AN APPLICATION OF STABLE ISOTOPE SCLEROCHRONOLOGY TO DETERMINE ANNUAL GROWTH CHARACTERISTICS FOR CHIONE ELEVATA

Stable isotope sclerochronology is commonly utilized to characterize growth patterns within the skeletal parts of bivalves. We applied stable isotope sclerochronology to determine the accretionary nature of hard-parts for the venerid clam, Chione elevata. Three live specimens of C. elevata were collected from a soft-substrate intertidal flat near Wilmington, NC. We drilled aragonite samples from the outer prismatic layer of each shell. Variation in oxygen isotope ratios (δ¹⁸O) was considered against each sample’s respective distance from the umbo of the specimen.

No discernable growth patterns were observed within the cross-sections of each shell. Dark growth increments in other bivalve taxa from the same geographic area, such as Mercenaria mercenaria, correlated with winter growth cessation. A similar accretionary pattern does not exist within C. elevata. Dark growth increments within C. elevata appear as randomly distributed. Their occurrence does not always correspond with positive peaks in δ¹⁸O values (winter growth cessation). Additionally, dark growth increments are not correlated with ornaments on the surface of the shell. Positive peaks in δ¹⁸O values correspond to indentations, or “notches”, on the surface of the shell. These surface notches form annually during the winter growth cessation. Therefore, we characterize surface notches as the physical markers of annual growth within C. elevata. Other species of Chione exhibit similar annual surface notches. However, those species deposit internal dark growth increments in correlation with the formation of notches on the surface of the shell.

C. elevata is common in the fossil record throughout the Southeastern United States. Knowledge of their life span and growth rates can provide insight into certain paleoenvironmental conditions for a large number of fossil assemblages.