TREE FIDELITY AND HOLE FIDELITY IN THE TREE CRAB SESARMA-LEPTOSOMA (DECAPODA, GRAPSIDAE)

Sesarma leptosoma is a small grapsid crab which lives on the roots of mangrove trees of the East African coast and migrates twice a day to a nd from the canopy, spending the hottest hours of the day and most of the night within the root system. When not feeding on the canopy this crab hides in small crevices which the aerial roots of Rhizophora mucr onata form on entering the mud surface, or it feeds, at low tide, by s craping the bark on the roots, Crabs marked and released on the tree o f capture remained on it throughout the observation period (6 days). S pecimens marked and then dislocated to trees 7 m distant were able to return to the tree of origin in a few days. Crabs dislocated 16 m away tended not to remain on the displacement tree, but they were never se en back on their own tree. Observations performed over a 16-day period showed that individually marked crabs, after each of the two daily mi grations, went back to one, or at most two, well-defined small root cr evices. The short-distance homing ability and the strong fidelity to a well-defined area shown by S. leptosoma are interpreted as behavioura l mechanisms evolved to avoid high predatory pressure.