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Helice tridens and Chasmagnathus convexus are grapsid crabs living in tidal flats . Both species construct burrows in the upper intertidal and supratidal zones, although they use also waterside areas in lower tidal zones during low tide. In this study, temporal variations in above-ground density and food items of the two species in different habitats (i.e., burrow areas and waterside areas) were investigated in the Shigenobu and Awai River estuaries of Ehime Prefecture, southwestern Japan, to examine the crabs' seasonal and die! activity patterns in relation to their food habits . Monthly investigations showed that above-ground densities of both species increased from spring and peaked in summer, and the densities were positively correlated with underground temperature. Die! activity patterns differed between the two species. Chasmagnathus convexus tended to be nocturnally active in both burrow and waterside areas . On the other hand, H. tridens exhibited high diurnal activity in their burrow areas during cool seasons (March , October), while at other times they tended to be active nocturnally in both the burrow and waterside areas . Helice tridens consumed various food items such as surface sediment, macroalgae, reeds , and macrozoobenthos. These crabs mainly consumed surface sediment in burrow areas, whereas they frequently consumed macroa!gae in waterside areas , where their foraging activity was greater. Food and habitat use of C. convexus differed from those of H . tridens; they primarily consumed reeds in burrow areas and rarely foraged in waterside areas. Overall results suggest that the differences in die! activity pattern between the two species are related to their different food habits and spatial variation in food availability.

Seasonal and Diel Activity Patterns of Two Burrowing Mud Crabs, Helice tridens and Chasmagnathus convexus (Crustacea: Decapoda: Varunidae)