The mole crab Emerita talpoida occurs in the swash zone of sandy beaches. Although crabs move with the tide, field studies found that the smaller crabs are distributed higher intertidally than the larger crabs. The present study tested the hypothesis that the rhythm, timing of activity, and activity amplitude with respect to tides would be different among megalopae, juveniles, small females, small males, mature males and large ovigerous females and underlie the observed field distributions. Activity of individuals in columns was observed under constant conditions with a video system and quantified as the number of ascents from the sand each 0·5 h. All life history stages had a circatidal rhythm in vertical swimming with median free running period lengths near 12·4 h and activity peaks after the time of high tide. Period length and time of activity peaks did not differ significantly among the life history stages. Activity amplitude was related to tidal amplitude of consecutive high tides at the collection site. Larger crabs had greatest activity after the lower amplitude high tides whereas small crabs had equal activity amplitude on consecutive tides. The test hypothesis was partially supported because the difference in activity amplitude predicts an oscillation between overlapping distributions of small and large crabs after low amplitude high tides, with the smaller crabs distributed higher on the beach after high amplitude high tides. The rhythm was not affected by a light:dark cycle and could be entrained by mechanical agitation.
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