HABITAT SELECTION AND ADULT-RECRUIT INTERACTIONS IN PECTINARIA-KORENI(MALMGREN) (ANNELIDA, POLYCHAETA) POSTLARVAL POPULATIONS - RESULTS OFFLUME EXPERIMENTS

The fate of recently settled populations of soft-bottom invertebrates depends not only on dispersal of pelagic larvae by tidal currents but also on other physical (e.g. resuspension) and biological mechanisms ( e.g. habitat selection and adult-recruit interactions) acting at the w ater-substratum interface. To assess the relative importance of such p rocesses under megatidal conditions in the Abra alba community of the eastern Bale de Seine (English Channel), flume experiments were conduc ted on post-larvae of the dominant polychaete species, Pectinaria kore ni, Habitat selection by post-larvae of P. koreni was determined in a first set of experiments, where individuals were sowed either on a sui table or on an unsuitable substratum. Once resuspended, post-larvae we re given a choice between two highly contrasting treatments with a nat ural organic-rich muddy sand and a bare flat PVC surface. P. koreni po st-larvae were able to leave an unfavourable substratum into which the y had initially burrowed and reach a more suitable substrate by drifti ng (induced by the secretion of mucus) before final settlement. The in fluence of adults on habitat selection and survival of P. koreni post- larvae was analysed in a second set of experiments, where individuals were sowed onto a suitable sediment with adults (test treatment) or wi thout (control treatment). The presence of conspecific adults induced a high resuspension rate of the post-larvae. Drifting occurred mainly just after the introduction of the current and affected the whole expe rimental population, regardless of size. Such a response seems to be r elated to the intense bioturbation caused by the sub-surface deposit-f eeding habit of the adults, which alters the boundary-layer flow. In c ontrast, the presence of adults of Owenia fusiformis, another dominant polychaete of the Abra alba community, led to an enhanced recruitment by a reduction in post-larvae resuspension. In fact, at low shear vel ocities, dense aggregates of tubes stabilize the sediment and so act a s particle traps. Thus, in the megatidal environment of the Dale de Se ine, physical conditions and behavioural responses interact and contri bute to structuring the benthic community.