EMERGENCE PATTERN AND SPATIAL-DISTRIBUTION OF THE COMMON COCKLE CERASTODERMA-EDULE

When groups of freshly collected cockles Cerastoderma edule L. were transferred to laboratory conditions and exposed to cycles of tidal emersion and immersion and a light regime of 12 h light: 12 h dark, up to 40% of the group 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 30 d in the laboratory. However, 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 0.5 m 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 3 low tides before the activity pattern disappeared. The intensity of the emergence pattern in continuous darkness was less than the nocturnal activity observed during light: dark conditions. Similarly, under continuous immersion, emergence during darkness was less pronounced than under simulated tidal emersion. In continuous illumination an emergence pattern was absent. The nocturnal pattern of emergence observed in the laboratory can be explained in terms of the cockles' reaction to burial with sediment. In laboratory experiments, cockles which were buried with a covering of sediment at the onset of emersion responded immediately by emerging onto the sediment surface. One group maintained in continuous darkness emerged at low water and continued to roam and plough across the surface when they were immersed, whereas a second group subjected to continuous illumination emerged upon burial and then reburrowed. Similar experiments carried out in the natural environment confirmed the biological significance of the laboratory-induced emergence rhythm. On the shore, replicate areas were covered with sediment during daylight, just prior to emersion of the natural cockle beds. Approximately 1. 5 h after burial, cockles were observed to emerge with peak activity at the onset of darkness, and again 24 h later. However, cockles did not emerge in any numbers in control areas in which no sediment was added. The sudden burial of cockles with sediment during a storm will stimulate them to move in order to maintain contact with the sand surface for feeding and respiration. The disturbance of cockles during collection and the sudden onset of darkness in the laboratory followed by the immediate emergence of the cockles, simulated the cockles' reaction to deposition of sediment in the field. The observed crawling and burrowing behaviour of the cockles during high tide and darkness represents a situation in which the cockle is continuing to try to emerge as if it was still covered in sediment.