Underwater television observations on the intertidal movements of shore crabs,<i>Carcinus maenas</i>, across a mudflat

Abstract

Closed circuit underwater television observations showed that, in summer, large numbers of Carcinus maenas performed daytime tidal migrations up and down a small mudflat in the Menai Strait, North Wales. At the observation point, 25 m above mean low water mark of spring tides, numbers of crabs moving upshore on flood tides averaged 84 m —1 of tideline, ranging from 48 to 126 m —1 . Most crabs were of 20–50 mm carapace width (1 and 11 groups); none smaller than 15 mm was seen. Peak movements occurred in at least 0.5 m depth of water. Over the high tide period, most crabs were dispersed over the upper half of the mudflat where crab population densities of 1·8–2·5 m —2 were indicated by diving observations.