Seasonal and tidal changes in foraging actlvity of the shore crab Carcinus maenas were investigated from August 1992 to May 1993 in Kerteminde Fjord, Funen, Denmark.Foraging activity was 15 to 20 times higher in summer and autumn than in winter and spring Desp~te the weakly tidal conditions at the study site a significant proportion of the crab population exh~bited tidal rhythmicity in foraging activity.Detailed examination revealed that it was predominantly early intermoult (green ventral carapace), male crabs that behaved in this way.Females constituted ca 20% of the crabs caught throughout the year and were active primarily at night, independent of the t ~d a l phase.In constant conditions in the laboratory, the majority of male crabs displayed endogenous heart rate rhythms in phase with expected tidal and die1 cycles at their collection sites.Non-green male crabs did not exhibit rhythmicity to the same degree.Juvenile crabs comprised a greater proportion of the total catch at low tide than at high tide.The findings are compared with earlier studies on C. estuarij (earlier C. rnediterraneus) in similar tidal conditions in the Mediterranean Sea.
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Aagaard et al. (1995) studied this question.