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

Sesarma leptosoma is the only species of crab inhabiting the mangrove swamps of East Africa which can climb to the top of tail mature trees, where it feeds on fresh leaves. Twice daily, in the morning and eveni ng, this crab performs mass migrations towards the canopy and spends a bout three hours on the tree-tops. At dusk it descends to spend the ni ght among the aerial roots of the tree. Two observation stations were built on a Rhizophora mucronata tree, up to 13 metres above ground lev el, to observe and record the feeding and spatial aspects of the behav iour of this species during its time in the canopy. Observations on th e daily migrations of marked specimens showed that the crabs are faith ful to a well-defined feeding site; moreover, the migration paths of t he marked specimens were exceptionally constant over the whole of the two-month study period. S. leptosoma was observed to feed on mature fr esh leaves by scraping the lower surface of the leaf with its chelae. It was also found to spend most of its time on leaf-buds in a particul ar stage of development. Water which accumulates among the scales cove ring these buds represents an important water supply for these crabs, which they presumably use to reduce physiological stress due to water loss. Feeding-site fidelity in S. leptosoma seems to be related more t o the chance of finding suitable leaf-buds, which are relatively rare and patchily distributed, than finding leaves, which are abundant and uniformly distributed all over the canopy.