CORE Metadata, citation and similar papers at core.ac.uk Provided by Helsingin yliopiston digitaalinen arkisto WALTER AND ANDRÉE DE NOTTBECK FOUNDATION SCIENTIFIC REPORTS No. 23 Food selection and feeding behaviour of Baltic Sea mysid shrimps MAIJU VIHERLUOTO Academic dissertation in Hydrobiology, to be presented, with the permission of the Faculty of Science of the University of Helsinki, for public criticism in the Lecture hall of the Department of Ecology and Systematics, P. Rautatienkatu 13, Helsinki, on March 16th 2001, at 12 noon. HELSINKI 2001 This thesis is based on the following papers, which are referred to by their Roman numerals: I Viherluoto, M., Kuosa H., Flinkman, J. & Viitasalo, M. 2000: Food utilisation of pelagic mysids, Mysis mixta and M. relicta, during their growing season in the northern Baltic Sea. – Mar. Biol. 136: 553-559. II Viherluoto, M., Viitasalo, M. & Kuosa, H.: Growth rate variation in the pelagic mysid, Mysis mixta (Mysidacea); effect of food quality? – Submitted manuscript. III Viherluoto, M. & Viitasalo, M. 2000: Temporal variability in functional responses and prey selectivity of the pelagic mysid, Mysis mixta, in natural prey assemblages. – Mar. Biol. (In press.) IV Viherluoto, M. & Viitasalo, M.: Effect of light on the feeding rates of pelagic and littoral mysid shrimps: a trade-off between feeding success and predation avoidance. – Submit- ted manuscript. V Engström, J., Viherluoto, M. & Viitasalo, M. 2000: Effects of toxic and non-toxic cyanobacteria on grazing, zooplanktivory and survival of the mysid shrimp Mysis mixta. – J. Exp. Mar. Biol. Ecol. (In press.) Papers I and III are reproduced by the kind permission of Springer-Verlag and paper V of Elsevier Science. 4 5 Food selection and feeding behaviour of Baltic Sea mysid shrimps MAIJU VIHERLUOTO Viherluoto, M. 2001: Food selection and feeding behaviour of Baltic Sea mysid shrimps. – W. & A. de Nottbeck Foundation Sci. Rep. 23: 1-35. ISBN 951-98521-2-3 nid.; ISBN 951-45-9828-8 PDF Mysids are an important link in the energy flow between primary and secondary producers and fish in the Baltic Sea. The present work contributes to mysid research by investigating the feeding and diet change of pelagic mysids (Mysis mixta and M. relicta) during their most intensive growth period during summer and autumn. The effects of light on the feeding rates of the pelagic (M. mixta) and the littoral (Praunus flexuosus) mysids and the effects of cyanobacteria on the feeding efficiency and survival of M. mixta were also studied. Pelagic mysids fed on various food items during their growth period and the diet clearly changed from phytoplankton and benthic material to a more carnivorous and pelagic diet towards autumn. Both the size of the mysids and the availability of food influenced the diet composition. Mysids of less than 7 mm in length were inefficient in capturing and handling larger zooplankters. Thus, 7-8 mm was a threshold size for zooplankton feeding. The mysids, which had attained this size, increased their zooplankton utilisation and grew faster than the mysids which grazed mostly on phytoplankton. Thus, omnivorous feeding habit may reduce intraspecific competition and therefore reduce juvenile mortality. Different zooplankton taxa are important at different stages of the mysids’ life cycle. Small mysids fed mostly on rotifers and gradually shifted to feed on cladocerans and copepods. Al- though they are omnivorous, they did have some prey preferences. The most selected species were the cladoceran Evadne nordmanni, and the copepods, Eurytemora affinis and Temora longicornis. The preference for E. affinis appeared to be dependent upon true selection, since E. affinis has good escape ability and is therefore a difficult prey to capture. Acartia sp. was mostly rejected although it was abundant throughout the study period. Ingestion rates followed sigmoidal func- tional response curves (Holling type III), with saturation levels at between 400 and 500 mg C l-1 depending on the month. This indicates that mysids cannot saturate their zooplankton feeding in natural feeding conditions, unless they are able to detect and forage in denser zooplankton patches. Light had a strongly negative effect on the feeding rates of pelagic mysids compared to feeding in total darkness, whereas no such effect was found on the feeding rates of littoral mysids. The habitats of mysids and thus their adaptation to natural light conditions, differ, which explains their different feeding rates. Light increases the predation risk and pelagic mysids migrate to deeper water when light increases, while littoral mysids hide among the macroalgal vegetation. The be- havioural patterns of pelagic mysids in the presence of light influenced their feeding. Mysids fed on cyanobacteria and were partly able to avoid the toxic strain, Nodularia spumigena. This might be an evolutionary adaptation in areas where cyanobacteria blooms are common. However, filaments of the cyanobacteria clogged the feeding appendages of the mysids and thus hampered their zooplankton feeding efficiency. Changes in the state of the Baltic Sea, such as eutrophication and changes in salinity level, may affect the plankton community and hence, the quality of food available to the mysids. Decreased salinity favours the prey species that mysids prefer, such as E. affinis and some cladocerans, though the increased occurrence of cyanobacteria blooms may alter their feeding success and decrease the quality of available food. Maiju Viherluoto, Department of Ecology and Systematics, Division of Hydrobiology, P.O. Box 17, FIN-00014 University of Helsinki, Finland. 6 CONTENTS INTRODUCTION INTRODUCTION ................................................... 6 Mysids – a link between lower trophic levels and fish Mysids – a link between lower trophic levels and fish .................................................... 6 Pelagic mysids: Mysis mixta and M. Mysid shrimps (Malacostraca, Peracarida, Mysi- relicta .......................................................... 8 dacea) are common crustaceans which inhabit Littoral mysids: Praunus flexuosus ............ 8 various aquatic environments, including oceans, Northern Baltic plankton community and estuaries and other brackish water ecosystems food availability of mysids .......................... 9 as well as freshwater lakes. They are highly adap- Previous studies on the feeding of tive species and therefore also good invaders of Baltic mysids ............................................... 9 Growth of mysids ...................................... 11 new areas (Ketelaars et al. 1999). Most of the Conceptual background .................................. 12 species are marine (~95 %), some live in brack- Optimal foraging theory ........................... 12 ish water and a few species occur in freshwater Functional responses ................................ 12 environments. Furthermore, some have become Predation cycle.......................................... 13 adapted to live in caves and wells and a few live Trade-offs .................................................. 14 in commensal association with other animals. OBJECTIVES OF THE STUDY .......................... 15 Some species burrow into the sediment, live just above it or migrate between bottom and surface MATERIAL AND METHODS ............................. 16 waters, a few are strictly pelagic species and Study area ........................................................ 16 some live in shallow water in the littoral zone Sampling .......................................................... 17 (Mauchline 1980). Field studies ..................................................... 18 In the Baltic Sea, there are currently at least Laboratory experiments .................................. 18 Statistical analyses and equations ................... 18 20 species of mysids, of which over half live only in the area near the entrance to the Baltic RESULTS AND DISCUSSION ............................ 19 Sea, where the salinity is close to oceanic salin- Food utilisation during growth ........................ 19 ity levels (Köhn 1992). Only 7 species occur east C:N ratio ................................................... 21 or northwards of the Arkona Sea. Mysis mixta Selective feeding ............................................. 22 Lilljeborg and the two sibling species of M. rel- Effects of environmental factors on feeding success ................................................ 24 icta Lovén (I and II; Väinölä 1986), are pelagic Abiotic factors: the effect of light ............. 24 species. The other four, Neomysis integer Biotic factors: the effect of (Leach), Praunus flexuosus (Müller), P. inermis cyanobacteria ............................................ 25 (Rathke) and Hemimysis anomala G.O. Sars, which is a recent invader from the Pontocaspian CONCLUSIONS ................................................... 26 region to the northern Baltic (Salemaa & Hieta- ACKNOWLEDGEMENTS ................................... 28 lahti 1993), live more or less in the littoral zone, among macroalgae, in crevices along rocky REFERENCES ...................................................... 29 shores, or on sandy beaches (Fig. 1). The distri- bution of mysids is mainly regulated by salinity, temperature and the depth of the water column, and they seem to avoid areas where oxygen con- centration is low at the bottom (Ackefors 1969, Salemaa et al. 1986). Mysids utilise a diversity of foods during their life cycle, which spans from a few months