Mating system of the symbiotic shrimp Chernocaris placunae (Decapoda: Palaemonidae) in Vietnam: social monogamy and differences between two sites with contrasting environmental conditions

In symbiotic crustaceans, social monogamy is favored in species that inhabit scarce, relatively small, and morphologically simple hosts in tropical environments where predation risk away from hosts is high. We tested this hypothesis in the shrimp Chernocaris placunae which inhabits the mantle cavity of the saddle oyster Placuna ephippium in the Indo-West Pacific. At two study sites in Nha Trang, Vietnam, C. placunae inhabits the mantle cavity of host individuals as heterosexual pairs more often than is expected by chance alone. At Tre Island, a tight correlation between the host and shrimp body size in both males and females was observed and pairing in shrimps was size-assortative; the carapace length was positively correlated between males and females forming pairs. Altogether, the information above suggests that C. placunae is socially and sexually monogamous in Tre Island. In turn, at Mun Island, no correlation between the host and shrimp body size was observed and shrimp pairing was not size-assortative, suggesting that C. placunae is socially but not necessarily sexually monogamous (and probably promiscuous) in this second study site. Host body size and abundance was lower at Tre Island compared to Mun Island suggesting that refuge (=host) trait drives, at least to some extent, the observed differences in the mating system of C. placunae . Tre Island is also a heavily polluted site suggesting that pollution might also be driving the mating system of the studied species. We have described shifts in the mating system of a marine invertebrate. Whether similar changes in the mating system of other marine organisms are occurring due to contemporary changes in seascapes is a relevant topic that deserves further attention. Manipulative experiments are needed to understand how shifts in community components driven by global change affects the social behavior of marine organisms.