VARIATION IN THE TIDAL MIGRATORY BEHAVIOR AND RHYTHMIC LIGHT-RESPONSIVENESS IN THE SHORE CRAB, CARCINUS-MAENAS

The distribution of shore crabs, Carcinus maenas (L.), within the inte rtidal and shallow subtidal zones of a rocky shore in the Menai Strait , North Wales, was recorded regularly by diving throughout the tidal c ycle. Samples were also obtained by searching the shore during low tid e and by collecting crabs in baited pots. It was found that the smalle r individuals (<35 mm carapace width) in the population tend to remain within the intertidal throughout the tidal cycle whilst others, mainl y the red-coloured adults, appear to remain permanently below low-wate r mark. The remainder of the population, predominantly green males lar ger than 35 mm carapace width, migrate upshore with the flood tide and downshore again with the ebb. The behaviour of C. maenas on the shore appears to be affected, at least in part, by a tidally rhythmic patte rn of responsiveness to directional light which is seen in freshly col lected crabs in the laboratory.