Settlement of Iceland scallop Chlamys islandica spat in response to hydroids and filamentous red algae: field observations and laboratory experiments

Abstract

Sampling carned out in the Baie des Chaleurs (Gulf of St. Lawrence, Canada) in 1990 and 1991 showed that newly settled Iceland scallop Chlamys islandica spat were not randomly distributed on the sea bottom. Various inorganic (sand, gravel, pebbles, shells of different species) and organic (algae and animals) substrata were examined. Twenty times more post-larvae were found on pensarcs of the dead hydroid Tubulana larynx than on hve hydroids and red algae (another preferred natural substratum). Maximum densities, up to 300 spat g' (dry wt) of hydroids, were observed on a mooring line at 2 m from the bottom. Laboratory expenments carried out to determine whether attachment behaviour of post-larvae could contribute to the observed distribution of spat showed that: (1) spat can resettle easily when dislodged from their substratum; (2) independent of their origin, over 60 % spat preferred to resettle on dead rather than live hydroids, filamentous red algae (Ptilota serrata and Phycodrys rubens) or fishing line (used as control); and (3) they also preferred to resettle on the type of substratum to which they were previously attached. Different hypotheses concerning active and passive larval settlement processes and post-larval redispersion strategy are proposed to explain this relationship between scallops and hydroids.