HOMING EXPERIMENTS IN THE TREE-CLIMBING CRAB SESARMA-LEPTOSOMA (DECAPODA, GRAPSIDAE)

Sesarma leptosoma an East African mangrove-dwelling crab, migrates twi ce a day from a system of known dens among the roots to well-defined f eeding areas in the branches of trees, reaching 15 m high. Field exper iments were performed to test whether chemical or visual cues are invo lved in the orientation and homing of this species to reach their feed ing areas. Manipulation of the substratum at branch junctions, in orde r to alter possible chemical cues, did not affect homing ability in S. leptosoma. Moreover, crabs trained to cross an asymmetrical artificia l wooden fork could still follow their preferred directions after (1) the fork branches had been su;itched, (2) the whole fork had been rota ted around the trunk, resulting in a right-left inversion, and (3) the inversion of two wide black and white screens hiding most of the cano py from view of the climbing crabs. These results suggest that S. lept osoma may not rely on reference systems such as chemical trail-followi ng and chemical or visual cues from the substratum, but probably depen d on complex visual information from the surroundings trunks and/or fr om the sun's position integrated with junction sequence memory.