Resilience and sensitivity to environmental stress in the American oyster, Crassostrea virginica

Filter-feeding exposes oysters to large numbers of microorganisms and contaminants when they occur in the surrounding water and sediment. Recent studies from our laboratory provide some insight to the strategy that oysters use against invading pathogens. Minutes after bacteria are injected into the adductor muscle, oysters can limit the ability of the bacteria to grow; however, bacteria remain intact in tissues for days. This same immune strategy may make oysters particularly sensitive to hypoxia. Hypoxia, accompanied by high levels of high levels of CO2 (hypercapnia) reduces bacteriostatic activity in oyster tissues, increasing the opportunity for bacteria to establish infections. In the present study we asked whether long term (28 d) exposure to low levels of a metal contaminant, cadmium (50 ppb), would alter bacteriostasis and/or degradation of bacteria in oysters. Simultaneously, we looked for impacts of Cd on cells of the immune system by assessing total hemocyte count (THC) in hemolymph, and in the gills and the digestive gland by assessing gene expression profiles using cDNA microarrays. We found that, although THC decreased with Cd exposure, neither bacteriostasis nor degradation of injected bacteria was altered. Genes associated with metabolism, detoxification, tissue damage and repair were differentially expressed in the gills of Cd-exposed oysters. In the digestive gland most transcriptional changes were associated with detoxification and the immune response. Thus, in response to environmental stress, each tissue examined mounted a distinct response but the oyster maintained an effective immune defense (NOAA’s Center of Excellence in Oceans and Human Health).