Asymmetric exploitation of two echinoid host species by a parasitic pea crab and its consequences for the parasitic life cycle

Exploitation of more than one host species may increase fitness of parasites, but it also shapes their life cycle and evolution. During its post-metamorphic stages, the pea crab Dissodactylus primitivus parasitizes the echinoids Meoma ventricosa and Plagiobrissus grandis with high prevalence. However, the present study provides evidence that P. grandis is infected by adult crabs only, attesting to an unusual asymmetry in host exploitation. Because of its low population densities, P. grandis could be a secondarily acquired host, and its addition to the crab life cycle could be adaptive. The latter hypothesis is tested in this study. Choice experiments revealed that adult and juvenile crabs were equally attracted to and able to settle on the 2 hosts. Most adult crab characteristics were not significantly different between the host species, but females living on P. grandis had higher fecundity. This asymmetric life cycle, where adult parasites infecting the 2 hosts show different relative fitnesses, looks like a transitory stage as predicted in models which predict the evolution of parasitic complex life cycles. However, this parasite asymmetric exploitation may also be a stable state, which occurs due to insufficient differences in crab fitness between the 2 hosts and the absence of host preference