DRIFTING AND DISPERSAL OF SMALL BIVALVES AND GASTROPODS WITH DIRECT DEVELOPMENT

Quantitative evidence that several marine benthic invertebrates lacking a planktonic larval stage disperse as juveniles and small adults by drifting in the water column has been obtained using off-bottom intertidal collectors. These collectors were mimics of a habitat commonly used by small molluscs in intertidal seaweed communities: red turf algae. Taxa caught in collectors include three brooding bivalves [Musculus sp., Lasaea sp., Transennella tantilla (Gould, 1853)] and small gastropods that produce benthic egg capsules and crawl-away juveniles (Barleeia spp.). Dispersal is accomplished by juveniles - and even sexually mature adults in the case of small-sized species - drifting with water currents using mucous threads in at least some of the taxa found. Some of these molluscs have also been caught in off-bottom collectors placed away from the shore, in the middle of a bay. Findings indicate that, contrary to what is suggested by their mode of development, these invertebrates have alternative and likely effective dispersal mechanisms (at least at a local scale). It is probable that frequent drifting excursions in these taxa enhance rafting opportunities, which in turn may favor long distance dispersal. Large numbers of juveniles of several other species of molluscs whose life cycles include a planktonic larval stage were also caught in off-bottom collectors. These results, coupled with those from previous studies, show that postmetamorphic drifting is a common feature of the life history and ecology of many marine bivalves and gastropods, regardless of their mode of development.