The mating system of the symbiotic pea-crab Dissodactylus crinitichelis (Brachyura, Pinnotheridae): monogamy or promiscuity?

Abstract

The mating systems of symbiotic crustaceans vary broadly, ranging from monogamy to polygamy to promiscuity. This disparity in mating systems coupled with wide differences in host ecology provides opportunities to study the effect of environmental conditions on the social behavior of marine organisms. In this study, we tested the prediction that symbiotic crustaceans inhabiting relatively large and abundant host species are promiscuous rather than monogamous. The host traits above constraint host monopolization and favor host-switching behaviors by males individuals and, thus, ultimately drive promiscuity as a mating system in symbiotic crustaceans. As a model system we used Dissodactylus crinitichelis, a small ‘pea-crab’ that inhabits the relatively large and abundant sand dollar Encope emarginata. We described the population distribution of D. crinitichelis at two intertidal localities in Sergipe, Brazil, and conducted laboratory experiments to infer the mating system of this species. Dissodactylus crinitichelis inhabit sand dollars as adult heterosexual pairs more frequently than expected by chance alone, but these heterosexual pairs did not exhibit size-assortative pairing. Sexual dimorphism was reversed in D. crinitichelis, with males attaining smaller average body sizes than females but exhibiting more elaborated weaponry (male > female claws). These findings disagree with expectations for monogamous symbiotic crustaceans. Laboratory experiments demonstrated that male and female crabs switch among host individuals rather frequently and heterosexual pairs retrieved from the field did not remain together for long times. Our results argue against social monogamy and in favor of promiscuity in D. crinitichelis. Our study supports the notion that host abundance and large body size (relative to that of symbiotic guests) favor promiscuity in symbiotic crustaceans.