Effect of light intensity on vertical migrations of the tree crab, Sesarma leptosoma Hilgendorf (Decapoda, Grapsidae)

Abstract

Sesarma leptosoma is a mangrove crab that lives among the mangrove roots and migrates twice a day to the canopy to feed on leaves and leaf-buds. An experiment was performed by covering the whole root system of mangrove trees with white and black tents, thus differentially reducing the light intensity reaching the crabs, in order to investigate whether 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 migration departure. Reaction to a light threshold is an efficient mechanism 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 intertidal crabs should regulate their migratory activity with the light level instead of the level of the tide. © 1995.