James Robert Muirhead

Graduate Department of Zoology

University of Toronto

Abstract

Predatory zooplankton detect prey with mechanoreception, visual cues or chemoreception. The objectives were to quantify swimming behaviour of Bytbnephes cederstroemi under the influence of prey, prey kairomones, and light and to mode1

Bythotrephes' encounter rate with prcy f?om Harp Lake. 1 used video capture and motion tracking to rneasure Bythrrephes swimming behaviour with Daphnia, Daphnia kairomnes and Polyphemus as prey. 1 also measured Bythotrephes' reaction distance to prey under different light levels. Bythoirephes swam fastest and in a more directeci fashion in the presence of light and prey than in treatments with Light or prey only. Also,

Bythotrephes' reaction distance increased in higher light levels and thus is likely to use vision as a detection rnechanism Results fkom the encounter mode1 show that small, slow-moving prey faced the greatest risk fkom Byrhomephes. Acknowledgements

1 sincerely thank my supervisior, Dr. W. Gary Sprules, for his advice and support when 1 rnost needed it, and for his encouragement throughout. I also thank my CO-supervisor, Dr. Rob

Baker, for helpful advice on animai behaviour. Special thanks go to Dr. Charles Rarncharan who provided me with the motion-tracking and analysis software he had personnaiiy written.

Additional thanks go to Dr, Hank Vandcrploeg at the Great Lakes Environmental Research Labs in Ann Arbr, Michigan for the use of his video setup for critical stages of my experiments; Dr.

Nom Yan and Bob Guard at the Ministry of Energy and the Environment (Dorset) for providing equipment and data on a moment's notice and Cristina Dumitru and Giliian Morgan for providing Bythutrephes abundance and diet preference data. Thanks also go to Sam Genova for his help with field collections of Bythotrephes.

The easy-going, fnendly atmosphere and personal support in the lab would not be possible without the presence of Dr. Stuart Whipple (Uncle Stu) and Agnes Blukacz (Rzapka).

Stuart: We taiked about issues arising from my thesis into the wee hours of the rnorning more often than 1 can remember. Agnes: 1 depended the rnost on Agnes while we were coliecting zooplankton fiom Harp Lake. I don't think either one of us is going to forget the trouble we went through to bring biick îive Bythotrephes. To Stuart and Agnes, 1 thank for completing experiments critical to my thesis; without their help I probably would not be able to finish.

Findy, 1 thank my famiiy Keith, Ga& Jackie and Karen who were always in my thoughts while 1 was away working on my Master's; I dedicate my thesis to them

iii Table of Contents Page

Abstract ii

Acknowledgements iii

List of Tables v

List of Figures vi

List of Appendices vii

Generai Introduction 1

Chapter 1 3

Inmductio n 4

Materials and Methods 7

Results 20

Discussion 30

References 38

Chapter 2 42

Introduction 43

Materials and Methods 45

Results 58

Discussion 76

References 83

General Discussion and Conclusions 86

References 88

Appendices 90

References 96 Page

Table 1. Second-order mean vector r and swimming velocity 23 with bootstrapped standard emrs.

Table 2. MANOVA summary tables for Daphnia kairomone and Light experiments.

Table 3. Mean of bootstrapped Sb for second-order angles, eigenvalues of ma& T and probabilities.

Table 1. Average prey swimming speeds.

Table 2. Prey risk factor (percentage in Bythotrephes diet) of species and lifestages.

Table 9. Interpretation of spherical distribution of directions based 94 on eigenvalues and eigenveçtors of matix T. Page

Figure 1. Video setup

Figure 2. Frequency histogram of swUnming behaviours 12

Figure 3. Second-order analysis of angles 18

Figure 4. Bythotrephes swïmming tracks under different treatrnents 21-22

Figure 5. Prey versus light interaction plot 27

Figure 1. Reaction distance Nming chamber 46

Figure 2. Rdonsphere for Byrhomphes 50

Figure 3. Circular distributions of Bythtrephes apparent reaction 59 distances under different light levels.

Figure 4. Relationship between Light level and reaction distance 61 for a third instar Bythotrephes.

Figure 5. Enco unter rate, to ta1 Bythotrephes-calanoid CO pepodid 62-64 encounters and prey risk.

Figure 6. Average encounter rates for Byrhotrephes and associated 66-68 prey risk at 0-10 m depth over 24 hows.

Figure 7. Encounter rate and prey risk wnsitivity responses to a 20% 70 change in surîàce Light levels.

Figure 8. Encounter rate and prey risk sensitivity responses to a 20% 71 change in Byrhotrephes swimming velocit y.

Figure 9. Encounter rate and prey risk sensitivity responses to a 20% 73 change in Dophnia swimming velocity.

Figure 10. Mean number and standard error of encounters of cyclopoid 74 nauplii for a single third instar Bythotrephes.

Figure 11. Mean number and standard emr of total cyclopoid naupüi 75 and Bythotrephes encounters. Page

Appendix 1. Description of motion aacking program 90

Appendix 11. Circulat and spherical gcarnctry and analysis 91

Appendix III. Effect of sample rate on Bythoîrephes swimming 95 parame ters.

vii General Introduction

One of the rnost ofien asked and pressing questions about invading species is what are some of the fùnctional characteristics of the species that permit a successful invasion. From individual case studies and literature surveys, many ecologists have proposed that successful invading species typically have r-selected traits, high dispersal rates. high genetic variabity. phenotypic plasticity, and polyphagy (Lodge 1993; Johnson and Carlton 1996; Rejmanek and

Richardson 19%). Although many successful invading species share cornmon traits, it is not possible to predict with certainty the invasive potencial of individual species in target comrnunities (Burke and Grime 19%). Predictive studies, according to Lodge (1993), require focused, quantitative studies on pYtrular invaders and comrnunity characteristics.

Bythoîrephes cederstroerni Schoedler (Onychopoda, Cercopagidae) is a large-bodied carnivorous hshwater moplankter with an elongated caudal spine that invaded the Great Lakes in the early-1980's kom Lake Lagoda, CIS (Bur et aL 1986; Evans 1988; Sprules et aL 1990) and has since spread to several inland lakes (Hall and Yan 1997). Bythotrephes' invasion success can be partly attributed to adaptations such as the barbed caudal spine which provides some protection against gape-lunited small fish (Branstrator and Lehrnan 1996). fast pmhenogenic reproduction and the production of resting eggs which serves as a source for restockïng the population after the winter (Rivier 1998). Adaptations for a predatory lifestyle include a large, media1 compound eye, fast swimming, reduced carapace and dedicated thoracic limbs used in prey capture. Despite the interest shown in Bythotrephes as an invasive species. there has been litt le work on Bythotrep hes' predatory be haviour. Predatory zooplankton typicaily search for prey by detecting hydrornechanical signals produced by their prey's swimming (Kerfoot 1978; Rice 1988). In addition, iike the predatory zooplankton Mysis relicfa (Ramcharan and Sprules 1986), Leprodora kindrii (Wolken and Gallik 1965) and Polyphemus pediculus (Odselius and Niisson 1983), Bythoh-ephes' compound eye is likely able to form images, in Chapter 1,I examine how Bythotrephes detects prey by rneaswing changes in

Bythotrephes swimming be haviour under various experimental conditions. By using vision,

Bythotrephes is able to detect prey at a greater distance in the presence of visible light than by mec hanoreception or direct enco unter and thus increase its foraging efficiency.

In Chapter 2,I measure the distance at whic h Bythotrephes reacts to prey under different light Ievels. 1 then model the encounter rate of Byrhotrephes with dflerent prey fÏom Harp Lake based on this reaction distance and swimming speeds fiom Chapter 1. B y incorporating a diunial light cycle and Lght attenuation in the water column into the model, 1 am able to identify spatial and temporal patterns of maximum risk faced by prey as well as qualify the relative rïsk among zooplankton species in Harp Lake.

Predation pressures exened by invading species such as Bythotrephes can have significant effects in stmcturing the zooplankton comrnunity (Lodge 1993; Yan and Pawson

1997). Impact models and encounter models baseà on static reaction distances (Gemtsen and

Strickler 1977; Giguère et aL 1982), may seriously underestirnate the number of encounters for a visual predator whose reaction distance wili Vary depending on ambient lig ht conditions influenced by the time of day and depth in the water column. Chapter 1

Swimming and predatory behaviour of Bythotrephes cedersimemi under

the influence of prey hydrodynamic cues, prey kaimrnones and light Introduction

The swirnming and feeding behaviour of herbivorous and planktivorous mpiankton has received considerable attention because of their pivotal role in energy transfer through aquatic food webs, When planktivorous £ish are scarce or absent, zooplanktivorous invertebrates can exert significant predation pressure on the srnailer members of the zooplankton community

(Dodson 1974). The quantification of swimming and feeding behaviour is essential for use in predictions of impact (Yan and Pawsun W98). bioenergetics and developrnent models (Lehman and Branstrator 1995; Yurista and Schultz 1995; Yurista and Schultz 1996) and predator/prey encounter models (Gerritsen and Strickler 1977).

Bythotrephes cederstroemi Schoedler (Onychopoda, Cercopagidae), whic h will be referred to as Byrhotrephes, is an invading predatory ciadoceran which was discovered in Lake

Huron in 1984 and has since spread to the other Great Lakes and inland Iakes (Bur et ai. 1986;

Evans 1988; Makarewicz and Jones 1990). Bythotrephes is a large-bodied, camivoro us keshwater cladoceran that swims continously through the water column and preys on a wide range of zooplankton, but prefers smali, slow-moving cladocerans based on maximum mortality and clearance rates (e-g., Polyphemus, Bosmina, Ceriodaphnia )(Mordukhai-Bo ltovskaia 1958;

Vanderploeg et ai. 1993; Rivier 1998) They also have ken reponed to consume copepod nauplü

(Vanderploeg et al. 1993). rotifers, and are also cannibalistic. At typical ephetic temperatures, a mature Bythotrephes kills approximately three-quarters of its body weight in prey per day (Lehrnan et al. 1997).

Whiie a considerable amount is known about copepod and Chaoborus feeding behaviour and mechanism. relatively little is known about the predatory behaviour of Bythomephes.

Investigations on the fecding mechanism of herbivorous copepods suggest that chemoreception is the sense most likely used in remote detection and capture of food particles (Stcickler 1982;

Buskey 1984; Paffenhofcr and Van Sant 1985). Poulet and Marsot (1978) demnstrated that copepods were able to discriminate between a homogenate of particles collecteci fkom a ph yt oplankton bloom and non-e~ched mirocapsuies. Chmbonrr increased their mvement fiequency in prey-conditioned water, thus increasing the probability of encounter (Berendonk and 0' Brien 1996). Invertebrate predators such as large caianoid copepods, insect larvae and chaetognaths use sensory setae to detect hydrodynaniic disturbances caused by the swinuning or feeding motion of their prey (Strickier and Bal 1973; Kerfoot 1978). Chaobuncs are able to detect such disturbances up to 4 mm (Reissen et ai. 1984; Kirk 1985) and are able to feed successfully without light (Newbury 1972). Visual detection of prey is Limiteci to a small group of carnivorous zooplankton kause most are unabIe to forrn images. However, the presence of compound eyes and increased feeding rate in the presence of light in the cyclopoid Corycaeus

(Gophen and Harris 1981) and Mysis (Ramcharan and Sprules 1986) suggests that vision plays an important de.

On the basis of its morphology, Bythomephes, like many predatory zooplankton, is most likely to use a combination of rnechanisrns for prey detection and feeding. The first antemules typicaily bear six aethetascs (sensory papiliae), five of which are probably responsible for chernoreception and one setae which acts as a mechanoreceptor (Martin and Cash-Clark 1995;

Rivier 1998). Additionai mechanoreceptors are located on the mandible, labnim, rnaxiliary process and thoracic limbs (Martin and Cash-Clark 1995). One of the most conspicuous features of Bythomephes is the presence of a large, medial compound eye which consists of at least 200 omrnatidia capable of forming images (Martin and Cash-Clark 1995). As a large, fast-swimming conthously moving predator, Bythrrephes foraging

behaviour is efficient only if its velocity is significantly higher than that of its prey and encounter

with prey increases fàster than the rate of energy expenditure associated with faster swimming

(Gemtsen and Stricklcr 1977). Ah, Bytiwtrephes maximizes the volume of water explored by

swimmùig dong highly tortuous paths; directed swimming is efficient for prey encounter only

once Bythotrephes has detected prey. The probability of encounter is ais0 dependent on the

reaction distance of Bytbn-ephes to prey. Visual detection range is approximately 12- 14 mm

based on O bserved changes in Bythotrephes swinuning velocity whereas remo te chemoreception

and rnechanoreception have a much smaller range (4 mm for mechanosensory detection in

copepods (Wong 1996) and 3 mm for Chaobom amencanus (Reissen et ai. 1984)). Thus,

visual detection is important on the basis that it provides information about prey at a maximum

range.

I hypothesize that Bythotrephes uses visual cues to detect prey at a greater distance

foLiowed by mechanoreception in order to increase prey encounter rates. Prey detection by

mechanoreception becornes most important deep in the water column and at night as

Bythotrephes feeds during both night and day (Vanderploeg et al. 1993). Like Chaoborus

(Berendonk and 0' Brien 1996), Bythotrephes should ahincrease its swhmhg velocity in the

presence of prey kairornones but this mechanisrn is probably the least important in terms of

rernote detection. Using video techniques and 3-dimensional motion analysis, 1present results of

Byrhotrephes swimming behaviour in the presence of Daphnia, Daphnia kairomones and

Polyphemus with and without visible light. Materials and Methods

Field collections and culture conditions

The experiments of Bythotrephes swimming behaviour in the presence of Daphnia,

Duphnia kairomones and Polyphemur wert run between July 1997 and Septembcr 1998. 1 collected zooplankton each surnmer between July and September 1997 and 1998 accordhg to need-

Bythotrephes were cokted fkom Harp Lake, Ontario (45" 05' N 61° 45' W) and prey were collected fiom Heney Lake (45" 23' N 79" 07' W) and Paint Lake (4S0 13' N 78" 57' W).

Harp Lake is a 37.5 m deep, 71.4 ha single-basin lake typical of Precambrian shield lakes (Yan and Pawson 1997). Heney Lake and Paint Lake were selected as they contain srnail cladocerans and copepods which Bythotrephes can consume (N. Yan, pers. corn), yet have not previously encountered Bythonephes. 1 also used laboratory stocks of Daphnia magna Straus.

1 collected both Bythotrephes and prey by towing a large plankton net (0.75 m diameter,

285 pm rnesh size) at a depth of 1-5 m for a distance of approximately 50 rn Prey were transferred into 20 L plastic carbuoys completely covered with heavy black plastic. 1 tapbd fkeezer packs to the sides of the carbuoys to reduce the activity and subsequent monaiity of the animals. In the laboratory, Bythoîrephes were kept in aerated carbuoys at a constant 14 OC in total darkness; 14 OC was within the thermal preferundurn (Yurista 1992; Lehman et al. 1997).

Prey were kept in total darkness pnor to the experiments in an aerated, 35 L aquarium at a room temperature of 18 - 22 OC Prey were fed large amounts of bakers' yeast for ease of culture.

However, the yeast diet may have affected the chemical composition of prey, but the chemical composition of kairomones produced by prey is largely unknown. Because of the difficulty in rnaintaining Bythoirephes cultures (Yurista 1992). 1did not feed Bythonephes and ran the experiments for only four days after each collection to reduce the effccts of starvation. Howevcr,

Bythoîrephes is highly cannibalistic and iïkely fed on its conspecifics.

A. Darihnia kairomone eyxxirnent

1. Video and motion analvsis

1performed the videotaping in a basement lab where the windows of the doors were covered and the seams were sealed with black electrical tape to prevent incursion of visible light,

A 40 x 40 cm posterboard platform and a backdrop of two 30 x 30 cm wds weze covereù with black velvet cloth. The Nming tank was an invened Pyrex bel1 jar (10 cm outer diameter and 15 cm taU at the center) positioned at the center of the platform (Figure 1). I used a container with rounded surfaces because my previous observations showed Bythotrephes gathered at corners.

Two black and white television cameras (Panasonic WV-1850 with a Canon 16-100 mm, fl.9 macro zoom lens) mounted on tripods were positioned 13 cm away fiom the wail of the tank and aligned at right angles to each other. 1 achieved right-angle alignment of the video carneras by ensuring that the sides and top of the video camera lens were equidistant fkom the sides of the tank and checked the camera position by rneasuring the size of the camera images on a video rnonitor (Sony PVM 127 1Q Trinitron Color Video Monitor). At a I:l magnification and at close proxirnity, the cameras viewed about 75 46 of the filming container, so the actual area of the tank filmed was the top 10 cm by 10 cm wide. The video signal was routed to a video quad unit

(Panasonic Quad System WJ 420), which presented the camera images side by side. Because the two images share a comnvertical axis (Y) and are at right angles, the three-dimensional position of an animal appears in the horizontal and vertical axis of one image (X and Y) and the horizontal axis of the other image (2).A date and time stamp was added with a Panasonic WJ-

8 10 Time-date generator and the final video signal was recordeci with a video cassette recorder

(Panasonic NV-8950). During nIming, the tank was illuminated by a rectangular 3 x 10 cm plate

of 36 infhred light-emitting diodes (minimum wavelength, 720 nm). hf3ared lighting should

reduce the orientation behaviour of zooplankton to directional lighting (Ramcharan and Spruks

1989) and &es zooplankton appear as bright white dots against a black background. The LED plate was hmed in a piece of dark blue polystyrene that restricted light scattering beyond the

walis of the filrning tank.

Video playback was digitized with a îÏarne grabber (Oculus 200A, CORECO, Inc.,

Montreal) which captures a fiame altemating at and 1/20&S. The motion tracking program of Ramc haran and Sprules (1989) rneasured distance travelled, velocity, mean angle and mean vector length r in vertical and horizontal planes at user-specified intervals, The horizontal angle ranges fiom O to 2x radians and the vertical angle ranges tiom d2 radians (90" straight up) to -d2 radians (90" straight dom). The mean vector r is a measure of the concentration of angles of unit length; a high concentration approaching 1 indicates a straight swirnming path while a low mean vector indicates a high degree of turning behaviour relative to the mean direction (Fisher 1993), Appendix II).

2. Ouantification of behaviour

1 observed twenty-five Byihotrephes swimming and separated swimming behaviour into four categories based on observed behaviour and breaks in the distribution of swimming velocities (Figure 2). "Pause" is characterized as swimming slower than 5.59 mm S-' .''Glide" between 5.59 mm d and 19.56 mm s-', "Fast" between 19.56 mm S-' and 35.87 mm S-' and

"Strike9*35.87 mm S' or faster. "Strike" corresponds to Byihonephes feeding behaviour as it grasps prey with its 6rst thoracic iimbs, and 35.87 mm S-' is the minimum jump distance of 7.17 mm between two hms (Le, 0.2 s). For the motion analysis, 1 noted Bythorrephes behaviour at

1 s intervals because Bythorrephes has at least a 1 s latent period between repeated feeding

"strikes".

Two hours prior to the experimtnt, a sample of Bythoirephes was removed from the carbuoys and allowed to acclimatize to room temperature (20 OC) in dechlorinatcd water. Effects of the presence and absence of chemosensory and rnechanosensory stimuli on mrimrrring wett tested with a 2 x 2 design. 1 assigned Bythoirephes randomly to four treatrnents to test the presence/absence of c hemical stimuli crossed w it h the presencJabsence of mechanical stimuli: unconditioned dechlorinated water only (WATER, no chernical, no mec hanical), dechio~ated water with Daphnia juveniies (DAPHNIA) as mechanosensory and minimal chemosensory stimuli, Daphniu-conditioned dechlorinated water (KAIROMONE) as maximum chemosensory stimuli with no mechanical stimuli and Daphnia-conditioned dechlo~atedwater with Daphnia juveniles (MIX) as both fonns of stimuli For each KAIROMONE and MIX replicate, 1 prepared the Daphnia-conditioned water by €iltering 1 L of aquarium water in which the

Daphnia were cdtured tluough a 0.45 pm MiIlipore filter and added 1 L of dechlo~atedwater to the aquarium. 1 was not concemed about the concentration of the kairomone because the purpose was to produce above-thresho ld cues. 1 assurned the above-thresho Id concentration was reached because a high abundance of Daphnia (at lest 10 c') was cultured for three weeks ùi a

35 L aquarium without water changes. However, the rate at which these Lairomones degrade is unknown. Smaii amounts of Daphnia kairomone were present in the DAPKNIA treatment as soon as the prey were added to the water. 1 assumed that the differences in kairomone [7 Pause Glide Fast Strike

14 18 22 Velocity (mm s*')

Figure 2. Frequency distribution of swimming velocity behaviours. The minimum speed for the cmising behaviour is 5.59 mm S-', 19.56 mm s" for fast cmising (fast), and 3 5-87 mm s-' for a strike. concentration among the DAPHNIA, KAIROMONE and MIX treamnts were large enough to produce differences in behaviour, The dechlorinated water was also passed through a 0.45 p m

MiIlipore filter. I pipetted six Daphnia juveniies into the oIming tank for the DAPHNIA and

MIX replicates. New Bythumephes and Daphnia were used for each replicate and the nIming tank was rinsed with distilled water between replicates-

Mer aiiowing Bytbfrephes to acclimatize in the nIming tank under darkmss for 30 min,

1 recorded eight repiicates of each treatment and taped each replicate for 20 min. Reüminary observations suggested that 20 mins were required to capture eight to ten viable swimming tracks that could be foiiowed by the motion analysis program. Swimming tracks were also discardeci if

Bythotrephes encountered the water surface or swam within 1 cm of the fî.lming tank wall as

Bythotrephes appeared to swim faster at the edges due to the higher refkaction of Light and rnay have been able to sense its own hydrodynarnic disturbance reflected fiom the waU For the

DAPHNIA and MIX treatments, fïiming began as soon as the prey was addeci- Because 1 used infiami illumination only, 1 videotaped the treamnts between 2100 and 0200 h to match natural nighttime conditions.

4. Data analvsis

For the purposes of analyses. a track must last for a minimum of 2 s in which there were no missing data. A replicate is considered one randornly chosen swirnming track per individual and individual Bythofrephes were used once only. There was only one Bythotrephes in the tank at one tirne. Within each treatment, replicates were pooled across instars and it was assumed that there is no interaction between instar and treatment. The sample sizes of first and second instars were too small to test this assumption (N = 6 pooled ftrst and second instars). 1 assumed that the three instars would react to the kairornones in the sarne fashion. A 2 x 2 factorial MANOVA was used to analyze differences in swirnming behaviour under the chemosensory stimuli (Chernical effect) or rnechanosensory stimuli (Hydrodynamic effect). Swimming velocity and mean vector r in the vertical (r-vert) and horizontal (r-horiz) planes were the response variables. Velocity was logio(x+l) transformeci prior to the analysis to equaüze the magnitude of its variance with the mean vector r. and al response variables were nodydistributeci with the exception of r-vert for the WATER treatment (Lilliefors p < 0.01).

B. Linht exDeriment

1. Video and motion Wvsis

1conducted the videotaping late September 1997 at the Great Lakes Environmental

Research Laboratories (GLERL)in Ann Arbor. Michigan. 1coliected Byrhoirephes as before with the exception that Polyphemur pediculus fkom Paint Lake (45" 13' N 78OS7 ' W) was used as prey.

The video system at GLERL consists of a high-resolution video carnera rnounted on a mo tor drive. Illumination is provided by an infiareci laser and the camera setup uses Schlieren brand optics. The camra position and focal length which provided XYZ coordinates were synchronized with each field in the video record through a tirne-code generator. For this experirnent, the position of the carnera was recordeci at a rate of 15 Hz The area of the camera view was 2.2 x 1.7 cm at a magnification of 10.86 x but could be rnoved to cover the entire 28 x

28 x 28 cm tank. 1 foilowed individuals in the 20 L filming tank controUing the camera position and focus with joysticks and a video monitor.

2. Ouantification of Behaviow

The quantification of behaviours used in this experiment follows that nom the Daphnia kairo mone experiment. Three hours prïor to the experiment, two groups of 40 Polyphemus were set aside to acclimatize in Petri dishes to mom temperature under darkness. 1 used a 2 x 2 design to test if the swimrning behaviour of Bythonephes was affected by the presence and absence of visible light and Polyphemus. There were four treatments: hhmi illumination only (CONTROL, no visible light. no prey), infrared illumination with Polyphemus (PREY)for mechanosensory stimuli with no visible light, hfked Uumination with visible Light (LIGHT) for visual stimuli with no prey. and infrared illumination with visible Light and Polyphemur (LIGHT and PREY) for both visual and mechanosensory stimuli. Water in all treatments came fkom the Huron River and was passed twice through a 0.2pm Filter. Kairomnes produced by Polyphemus were present in the PREY,and LIGHT and PREY treatments; however, 1 assumed that differences in kairomone concentration among treatments were negligable as kairomones from other zooplankton would be present in the Huron River water although the rate of decay for Humn

River prey kairomones is unknown. Although Polyphemus represents novel prey for the Harp

Lake population of Byrhotrephes (Yan and Pawson 1997). Polyphemus was chosen because it is smail(l.2 f 0.07 SD mm) and relatively slow moving. A microscope lamp covered with a doubled KimWipe brand tissue provided diffuse visible Light. 1 approximated Light intensity to be 30-40 pE m-' S-' at the bottom of the filming tank based on a Light Uitensityldistance calibration from preliminary experiments-

The size of the container had some effect on Bythotrephes swimming. In the water only treatrnent of the kairomone experirnent, Bythotrephes swam at an average of 22.1 f 1.14 SE mm

S-' and 14.8 f 1.1 SE mm s" in the water only treatrnent of the prey and Light experirnent, so

Bythonephes' higher swimming speed in a srnalier container may have biased results. Differences in Bythotrephes' tirnung behaviour ktween the two treatmenu can not be cornpucd due to the different methods of motion analysis.

For each treatmnt, eight randomly chosen Bythotrephes were piaced in the 8 L tank and aiio wed to acclirnatize for a minimum of 30 min. This abundance of Bythotrephes is within the range found in Harp Lake (G. Sprules, pers. corn). Filming for the PREY and LIGHT and

PREY treatments began as soon as Polyphemus was added. 1judged negligable disturbance of adding prey to the tank based on the disturbance fiom a small volume of water (5 mi,) comparai to the 8 L of water so 1did not control disturbance by adding water to the non prey ueatments-

Due to tirne constraints, 1fïimed eight Bythotrephes in the tank at the same ticne therefore there may have been interference between the replicates. 1 Nmed each treatment for 2 h and each individual Bythotrephes was foiîowed for a minimum of 10 min. Al1 fïiming took place between

1200h and 1800h.

4. Data analvsis

For each replicate (one randomIy chosen 2 s crack per animal), 1 obtain the mean swimming speed, mean vector r, both the vector in three dimensions as well as its horizontal and vertical components and mean direction. The mean vector r-horiz was not normaily distributed within each treatment @ < 0.01) and was negatively skewed in al1 treatments due to sarnpling bias. This bias is a result of tracks chosen with the animal in focus to ensure the z-coordinate is accurate. Swimming tracks with overali hear motion were more likely to be in focus as a result of operator-controlled focus than tracks which spirailed. Thus, increased Bythotrephes tuming behaviour may less likely be detected in treatments in which there was less directed swimming.

However, a parametric 2 x 2 factorial MANOVA was still used because the bias in the horizontal position is consistent across treatments. The main effects tested in the MANOVA were visual cues, hydrodynamic cues, and visual and hydrodynarnic interaction; the response variables were loglo-transformedswimming velocity and the horizontal and vertical cornponents of the mean vector r. 1refer to the MANOVA as a kt-order analysis because it does not cake into consideration the mean direction of the swunmuig track for each animal (Figure 3). only the horizontal and vertical components of its turning behaviow.

Variability of turning behaviour between animals can be measured with second-order analysis. A second-ordcr analysis of angles simply involves the analysis of a set of replicate rnean angles (Appendix Il) where each replicate has a rnean and variance associatcd with it. I obtained the mean direction (in three-dimensional coordinates) and the length of the rnean vector r fkom a randornly chosen track of 2 s duration for each animal (Figure 3). The mean angles are then weighted by the rnean vector r to give the second-order directions. To test second-order directions for no response to treatments, or random mean swimrning direction, 1 used a modification of a rotationaily-invariant test fiom Watson (1966) in whic h eigenanalysis is performed on a matrix T of the sums of squares and products of x-y-z coordinates of each rnean angle and mean vector (Appendix II). If the three eigenvalues are equal, then the points are randomly distributed; if unequai, the points are concentrated either at one or both poles or they form a girdle distriôution (Watson 1966; Mardia 1972). For second-order angles where the mean vector r is heterogeneous, the points no longer fall on the surface of a unit sp here but are contained within the volume of the unit sphere (Figure 3)-

Iused

as the test statistic where r are the eigenvalues under the nuU hypothesis that ail three eigenvalues are equal, Le., the directions are randomly distributed within the unit sphere. 1 Verticai axis

I I

L Horizontal axis A) Swimming paths of individual animais in one treatment with a randomly chosen track of 2 s duration- Arrows indicated direction of travel and hash marks are placed at 1 s intervals.

B) Mean angle for one randomly chosen track for each animal.

4 C) Mean angle weighted by mean vector length, r.

Largest eigenvector _ eigenvectors

D) Eigenanalysis of second order angles for a single treatment. Second order angles wen bwtstrapped and eigenanalysis was repeated for each iteration of the bootstrap. Lines with arrowheads indicate mean angles; Heavy dashed lines indicate first, second and thi rd eigenvectors.

Figure 3. Flowchart of steps in the first and second order analysis of angles. Only three animals are show. For each randody chosen track, (Step A), speed, and length of the vertical and horizontal components of mean vector. r, (r-vert and r-horiz) were used in the first order analysis. In the second order analysis of angles, (Steps B-D), eigenanalysis was performed on the mean angles from each track weighted by the mean vector. r. bootstrapped the sample mean angles and mean vector r and recaiculated this statistic 1000 hs to derive the probability distribution of Sb under the nuil hypothesis of a random dismbution.

The test statistic itseif can not be used as a general diagnostic tool to compare treatment effects because each second-order mean swimming direction is weighted by variability within each replicate. The probability associated with this stahtic can be used as a generai inference tool: if only a small proportion of randomized test statistics are larger than the originai, the eigenvalues

&om the original sample are sufficiently unequal and hence, there is some directionality to the swimming between replicates for the particular treatment. The standardized ciifference shows the sarne information as the probability: the standardized ciifference is simply the ciifference of the sample statistic and the mean of the bootstrapped statistics divided by the standard error of the bootstrapped statistics, It is important to note that this is not a formai statistical test that compares the variability in rnean direction of swimming tracks among treatments, In essence, the sample from each treatrnent is compared to a population generated fiom the same randornized sample. If the angles were not randomly distributed then standard errors and 95% confidence limits for the second-order rneans were calculateci with a bias-correcteci bootstrap of the sample mean angles (EfÏon and Tibshirani 1986; Fisher 1993). Swimming velocity was also bootstrapped with 1000 iterations. Results

A. Da~hniakairornone experiment

Bythoîrephes exhibited a swimming pattern of tortuous paths and slow and fast cruking with speeds ranging from 15 to 23 mm s" (Table 1, Figures 4 a, b). Of the chemical, hydrodynamic and intedon eEects, only the chemicaî main effcct was significant at the a = 0.05 level for the MANOVA (Wilks*1 = 0.702, d.f- 3,26, p = 0.025) Vable 2 a). hdividuals in the treatrnents with kairornones displayed more directed swirnming in the horizontal plane (r-horiz) (man = 0.65 f 0.23 SD) than the treatments without kairornones

(mean = 0.37 f 0.29 SD) flable 2 b). Bythrrephes swirnming velocity and the verticai component of turnllig khaviour (r-vert) were not affected by the presence of Daphnia kairornones however. Swimmllig velocity was only weakly correlated with increasing directedness in tdgfor both the horizontal (total correlation r = 0.078) and vertical (total correlation r = 0.018) components in the MANOVA. The second-order analysis of mean vectors shows a similar trend where Bythotrephes turns less (Le,, higher mean vector, r) in both a horizontal and vemcal components while swimmhg faster across ail treatments (Table 1 a). The second-order mean vectors r and swimming velocities (Table 1 a) do not refkct the pattern of

MANOVA results because the MANOVA compared the means of fast-order mean vectors whereas the second-order mean vectors are based on the fust-order mean vectors and mean angles. In other words, the MANOVA results reflect the variability of direction within a swimming track (mean vector r) whem the second-order analysis shows variability within Treatment

Kairomone

Absent Vertical axis

Absent

il Horizontal axis

Start Start

Present

iii) iv)

Figure 4a. Diagram matic representations of Byfhotrephes swi mmi ng tracks under the four treatments for the Daphna kairomone experiment. i) WATER, ii) KAIROMONE, iii) DAPHNIA, iv) ME. Scale is on1y approximate and only one horizontal axis is shown. Treatment Light

Absent Vertical axis Preseat

Absent

Prey i) Horizontal axis

Start

iii) iv)

Figure 4b. Di agrammatic representations of Bythoirephes swimming tracks under the four treatments for the Light experiment. i) CONTROL, ii) LIGHT iii) PREY, iv) LIGHT AND PREY. Scale is only approximate and only one horizontal axis is shown. Table 1 a) Dophnia kairomone experiment Second-order man vector r for the vertical (r-vert) and horizontal (r- horiz) components and swimming velocity with standard errors. WATER is dechlorinated water only, DAPHNIA is Daphnia ody, KAIROMONE is kairornone only and MIX is the Daphnia and kairomone mk. N = 8 replicates for each treatment.

Treatment

WATER 0.70 * 0.089 0.25 f 0.127 22.54 i 2.637 DAPHNIA 0.66 I0.080 0.06 f 0.076 15.57 f 1.491 KAiROMONE 0.67 k 0.074 0.13 f 0.122 14-55f 1.641 MIX 0.90 * 0.014 0.30 f 0.1 10 20.00 f 3.584

b) Light Experiment Second-order mean vector r for the vertical (r-vert) and horizontal (r-horiz) components and swirnming velocity with standard errors. CONTROL is the no light/no prey treatment, PREY is prey only, LIGHT is the visible iight treatment and LIGHT AND PREY is the visible light and prey treatment. N = 8 replicates for each treatment,

Treatrnent R-vert R-horiz Velocity (mm s") (* SE) (* SE) (* SE) CONTROL 0.83 * 0.036 0.11 f 0.174 14.30 i 1.296 PREY 0.74 f 0.041 0.41 f 0.206 9.12 i 1.218 LIGHT 0.80 f 0.064 0.23 î 0.129 13.25 f 1.801 LIGHT AND PREY 0.70 f O. 11 1 0.42 f 0.200 15-74f 1.530 . tracks plus the variability ktween anirnals (mean direction weighted by mean vector r) (Fi-

Test for random direction

There was no effect of treatment on the preference of swimming directions. However, swimming tracks in the MLX treatrnent were the most directed (p = 0.906) and least directed in the DAPHNlA treatment (p = 0.754) VabIe 3 a).

B. Lipht experirnent

Bythotrephes exhibited ody slow cruising behaviour in ail treatrnents. Visual, mechanosensory main effects and interaction were significant at the a = 0.05 level for the

MANOVA (interaction Wilks' k = 0.656, d.f. = 3.26, p = 0.01 1) cable 2 c). Further interpretations of the main effects were avoided because of the significant interaction effect. The vertical component of Bythofrephes turning behaviour (r-vert) and velocity were signifïcantly different in the interaction of mechanosensory and visual cues (Table 2 d, Figure 5). Ln trials w ith Polyphemus, Bythotrephes' swimming velocity increased signif icantly in the presence of visible light pukey HSD, p = 0.090, correcteci for multiple variables) than in the dark. R-vert was also significantly lower in the treatment with Polyphemus in the dark than the other three treatments (Tukey HSD, p < 0.002 for all post-hoc comparisons). The correlation between swimming velocit y and directedness in the vertical plane (total correhtions r = 0.45) was greater than in the Daphnia kairomone experiment. In the second-order andysis of angles, however, the slower swimming speed did not reflect more turning behaviour nable 1 b). Table 2. MANOVA tables for Daphnia kairomone and Light experiments a) Daphnia kairomone experiment Sununary of al1 effeçta

EEect Wilks'sk Rao'sR d.f. 1 df. 2 p-ievel

Chernical 0.702 3.686 3 26 0.025* Hydrodynamic 0.8 12 2.003 3 26 0.138 Interaction 0.890 1.066 3 26 0.380

b) Chernical main effect LogioVel represents Loglo(Ve1ocity +1), R-Horiz and R-Vert are the horizontal and vertical components of the mean vector r.

Effect MS Effect MS Error F1.28 p-level

LogloVel 0.032 0.039 0.820 0-373 R- Horiz 0.61 1 0.064 9.49 1 O.OOSf R- Vert 0.009 0.008 1.O76 0.308 -c) Light Expriment Effeçt Wi's 3c Rao's R d.f. 1 d.f. 2 p-kvel

Visual 0.659 4.479 3 26 0.0 12* H ydrod ynarnic 0.690 3.894 3 26 0.020* Interaction 0.656 4.550 3 26 0.01 l*

dl Interaction effecr Loglove1 represents Loglo(Ve1ocity +1), R-Horiz and R-Vert are the horizontal and vertical components of the man vector r.

Effect MS Effeçt p-level 0.7 No LQht Ught No üght üght No Prey Prey Figure 5. Mean velocity and man resultant vectors r in the horizontal and verticai danes with f 1 standard errors. N = 8 for each treatment. Table 3a. Mean of bootstrapped Sb for second-order angles, eigenvalues of directionai cosine matrix T, probabiiities and standardized difftrences, D, for the Daphnia kairornonc experimtnt.

WATER 1.62 [0.38, 1.06,2.381 0.867 4.9 1 DAPHNIA 1.57 [0.49,0.85,2.34] 0.754 3.22 KAIROMONE 2.5 1 [1.00,0.84,3.45] 0.837 4.5 1 MIX 1.69 [2.01, 1-00. O. 131 0.906 5.76

b. Mean of bootstrapped Sb for second-order angles, eigenvalues of directional cosine maaix T, probabilities and standardized merences, D, for the Light experiment.

Treatrnent Sb eigenvalues (7) P D

CONTROL 4.14 [4.86,2.73,0.14] 0.641 1.58 PREY 3.56 [3.58, 1.65,0.05] 0.847 2.34 LIGHT 3.84 [2.08,4.94,0.60] 0.734 2.21 LIGHT AND PREY 3.24 [2.06,4.65,0.7 11 0.724 2.18 Tests for raridom direction

Swirnming directions were randomly distributed in ail treatments (Table 3 b). Directions in the CONTROL treatment were least concentnted (p = 0.641) and highiy concentrated in the

PREY treatment (p = 0.847). The eigenvalues in the light and prey experiment are generally higher than those in the Daphnia kairomone experiment because the first-order mean vectors r in the light ard prey experiment were higher. In both experiments, the low second-order mean vectors r also suggest a random distribution (Table 1). Discussion

This is the fkst report of Bythephes swimming response to prey kairomones and visibility of prey. However, the swirnming response to these cues is only indirect support for the types and contributions of detection mechanisms. Changes in feeding rate under different light and chernical conditions and physiological studies of the eye, chemoreceptors and mec hanoreceptors are required for a compre hensive under standing of feeding be havio ur.

Chemosensory detection

The results support the view that Bythoirephes responded to Daphnia chemical cues;

Bythoirephes swam in a more directed Edshion in the horizontal plane in the presence of Daphnia kairomones but there was no significant difference in the swimming speed contrary to predictions. However, Byrhorrephes' directed swimming may simply be an artifact if

Bythoirephes was overwhelmed by the large concentration of kairomones as there was no additional increase of swimming speed. In addition, directed swimming to follow chemical cues wodd only rnake sense ifthere was achemical gradient in the water. In the Daphnia kairornone expenment, the Dap hnia-conditioned water was Ntered and mixed to inswe homogenous kairomone concentrations. Although optimal foraging theory suggests that Byrhorrephes should tum more when they have found prey so they will not leave a profitable area (Le., area-restricted searching)(Smith 1974). hydrodynamic cues in addition to chemical cues may be required to indicate the patch is profitable. Buskey (1984) reponed that the addition of plastic spheres to phytoplankton conditioned seawater was necessary to elicit more turning behaviour h the copepod Pseudocalanus minutus. Reduced turning behaviour in the presence of kairomones only would alïow Byrhomephes to explore new areas and discover the source. Similar behaviour has been nported by Young and Taylor (1990) where the straight swimming tracks of Daphnia

lenphened in food rich conditions, presumably to avoid water Daphnia had already £ütered.

Kncreased activity and directed movement in response to prey kairomones has been reported in a

variety of marine and freshwater crustaceans. Berendonk and 0' Brien (1996) demonstrated that

Chnooorus increased its movemnt 6nquency in the presence of daphnid and copepod

kairomones, thus increasing the probability of encounter (Gerritsen and Strickler 1977).

Weissburg and Zir-Faust (1993) established that the blue crab, Callinectes supidus,

increased its predatory success with straight search paths parallel to the direction of the flow

produced by their bivalve prey. Buskey (1984) found that the copepod Pseudocalanus minutus

increased its swimming velocity and reduced turning behaviour in the presence of phytoplankton

conditioned seawater. In this study, however, Bythoirephes swimming velocity decrea~ed

slightly with a decrease in tdgbehaviour when exposed to the Daphnia kairomone.

Remote perception of prey by kairomones in Bythonephes is probably not a biologically

significant detection rnechanism; for fast swimming zooplankton predators such as Bythofrephes,

chernoreception may be used in the "tasting" of prey to determine prey quality. (R. Baker, pers. corn). Bythotrephes is unlikely to orient itseIf to chemical trails and gradients due to its high

swirnming speed and Ioopuig turns; such swimming behaviour wouid disperse chemical trails

and gradients and would provide Limited information about prey location and qudity.

M-n M-n

Both the Dophnia kairomone and the Polyphemus with visible light experiments were

performed to test the effect of hydrodynamic stimuli on the swimming behaviour of

Bythotrephes. In the first experirnent, Bythoirephes did not show any change in swimming

behaviour between the plain water and Daphnia only treatments. In the second experiment, Bythorrephes responded to the presencc of Polyphemus in the absence of visible light with a sharp decrease in velocity and increase in tuming behaviour (Figure 4 b). Differences in the swimming behaviour of Daphfiia and Polyphemus may have affected Bythephcs' response.

Dophnia swims with smooth, regular strokes whiie Polyphemrrs swims with an irregular mtion

(Young and Taylor 1990). Chemoreception was uniikely used as a detection mechanism as there were zoopiankton kairomnes in the Huron River water for aU four treatments; the addition of

Polyphemus kairomones would probably have Little effect. Differences between juvenile

Daphnia and Polyphemus swimming behaviour may be responsible for their susceptibility to

Byrhomephes mechanoreception. Daphnia swimming consisted rnostiy of pauses, short hops and sinking behaviour whereas Pofyphemus constantly darted in different directions, thus creating a greater hydrod ynarnic dist urbance. Bythtrephes' decrease in velocit y may be necessary to irnprove detection of prey by mechanoreception as waves fÎom Bythotrephes' swimming strokes would create interference (Gerritsen and Strickler 1977). A reduction in swimming speed may represent a tradeoff between the ability to detect prey via mechanosensory rneans and direct encounter with prey because Bytho~ephesis a cruising predator with a high swimming speed and is in constant motion. Browman et aL's (1989) observations on the continuous swimming behaviour of Leptodora kindtii indicate that prey location is entirely dependent on direct tactiie contact; Leptohra never fed on sessile or semi-sessile prey except when contacted directiy.

Buskey (1984) observed that the copepod Pseudocalanus minutus swam significantly slower with fewer swimming bursts and more pauses in the presence of plastic spheres than in the presence of fihered seawater. Unlike Bythotrephes, Pseudocalanus did not show any change in tuming behaviour in the presence of mechanosensory stimulation. Detection of the walls and corners of the fdrning charnbers is further evidence of the role

of mechanoreception in Bythotrephes swunmllig and feeding behaviour. In rectangular

containers, Bythotrephes tended to accumuiatc in the corners cven whcn the container was covered to rcstrici light. Surfies and corners wouki represent a novel hydromhanical stimulus

because Bythotrephes is a pelagic animal. Zaret (1980) suggested t hat zooplankton could detect the presence of a wall and corners by sensing a deçrease in their O wn velocity.

Mechanoreception is thcrefore iikely to be a major component of the feeding mechanism in

Bythotrephes especially at night when visual predation by fish is minirnized.

Visual detection

Changes in Bythotrephes swimming behaviour under the influence of visible light and the presence of prey suggests that vision plays an important role in the remote detection and encounter of prey. As predicted, Bythotrephes increased its swimming speed and reduced its tuming behaviour in the presence of Polyphemus in visible light, thus increasing the probability of encounter and foraging efficiency. Observations of Bythotrephes' rap id turning and attacking behaviour in light indicate a rriaximum reaction distance around 12-14 mm. At this distance, mechanoreception is unlikely due to the rapid attenuation of pressure waves f?om zooplankton swimming and feeding cumnts (1.93 mm at 1 mm s-' for Dophnia pulux, (Krk 1985)).

Evidence for visually guided feeding behaviour has been noted for a variety of zooplankton. Increased swimming speed and greater turning behaviour of Polyphemus in visible light and prey mixtures of Bosmina and juvenile Daphnia magna has ken reported by Young and Taylor (1990). Visual inputs are also able to explain features of chasing and shadowing behaviour in Polyphemur whose chases ceased in the absence of visible light (Young and Taylor

1988). Feeding rate is affected by light intensity because the crustaceans Bythotrephes and Lqtodora eat Lss in the dark than they do under nodlight (Mordukhai-Boltovskaia 1958).

This phenomenon has also been observcd in Mysis relicta (Ramcharan and Spdes 1986) and the

cyclopoid copepod Corycaeus anglicus (Gophen and Hams 1981). Ramcharan and Spdes

(1986) report& that Mysis reduced iîs swimming speed in moderate light (4.3 x 10~'to 4.3 x IO-'

W m-2) when fed Daphnia magna and Diacyclops bicuspidatus thornasi, whereas I O bserved an

increase in Bythotrephes swimrning speed in the presence of prey.

Changes of Bythtrephes' swïmrning velocity in light may be înterpreted as a Éradwff

between avoidance behaviour fkom other visual predators such as fish when prey are absent and

the detection of prey when prey are present; Le., Bythotrephes may be more likely to risk

deteçtion by fish by swimming faster (O' Kede et al. 1998) if there is the benefit of increased

prey capture (O' Brien 1987). The dark pigmentation in Bythotrephes' eye is highly visible

against iliuminated water and is partly responsible for Bythotrephes' susceptibility to visual

predation (Kerfoot 1980). However, in Harp Lake where Bythotrephes is protected in an

epilimnetic thermal refuge from lake herring (G. Sprules, pers. corn), Bythotrephes does not

face as great a risk and is apt to swim faster to maximize prey encounter. Mysis f?om Gu11 Lake, on the other hand, appears to prefer cooler temperatures (5 OC, (DeGreave and Reynolds 1975)) in the hypolimnion where it coexists with planktivorous fish and is likely more susceptible. The evolution of Bythotrephes* intense multicolored pigmentation in Harp Lake also supports the absence of the need to be inconspicuous. Bythotrephes has dark blue/green legs, yeilow brood pouch and red caudal spine. Such intense pigmentation may serve as protection against UV radiation (Ringelberg 1980) Analvsis of Experimental Desim

Assessrnents of swimming and feeding behaviour of planktonic organisms are difflcult at best and supplementary expefiments and oôsewations on feeding rate are necessary for a complete picture. Tradeoffs exkt between the size of the container and the focal depth of the camera. Container effects, such as reduced swimming rate have bten reporteci for Daphnia

(Dodson et aL 1997). Because the filming chamber for the Daphnia kairomone experiment was dy10 x 10 cm, Byfhafrephesswirnming and turning may be affected. Dodson (199'7) recommends an observation chamber that is large enough so animais would be at ieast 10 cm away fkom any wail when filmed, Consequently, ciifferences in the fïiming chamber sizes between the Daphnia kairomone and Light experiments rnay affect the results- Idedy, swimming behaviour of zoopIankton should be Nmed in situ; however, it is nearly impossible to conuol environmental variables such as light, food, temperature, etc. Additional sample sizes are required for a ro bust statistical analysis, especially for directional statistics where Iarge sample sizes are recommended for rotationaily invariant tests (Le., tests for randornness in whic h results do not depend on the orientation of the axes, (Mardia 1972)). For this study, this represents a difficult task as a single replicate cm take up to two hours for acclimatization and fdrning. Despite the randomness of sw imming direct ions in bo th experirnents, the proposeci method of testing of second-order directions for randomness is effective for providing a cornparison of directional variability between ueatments. An addit ional experiment to test

Bytbirephes' orientation behaviour to the presence of prey kairornones would be beneficial and would require a large observation chamber and a point source of prey kairomones. Further experirnental modifications such as the oblation of Bythonephes' eye would provide concrete information about the role of vision in feeding behaviour; however, this is lilcely to cause abnormal behaviour.

The sampling rate of behaviours can have significant effect on the results (Appendix III).

At a higher sampiing rate, parameters that measure turning behaviour such as the average resultant vector in the horizontal plane can decrease rapidly. At low rates, the sampling points on the swimming path are spread far apazt and measurements taken in straight Lines between those points give the impression of less turning behaviour. At high sampling rates, the measured swimming path consists of more points with fewer straight line segments between the points. In this experiment, a sampling rate of one second underrepresented the actual tuming motion of

Bytbfrephes sw immuig. Hence, significant ciifferences in tuming measurements between treatrnents are conservative.

My experiments provide evidence that visual and hydrodynamic cues are the two main methods of remote detection used by Bythotrephes. In the treatment with Polyphemus and no visible light, Bythotrephes increased turning behaviour in bo th horizontal and vertical planes and reduced swimmuig speed as compared to the controL Bythoîrephes' visual detection of prey is evident fiom the increase in the average swimming speed and reduction of turning behaviour in the Polyphemus and Light treatment as compared to the Polyphemus in dark treatment. Prey detection by chemoreception is not as evident as Bythotrephes did not show signifkant changes in speed, only decreased tuming behaviour. In the light experiment, chemoreception, however unlikely, can not be ruled out completely as a detection mechanism.

People's perceptions of simple chemosensory versus hydrodynamic versus visual feeding mechanisms are now king replaced by multiple feeduig mechanisrns &ected by a variety of environmental characteristics. Models that predict encounter rates are only effective when parameters such as swunming speed and turning behaviour are assigned accurate vnlues.

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Stnckler. J. R, 1982. Calanoid copepods, feeding currents, and the role of gravity. Science 218: 158-160.

Strickler, J. R. and A. K. Bal, 1973. Setae of the first antennae of the copepod Cyclops scuhrer (Sars): Their structure and importance. Proc. Nat. Acad. Sci USA 70: 2656-2659.

Vanderploeg, H. A., J. R. Liebig and M. Omair. 1993. Bythotrephes predation on Great Lakes' zooplankton measured by an in situ method: implications for zooplankton cornrnunity structure, Arch. HydrobioL 127: 1-8.

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Yan, N. D. and T. W. Pawson. 1997. Changes in the crustacean zooplankton comrnunity of Harp Lake, Canada, foliowing invasion by Bythoîrephes cederstroerni. Freshwater BioL 37: 409- 425. Yan, N. D. and T. W. Pawson. 1998. Seasonal variation in the size and abundance of the invading Bythonephes in Harp Lake. Ontario, Canada. HydrobioL 361: 157- 168.

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Yurista, P. M. and K. L. Schultz. 1996, Comments on "A mode1 for growth, development, and diet seiection by the invertebrate predator Bythotrephes ce&rsîroemi" by J. T, LEHMAN AND D. K BRANSTRATOR.J. Great Lakes Res. 21:610-619. J. Great Mes Res. 22: 925- 929.

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Bythobephes enmunter rata and prey nsk for Harp Lake, Ontario Introduction

Physical encounters among organisms in aquatic systems mediate many bio tic

interactions such as predaaon, cornpetition, parasitism and mting. The rates of

encounters and their associateci pro babiiities influence these bio tic interactions and can

have a signifcant effet on the evolution and ecology of organisms.

Bythotrephes cedersîroemi is a cruising invertebrate ptcdator (Gemtsen and

Strickler 1977) as it swims con~uousiythroughout the water column and strikes at

zooplankton prey as they pass close by. Bythorrephes detects prey by mechanoreceptors

located on the fkst antemules, mdible, labrum, maxillary process and thoracic limbs

(Martin and Cash-Clark 1995) or visually by the large medial compound eye. Similar to

Leptodora, (Browman et aL 1989) Bythotrephes arches dorsally at the abdominal region

and throws out the first pair of thoracic appendages to grasp prey and bring them into the

"feeding basket" formed by the thoracic appendages (Rivier 1998). Prey are manipulated headfust to wards the mouth, tom apart by mandibles and their Liquid contents are sucked out. Bythotrephes has a high feeduig rate and kills approximately tluee-quarters of its body weight in prey pet day (Lehman et ai. 1997). Therefore, Bythotrephes can have a

~ig~cantimpact on the structure of zooplankton communities flan and Pawson 1997).

Earlier models of encounter rate are often based on the kinetic theory of gas

(Volterra 1926), yet planktonic animals do not move about passively or coude randomly with each other in space (Gemtsen 1980). Thus, there is a need for descriptive rmdels of encounter rate which incorporate certain aspects of specific animal physiology and behaviour. Planktonic animals have directeci predatory and escape behaviour and sensory systems which enable them to detect and pursue potentiai prey or mates, or avoid predators. Three-dimensional models for these animals have been developed by

Gemtsen and Strickler (1977) for predators with sp herical encounter fields and by

Giguère et ai. (1982) assuming a cyLindnca1encounter field around the predator.

In this paper, I delimit the encounter field for Bythotrephes feeding on zooplankton under different light regimes and apply Gemtsen and Strickler's (1977) encounter mode1 to Bythotrephes predation to quantify prey risk over two midsummer die1 periods. With this approach, 1 am able to predict the number of prey Byrhofrephes can encounter and show watial and temporal patterns of high prey risk. As Bythotrephes is adapted p hysio logicaUy to be a visual predator iike Polyphemus pedicufus (Young and

Taylor 1988) and Mysis reliera (Rarncharan and Spniles 1986). 1predict that the highest rates of encounter and high prey risk occur during the day in the epilimnion at maximum

Light levels. Materials and Methods

Reaction distance experiment

1ran the experiment of Sytbtrephes*reaction distance to prey fkom August to October

1998. 1collected Bythoîrephes fiorn Harp Lake, Ontario (45" 05' N 61" 45' W) and prey fiom

Heney Lake (45" 23' N 79" 07' W). 1 cokcted both Bythotrephes and prey by towing a large piankton net (0.75 m diameter, 285 prn mesh size) at a depth of 1-5 m for a distance of approximately 50 m Prcy were transfcmd into 20 L plastic carbuoys cornpletely wvedwith heavy black plastic. 1 taped hzerpacks to the sides of the carbuoys to reduce the activity and subsequent mortality of the animals. in the laboratory, Bythoîrephes were kept in aerated carbuoys at a constant 14 OC in totai darkness; 14 OCis within the preferred the& range

(Yurista 1992; Lehman et ai. 1997). Rey were kept in total darkness prior to the experirnents in an aerated. 35 L aquarium at a room temperature of 18 - 22 OC and were fed large amounts of bakers' yeast. 1 did not feed Bythotrephes and ran the experirnents for only four days afkr each collection to reduce the effects of starvation because of the diffïculty in maintainhg

Bythotrephes cultures (Yurista 1992). However, Bythotrephes is highly cannibalistic and Likely fed on its conspecifics.

1. Video se tu^ and reaction distance rneasurernent

The acryiic fîiming chamber was 3 cm high, 15 cm long and 10 cm wide tapering to about 5 cm at one end (Figure 1). A rectangular fiame, for wuring Bythotrephes. was inserted into the chamber and a small metal loop was anached at its rniddle. A millimeter resolution grid was fixed to the bonom of the chamber to provide scale during filming anaiysis. Initially, a srnali current was generated with a peristaluc pump to draw the prey towards Bythotrephes. Stand with clamp for LEDs

Microscope light sources Bythofrephes tethered with sik thread

A

Figure 1. Diagram of reaction distance filming chamber with video camera removed. However, Bythotrephes reacted to the current in conjunction with prey and accurate

measurements of reaction distance could not be obtained. A single black and white television

camera (Panasonic WV-1850with a Canon 16-100 mm, fl.9 macro zoom lens) was mounted

directly above the fhing chamber. At a rnagnification of 3.4: 1, the camera viewed a 5.8 x 6.9

cm wide area of the fiiming chamber. A date and time stamp was added with a Panasonic WJ-

810 Time-date generator and the final video signal was recorded with a video cassette tecorder

(Panasonic NV-8950). hfhred iiiumination was provided by a rectangular 3 x 10 cm plate of 36

inhed light-emitting diodes (minimum wavelength, 720 nm) fkom behind the tethered

Bythotrephes (Figure 1). Diffuse visible illumination was provided by microscope light sources

that were covered with Kimwipe brand tissues.

Six light levels were used as treatments for this experiment: infrared only, and idked

with 2. 5, 10, 150 and 500 pE m-* sec-' of visible Light (LiCor LI-185B

Quanturri/radiometer/photometer). 1chose these light levels as they represent conditions at 2- 10

m depth in Harp Lake (Nom Yan, unpublished data). I randomly assigned ten animals per

treatment for a total of 60 animals. For each replicate, 1tethered individual Bythotrephes wit h a

black silk thread to theù spine by Krazy glue brand adhesive and left them to acclirnatize in the

filming container for at least ten minutes with the light set at one of the six light levels. 1

pipetted two mL of live prey consisting of a variety of smail cladoceran and copepod species

(about 20 to 30 individuals) coliected fkom Heney Lake into the container. Individuai

Byrhotrephes were filmed for five minutes. 1replaced the dechlo~atedwater in the filming

chamber and pipetted fiesh prey after each animal was flmed. The temperature of the water

remaineci constant at 20 OC before and after each Nming episode, The maximum reaction distance is dehed as the distance fkom the centre of

Bythotrepthes' eye to the prey the instant before Bytbîrephes displayed parallax tracking behaviour. 1 define parallax tracking behaviour as Bythotrephes moves sideways to a new position for a distance of at least two body widths during whic h Bythotrephes is onented towards the prey as it swirns past. The sideways swimming behaviour is simply an artifact of the tethered

Bythotrephes; in natural conditions, Bythonephes would simply turn towards its prey. 1digitized the video image with a Mu-Tech MVlOOO video capture card and measured reaction distance and angle fkom video stïlls with Scion Image (version 1.62a. Scion Corp.). The apparent vertical or horizontal angle of encounter is the angle in either the predominantly horizontal or vertical plane formed by the prey and Bythotrephes' main body axis (long axis of Bythotrephes is forward directed = 0"). If Bythtrephes was oriented so that the brood sac was facing up or down, the encounter was classified as either left or right in a horizontal plane relative to

Bythotrep hes. Likewise, if Byfhotrephes' side was facing up, the encounter was classified as occuring in an apparent vertical plane relative to Bythotrephes.

Encounter model and Drev risk

The most usehl approach for the development of a predation model of Bythotrephes and prey in Harp Lalce is the separation of predator-prey interactions into separate components as pioneered by C. S. Holling (1959). With this approach, predator behaviour can be divided into four conditional events: encounter, anack, capture, and ingestion (Gemtsen and Strickler 1977;

Giguère et ai. 1982). In my model of Bythotrephes predation, 1 am primarily concerned only with predator-prey encounter rates and the potential risk faced by prey. 1 used Mathcad 7

Professio na1 version (Mathso fi, Inc.) to develop the model. As Bytbtrephes swims around in a volume of water, the likelihood that it will encounter prey depends on Bythoîrephes' higher swinnning speed. Bythofrephes' reaction distance and prey abundance (Gerritsen and Strickler 1977). In the modei, reaction distance can be visuaked as a sphere mving through a volume of water (Figure 2). Evaluating for the azimutha1 angle $

(angle formed by Bythotrephes and prey on the horizontal plane) gives:

Z, = xR2N~luJiï2 + v2 -~iïvsos~sin WB 2 where Z, is the theoretid number of prey encountered by a single Bythoîrephes per hour and per m3, R is the reaction distance to prey (radius of the sphere) (rn) ,NH iS the number of prey per m3, Ü is the average velocity of prey (m h-') and v is the velocity of Bythotrephes (m Y') (Fi-

2). The subscripts P and H refer to predators and prey, respectively. The total number of

Bythotrephes-prey encounters (Q) is given by the encounter rate for a single Bythotrephes by the abundance of Bythotrephes (N,):

Q=Wp The potentiai risk faced by prey is the prokbility that a prey will be encountered by

Bythotrephes. Factors such as prey motion (&et 1980) and dark eye pigmentation (Kerfoot

1980) contribute to the hydrodynamic and visual conspicuousness of prey and increase the likelihood of eliciting a reaction nom Bythotrephes and thus the risk of king ingested. On the other hand, prey escape responses during the attack phase will decrease the chance that prey wiU be ingested. As a means of avoiding predation, prey have two inunediate choices: they can avoid encounters with Bythotrephes by swimming when and where Bythotrephes is absent, and reduce hydrodynarnic conspicuousness by swimming slowly. Direction of Bythotrephes traveI

Figure 2. Reaction sphere for Bythoîrephes 1 used a measure of prey risk (Y), which 1defined as:

Y = QN~P, where Q is the total number of Byfbtrephes-prey encounters, NH is prey abundance

(no. ma3)and PH is the proportion of the type of ptey Ui Byfhomephes diet (G. Morgan. unpublished data). Although diet proportion is a result of the four components of predator behaviour hmencounter to ingestion (Holling 1959), 1 includcd weighting Bythorrepks-prey encounters and prey abundance by diet to include physiological and behavioural aspects of prey conspicuousness and escape behaviour. For example, weighting by the prey abundance in

Bythotrephes' diet incorporates the effects of Bythotrephes' greater ability to handle srnall prey than large as weil as effective escape behaviour of sorne of the copepods.

Encounter rate is a function of predator and prey abundance and speed, with reaction distance depending on Light leveL As the season progresses, light levels will change in the diel cycle, predator and prey abundance and vertical distribution will change and affect the outcome of encounter rate and prey risk.

Mode1 Parameters

To apply Gerritsen and Strickler's model (1977) to Bythomephes encounter rates, 1 must choose appropriate values for the model. The parameters required are reaction distance,

Bythotrephes abundance and ve!ocity, and prey abundance and velocity.

Reaction distance

Reaction distance is the most sensitive variable in the mode1 as it is the only squared term. Bascd on my observations fkom previous experiments, Byfhotrephes primarily uses vision as a remote detection mechanism foilowed by rnechanoreception. Consequently, reaction distance should be a function of ambient light level. From the data coilected from the reaction distance experiment, a piecewise regression on reaction distance as a fbnction of Light kvel was caiculated for visibk Light only (> O pE m2d).

The piecewise regressio n mode1 estimated two linear components and a single beakpbint in the y-axis by a lest-squares method. As the breakpoint between the two slopes corresponded to the area between 2 and 10 pE m-2 s-1 of üght inclusive (0.48 and 1.04 logio+lpE m'2 se'), a separate hear regression was performed on this area only to provide a more accurate visible Light level which corresponded to the breakpoint in reaction distance. Reaction distance in the da& was rneasured as the average reaction distance under infrated illumination only

-2 (Le., O pE m s-1 ).

Bythotrephes' reaction distance as a function of Light in the fdming chamber can be extrapolated to Harp Lake over a 24-hour period at various depths given the arnount of soiar radiation reac hing the lake and the attenuation coefficient of Lig ht. Photosynthetically available radiation (PAR) measured at the surface was provided by Norm Yan at the Ministry of Energy and the Environment (MOEE,Dorset) for July 18-19 and August 28-29,1995 at the Paint Lake

(45" 13' N 78O 57' W) and Heney Lake stations. Both are in close proximity (c 27 km) to Harp

Lake. PAR radiation (W ~2 s-1 ) was converted to Light quanta using the relationship PAR pE m-

S-l = 4.2 pE W*s-l (More1 and Smith 1974). PAR profiles of Harp Lake were provided by

Bob Girard and Norm Yan for July 14 and August 12,1998 and the vertical attenuation coefficient k was averaged for these two dates. Visible Light at each mew depth over the 24- hour period was calculatecl assuming an exponential decay with depth (Voilenweider 1974) and die1 PAR levels at 0.1 m depth. PAR levels measured in air were converted to PAR at O. 1 m depth by multiplying with the average transmission between the reference light masurement in air and at 0.1 rn for July 14 and August 12, 1998. Bvthotrephes and Drev abundance

Die1 Bythotrephes and prey abundance data in Harp Lake were provided by Chris

Charron and Cristina Durnitru (1998) for July 18-19 and August 28-29.1995.

There were only six theperiods for which there were actual abundance data: 16:ûû. 20:00,

24:00, 5:00.9:0, 12:ûû. Redator and prey abundance (no. m-2) were calculateci for four depth zones: 0-5 m, 5-10 m, 10-15 m and 15-33 m In this model, it is assumed that the zooplankton were spread equally among each metre for each zone, and values for abundance were rounded to the nearest integer. Linear interpolation was used to estimate abundance between the sampling times to obtaïn a complete die1 cycle. This method of interpolation assumes that there is a hear rate at which individuals arrive and depan the depth zones at each hour. Therefore. the model is spacially and temporaily explkit to metre intervals over 24 hours for the two die1 periods.

1 used average swimrning velocities for each Bythotrephes instar from the Daphnia kairomone experiment (Chapter 1). Bythorrephes' encounter rate for Harp Lake niay be slightly overestimated as Bythtrephes swirns faster in smailer containers (Chapter 1). 1pooled swimming velocities fiom first and second instars for an average of 16.03 mm s" due to a 10 w sarnple size of only two first instars. Third instars had an average velocity of 20.28 mm S-'. 1 used literature values for average prey velocity (Table 1).

The prey risk factor, or percentage in Bythephes diet. was used to weigh the proportion of total Bythoîrephes-prey encounters and prey abundance to include a measun of prey electivity

(Table 2). In the mode4 taxonomic resolution went as &as order for naupliar and copepodid

Lifestages and species for adlilt stages.

Encounter rate and prey risk sensitivity were tested with 20% changes in surface light levels, Bythotrephes and prey abundance, and Bythotrephes and prey swimming velocity. 1 used third instar Bythotrephes and Daphnia galeuta mendotae as typical predator and prey, respec tively.

Assum~tionsof the Mode!

The assumptions of the mode1 are as foliows: 1) the animals are randomly disaibuted in space, 2) the anirnals are swimming in random directions, 3) the average swimming speeâ of each species is use& 4) Bythotrephes has an encounter radius, R, which is constant in aii directions but varies according to tight level, 5) the three Bythoqhes instars have the same diet and ability to capture prey; and conversely, prey face the sarne risk regardless of which instar they encounter, 6) the animals arrive and depart each depth at each hour at rr linear rate.

To evaluate the effect of heterogeneous zooplmkton distributions within each depth zone,

1 generated 22 random data sets of third instar Bythomphes and cyclopoid nauplii encounters.

For each depth block (e.g., 0-5 m) per hour, a nornially dismbuted random number with mean abundance of prey in the block and standard deviation of 500 was generated. 1assumed that this standard deviation was large enough to create a heterogeneous abundance within that block as the standard deviation was ofien larger than the mean. 1 generated another uniformly distributeci random number corresponding to each depth and assigned the randornized abundance to that dep th. This process was repeated until there was oniy one depth in the block without an assigned value lefi. The value assigned this depth was the rernainder f?om the total abundance for this depth block subtracting the sum of the randomized values of abundance. If this number turned out to be negative, more values for abundance were generated until ail depths within the block were assigned positive values. This method was repeated for each of the 24 hours.

From my earlier experimcnts with changes in Bythotrephes behaviour under the influence of prey kairo rnones, Bythoirep hes sw imming directions in d four treatments were randomly Table 1. Average prey swimming vebcitics used Li the modeL Calanoid nauplü and copepodids are assumeci to have similar swimming speeds compared to cyclopoid nauplii and copepoàiàs.

Species Velocity Reference (mm s-'1

Bosmina longirostris 1.5 (Zaret and Kerfoot 1980) Calanoid nauplii 0.3 Calanoid copepodàds 1.4 Cyclopoid nauplii 0.3 (Gemtsen 1978) Cyclopoid copepodids 1.4 (Gemtsen 1978) Diacyclops bicuspidatus rhomasi 1.7 (Gemtsen 1980) Daphnia galeata mendotae 1.2 (Gemtsen 1980) Epischura lacustris 3.04 (Wong and Spruies 1986) Leptodiaptomus rninuîus 3.4 (Ramc haran and Sprules 1989) Leptodiaptomus sicilis 1.68 (Ramcharan and Sprules 1991) Senecella calanoiàès 4.47 (Wong and Spruies 1986) Table 2. Rey ria factor @acentage in Bythotrephes diet) of viesand Mestages used in the encounter rnodel Data provideci by G. Morgan. Values do not add up to 100% due to the exclusion of some animais.

Percentage in diet (46)

Cladocerans Daphiu galeata mendotue 5.27 1 35.230 Bosmina longirostn's 2,165 0.270

Calanoid adults Epischura lacustris 2.269 3.523 Senecella calanoides 2.269 3.523 Leptodiuptomus minutus 2.269 3.523 Lep todiup tomus sicilis 2.269 3.523

Cyclopoid adults Diacyclops bicuspidatus thornari 2.269 3.523 Tropocyclops extensus 22.526 7.720

Calanoid copepodids 1.004 12.280

Cyclopoid copepodids 0.502 6.140

Calano id nauplii 37 -624 13.644

Cyclopoid nauplii 18.812 6.822 distributcd. The assumption of a constant reaction radius, R, that varies in relation to üght level was tested in the reaction distance experiment, Results

Reaction distance

Byfhoîrephes reactions to prey were rnainly distributed within a 180° arc in both apparent horizontal and vertical planes under ali light treatments (Figure 3). Reaction orientations in the apparent horizontal and vertical planes were not randomly distributed (V-test for horizontal plane, u = 5.35, n = 39, p < 0.0005; V-test for vertical plane, u = 2.61, n = 19, p 4.005) (Durand and Greenwood 1958) but were significantly oriented towards O O (horizontal mean angle =

-3.14 O, vertical mean angle = 4.26 O). There were probably more reactions directly in 6ront of

Bythotrephes; in this case, lateral movement was not necessary to follow prey and so there was no behaviourai indication of padax tracking.

The piecewise regression mode1 explained a large proportion of the variability

(R~=0.78) and the additionai iinear regression fit on the 2 to 10 pE m-2 s 1 was also highly signïficant (Figure 4) (~~4.70,ANOVA, F548.64, d. f.=l, 19, p 4.0005). Reaction distances at

150 pE m-2 <1 did not Vary significantly among the three instars (Single factor ANOVA, F =

1.154, d.f .= 2,18, p = 0.337) although in generai, reaction distance increased fkom 6rst to third instar. First instar Bythfrephes had a rnean reaction distance of 7.77 mm 10.788 SE; second instars, 7.80 mm f 0.780 SE and third instars, 9.21 mm f 0.564 SE. Power of the performed

ANOVA was low at 0.4 due to the large variability in reaction distances for each instar. The

2 1 distribution of maximum reaction distances at 150 pE mm S- was norrnally distributed for al1 three instars (Lilliefors, p > 0.20, n = 7 for each instar). Horizontal Angle (degrees) 100 80

Distance (mm)

Light Level + O p~rn-~s" r 2p~rn'~s" i 5 p~rn-~s-' Vertical Angle (degrees) A i~~~rn-~s*' 0 i~op~rn-*s~~ 100 80 m 500 pE m-2~-1

Figure 3. Apparent reaction distance to prey relative to Bythotrephes main body axis under different light levels. The origin of the gnph represents eye position and the zero angle represents the direction Bythotrephw was facing. Negative horizontal angles refer to angles to the right of the midline and negative vertical angles refer to angles below the horizontal. Reaction distances (R, in mm) based on lïght level (L,in pE rrï2 5' ) for aii three instars wcre used in the encounter mdel as follows:

The breakpoint in light level at 6.553 pE m-2 s-' predicted from the linear regression correspondeü to the breakpoint in reaction distance (9.05 mm) estimatd fkom the piecewise regression (Figure 4).

Encounter rate and mey risk

The encounter rate for Bythotrephes and prey increases with increasing reaction distance

(R), prey abundance (NH)and Bythotrephes and prey velocity (v and u).

This means that Byrhotrephes may increase its encounter rate with prey by feeding in an illurninated environment which maximizes Bythotrephes' reaction distance, by feeding in areas where there is maximum overiap in Bythotrephes-prey spath1 and temporal distributions and by swimming faster.

Example of diurnal Dattems of encounter rate and Drev risk

In Harp Lake, Byfhorrephes encountered most prey (e.g., calanoid copepoâids) throughout the die1 periods at depths of 5-10 m (Figure 5 a,b,c); there were few encounters below

10 m and encounters are almost non-existent below 15 m. The maximum rate of encounter occuffed at 1600 h and 0900 h at the 5-10 m zone when light levels were between 6 to 68 pE m-2 s-'; however, a large propomon of encounters occurred between 2400 h and 0500 h where conditions favour mechanoreception as Bythomphes' detection mechanism. Despite a similar pattern of encounters between the July and August die1 periods, the number of prey encounters by a single Bythotrephes decreased three-fold in August due to declining copepod populations. N=39 . O 14 a n

E ' l2 C vE 8 O

5V) Io: a 0 A a C .-O Regression equations a C, % 6 A) RD = 6.610, if Light = O 6) RD = 3.902 + 0.84Z0Light, 2 if Light > O and < 6.553 4- C) RD =11.896-0.002'Light, if Light >= 6.553 D marks the breakpoint for the piecewise regression

Figure 4. Third instar Bythofrephesreaction distance (RD) to prey at di Eerent light levels. The breakpoint in reaction distance (9.047 mm) estimated from the piecewise regression corresponds to a light level of 6.553 pE mJ 5'. Illumination conditions in the water are considered dark when the light is equal to zero; othenvise, reaction distance is predicted based on regressions in visible light. lv) -752.45 rn4

L ii) 0.06 % risk

7- ; !me j h~ursj Figure 5 A. Encounter rate, total Bythotrephes-calanoid copepodid encounters and prey risk for July and August populations of calanoid copepodids. i) Encounter rate for a single first instar Bythotrephes. luly; ii) Total Bythotrephes-calanoid copepodid encounten, July; iii) Calanoid copepodid prey nsk. July; iv) Encounter rate for a single first instar Bythoirephes, August; v) Total Bythotrephes-calanoid copepodid encounters, August; vi) Calanoid copepodid prey risk, August. ii) v> ! 0.20 O/O risk -1.46 % risk û !- -,

vi) !500 2000 2400 0500 0900 1200

Figure 5 B. Encounter rate, total Bythotrephes-calanoid copepodid encounters and prey risk for July and August populations of calanoid copepodids. i) Encounter rate for a single second instar Bythotrephes, July ; ii) Total Bythotrephes-calanoid copepodid encounters, Jul y; iii) Calanoid copepodid prey risk, July; iv) Encounter rate for a single second instar Bythotrephes, August; v) Total Bytlrotrephes-calanoid copepodid encounters, August; vi) Calanoid copepodid prey risk, August- ii) v) 0.36 Oh risk 1.36 % nsk r - - l ni- r

Time (hours) Figure 5 C. Encounter rate, total Bythorrephes-calanoid copepodid encounters and prey risk for July and August populations of calanoid copepodids. i) Encounter rate for a single third instar Bythotrephes, July; ii) Total Bythotrephes-calanoid copepodid encounters, July; iii) Cdanoid copepodid prey nsk, July; iv) Encounter rate for a single third instar Bythotrephes, August; v) Total Bythotrephes-calanoid copepodid encounters, August; vi) Calanoid copepodid prey nsk, August. A 4 mm difference between third instar and secondlfirst instar Bythotrephes swimming velocity resulted in 230 more encounters for the third instar. Total encounters, where the encounter rate of a single Bythotrephes with calanoid copepodids is weighted by Bythotrephes abundance, was higher in August for est hstars than for second or third instars. Rey Mt for calano id copepodids facing predation fiom aïï three instars was higher in August than July despite a decrease in the number of total encounters for second and third instars due to a 12-fold increase in the proportion of calanoid copepods in Bythonephes diet (Table 2). Overail prey risk was highest in the upper five metres of the water column for most of the prey except Diacyclops bicuspidatus thornasi whose Bythotrephes encounters occurred in the 5- 10 m depth zone.

Average encounter rate and Drev risk comarison

As most of the encounters between Bythomephes and prey occur in the depth zone of 10 m to the surface, average encounters and prey nsk within this zone provide a rough comparison of diurnal patterns among prey (Figure 6)- The average encounter rate for a single Bythotrephes was highest for cdanoid copepods in July followed by encounters with Daphnia galeata mendotae in August. The high proportions of calanoid copepodid and D. g. mendotae encounters are essentially due to high prey abundance; there is little difference among average prey swimming speeds (Table 1). Bythotrephes encounter rate of C. 6. thornasi, Epischura fucustris, Leptodiaptornus minutus, Leptodiaptomus sicifis and Senecella calanoides was negligible in July and August. As the season progresses, there is a change in the encounter rate arnong prey. Ln July, Bythotrephes encountered calanoid copepod, calanoid nauplii, C. 6. thmasi, and cyclopoid nauplii in high proportions; in August, Bythotrephes encountered mainly cyclopoid copepods and D. g. mendotae (Figure 6). Patterns of prey risk differed among First Instar Byfhotrephes

CaiCope DiaBiThorn CycNaup EpiLac LepSic CalNaup CycCope DapGal LepMin Sene Prey Figure 6a. Average encounter rate for first instar Bpho~rephesand associated prey risk at 0-10 m depth over 24 hours. Legend: CalCope, Calanoid copepodids; CalNaup, Calanoid nauplii; DiaBiThom, Diacyciops bicuspi&tus thornasi; CycCope, Cyclopaid copepodids;CycNaup, Cyclopoid nauplii; DapGal, Daphnia galeata mendotue, EpiLac, Epischuru facustns; LepMin, Leplodiaplmus ntimtus. LepSic, teptodiaptomus sicifis; Sene, Senecella caIunoides. Second Instar Byfhotrephes

July Encounters 0August Encounters - - * - - July Risk -August Risk

CalCope DiaBiThom CycNaup EpiLac LepSic CalNaup CycCope DapGal LepMin Sene Prey Figure 6b. Average encounter rate for second instar Byihotrephes and associateci prey risk at 0-10 m depth over 24 hours. Legend: CalCope. Calanoid wpepodids; CalNaup, Calanoid nauplii; DiaBiThom, Diacyclops bicuspichius thornasi; CycCope, Cyclopoid copepodi ds;CycNaup, Cycl opoi d naupl ii; DapGal, Daphnia gaieuta mendotue. EpiLac, &ischuro Ipnstriq LepMi n, Leptadiaptomus rnirn~tus;LepS i c, Leptaiiaptomus sicifiq Sene, Senecelfa cafanoides. Third Instar Bythotrephes

-- - July Encounters OAugust Encounters - - * - - July Risk *August Risk

CalCope DiaBiThom CycNaup EpiLac LepSic CalNaup CycCope DapGal LepMin Sene Prey Figure 6c. Average encounter rate for third instar B'hotrephes and associated prey risk at 0-10 m depth over 24 hours. Legend: CalCope, Calanoid copepodids; CalNaup, Calanoid nauplii; DiaBiThom, Diacycfop bicuspi&tus thornasi; CycCope, Cyclopoid copepodids;CycNaup, Cyclopoid nau pl i i ; DapGal ,Daphnia gaieata mendotue, EpiLac, Qischura lacusfris; LepMin, Leptodiiapiornus mim~tus;LepSic, Leptodiaptomus sicik Sene, Bythotrephes instars and were generally correlateâ with encounter rate with the exception of caianoià copcpodid encounters. That is, in general, relatively higher tncounter rates in July than

August werc associated with higher risk hced by prey in July. The potential risk faced by calanoid copepodids in July is almst negligible compared to the relative rate of encounters with other prey due to the low proportion in Bythotrephes diet (Table 2). Fit instar Byrhotrepks posed the greatest risk to August populations of D. g. mendotue foiiowed by July populations of calanoid naupiii. Calanoid and cycbpoid nauplü, D. g. mendUtae faced the highest risk from second and third instar Bythotrephes.

Sensitivitv Analvsi~

Effect of lieht level on reaction distance

As the only squared term in the encounter modei, variation in reaction distance causes the greatest change in the encounter rate. Reaction distance, and thus encounter rate for a single

Bythotrephes and prey risk are rnost sensitive to underwater light levels whic h range f?om 2 to 10 pE m-' s" (Figure 4). Liewise. negative sensitivity occurs at high light levels due to the negative slope of the piecewise regression. In terms of a die1 pattern, the encounter model is highly sensitive at 7-10 m depth firom O800 h to 2000 h (Figure 7).

Effect of Bytbtrephes and Drev velocity

The encounter and prey risk model responded proportionally to the change in

Bythtrephes swimming vebcity; i.e., encounter rate and prey risk changed 20% as a response to a 20% change in swimming velocity (Figure 8). Differences between the 10% response and 20% response are strictly due to rounding emor. The contour in the prey risk sensitivity graph (Figure

8) reflects third instar Bythotrephes abundance; blank areas are an artifact of Bythofrephes absence and not zero percent sensitivity. Time of Day (hours) Legend: OcO%OO%a10%120%.30% Figure 7. Encounter rate and prey nsk sensitivity responses to a 20% change in surface light levels. A) Encounter rate, light+20%, B) Encounter rate, light-20%, C) Prey risk, light+20%, D) Prey risk, light-20%. Tirne of Day (hours) Legend: O

Unlike for Bythoîrephes, a 20% change in prey (e-g., D. g. mendotae) swimming velocity caused only a O to 0.1 % change in encounter rate and prey risk (Figure 9). Thus encounter rate and ptcy risk are more dependant on the swimming velocity of cruising prcdators such as Bythrephes than that of the prey (Gerritsen and Striclder 1977).

Rey risk changed by 20% when Bythotrephes abundance was changed by 20%.

Likewise, encounter rate for a single Bythofrephes changed by 20 % when D. g. mendotae abundance was changed by 209. Patterns in prey Nk are due to Bytbtrephes distribution.

Monte Carlo anal~sig

A heterogenous distribution of cyclopoid nauplii within a depth block had Little effect on the encounter rate for single third instar Byfhonephes and totd Bythotrephes-cyclopoid nauplü encounters (Figures 10, 11). With the exception of oniy a few depths, the original encounter rate based on average abundances of a depth block was within one standard error of the mean. The assumption of a homogenous distribution at the scak of lm h-' for the six tirne periods is therefore reasonabIe (Gerritsen and Strickier 1977). Time of Day (hours) Legend: 0O % El 0.01 % H 0.05 % HO.1 % Figure 9. Encounter rate and prey nsk sensitivity responses to a 20% change in Daphnia swimMng velocity. A) Eccounter rate, velocity+20%, B) Encounter rate, velocity-20%, C) Prey risk, velocity+20%, D) Prey risk~elocity-20%. 1600 2000 2400 0500 0900 1200 Time of day (hours) Figure 10. Mean number and standard error (SE) of encounters of cyclopoid nauplii for a single third instar Bythotrephes. Black lines represent original means and shaded grey areas represent means +/- 1 SE from the monte car10 simulations. N = 22 simulated data sets and the scale bar is set to the maximum number of encounters. -- 1600 2660 2400 0500 0900 1200 Time of day (hours)

Figure 1 1. Mean number and standard error (SE) of totai cyclopoid nauplü-third instar Bythotrephes encounters. Black lines represent original means and shaded grey areas represent means +/- 1 SE frorn the monte car10 simulations. N = 22 sirnulated data sets and the scale bar is set to the maximum number of encounters. Discussion

Bythotrephes prcûation can be described by four successive events: encounter, attack, capture and ingestion, where each event has an associateci probability of occurrence (Holiing

1959). In this model, 1address how Bythotrephes' encounter rate is influenced by rnechanoreception and vision depending on the time of day and depth within the water colurnn.

The encounter rate is affecteci by differences in predator and prey swimming speeds

(Gerritsen and Strickkr 1977). Thus, a prcdator cm niaximize its encountcr rate with prey by swirnming faster than i's prey. On average, Bythotrephes swims so much faster than its prey, especiaiiy srnalier genera such as Bosmina and immature life stages of zooplankton in gencrai, that encounter rates based on speed should contribute Little to prey selectivity (Table 1,

Ramcharan 1984). Models of prey encounter rates, however. which ignore the cffect of prey swimming velocity because the predator swims significantly faster fail to include the effect of prey swimming speed on the hydrodynamic disturbance they create (Kiorboe and Visser 1999).

In my experïment on reaction distance, 1 measured Bythotrephes' maximum reaction distance to an assortment of prey with different swimming speeds. Thus, while 1 incorporated the prey's swimming speed by way of hydrodynamic disturbance into Bythotrephes' reaction sphere under the assumption that Bythotrephes' reaction distance increases with increasing hydrodynamic disturbance fiom the prey, 1 made no distinction among prey as how they affect reaction distance.

The proportions of prey in Bythotrephes' diet indïcates that prey swimrrring speed alone does not underlie susceptibility. Much of the interaction between Byrhotrephes and its prey depends on Bythotrephes' handling and ingestion. Handling thne is kept to a rninimum:

Bythotrephes seizes the prey, manipulates it headfirst, tears it apan with the mandibles and sucks out the iiquid contents (Rivier 1998). As a consequence of reduced handihg th,the tirne spent searching for and capturing prey is greater as shown for Mysis reficta (Ramcharan 1984).

Handling tirne for larger prey is predicted to last longer than for small prey which may explah the high percentage of nauplii in Bytbtrephes*diet fiable 2). However, this does not explain the high proportion of D. g. menabtae in Bythotrephes diet for August because D. g. mendotue is a relatively large-bodied species. With dcrate ingestion (0.60) and assimilation rates

(0.62)(YunSta and Schultz 1995), a third instar Bythotrephes wodhave to cncountcr the equivalent of 1585 calanoid nauplii on a daily kisto fuIfill consumption needs. Based on my rnodel, a single third instar Bythonephes could theoreticaiiy encounter 6495 calanoid nauplii in the upper 10 m in July over a 24hour period.

Natural lipht levels and vision in Bvthotrephes

The use of vision as a major prey detection mec hanism in Harp Lake is probable, given the oiigotrophic nature of Precambrian shield Mes. Light levels sufficient for Bythofrephes vision (1 2 pE m'2 s'l) are present at depths of 10-11 m du~ga rnidsurnrner mornïng. The assessrnent of Bythofrephes vision in Mes, however, is complicated by diurnal and lunar iight cycles, changes in lake turbidity and primary productivit y. Rarnc haran and Sprules (1986) estimated that illumination provided by the rnoon in northern Ontario lakes was sufficient to permit Mysis relictu to search fkom 6-16 an, whereas there is no illumination at night in this modeL Therefore, both the encounter rate and prey risk would be underestimated in this madel, especially because the mode1 is highly sensitive to changes in lower light levels. The effectiveness of vision is also reduced due to increased light attenuation and scattering caused by elevated turbidity during sp~gand fa11 ovenurn. Twice a year. Harp Lake mixes completely within a week as soon as surulce temperature reaches 6 OC (B. Girard, pers. corn). Occasional midsummer algal blooms absorb more light to the point where light penetration is alrnost non- existent (Secchi depth 0.3 m; B. Girard, pers- corn) and can last upwards to a rnonth.

Prev Risk

Prey conspicuousness is best describai in terms of the detection mechanisms used by their major predators. Zooplankton prey have essentially two options to reduce their conspicuousness to invertebrate and fish predators: either they reduce their visual or hydromechanical signature. Generaîly, prey with low contrast experience lower mortaiity rislr ffom visual predators and this benefit is influenced by light intensity, zooplankton size and water cIarity in order of decreasing impottance (Giske et al. 1994). Morphological adaptations in prey to reduce contrast such as reduced eye size and amount of eye pigmentation has been shown in

Bosmina longirostn's (Kerfoot 1980); the eye pigment region is usually the darkest area on zooplankton. DBerences in overd body size can also affect detection by predators; larger prey are detected by visual predators at a pater distance than srnaller prey (Giske et al, 1994;

Kiorboe and Visser 1999). Prey motion, usually described in ternis of a hydrodynamic disturbance, has a definite visual component. Zaret (1980) suggests that pianktivorous fish such as Meluniris chagresi recognizes prey items according to motion and selects Bomina longirostns over Bominopsis deitersi and Eubosmit~arubicen even though the latter two species have relatively larger man body size and eye-pigmentation diameter. Buskey (1994) hypothesized that prey with an irregular swimming motion were more Wrely to be selected by planktivorous fish than prey with periodic motion, Personal observations of greater tuming in

Bythotrephes tracking behaviour deran escape jump by a copepod support this hypothesis. The copepods' escape jump was uiitiated at least 8 mm from Bythonephes so visual cues were most likely responsible for Bythorrephes' accurate tracking behaviour. For invertebrate predators such as Bythotrephes, the nature of visuai cues (Le., frequency, amplitude) which elicit attack responses is unknown at this point aithough Bythotrephes' compound eye is adapteci to fom images.

Adaptations in prey to reduce risk by reducing their hydrodynamic disturbance or altering their swimming motion are cornmon in many mopiankton. Wong et al. (1985) present evidence of decreasing fkequency of jumps of the calanoid copepod Diaptomu minufus in the presence of predatory copepods. Linutocalanus rnacm, Cyclops sccutifer and Diacyclops bicuspidatirr.

Similar results have been reported by Ramcharan et aL (1991) for the copepods Diaptomus sicilis and Dioptomus oregonensis. In the presence of the predator. Limnocalanus macrulus, D. sicilis swam more slowly with fewer jumps and hops than D. oregonensis and consequently, experienced lower vulnerability to predation. An early study on Bythotrephes (Mordukhai-

B oltovskaia 1958) demonstrated that Bythoîrephes avo ided sessile or semi-sessile cladocerans.

An additional adaptation to reduce prey risk is the spatial or temporal avoidance of predators by way of vertical migration. However, in Harp Lake, potential cladoceran and copepod prey coexist with Bythoîrephes presumably to minimize risk from another predator, the lake herring.

Discussion of mode1 assumtions

1 have shown that Bythotrephes has an approximately hemispherical reaction volume which at first, appears to contrast with Gerritsen and Suicker's (1977) mode1 assumption of an encounter radius which is constant in aii directions. However, tether effects and prey choice depending on prey location relative to Bythotrephes may bias the reaction volume. Typical

Bythotrephes swimmuig patterns consisted of loops, spirals and tight turns with diameters slightly larger than its total body length, uniike in the reaction experirnent where Bythonephes swimming was restricted by a tether. Although the tether resPicted Bytiwtrephes movement, there was no apparent effect on Bythotrephes swimming activity. Bythumephes instars had an average of 10.2 swimming strokes pet second when tethereû in the treatment with infiarcd îight only as compared to an average of 10.4 swimming strokes per second of a free-swimming

Bythotrepks under infrared light conditions (Chapter 1, Light and prey experiment). Secondly, given a choice between prey that creates a hydrodynamic disturbance behind Bythotrephes to prey that can k detcctcd visuaiiy in front, Bythorrephes' visual recognition of prey has an advantage in selecting for prey quality. Time spent handling overly large prey or avoiding potential cornpetitors and dangerous contacts is saved.

The tight looping and tuming behaviour in Bythotrephes, in essence, rotates the hernispherical reaction volume to fonn a reaction sphere and the assumption of a constant reaction volume thus depends on the spatial distribution of prey. If Bythofrephes is able to complete a loop before it encounters a prey, the reaction volume is spherical; likewise, the reaction volume is restricted to a hemisphere if Bythtrephes encounters another prey before complethg a tum. For Bythomphes to encounter prey before it cornpletes a turn within a volume of 125 cm3, prey abundance would have to be 8.00 x 1o3 m-3 if they were equaUy spaced in a cubic metre of water. In 1995, Harp Lake had an average zooplankton abundance of 2.00 x

IO* mW3(Yan and Pawson, 1997). Thus, the assumption of a reaction sphere constant in di directions but depending on Light level is not met. De Vita et aL's (1982) two-dimensional mode1 for terrestrial mite encounrers found that the turning behaviour of animals versus straight- line motion did not affect encounter rate. However, turning behaviour has ken shown to increase the variance in encounter rate (Skellam 1958). Spatial heterogeneity in the distribution of predator and prey populations are a crucial component in stabilizing population interactions (Hassell and May 1974; Hasseil 1979). Most prey populations exhibit clumped distributions and random search impfies that predators search each area eqdy(Hassel1 1979). However, to enhance their fitness. predattors will spcnd more of their searching time where prey are abundant, Thus the assumption of random search depends grealy on the whethtr the predator is scarching within a paîch or bctween patches (De Vita et al,

1982). For fast cruising predators such as Bythotrephes, the relative merence in swimming speed with prey is more important than the direction of travel or prey patchiness (Gemtsen

1980).

Prey patchiness, like the turning behaviour of predators, creates variability in the encounter rate for single predators and total predator-prey encounters but has Little effect on the mean encounter rate.

The assurnption of qua1 prey risk regardless of which instar they encounter is not realistic due to ciifferences in the abiiity of Bythotrephes instars to detect and capture prey. Third instars iikely have a larger diet range because they are able to handle a larger range of prey sizes.

Experimental modifications

Idedy, rneasurements of Bythonephes reaction behaviour should be made on free- swimming animds in lieu of tethered animals. This wouid require the exact position of both

Bythotrephes and prey in three-dimensions as weii as the orientation of Bythtrephes at the moment of reaction. Second, there is an assumption that prey behaviour did not change at different iight levels. In addition, maximum reaction distances in which there were indications of Bythoîrephes tracking behaviour were used in the modeL Refinements to measunmenu of reaction volume would include probability fields where the probability of Bythonephes reaction would be mapped out at specified distances for various prey. Only by comparing reactions to different prcy coupkd with dctailed informadon about fâctors that infiuence prey risk wouid our knowledge of Bythotrephes sensory mechanisms improve.

Future stu-

To refine potential impact models of Bythorrephes on other zoophnkton communities, additional information is required on bchaviour of Bythatrephes and prey. Such exprrimcnts would include die1 enclosures in Harp Lake which would couple ambient light conditions with

Bytbfrephes feeding rate for each instar. In addition, multivariate measutes of prey risk which incorporate body size, amount of pigmentation and swimming movernent would be an asset for encounter models.

Su- and conclusion^

For maximum Bythotrephes encounter rates, vision is the most important detection rnechanism followed by rnechanoreception. Reaction distance increases at low leveis of ambient light to a maximum and highest rates of encounter occur withïn the upper 10 m throughout the diel period. Prey risk in the epilimnion at night is still high due to reaction distances based on

Bythotrephes mechanoreception. Predicted species at risk are likely to be small-bodied with slow swimming speeds (e.g., calanoid and cyclopoid nauplii) and overlap spatially and temporally wit h Bytbtrephes (e.g., D. g. mendotae). The zooplankton community composition in Harp Lake has already been altered such that populations of srnalier, slow-moving species have been dramaticaîiy reduced and muc h of the biomass is found in larger zooplankton such as

D. g. mendotae (Yan and Pawson 1997). Future encounter models must include complex reaction distances that incorporate elements such as ambient Light and predator/prey behaviour in order to bridge the gap between behavioural and community level dynarnics. References

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Bythtrephes' succcss as an invader into Harp Lakc and other inland lakcs can be

partiaiiy ahbuted to its position as an intermediate predator within the zooplanhon

community; Bythtrephes feeds efficiently on a range of smaller zooplankton while

enjoying a size refuge because of its dorsal spine and a thermal refuge fiom its own

predators. In lakes whcrc zooplanktivorous fish and Byîhotrephes overlap, Bythoîrephes

is itself suppressed by predation (Keilty 1990). Bythotmphes' reüuction of the smaller

zooplankton populations in Harp Lake foilowing invasion (Yan and Pawson 1997) is

understandable given its adaptations as a predator: fast swimrning compared to its prey

and visuaiiy-guided predation.

Visual prey detection has several advantages over mechanoreception during encounter and capture. Visual detection in Harp Lake is an advantage for Bythotrephes

as it spends most of the time in the meta- and epilimnion throughout the die1 cycle.

Visual predators such as Bythotrephes may be able to detect prey more accurately than by foilowing the hydrodynamic wake cfeated by swimrning prey (Kerfoot 1978). Due to the rapid attenuation of hydrodynamic vibrations in the water, visual cues enable the predator to detect prey before the prey senses a predator. Ramcharan (1984) comments that fast

swimrning and visual predation rnay have CO-evolvedas complimentary behaviours.

Although fast swimmllig ailows for increased prey encounter (Gemtsen and Strickler

1977) Chapter 2, it also interferes with the mechanoreceptive abiiity of the cruising predator and cornes at a high metabolic cost. The energy spent swimming increases with the square of the swirnming speeà but the energy gained during feeding is Mted

(Gemtsen 1980). Additional rnoipologicd adaptions for a fast swimming &style include a reduced carapace, large antennae used in swimming, and a caudal spinc which may serve as a stabiliter and to aid buoyancy. The role of the caudal appendage in swimming is stiU contmversial, however.

Conclusiom

In both chapters, 1have presentcd evidence that Bythotrephes uses vision extensively in predation. Bythotrephes swims faster and in a more directed fashion in tk presence of light and prty (Chapter 1) than in treatmnts with light or prey only. As mentioned earlier, faster swimming cornes at a higher metabolic cost and is only an advantage if Bythonephes encounters suitable prey. Information about prey suitability such as size and rnovement rnay be transmitted by visual cues at greater distances than by h ydrodynamic cues. Secondly, Bythotrephes' forward directed reactio n volume to prey

(Chapter 2) increased to a maximum in higher light levels. Bythotrephes' parallax tracking behaviour of prey at distances too large for mchanoreception also supports visuaiiy guided predation-

Predicted impact models for invaders which are visualiy guided predators must incorporate behavioural information about the invader as well as characteristics that affect the visibüity of prey. Of ail successful invaders, the predators are the most Wely to have a large impact on the native community (lodge 1993). References

Branstrator, D. K. and J. T. LRhman. 1996. Evidence for predation by young-oGthe-year Aiewife and Bloater chub on Bythotrephes cederstroemi in Lake Michigan. J. Great Lakts Res- 22: 9 17-924. Bur, M. T., D. M. Klarcr and K. A. Krieger. 1986. Fmt records of a Ewopean cladoceran, Bythoîrephes ceàèrsîroemi, in Lakes Erie and Huron. f. Great Lakes Res. 12: 144-146.

Burke, M. J. W. and J. P. GNne. 1996. An experimental study of plant community invasibility. Ecology 77: 776-790.

Evans, M. S- 1988. Bythtrephes cederstroerni: its new appearance in Lake Michigan. J. Great Lakes Res. 14: 234-240.

Gemtsen, 1. 1980. Adaptive responses to encounter problems, p. 52-62. In W. C. Kerfoot W.], Evolution and ecology of zooplankton cornmunities. University Press New England

Gemtsen, 1. and J. R. Strickler. 1977. Encounter probabilities and comrnunity structure in zooplankton: a mathematical modeL J. Fish. Res. Board Can. 34: 73-82.

Giguère, L. A., A. Delàge, L. M. Diii and J. Gerritsen. 1982. Predicting enounter rates for zooplankton: a mode1 assurning a c ylindrical encounter field. Can. J. Fish. Aquat. Sci 39: 237-242.

Haii, R. 1. and N. D. Yan. 1997. Comparing annual population growth esthates of the exotic invader Bythtrephes by using sediment and plankton records. LimnoL Oceanogr. 42: 112- 120. Johnson, L. E. and J. T. Carlton. 1996. Post-establishment spread in large-scale invasions: dispersal rnechanisms of the zebra musse1 Dreissena polymorp ha. Ecology 77: 1686- 1690. Keilty, T. J. 1990. Evidence for alewife (Afosa pseudoharengus) predation on the european cladoceran Sytbtrepes cederstroemi in northern Lake Michigan. J. Great Lakes Res. 16: 330-333. Kerfoot, W. C. 1978. Combat between predatory copepods and their prey: Cyclops, Epischura, and Bosmina. LirnnoL Oceanogr. 23: 1089-1102.

Lodge, D. M. 1993. Biological invasions: lessons for ecology. TREE 8: 133-137. Odselius, R. and D. E. Nilsson. 1983. Regïonally different omrnatidial structure in the compound eye of the water-flea Polyphemw (Cladocera, Crustacea). Proc. R. Soc. Lond. B 217: 177- 189.

Price, H. J. 1988. Feeding rnechanisms in marine and frcshwater zooplankton. Bull Mar. Sci 43: 327-343. Ramcharan, C. W. 1984. Visual and mechano-receptive predation in Mysis relicra. M. Sc. thesis. University of Toronto. Toronto, Ontario.

Ramcharan, C. W. and W. G. Spruies. 1986. Visual predation in Mysis reficta Lovén. LimnoL Oceanogr. 31: 414-419.

Rejmanek, M. and D. M. Richardson. 19%. What attributes niake some plant specks more invasive? Ecology 77: 16%- 1661.

Rivier, 1. K. 1998. The Redatory Cladocera (On ychopoda: Podonidae, Polyp hemidae, Cercopagidae) and Leptodorida of the world. Backhuys Publishing

Spruies, W. G., H. P. Riessen and E. H. Jin. 1990. Dynamics of the Bython-ephes invasion of the St. Lawrence Great Lakes. J. Great Lakes Res. 16: 346-351.

Woiken, J. J. and G. J. Gallik. 1965. The compound eye of a crustacean, Leptodora kindtü J. Ceii BioL 26: 968-973.

Yan, N. D. and T. W. Pawson. 1997. Changes in the crustacean zooplankton community of Harp Lake, Canada, foîiowing invasion by Bythotrephes cederstroerni. Fresh water BioL 37: 409- 425. Appendix 1- Description of motion tracking program

The fkst program, writttn in asscmbly language, tracks a single anunal by locating the brightest three adjacent pixels on a meen within two sd(80 x 80 pixel) windo ws. Tracking of multiple animals cm be perfomed with the analysis of repeated video playback. The operator can isolate an individual by selecting it if multiple animals are present in the searc h window before the tracking ûcgins or can isolate a single animal by shifting the direction of the search windows interactively wMe tracking is occuring (Ramcharan and Spruks 1989). The outputs of this program are the X and Y cornponents of the zoopiankter's position in both the left and right images.

The second program written in TurboPascal 3.0 (Borlund) allows the operator to remove portions of the swimming track if the search window had jumped erratically during tracking.

The third program also written in TurboPascal 3.0 first snioothes the coordinates to remove spurious motion (Young and Getty 1987) by running a 5-point moving average. Missing data or erratic portions of the swimming track are removed before analysis and measurements of swimmhg velocity and other measurements are taken at user-specified intervals. Appcndix II - Circular and spherical gwrnetry and analysis

The mean vector r is calculated in bo th horizontal and vertical planes and is, in essence, a masure of concentration of the cornponent vectors of of that path (2ar 1996). It is calculated as:

n cos ai where X =- n and Y =- n for a first-order analysis of angles (Zar 1996). A high concentration of angles would result in an r appmaching 1 and high dispersion of angles would give an r near O indicating the possible random distribution of angles around a circle. Fisher (1993) cautions about the use of r as an indicator of concentration if the data is multimodal.

The rnean angle is then caiculated as:

S~hericaidata

For sphencal data, the position of a point can be represented in Cartesian coordinates with axes, X, Y and 2, or in spherical polar coordinates with azïmuthal and polar angles, @ and 8, and vector, r. Unlike the convention used in geology, in this case X and Z refer to the axes in the horizontal plane and Y is the axis in the vertical plane; Wtewise, the polar angle 0 is masured fiom the horizontal and not fkom the vertical axis. The mean vector, r, is calculated as: where the average directional cosines are

n Ccoseisin Oi

2 sin en

and Z = >'' n for a point on the surface of a unit sphere. For the second order mean angles and vectors, the directional cosines, X, Y and Z are weighted by the mean vector, r to give:

n r, * Ecosû, sin $i

n f * Ccose, cos*, mdZ= I=I

There is no formai test of randomness for second-order angles with sphefical data.

However, a rotationally invariant test of randomness for first-order angles may be mociified nom

Watson (1966). The test is based on the moment of inertia of n points where (Xi. Yi. Z),i = Ln are the directional cosines and u (U,V, W) is a fured direction. The moment of inertia of the directions O(, Y, Z) about u is are the directional cosines and u (U, V, W) is a fixed direction. The moment of inertia of the directions (X, Y, Z) about u is

M=u'Bu (2-7) where B = nI-T (2-8) and T is the 3 x 3 rriatrix of the sums of squares and cross-products of (X.Y, 2). ie-

where X, Y, Z are the weighted directional cosines (2.6a-c).

Let 71, Q and ~3 be the eigenvalues of the ma& T in ascending order and ti, t2 and ts be the corresponding eigenvectors for each eigenvalue. If aii the eigenvalues are distinct, the eigenvectors can be regarded as the direction cosines of a set of coordinate axes (Mardia 1972).

Hence, if the three eigenvalues are equal, the moments of inertia are equal and distribution of points is isotropie or random Points are concentrated either at one or both poles or they form a girdle distribution if the three eigenvalues are unequai. Interpretations of the eigenvalues that follow are from Mardia (1972) and Watson (1966). Table 9. Interpretation of spherical distributions based on eigenvalues ('ri , Q, ~3)and eigenvectors (tl, t2. t3) of mtrix T .

Eigenvalues Distribution

Concentration at one end of t if R is large (unimodal), otherwise concentration at both ends of t3 Rotational symmetry about tj

Gidespanned by t2, t3

Symmetric girdle, rotationaliy symmetrical about t 1

In the first-order analysis of angles, the trace of the matrix T. (i.e., ri+?2 + ~3 ) will sirnply equal the number of points. n. given that they al have a unit vector and therefore a suitable statistic depends on the difference .ti-n/3. However, for second-order angles where r is heterogeneous, the points no longer faU on the surface of a unit sphere but are contained within the volume of the unit sphere. 1 used

as the test statistic in the spirit of the chi-square distribution and bootstrapped the original spherical data to simulate the population for hypothesis testing (Efion and Tibshirani 1986). Appendix III. Effect of -le rate on the behaviour parameters for a replicate under the Daphnia only treatxnent. R-vert and r-horiz represent the length of the average resuitant vector in the vertical and horizontal planes, repectively.

Samle rate (SI Velocitv (ds) r-vert Referenccs for Appendices

Efion, B. and R. Tibshirani 1986. Bootstrap methods for standard errors, anfidence intcrvals, and other measures of statistical accuracy. Stat. Sci 1: 54-77.

Fisher, N. 1. 1993. Statistical Analysis of Circular Data. Cambridge University Ress.

Mardia, K. V. 1972. Staiktics of Directional Data. Academic Press.

Ramcharan, C. W. and W. G. Spniles. 1989. Preliminary rcsults fiom an inexpensive motion analyzer for free-swimming zooplankton. Limnot. Oceanogr. 43: 457-462.

Watson, G. S. 1966. The statktics of orientation data. J. GeoL 74: 786-797.

Young, S. and C. Getty. 1987. Visuidly guided feedùig behaviour in the Nter feeding cladoceran, Daphnia magna. Anim. Behav. 35: 541-548.

Zar, J. H. 1996. Biostatistical Andysis. Prentice-Hall, Inc.