Emergence pattern and spatial distribution of the common cockle Cerastoderma edule

Abstract

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.

Food sources of an infaunal suspension-feeding bivalve Cerastoderma edule in a muddy sandflat of Marennes-Oléron Bay, as determined by analyses of carbon and nitrogen stable isotopes

“…This phenomenon could not be excluded in the case of Cerastoderma edule because adult cockles are known to stretch their short siphons up into the water in response to oxygen depletion (Jmgensen 1980). C. edule is also known to be able to actively emerge onto the surface of the sediment in response to the instability of the sandy bottom (Richardson et al 1993). In Marennes-Oleron Bay, the study site is characterised by relatively strong current velocities (up to 0.60 m S-') compared to the inhalant jet flow of adult-size cockles in the range 0.005 to 0.010 m S-' given by Andre et al (1993).…”

Intertidal facilitation and indirect effects: causes and consequences of crawling in the New Zealand cockle

“…The exact nature of the disturbance appears less important, since bioturbation by lugworms Abarenicola affinis, coverage by macroalgae and to some extent also by shells, and even the vicinity of the mud whelk Cominella glandiformis can also induce crawling behaviour similar to that induced by ghost-shrimp bioturbation. Richardson et al (1993) also showed that surfacing activity of C. edule is positively influenced by the presence of conspecifics, in that crawling individuals induce surfacing in buried specimens they encounter along their path. Such intraspecific interaction could not be demonstrated in the present study, however, whith the crawling activity of Austrovenus stutchburyi being unaffected by cockle densities of > 200 ind.…”

“…Escaping such unconsolidated sediments through surface crawling would therfore seem to be an adaptive behaviour. Richardson et al (1993) observed a similar response in the European cockle Cerastoderma edule experimentally exposed to burial, and suggested that crawling could be a widespread escape-behaviour of short-siphoned suspension-feeding bivalves living in habitats where the upper sediment stratum is regularly disturbed (see also Ansell 1994). The exact nature of the disturbance appears less important, since bioturbation by lugworms Abarenicola affinis, coverage by macroalgae and to some extent also by shells, and even the vicinity of the mud whelk Cominella glandiformis can also induce crawling behaviour similar to that induced by ghost-shrimp bioturbation.…”

“…The crawling activity of the European cockle Cerastoderma edule appears to be influenced by its own density (Richardson et al 1993). Such intraspecific interaction could be relevant also in the New Zealand cockle, and since Expt 1 examined different parts of the transect with different cockle densities, an additional experiment was performed on a short stretch of the transect to separate the impact of shrimp and cockles on the cockles' crawling activity.…”

“…However, C. filholi also coexists with larger macrofaunal organisms such as the New Zealand cockle Austrovenus stutchburyi (Veneridae) that may also be negatively affected by the presence of ghost shrimps. For instance, the European cockle Cerastoderma edule travels longer distances upon the sediment surface as a response to deep burial in sediment (Richardson et al 1993), and observations of crawling tracks made by A. stutchburyi in association with ghost-shrimp beds or other sources of biogenic disturbance suggest that the New Zealand cockle acts in a similar manner. However, surfacing may be particularly detrimental to A. stutchburyi, because benthicfeeding fishes (Labridae) exploit this behaviour by cropping off the cockle's foot (Mouritsen & Poulin 2003a,b).…”