ACTIVITY PATTERNS IN THALAMITA-CRENATA (PORTUNIDAE, DECAPODA) - A SHAPING BY THE TIDAL CYCLES

The activity rhythms of the swimming Portunid Thalamita crenata have b een studied on the basis of crab sightings in surveys along three tran sects of a mangrove swamp in Kenya. Activity is well matched to the en vironmental regime and is prevalent during the high tides. However, ac tivity does not occur over the whole of the high tide, but is confined to ''temporal windows'' corresponding to a water level between 10 and 30 cm. Our findings suggest that activity in Thalamita crenata is sha ped by the tidal cycles and we propose that variation in hydrostatic p ressure as the tide changes is the cue which elicits activity.