Date Awarded


Document Type


Degree Name

Doctor of Philosophy (Ph.D.)


Virginia Institute of Marine Science


Roger Mann


Studies were carried out to quantify abundance, mortality, and variability in these parameters, during settlement and recruitment of bivalve mollusks, using the oyster, Crassostrea virginica, as a primary model species. Most work was undertaken in the York River, Chesapeake Bay, Virginia, with additional work in the Indian River, Florida. The period chosen, in the bivalve early life history, was from the late planktonic larva to the early benthic juvenile. Studies were designed to specifically examine (a) abundance of late-stage larvae in the plankton, (b) the relationship between larval abundance and settlement, and (c) mortality immediately following settlement. Variability in abundance or mortality was also examined at each of these stages. It was found that, of larvae in the plankton with the morphological characteristics of competency-to-settle, about 80% would settle within 24 hours, under laboratory conditions. Temporal variation in abundance of planktonic larvae was high and apparently random, but separate species covaried in observed abundance. Time of day and tidal phase had little or no effect on larval distribution, but late stage larvae showed a consistent depth preference, which varied depending on species. Crassostrea late larvae tended to be most abundant near the benthos, both at the Virginia site and at the Florida site. The relationship between planktonic abundance and settlement density of Crassostrea was weak, within one order of magnitude of variation in larval abundance, although the same relationship for a sympatric species in Florida, Ostrea equestris, was significant. Settlement onto a natural oyster reef was more variable but not significantly lower than settlement onto adjacent defaunated shell substrate. The increased variability could not be accounted for by coverage by dominant fouling macroorganisms. Mortality of newly-settled juvenile Crassostrea was high, with near 100% mortality within 28 days on a natural oyster reef, with high but significantly lower (about 96%) mortality on previously defaunated shell substrate over the same time.



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