Further Studies on Predator Induction of Crests in Australian Daphnia and the Effects of Crests on Predation

Abstract

The effects of notonectid predation on populations of Daphnia in a controlled seminatural environment were studied. Log crest size was positively correlated with predator density, suggesting that crest induction was not an all-or-nothing type of response. The size of the Daphnia population varied inversely with notonectid density. Laboratory studies indicated that a crest-induction factor was produced by all Anisops spp. and the one species of Enithares that were tested. Two forms of D. carinata s.1. differed in their ability to respond to the same species of notonectid. Non-notonectid predators did not induce crests. Results of predation experiments supported previously published indications that helmets reduce the efficiency of notonectid predation. The effect of the helmet varied, depending on the species of notonectid and the size and form of D. carinata s.1. The crest acted primarily to increase predator avoidance by evasion rather than by escape. A comparative study of an ecophenotypic morph and a permanently crested morph showed that the presence of a crest was associated with reduced fecundity. The growth rate of the permanently crested morph during this experiment was slower, although its intrinsic mortality was considerably higher.