Effects of predation by Hydra (Cnidaria) on cladocerans (Crustacea: Cladocera)

Planktonic cladocerans have evolved different strategies to avoid predation from vertebrates; these include changes in morphology, behavior, physiology, and/or life-history traits. However, littoral cladocerans are better adapted to avoid invertebrate predation particularly from insect larvae by evolving morphological and physiological adaptations. Nevertheless, this has not been proven for some littoral predators such as <em>Hydra</em>. In this study, we provide quantitative data on how <em>Hydra</em> affects its zooplankton prey. We studied the predation behavior on <em>Alona glabra</em>, <em>Ceridodaphnia dubia</em>, <em>Daphnia pulex, Daphnia </em>cf. <em>mendotae, Diaphanosoma birgei, Macrothrix triserialis, Moina macrocopa, Pleuroxus aduncus, Scapholeberis kingi, Simocephalus vetulus, Elaphoidella grandidieri, Brachionus rubens </em>and <em>Euchlanis dilatata</em>. We also tested the indirect effect of allelochemicals from <em>Hydra</em> on the demography of <em>Daphnia </em>cf. <em>mendotae.</em> Littoral cladocerans are specially adapted to resist nematocyst injection and discharge of toxic substances from <em>Hydra</em>. A significant decrease in the population growth rate from 0.21 to 0.125 d^-1 was observed at densities of 2 ind. ml^-1. The role of carapace thickness as an adaptive strategy of littoral cladocerans against <em>Hydra</em> predation is discussed.