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INORGANIC ELEMENTS AND DISTRIBUTION OF EASTERN OYSTERS.
Fisher, W S. INORGANIC ELEMENTS AND DISTRIBUTION OF EASTERN OYSTERS. Presented at Nat'l Shgellfisheries Association, Honolullu, HI, Mar 1-5, 2004.
Fisher, William S. In press. Inorganic Elements and Distribution of Eastern Oysters (Abstract). To be presented at the 96th Annual Meeting (Aquaculture 2004) of the National Shellfisheries Association, 1-5 March 2004, Honolulu, HI. 1 p. (ERL,GB R962).
For over a century we have marveled at the high concentrations of inorganic elements, particularly zinc and copper, in oysters. Many have suggested that accumulations were inadvertent because the concentrations were too high to serve any known physiological function. Recent evidence, however, has led to a hypothesis that eastern oysters, Crassostrea virginica depend on substantial quantities of inorganic elements to serve in antimicrobial defense and shell deposition. Integral to this hypothesis is the understanding that ameboid blood cells (amebocytes) and adult shells can actively concentrate inorganic elements thousands of times over ambient.
Amebocytes in the mucus and gut of oysters appear to concentrate zinc, copper, and possibly tin, iron, manganese and other elements from food and the water column. Discounting shell concentrations, zinc and copper are retained almost exclusively (approximately 90%) in the amebocytes. Sequestration in membrane-bound amebocyte granules prevents toxicity and may provide storage until the elements are released in a defensive capacity. Extracellular release of copper and zinc (via degranulation) has been demonstrated during repair of wounds; they may serve as antimicrobial agents and initiate extracellular clotting. High metal concentrations released within a small area would provide a potent cytotoxic action against either potential pathogens or food organisms. An intracellular release of metals may occur similarly during phagocytosis when granules (lysosomes) fuse with particle-laden phagosomes. Adult oyster shells also contain high concentrations of the same inorganic elements that are retained in the amebocytes. High shell concentrations support a hypothesis that release of the elements from amebocytes initiates precipitation within the matrix of the organic conchiolin, leading to shell calcification. Amebocyte involvement in shell repair and pearl formation has been demonstrated in other molluscan species.
If eastern oysters are, in fact, dependent on high concentrations of inorganic elements, then they must inhabit areas that provide a source. Oysters are marine animals, but the elements are derived from terrestrial sources. Hence, oysters must live in close proximity to land or to freshwater flows that transport the elements. Oyster distributions match this supposition; they exist close to land and venture into deeper waters only within zones of freshwater influence.