OXYGEN ISOTOPE VARIATION WITHIN RANGIA CUNEATA POPULATIONS

Rangia cuneata is an endobenthic clam typically found in brackish water estuaries of the Atlantic Ocean and Gulf of Mexico coasts of North America. Its shell may potentially be useful as an archive for environmental conditions, including salinity and temperature, for many coastal areas due to its wide salinity tolerance. However before R. cuneata shells can be developed as a proxy archive, some fundamental characteristics need to be better assessed. For example, its shells usually contain sharp and irregular growth breaks suggesting that their growth is frequently interrupted, thus, it is uncertain if individuals within a population record the same environmental conditions. To address this issue, samples of living R. cuneata were collected in two sites in the Mobile-Tensaw Delta (Gulf of Mexico coast, Alabama, USA) where previous studies that focus on this species have taken place and where time series environmental data exist. Time-equal sections of shell as identified through growth increments were analyzed for oxygen isotope (δ¹⁸O) ratios to assess if the values on the growing edge of the different shells are statistically similar. Presumably populations would have identical δ¹⁸O values (within precision limits) if all individuals were precipitating new shell at the time of capture.

Results show that in one population a majority of the shells were growing at the time of capture as δ¹⁸O values corresponded to values predicted based on local water temperature and δ¹⁸O in the water taken at the time of capture. The second population, however, had few individuals with similar δ¹⁸O values at the shell margin. The second population had several individuals that appeared to have ceased precipitating shell several days to weeks before capture (based on comparison to on-site time series data). It is uncertain what causes individuals in a population to cease shell precipitation for periods of time while other do not. These issues must be address if R. cuneata is to be used as a time-series environmental proxy.