Studies on the life cycle and reproduction of the parasitic amphipodHyperia galba in the North Sea

Abstract

The structure of aHyperia galba population, and its seasonal fluctuations were studied in the waters of the German Bight around the island of Helgoland over a period of two years (1984 and 1985). A distinct seasonal periodicity in the distribution pattern of this amphipod was recorded. During summer, when its hosts—the scyphomedusaeAurelia aurita, Chrysaora hysoscella, Rhizostoma pulmo, Cyanea capillata andCyanea lamarckii—occur in large numbers, supplying shelter and food, a population explosion ofH. galba can be observed. It is caused primarily by the relatively high fecundity ofH. galba which greatly exceeds that of other amphipods: a maximum of 456 eggs was observed. The postembryonic development is completed in the medusae infested; only then are the young able to swim and search for a new host. The smallest fréely-swimming hyperians obtained from plankton samples were 2.6 mm in body size. The size classes observed as well as moult increment and moulting frequencies in relation to different temperatures suggest that two generations are developed per year: a rapidly growing generation in summer and a slower growing generation in winter that shifts to a benthic mode of life and hibernation. For short periods, adult hyperians may become attached to zooplankters other than scyphomedusae. However, when releasing the progeny, they are dependent on the presence of these coelenterates. Apparently, a host specificity does not exist. During daytime, the hyperians seem to avoid a host change; only 0.2% of all the individuals sampled in the plankton during the day were not associated with medusae. The heavy infestation of medusae by this crustacean leads to a weakening and a progressive breakdown of these important predators on fish larvae.H. galba occupies a specific position in the marine food web which is discussed in detail.