Emergence pattern and spatial distribution of the common cockle Cerastoderma edule

When groups of freshly collected cockles Cerastodema edule L. were transferred to laboratory conditions and exposed to cycles of tldal emersion and immersion and a light regune of 12 h light. l 2 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 quescent 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 hlgh densities cohsions with other buried cockles occasionally occurred, which in turn stimulated emergence and a change of posihon. Groups of cockles previously exposed to a hdal cycle and 1ight: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 activlty observed during light :dark conditions. Similarly, under contmuous immersion, emergence during darkness was less pronounced than under sunulated tidal emersion. In continuous illummahon 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 buned with a covering of sediment at the onset of emersion responded immediately by emerglng onto the sedvnent 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 sublected to continuous ~Uumination emerged upon burial and then reburrowed. Sirmlar experiments carried out in the natural environment confirmed the biological sigmficance 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 agaln 24 h later. However, cockles &d not emerge in any numbers In control areas in which no sediment was added. The sudden burial of cockles with sediment during a storm w l l stimulate them to move in order to maintain contact with the sand surface for feeding and respuation. The disturbance of cockles dunng collection and the sudden onset of darkness in the laboratory followed by the immediate emergence of the cockles, simulated the cockles' reactlon to deposition of sedlrnent in the field. The observed crawhng and burrowing behaviour of the cockles dunng high tide and darkness represents a situation in which the cockle is continuing to try to emerge as d it was still covered in sediment.