LARVAL HABITAT CHOICE IN STILL WATER AND FLUME FLOWS BY THE OPPORTUNISTIC BIVALVE MULINIA-LATERALIS

Competent pediveligers of the coot clam Mulinia lateralis (Say) clearly preferred an organically-rich mud over abiotic glass beads in 24-h flume-flow experiments, and often demonstrated the same choice in still-water experiments. We hypothesize that pediveligers with characteristic helical swimming paths above the bottom can exercise habitat choice in both still water and flow, but that the limited swimming ambits of physiologically older pediveligers require near-bottom flows to move the larvae between sediment patches so that they can exercise habitat choice. Although M. lateralis larvae are planktotrophic, their ability to delay metamorphosis in the absence of a preferred sediment cue is limited to about five days, a shorter time than the lecithotrophic larvae of the opportunistic polychaete species, Capitella spp. I and II. Field distributions of all three opportunistic species may result, at least in part, from active habitat selection for high-organic sediments by settling larvae.