Cerastoderma edule and polychaetes in Atlantic littoral muddy sand

Description (English)

Extensive clean fine sand or muddy sand shores with abundant cockles Cerastoderma edule. The community consists of the polychaetes Eteone longa, Scoloplos armiger, Pygospio elegans, Spio filicornis and Capitella capitata, the crustaceans Bathyporeia sarsi, Bodotria arenosa arenosa and Crangon crangon, the spire shell Hydrobia ulvae, as well as the cockle C. edule and the baltic tellin Limecola balthica (Macoma balthica). This biotope carries commercially viable stocks of C. edule, and it is therefore possible to find areas of this habitat where the infauna may have been changed through recent cockle dredging. Cockle dredging can result in a reduced bivalve abundance and reduced densities of some polychaete species, including P. elegans (Moore, 1991). At the outer edges of large flats, there may be a zone between the cockle beds and more exposed sands, where there are fewer cockles and B. sarsi is the commoner species.
Situation: The community is found mainly on the mid and lower shore where the sediment is water-saturated most of the time. Where it occurs in muddy sand, this unit has broad transition areas with units MA5-251 and the MA6-22, and where it occurs on clean sand shores, it may have broad transition areas with MA2-2712. Higher on the shore, adjacent to this biotope, MA5-254 is found, with fewer polychaete and bivalve species due to the drier sediment found on the upper shore.
Temporal variation: A layer of mud with dense spionid polychaetes may build up on cockle beds in sheltered areas, creating a cohesive muddy layer 10-15 cm thick overlying the whole area. This may break up leaving a series of pits and patches with miniature cliffs, giving it an appearance similar to a stony shore when seen from a distance. It should be noted that where it occurs, Hydrobia ulvae tends to move a lot and may be highly variable in abundance.

Source: EUNIS habitat classification