Movements between coral colonies in Trapezia ferruginea (Crustacea: Brachyura), an obligate symbiont of scleractinian corals

Trapezia ferruginea appears to depend entirely on its coral host (Pocillopora spp.) for shelter and food. Field and laboratory experiments have shown that the brightly colored crabs move between colonies at night. Analysis of movements in crabs randomly distributed among randomized Pocillopora damicornis colonies under laboratory conditions showed that after 6 days there was a significant increase in the number of heterosexual pairs (P<0.001) and a significant correlation between the number of crabs and the size of colonies (P=0.003), as well as between the size of crabs and the size of colonies (P=0.020) — conditions which are also observed in the field. No such correlations were obtained (P>0.05) in a similar experiment where very intense predation by carnivorous fishes was present. This host selection by adults appears to be elicited by the lack of a suitable partner and the inadequate size of their established territory in the colony.