The distribution of eastern oysters Crassostrea virginica near terrestrial watersheds has led to a general impression that low or variable salinity is imperative for survival. However, freshwater runoff contains numerous mineral elements from geologic deposits that could play significant roles in oyster physiology. Two metals of terrestrial origin, copper and zinc, are accumulated to extremely high concentrations in eastern oysters, even in the absence of anthropogenic sources. As yet, there has been no defendable demonstration of a physiologic function for such high concentrations. Both copper and zinc, however, are accumulated almost exclusively in the amebocytes and calcareous shell of oysters, a unique distribution that implicates a role in the functions of amebocytes. Amebocytes are migratory, diapedetic cells generally recognized to provide nutriment and defense through phagocytosis, killing, and digestion of invading or ingested microorganisms. There is sufficient evidence in existing literature to suggest that copper and zinc directly contribute to these antimicrobial activities. This review presents historical and recent findings that demonstrate a strong affinity of oyster amebocytes for copper and zinc (even in low ambient concentrations), prolonged retention of the metals despite a potential route of elimination, and strong circumstantial evidence of antimicrobial activity by accumulated copper and zinc. It is proposed that oysters actively concentrate copper and zinc as antimicrobial agents to be used in intracellolar and extracellular killing (direct toxicity) as well as extracellular clot formation (precipitation of hemolymph). This potential, combined with evidence of amebocyte involvement in deposition of oyster shell, provides an alternative framework for understanding amebocyte functions, defense activities, and coastal distributions of oyster populations. It also affords some resolution to the apparent contradiction of eastern oysters thriving at seemingly polluted locations.

KEY WORDS: oysters, Crassostrea virginica, marine bivalves, copper, zinc, metals, contaminants, amebocytes, leucocytes, hemocytes, internal defense, immunomodulation, immunosuppression, metal toxicity, sentinel species

INTRODUCTION

Eastern oysters Crassostrea virginica are distributed across the North American coast of the Atlantic Ocean and Gulf of Mexico in bays, estuaries, and coastal zones, most often near sources of fresh water. The proximity to fresh water has often led to a conclusion that oysters require the low or variable salinity provided by freshwater inflow (Cake 1983, Soniat & Brody 1988, Berrigan et al. 1991). Intermittent salinity reductions can deter stenohaline predators such as oyster drills (Loosanoff 1955, Lunz 1955, Hopkins 1956, Wells 1961, Menzel et al. 1966, Berrigan et al. 1991) and can slow incidence of disease caused by the protozoans Perkinsus marinus and Haplosporidium nelsoni (Sprague et al. 1969, Ford 1985, Ford & Haskin 1988, Chu & Greene 1989, Chu et al. 1993, La Peyre et al. 2003). However, there is ample evidence of near-shore, intertidal oyster beds in high-salinity estuaries and coastal zones (Beaven 1955, Gunter & Geyer 1955, Lunz 1955, Nelson 1955, Copeland & Hoese 1966). It is possible then, that eastern oysters are influenced by other factors that accompany freshwater influx, such as terrestrial elements, rather than reduced salinity. Copper and zinc, like many elements in the marine environment, originate from natural terrestrial sources and are delivered to oyster beds in watershed runoff (Prytherch 1934, O'Connor 2002). Runoff with sufficient volume (i.e., streams and rivers) can generate flows that bring elements far and deep into the receiving water where subtidal oyster beds exist. Without such runoff, oysters may be confined to shallow intertidal areas adjacent to land where the elements originate. If this is correct, the availability of terrestrial elements may well be a principal determinant in the distribution of eastern oysters.

Copper and zinc are terrestrial elements of special interest because they accumulate to extraordinary concentrations in eastern oysters. They are accumulated against chemical gradients, even from low ambient concentrations, and are retained within the oyster longer than other metals, despite the apparent availability of an elimination mechanism. Moreover, zinc and copper are exclusively sequestered in oyster amebocytes (Fig. 1). This brings them into direct contact with a cell type credited with many indispensable responsibilities for oyster survival, including antimicrobial activities for defense and nutrition. These considerations implicate a physiologic reliance on copper and zinc that, if true, would support the concept that terrestrial elements are key factors in oyster success and distribution. Furthermore, it would resolve the apparent contradiction that eastern oysters often thrive at relatively polluted locations (e.g., Abbe & Sanders 1986). Fortunately, there is ample information in the literature to examine such a possibility. Because of the highly accumulated concentrations and unique association with amebocytes, uptake and disposition of copper and zinc in oysters has been widely studied.