Secondary settlement of cockles Cerastoderma edule as a function of current velocity and substratum: a flume study with benthic juveniles
Résumé
Some newly-settled bivalve molluscs can experience a second dispersal stage in the water column and colonize areas distant from initial settlement zone ('secondary settlement'). To document mechanisms involved in such a process, experiments were conducted in a recirculating 13-m flume, using juvenile cockles Cerastoderma edule (shell length between 0.8 and 5.7 mm). After 4 h and under current surface velocities of 10, 20 and 24 cm s−1, all juveniles left the plexiglass substratum (site of initial introduction) and 42.4, 58.6 and 76.2% of juveniles, respectively, were retrieved from a downstream sand area (which only represented 7.7% of the total flume surface). Naked-eye observations showed that smaller individuals were borne within the water column, whereas larger individuals tended to roll or slip on the substratum. Byssus threads produced by the juveniles were often seen, sometimes covered in sand grains. These observations were confirmed by finding larger juveniles in the upstream part of the sand area. When sand was replaced by mud, a reduced proportion of cockles was retrieved in the experimental substratum (18.4, 20.0 and 16.4%, respectively). Observations showed that juveniles rarely succeeded in anchoring themselves in mud. When initially introduced on a favourable substratum (medium sand), more than 87% of juveniles were retrieved from that sand array at all flow velocities. This study shows that secondary settlement occurs for juvenile cockles up to 5.7 mm in shell length and depends not only on flow velocity but on substratum type.