Natural Sources and Properties of Chemical Inducers Mediating Settlement of Oyster Larvae: A Re-examination.

Live adult oysters and biofilms were separated experimentally as potential sources of waterborne chemical inducers of settlement in oyster larvae (Crassostrea virginica). Bacteria films growing on external shell surfaces were removed by mechanical agitation and chemical oxidation. This technique removed >99% of the viable bacteria without disrupting the normal production of metabolites by the oysters, measured as the weight-specific production of ammonium and dissolved organic carbon (DOC). In comparison to the external biofilms, microfloral abundances in oyster tissues and on internal shell surfaces were numerically insignificant ({le}0.1% of total). Biofilms growing on aged shell material without the living oyster served as a source of bacteria metabolites. Metabolites released in particle-free, artificial seawater (ASW) medium by biofilms and by adult oysters (lacking biofilms) were tested for effects on larval behavior, relative to ASW (control). The larvae were exposed to solutions in a Plexiglas[R] microcosm (30 ml capacity). Locomotory responses were video recorded under infrared illumination, then subjected to computer-video motion analysis. Oyster larvae responded similarly to waterborne substances released both from adult conspecifics and from biofilms. The responses included: larvae rapidly swimming vertically downward in the water column; their horizontal swimming speed then slowed while their rate of turning increased, which focused activity near the bottom; and finally, the larvae contacted the bottom and attached with their foot, indicating settlement. Further analysis demonstrates that the settlement-inducing compounds of each source have a molecular weight between 500 and 1000.