The process by which molluscs form their shells is complex and not well understood. The bulk of the mineral component of the shell is calcium carbonate, appearing either in calcite form or as aragonite, and sometimes vaterite. Various models have been proposed to explain how the mineral crystal is produced, including direct secretion and also formation in hematocytes. When other metals are present in the environment around the oyster, they can be incorporated into the mineral crystal structure as much as their concentration and the steric constraints of the crystal will allow. The nature of how these metals get incorporated is also not well understood. The purpose of this study was to determine if the relationship between the concentrations of trace metals in the shells of the eastern oyster Crassostrea virginica reveals anything about the process of biomineralization in molluscs. Oysters were harvested from marked locations and selected to be of similar age and development. Their shells were cleaned and dried, and the mass and dimensions of each shell was measured. The shells were crushed to a fine powder, and 25 g of shell was dissolved in a 5:1 mix of metal-free concentrated nitric acid and 30% hydrogen peroxide, to liberate the metal constituents of the shell. The metal concentrations were measured by inductively coupled plasma optical emission spectroscopy. Metal concentrations were plotted against three separate health condition indices and against one another. Metal concentrations were also grouped by the oyster's geographic location and regression analyses were conducted to identify or eliminate site as a potential explanatory variable. There was no significant correlation between any metal concentration in the shell and the health of the oyster. Differences in metal concentration with geographic origin were very slight, with a single exception. Oysters taken from the Lynnhaven River had lower amounts of silicon and nickel than elsewhere, by a slight margin. Four groups of metals appear in proportional amounts. Silver and iron always appear in trace quantities but in a 1:6 ratio. Copper and boron also appear in trace quantities in a 1:4 proportion. A group including vanadium, thallium, manganese, strontium, magnesium, sodium, and potassium appear in proportion to one another, and together can account for as much as 3% of the metal mass of the oyster shell. A fourth group including antimony, nickel, and silicon also appears in constant proportions to one another, and also can account for 3% of the metal mass in the shell. The third and fourth groups of metals are totally exclusive of each other, such that if one group is present in the oyster, the other group is never detected. Proportional amounts of metals and exclusivity of groups of metals strongly suggests metabolic control of metal incorporation into oyster shells. It is also possible that more than one mechanism for shell formation exists.
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Vol. 35 • No. 1