Geochemical analysis of Crassostrea virginica as a method to determine season of capture

A group of American oysters (Crassostrea virginica), collected monthly for 1 year, was incrementally analysed via laser ablation mass spectrometry and reflected light microscopy in order to create a method of determining season of capture. Environmental data taken at the collection site (Altamaha Sound, GA) are compared to these incremental analyses. Causal factors in growth band development area defined and related to seasonal growth. Biannual growth bands are visually distinct and consist of alternating dark and light layers in cross-sections of the chondrophore. Variation in water temperature is the principle mechanism producing this variation, with secondary stressors, such as salinity, creating less regular overprints on these layers. Incremental analyses of O-18 indicate that dark bands are usually precipitated during cool months and light bands during warm months. Notable exceptions to this pattern occur during flood events and extreme heat that may cause precipitation of dark bands at any time. These patterns will vary according to local climate and latitude. Absolute values of C-13 and O-18 and time of increment formation vary between individuals of the same bed, therefore isotopic and visual analyses should be performed on large populations. The pattern of isotope variation through ontogeny differentiates between seasonal banding and banding caused by secondary stressors. Visual analysis alone cannot detect such irregular variation, thus the regular patterns of isotope distributions must be employed in conjunction with visual analysis and local comparative collections in large samples to determine seasonality.