Condition index of the Eastern Oyster, Crassostrea virginica in Sapelo Island Georgia—effects of site, position on bed and pea crab parasitism

Journal of Shellfisheries Research, Jan, 2005 by Norman Mercado-Silva

ABSTRACT The condition index (CI) is a tool that has been used to estimate the effect that different environmental factors have on oyster meat quality. The CI of oysters in five beds in Sapelo Island and the Doboy Sound (Georgia, United States) were calculated using three different methodologies. Values from each site were used to: (1) compare two sites located in relatively large systems (river/ sound) and three sites in a small creek to test for differences due to system size and related nutrient availability; (2) determine the effect of a parasite, the pea crab (Pinnotheres spp.), on oyster CI; and (3) determine the effect of differential position of oysters on a bed, with respect to CI (oysters were collected from areas of the bed that are permanently submerged and areas with the longest dry periods between tidal cycles). With respect to the three analyses mentioned earlier, it was found that oyster CIs for sites in larger systems (river and sound sites) were significantly higher than those in sites with lower flux and width (creek sites) and that the presence of pea crabs as parasites can reduce CI by ~50%. In addition, no difference in CI was found between oysters collected from different positions in the oyster bed, indicating that the CI was not altered within a site from effects of exposure. This relatively unusual finding is discussed but suggests the robustness of CI as an intersite analysis tool. Not all differences found in the comparisons were significant for all CIs used, but qualitative results among them were consistent. These variations in significance and the adequacy of each method for analysis are briefly discussed.

KEY WORDS: Crassostrea virginica, Sapelo Island, condition index, parasitism, tidal exposure, Pinnotheres, pea crab

INTRODUCTION

The condition index (CI) of the Eastern Oyster, Crassostrea virginica (Gmelin 1791) has commonly been used to evaluate how these organisms are affected by their environment (Van Dolah et al. 1992, Rheault & Rice 1996). The index has been most important in the use of oysters as indicators of environmental pollution (Lauenstein & O'Connor 1988, Lytle & Lytle 1990, Yevich & Zaroogian 1990, Pridmore et al. 1990, Palmer et al. 1993). The use of CI as an environmental monitoring tool is based on the effect that environmental conditions and different pollutants have on the oyster's growth (Lawrence & Scott 1982, Scott & Lawrence 1982, Rosas et al. 1983, Marcus et al. 1989). It compares the dry meat weight of the animal to the interior volume of the shell, and given the physiologic changes suffered by the oyster in terms of carbohydrate and protein fractions, and lipid and mineral contents, it has been related to pollution effects (Austin et al. 1993). The CI is an inexpensive, quick, representative and responsive tool for monitoring pollution (Scott & Lawrence 1982) and has also been used to estimate growth differences among oysters living in environments subject to different salinities and temperature regimes (Austin et al. 1993, Schumacker et al. 1998). Other conditions that could cause differences in oyster CI are the presence of parasites or commensal organisms living in association with oysters and the relative exposure to desiccation of oysters located at different tidal heights (Cheung & Tse 1993). Parasites and commensals are known to reduce the input of vital resources to a host, thereby reducing its growth and reproductive potential (Stauber 1945, Bierbaum & Shumway 1988, Schmidt & Roberts 1996). The amount of time that an oyster remains above water during tidal lows relates to the period of food intake available to the individual; consequently, oysters that remain under water for longer periods of time could potentially have increased growth (Littlewood 1988, Crosby et al. 1991, Bartol et al. 1999).

The evaluation of the CI of oysters in and around the Sapelo Island National Estuary Research Reserve (SINERR) on Sapelo Island (Georgia, United States) was the main focus of this study. SINERR is part of a protected barrier island with plentiful oyster beds in the rivers and creeks that surround it (Walker & Cotton 2001). Oyster beds in 3 locations were studied: Marsh Landing Dock, Meridian Dock, and Dean Creek. The Marsh Landing Dock in the Dulpin River on Sapelo Island and the dock in Meridian are located in large water systems and are subject to constant boat traffic. The Doboy Sound and the Dulpin River receive nutrient inputs from a large system of marshes. Dean Creek in the SINERR is a smaller tidal system that receives nutrient inputs from one marsh on the island and has a comparatively low water flow. Sapelo Island as a whole has no obvious source of contaminants to the surrounding estuaries and although sections of the island were exposed to varying degrees of human activity in the past, it has remained in a natural state for the past 100 y. Therefore, impacts of anthropogenic origin on oysters are not suspected.

Given the variety of conditions that different oyster beds are subject to in the systems described earlier, the CI was used to observe differences in the fatness of oysters in 3 locations on Sapelo Island: Dean Creek, the Doboy Sound, and the Dulpin River. In addition, CI values were compared between oysters hosting pea crabs (Pinnotheres spp.) and "uninhabited" oysters to test the effect of parasitism on oyster meat quality. The pea crab gains shelter and food from its host oyster and reduces oyster food availability (Warner 1977). The presence of this parasite has been suspected of having effects on oyster growth (Kennedy et al. 1996) and meat content (Haven 1959) and has been related to a reduced gametic development in C. virginica (O'Beirn & Walker 1999) and Mytilus edulis (Bierbaum & Shumway 1988), but little else is known about other effects it could have. Finally, differences in CI values from oysters located at different heights of an oyster bed (i.e., subtidal vs. highest intertidal) were also investigated.