ABSTRACT
The evolutionary history of decapods and their parasites is assessed with particular reference to the use of parasites as proxies for host phylogeny. We focused on two groups of obligate parasites that use decapods as their definitive hosts: parasitic isopods of the family Bopyridae and parasitic barnacles of the superorder Rhizocephala. Bopyrids and rhizocephalans differ in that the rhizocephalans have a direct life cycle whereas bopyrids require an intermediate host. In addition, rhizocephalans cause drastic impacts on hosts (including castration and behavioral modification) whereas bopyrids have less pronounced impacts but often also castrate hosts. The diversity and host specificity of both groups are reviewed and their patterns of association with decapod hosts are analyzed. Aside from the Dendrobranchiata (with 39 bopyrid species) and the Caridea (with 8 rhizocephalan and 203 bopyrid species), the more basal decapods are relatively unparasitized or completely lack representatives of these parasites. In contrast, the most derived decapod taxa (Anomura and Brachyura) host the largest number of parasites (233 rhizocephalan and 282 bopyrid species). Counterintuitively, when the phylogenies of the decapods and parasites are compared, some of the most basal parasite groups are found associated with more derived host groups. Our findings indicate a degree of cospeciation but suggest that host switching has been frequent in these parasites, with colonization of caridean shrimp occurring in both groups. Conclusions based on the coevolutionary analyses are complicated by the fact that comprehensive cladistic analyses of the parasites are presently lacking; our review can act as a catalyst for more directed studies analyzing the coevolution of these groups and testing particular hypotheses on their evolutionary history. Although the value of parasites in the elucidation of the phylogeny of decapods as a whole may be limited due to host switching, parasites may be informative within particular decapod taxa. We explore an example of this within the Anomura and indicate how such coevolutionary analyses may show host taxa that we would predict to have parasites but presently appear to be lacking them, likely due to limited sampling or evolution of anti-parasite defenses. In addition, these analyses are important in applied areas of decapod ecology (e.g., fisheries) and a brief discussion is provided on the role of coevolutionary studies in the use of bopyrids and rhizocephalans as biological control agents of invasive and/or pest decapod species.
