Life-history responses to Chuoborus of spined and unspined Daphniu pulex

Abstract

Water-borne chemicals released by the larvae of the predatory phantom midge Chaoborus are known to induce morphological modifications in its prey Daphnia pulex: these cladocerans develop neck spines which may carry several teeth. Some work has shown that these morphological variations enhance the prey's chances of escape. but since these neck teeth are not fixed defence reactions, they are thought to entail some form of cost, such as delayed maturation and reduced fecundity. In this study. the relationship between morphological and life-history changes in four clones of Daphnia pulex reared in the presence and absence of Chaoborus flavicans was examined. Special emphasis was placed on the genotypic comparison of the modifications. While all four clones showed a delay in maturation time in the presence of Chaoborus, the neck spine responses differed markedly among the genotypes: one clone never had any neck teeth, another always produced one single tooth, and two clones produced varying numbers of teeth per spine (means 2.9 and 4. respectively). These results indicate that there is no general pattern of neck teeth production corresponding to delayed maturation. What there appears to be is genetic variability in two independent and possibly adaptive responses. However, the clone without neck teeth was the only one which showed no predator-induced reduction in fecundity. Another common morphological response to Chaoborus was that juveniles of all clones developed elongated tail spines.