Activity rhythms and the tide in a saltmarsh beetle Dicheirotrichus gustavi.

Field observations at Mersea Island, Essex confirm previous observations at Scolt Head Island, Norfolk that the intertidal beetle Dicheirotrichus gustavi Crotch (Coleoptera: Carabidae) has a rhythm of activity on the saltmarsh surface which is suppressed during periods of submerging tides. Although the timing of the tides at the two sites is 180° (6 h) out of phase, the timing of beetle activity at the two sites is the same, with a peak of activity shortly after dusk. Beetle activity therefore shows no special phase relationship with the "critical" tide - the first high tide that covers the beetle zone after a period of emergence. At Mersea, the peak of beetle activity coincides with the critical high tides, but the beetles were observed to escape from the seawater by scrambling up the mud and vegetation.