Abstract
Although most invertebrate predators are size-selective, two species of Hydra, H. oligactis and H. pseudoligactis, are not. A marked preference for Daphnia pulex over Simocephalus vetulus, similarly sized prey items, is observed in feeding trials. S. vetulus is virtually ignored and swims among the tentacles of Hydra, whereas D. pulex is rapidly attacked and captured. However, normal feeding responses are induced in the presence of homogenates of both Daphnia and Simocephalus. This suggests that the lack of response to live Simocephalus as prey items may be due to defense mechanisms evolved during the course of long-term coexistence in the shallow, weedy littoral zone of lakes and ponds. Such mechanisms could involve reduced activation of nematocysts, immunity to the toxin, or lack of penetration of nematocysts through the carapace of Simocephalus. The impact on the structure of the zooplankton community of this differential susceptibility to predation by Hydra is discussed.
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