Effects of inorganic mercury exposure on growth, survival, and reproduction of eastern oysters, Crassostrea virginica Gmelin, and performance of larvae

Mercury pollution is a serious global environmental threat. As such, it has been included in the US Environmental Protection Agency's national strategy for persistent, bioaccumulative, and toxic pollutants and is the subject of continued attention in the national media. It has been recognized as a public health threat ever since the Minamata Bay disaster in Japan in the 1950s, where consumption of contaminated fish and shellfish resulted in numerous fatalities and illnesses. Part of the difficulty with understanding consequences of mercury pollution is the complex biogeochemical cycling of mercury in the environment. Precise mechanisms of biotransformation and cycling of mercury are still not well known.; Mercury and other nonessential heavy metals are highly toxic to organisms, although mechanisms for sequestration exist which may allow tolerance of very low levels of exposure. Some organisms, especially bivalves, are tolerant of high body burdens of mercury without any apparent ill effects. The eastern oyster, Crassostrea virginica Gmelin, is one such organism. Over the last two decades, research on heavy metal exposure in oysters has demonstrated that although oysters survive very high body burdens, numerous sublethal effects might result. Adverse effects include lysosomal destabilization, glutathione depletion, decrease in hemocyte numbers and activity, reduced fecundity, reduced growth, and protein damage. These effects can collectively result in severe adverse consequences for oyster populations, but the ecotoxicological consequences of mercury pollution are poorly understood.; In this study, eastern oysters were exposed to sublethal concentrations of mercury in a three-week, static renewal exposure in Fall 2002 and overwintered. In Spring 2003, oysters were transferred to the Auburn University Shellfish Laboratory for maturation and spawning. Body burdens were analyzed over the depuration period to evaluate depuration rates. Unlike previous studies on mercury depuration in oysters, oysters in this study excreted most of their body burdens. Oysters were examined to determine if mercury exposure had any effects on growth, survival, and reproduction. Mercury exposure had an adverse impact on growth of oysters, but effects on survival were unclear. Oysters were then manually spawned, and larvae were evaluated to determine whether effects of mercury exposure to adult oysters affected offspring. There was no apparent effect on larvae. However, some results of the present study may be artifacts of overwintering conditions, particularly nutritive stress. In nature, periodic exposure of mercury may inhibit growth of oysters, thus reducing fecundity and increasing the impact of predation on oyster populations. Assuming a lack of nutritive stress in natural oyster populations, mercury may also result in long-term adverse effects on oyster resource productivity.