REVISITING GROWTH INCREMENT COUNTING AS A METHOD FOR BIOLOGICALLY AGING CRASSOSTREA VIRGINICA FROM THE U.S. MID-ATLANTIC

Demographic and population data are difficult to interpret from the fossil record. Diagnostic features used for assessing the biological age of an organism are rarely preserved in the fossil record or, in the case of many invertebrates, are absent altogether. This poses a problem for conservation paleobiology, which could use population data from the fossil record as a natural baseline to compare to modern ecosystems.

In the case of the eastern oyster, Crassostrea virginica, the fossil record presents an opportunity to assist in conservation efforts by studying ancient oyster populations. C. virginica has played an important role in estuaries along the eastern coast of the U.S. for millions of years. However, modern oyster populations have suffered significant losses from over-fishing and disease. Conservation efforts have fallen short of restoring populations to pre-1900 levels.

One method for biologically aging oysters, growth increment counting, relies on the assumption that the growth increments within the hinge are accreted bi-annually. We tested this hypothesis by conducting stable oxygen isotope sclerochronological analyses of four Pleistocene-aged oysters from Virginia and Delaware. Bisecting the left hinge of each oyster, we collected powdered carbonate samples (N = 100) from transects drilled into the growth increments beneath the outer prismatic layer of the hinge. Powder samples were then run on an isotope ratio mass spectrometer to obtain an isotope profile for each hinge.

Our results demonstrate that, in high-resolution profiles, grey growth increments correlate with δ¹⁸O maxima, while white growth increments correlate with δ¹⁸O minima. Resampling the data to include fewer data points, we find that lower resolutions display no such correlation. We therefore believe that there is sufficient evidence to support growth increment counting as a reliable method for biologically aging C. virginica from the U.S. Mid-Atlantic; however, we would not expect the correlation to hold at higher or lower latitudes due to changes in seasonality. By confirming a basic method for aging oysters, our results will help us to better understand both Pleistocene and modern oyster populations, as well as to interpret records from archaeological and historical deposits.