Sediment choice by settling larvae of the bivalve, Spisula solidissima (Dillwyn), in flow and still water

The surfclam, Spisula solidissima (Dillwyn), is a common species in many sandy locales off the east coast of North America from Labrador to North Carolina, yet it rarely occurs in muddy areas. In order to determine if this distribution may result, at least in part, from larval habitat selection, 24-h laboratory still-water and flume-flow experiments were conducted using a sediment-filled array. Sediment treatments included low-organic sand typical of adult habitat and a contrasting organic-rich mud. In flow experiments, larvae consistently chose sand over mud. In still water, however, results were variable, with higher settlement sometimes observed in mud. Measurements of settled individuals from four pairs of still-water/flow experiments indicated that mean shell lengths of clams in sand treatments were generally similar to those in mud for flow experiments, but in three of the corresponding still-water experiments clams in mud treatments were significantly smaller than those in sand. This difference may be a result of a deleterious effect of the mud on the larvae, resulting in a disproportionate representation of small, precompetent individuals in the mud treatments. These flow and still-water experiments indicate that larval habitat selection may contribute to the sediment-specific adult field distributions of the surfclam. (C) 1998 Elsevier Science B.V. All rights reserved.