EFFECT OF LIGHT-INTENSITY ON VERTICAL MIGRATIONS OF THE TREE CRAB, SESARMA-LEPTOSOMA HILGENDORF (DECAPODA, GRAPSIDAE)

Sesarma leptosoma is a mangrove crab that lives among the mangrove roo ts and migrates twice a day to the canopy to feed on leaves and leaf-b uds. An experiment was performed by covering the whole root system of mangrove trees with white and black tents, thus differentially reducin g the light intensity reaching the crabs, in order to investigate whet her the light level could be responsible for the onset of the morning upward migration. A correlation was found between the light reduction induced by the tents of different colours and the delay in the crab mi gration departure. Reaction to a light threshold is an efficient mecha nism to synchronise perfectly the activity of a whole crab population. The questions under discussion are why the migration should start in such a synchronised and explosive way and, more generally, why these i ntertidal crabs should regulate their migratory activity with the ligh t level instead of the level of the tide.