VVESTERNHAGEN and ROSENTHAL: PREDATOR-PREY RELATIONSHIP 



behavior (Murdoch and Marks 1973), might be 

 either caused by different distribution of prey 

 species (Oaten and Murdoch 1975), differences in 

 palatibility (Rolling 1965), avoidance behavior of 

 the prey, or conditioning and/or training of the 

 predator (Murdoch 1969; Oaten and Murdoch 1975) 

 in cases of weak preferences. 



Although generally H. medusaritm was con- 

 sidered to lead a parasitic life on medusae (Sars 

 1895) such as Cyanea capillata (Bowman et al. 

 1963) or Plenrohrachia pileus (Evans and Sheader 

 1972), the results of our experiments show that 

 even in the presence of alternate prey this am- 

 phipod displays considerable predation on herring 

 larvae. 



Unlike another carnivorous hyperiid, Pam- 

 themisto gaudichaudi, which hunts moving plank- 

 ton visually (Sheader and Evans 1975), H. medu- 

 sarum depends on random encounters with its 

 prey. Many carnivorous copepods display the same 

 behavior (Dziuban 1937; Fryer 1957; Lillelund 

 1967; Rosenthal 1972; Brandl and Fernando 1974; 

 Ambler and Frost 1974). This mode of hunting 

 requires a relatively high density of prey in- 

 dividuals which at times is provided by the enor- 

 mous numbers of newly hatched herring larvae. 

 During this investigation, herring larvae density 

 during the day at the water surface was 

 frequently above 2 larvae/ 100 cm'-' (direct obser- 

 vations). Simultaneous mass occurrences of H. 

 medusarum suggest that the amphipods could 

 possibly contribute considerably to herring larvae 

 mortality, especially since conditioning to abun- 

 dant prey organisms is comprehensible as could be 

 shown by Sheader and Evans (1975) for P. gaudi- 

 chaudi and its feeding on fish larvae. In fact 

 stomach-content analyses of H. medusarum cap- 

 tured during this study period revealed that the 

 amphipods had eaten considerable amounts of fish 

 larvae (Westernhagen 1976). 



ACKNOWLEDGMENTS 



We are indebted to D. F. Alderdice for providing 

 laboratory space, to J. Klinckmann and G. Fiir- 

 stenberg for expert technical assistance and to M. 

 Blake for advice on the preparation of the 

 manuscript. 



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