Characterization of the symbiotic crab Lissocarcinus orbicularis (Dana, 1952) population in Toliara bay, Madagascar

Arlequin crabs (Lissocarcinus orbicularis) are symbionts found exclusively on the integument or in the buccal/cloacal cavity of several species of sea cucumbers. The population of these crabs was investigated on holothuroids of the barrier reef of Toliara (South-West of Madagascar) from December 2006 to December 2007. Eleven holothuroid species were observed and seven were infested. There is generally only one crab on infested sea cucumber but up to ten individuals were recorded on a Thelenota ananas, in that case, most crabs were juveniles. The length of the observed L. orbicularis was of 0.3 to 1.4 cm from the tip of the rostrum to the end of the cephalothorax, with a mean length of 0.85 cm for the population. L. orbicularis is characterized by a sexual dimorphism (females are bigger than males) and the presence of pereiopods morphologically adapted to fixation on the integument. They harbour different colour patterns depending on the colour and pattern of their host. Gravid females were observed at each period indicating that the crab reproduce all year round.