Emergence pattern and spatial distribution of the common cockle Cerastoderma edule

Abstract

When groups of freshly collected cockles were exposed to cycles of tidal emersion and immersion and a light regime of 12h light:12h dark, up to 40% emerged onto the surface of the substratum during emersion at the onset of darkness. This pattern of emergence decreased in intensity from 37% to 10% of all individuals emerging after 30d in the laboratory. Not all the same cockles emerged each day. After emergence, the cockles remained quiescent until immersion, when they actively roamed and ploughed across the sand surface and then reburrowed. During movement the cockles produced furrows up to 0.5m in length; at high densities collisions with other buried cockles occasionally occurred, which in turn stimulated emergence and a change of position. Groups of cockles previously exposed to a tidal cycle and light-dark regime responded to continuous darkness by emerging onto the substratum for three low tides before the activity pattern disappeared. The nocturnal pattern of emergence observed in the laboratory can be explained in terms of the cockles' reaction to burial with sediment. -from Authors