VERTICAL MIGRATIONS IN THE TREE CRAB SESARMA-LEPTOSOMA (DECAPODA, GRAPSIDAE)

Sesarma leptosoma, somewhat similar to the Atlantic related species, A ratus pisonii, is an East-African mangrove crab which spends its entir e life on the roots and branches of mangrove trees (mostly Rhizophora mucronata, Bruguiera gymnorhiza and Ceriops tagal). S. leptosoma never enters the water, nor does it ever venture onto the free mud surface at low tide. Part of the day and night it remains on the lower parts o f the mangrove aerial roots, which are often encrusted with a wet alga l mat of Bostrichia spp., searching for food and water. Twice a day, f rom ca. 06:00 to 08:00 hrs in the morning and 16:00 to 18:00 hrs in th e afternoon, many of the crabs migrate as far as the leaves at the top of the tree on which they feed. However, they only spend a brief peri od among the leaves, from ca. 07:00 to 10:00 and 17:00 to 19:00 hrs, a fter which time they make their way back towards the roots again in tw o downward migrations. In the morning, the downward migration brings a ll the crabs back to the roots, but in the evening not all the crabs t ake part in the downwards migration and some of them pass the whole ni ght in the tree tops. A comparison of the migration time patterns for two different periods of the year (June-July and November) shows that the number of crabs migrating along the tree trunk is modulated by the spring-neap tidal cycle, while the onset of daily migration seems to be controlled mostly by the light level and/or other climatological cu es. The adaptive significance of this migratory behaviour is discussed . Observations reported in our study were made in Mida Creek, Dabaso, Kenya in 1991 and 1992.