Host-use pattern and sexual dimorphism reveals the mating system of the symbiotic pea crabAustinixa aidae(Crustacea: Brachyura: Pinnotheridae)

Abstract

Austinixa aidaeinhabits burrows of the ghost shrimpCallichirus majorat Perequê-açu beach, Ubatuba, Brazil. We described the host-use pattern and sexual dimorphism ofA. aidaeto test for monogamy given the generality of this mating system in the subfamily Pinnothereliinae (family Pinnotheridae) to whichA. aidaebelongs. Against expectations,A. aidaelives as solitary individuals within burrows more frequently than expected by chance alone. Additional observations suggested thatA. aidaeexhibits a polygynandrous mating system with males moving among burrows in search of receptive females. First, only 21% of the burrows harboured heterosexual pairs of crabs and the body size of paired crabs was poorly correlated. This suggests pair instability and frequent shifts among burrows by male and/or female crabs, as reported before for other symbiotic crustaceans in which the body size of paired crabs is poorly correlated. Second, males paired with females that were sexually receptive (without embryos) or that have been receptive recently (carrying early embryos) were found more frequently than expected by chance alone. The above agrees with that reported for species in which sexual pairing does not last long. Third, sexual dimorphism in terms of claw size and coloration was evident. Claws were larger in males than in females, a condition that argues in favour of male–male competition inA. aidae. In addition, the body coloration of males was more similar to the sand grains of the beach than that of females. This sex-specific coloration suggests that males are ‘better adapted' than females to roam on the surface of the beach in search of burrows because their coloration should diminish the risk of detection by predators. Experiments are needed to reveal the details of the polygynandrous mating system herein inferred forA. aidaeand to understand those conditions favouring particular reproductive strategies in symbiotic decapod crustaceans.