Relationship of amebocytes and terrestrial elements to adult shell deposition in eastern oysters

Journal of Shellfisheries Research, August, 2004 by William S. Fisher

ABSTRACT Freshwater runoff contains terrestrial elements from geologic deposits that may be vital to survival of eastern oysters Crassostrea virginica, a factor that could account for their distribution near terrestrial watersheds. Some of these elements are accumulated in granules of oyster amebocytes, which are highly mobile, diapedetic cells with widely-recognized roles in oyster defense and nutrition. Amebocytes are known to discharge antimicrobial substances from membrane-lined cytoplasmic granules into extracellular spaces, a process referred to as degranulation. A similar, if not identical, process has been described for discharge of copper and zinc from cytoplasmic granules to precipitate hemolymph proteins and initiate clotting. This review examines evidence that supports amebocyte involvement in eastern oyster shell deposition and a potential role for accumulated terrestrial elements in crystallization. It is proposed that metal-bearing amebocytes discharge inorganic elements that precipitate proteins and initiate or facilitate shell crystallization. Support for this conjecture derives partly from high concentrations of terrestrial elements in adult shell. changes in shell morphology at metamorphosis, hypothesized mechanisms for shell repair and pearl formation, and the absence of other known mechanisms for shell crystallization.

KEY WORDS: oysters, Crassostrea virginica, marine bivalves, copper, metals, contaminants, amebocytes, leucocytes, hemocytes, larval setting, larval metamorphosis, shell deposition, metal toxicity, oyster management, oyster reef restoration

INTRODUCTION

Population distributions of eastern oysters Crassostrea virginica near terrestrial watersheds has led to a general impression that low or variable salinity is vital to their survival (Cake 1983, Soniat & Brody 1988, Berrigan et al. 1991). However, freshwater runoff contains numerous mineral elements from geologic deposits that could perform significant functions in oyster physiology. Copper and zinc, for example, are accumulated to extraordinarily high concentrations in eastern oysters, even in the absence of anthropogenic sources. There is no known physiologic function for such high concentrations of these terrestrial metals, but it has been proposed that they are actively assimilated and stored in amebocyte granules for antimicrobial activities related to defense and nutrition (Fisher 2004). Both metals are accumulated in the amebocytes and calcareous shell of oysters, a unique distribution that indicates a link between amebocytes and deposited shell, the initial and final repositories for the metals.

Because amebocytes (blood cells, hemocytes) are known to recognize, locate, ingest, and digest a variety of biotic and abiotic foreign particles (phagocytosis), the role of amebocytes in microbial defense has been widely investigated (Metchnikoff 1891. Yonge 1926, 1937, 1946, Stauber 1961, Cheng 1975, 1977, 1996. Feng et al. 1977, Feng 1988). A hypothesis that copper and zinc are instrumental in amebocyte antimicrobial activities was generated from evidence that the metals are avidly concentrated by eastern oysters from low ambient concentrations, and are retained in amebocyte granules for relatively long periods of time despite available elimination mechanisms (Fisher 2004). There is evidence that copper and zinc, stored at high concentrations in amebocyte granules, can be discharged into phagosomes for intracellular killing or into extracellular spaces (degranulation) at wound sites for microbicidal action and clot formation (Ruddell 1971, Sparks 1972, Brown 1975, Ruddell & Rains 1975). Here, discharge of metals is proposed as a mechanism to initiate or facilitate shell crystallization.

The importance of the oyster shell is indisputable; shell structure, strength, and growth are critical factors in oyster survival. According to Melbourne R. Carriker, an authority on eastern oyster shells and shell formation,

   "Because the oyster lacks an internal skeleton. its valves
   serve as an exoskeleton, providing support for the soft internal
   organs and preventing collapse of the mantle cavity.
   Consequently, vital physiologic functions of the mantle cavity,
   i.e., circulation of seawater; gaseous exchange, discharge
   of gametes, and removal of pseudofeces and fecal
   and catabolic wastes, can be carried out without impediment.
   Valves also protect internal organs from mechanical
   impacts, osmotic stress, and many predators, and from desiccation
   in the intertidal zone." (Carriker 1996, p. 102),

The greatest attributes of shell, its strength, and stability, are obtained through crystallization, which binds disassociated elements into an inflexible crystalline lattice as the shell is deposited. In this overview, evidence is presented to link shell deposition with activities of amebocytes and with the discharge of metals from amebocyte granules that may serve to initiate or facilitate crystallization. As crystallization proceeds, the metals are irreversibly bound and accrue to high concentrations in the shell. Amebocyte involvement in shell deposition is supported by the existence of high concentrations of terrestrial metals in deposited shell, differences in larval and adult shell structure, and the potential role of metal deposition in shell crystallization. Like defense and nutrition, deposition of shell serves an important survival function for oysters, meriting the expenditure of energy necessary to accumulate and deposit the metals. A dependence of oysters on terrestrial metals could account for their exclusively estuarine and near-coastal distribution.