In many symbioses involving marine crustaceans the nature of the relationships and the population biology remain overlooked, obscuring the understanding of their ecology and evolution. The association between the pea crab Dissodactylus primitivus and the irregular sea urchin Meoma ventricosa was investigated in the Caribbean Sea. The work tested the predictions provided by an evolutionary model, in which host-switching behaviour and polygynandry are expected for the mating system of D. primitivus. The crab prevalence (infected sea urchins) and mean burden were high and variable between sites and/or years. All post-metamorphic stages of crabs were found on the host, and no exclusion effect was observed between juvenile and adult crabs. In adults, although a general 1:1 sex ratio was stable over the years, there was no evidence for social monogamy. Adult males occurred more often on host-harbouring non-ovigerous females, suggesting that they search for females available for mating. This assumption was supported by a recolonization experiment showing that the males move between hosts. However, both juveniles and females also moved between hosts, indicating that this behaviour could occur due to causes other than mate searching. D. primitivus inflicts external lesions on M. ventricosa teguments. The most severe lesions were associated with a lower gonad volume during spawning, suggesting a reduction in fitness of the host. The demography of D. primitivus and the recolonization experiments indicate that the mating system of this crab is the 'pure-search polygynandry of mobile females', but the parasitic nature of the relationship could confound this assumption. |
