The Effect of Hydra on the Outcome of Competition Between Daphnia and Simocephalus

¹ Department of Biological Sciences, University of Windsor, Windsor, Ontario, Canada N9B 3P4

The cladoceran genera Daphnia and Simocephalus often co-occur in nature. In laboratory experiments, populations of the two genera had similar growth rates when grown separately, but when cultured together Daphnia invariably excluded Simocephalus. However, the added presence of the littoral zone predator, Hydra, reversed this trend with Simocephalus remaining after Daphnia had been eliminated. This result was robust in culture vessels as small as 100 ml and as large as 85 l. It is hypothesized that Simocephalus has evolved a suite of energetically expensive traits to deter littoral zone predators, whereas Daphnia, which are planktonic, have not evolved such costly traits and hence have more energy available for reproduction and are able to exclude Simocephalus.