Age determination in bivalves

Individual age is a key parameter when studying population dynamics of a species. However, accurate age determination in bivalve shells is not always straightforward due to the difficulty of using external growth lines on the shell surface as a reliable method to identify annual growth patterns.

Age validation

Age validation in bivalves can be done by validating the seasonality in growth lines in the shell using stable oxygen and carbon isotopes (δ18O and δ13C) as well as trace elements (Sr, Mn, Mg and Ca). During a 1-year growth experiment on Limecola balthica (formerly Macoma balthica), δ18O values in the shell carbonate peaked in February/March and matched the annual line visible in the acetate peel of the shell's cross-section. Most carbonate was deposited in the months March to May, which was supported by the observed fast growth in the experiment during this period. Growth stopped between October and December indicating that annual growth lines are formed due to autumn growth stop.

Conclusion

δ13C profiles did not show any clear seasonality related to the position of the growth lines on the valve or acetate peel and, therefore, cannot be used to estimate age. Along the hinge, Sr was the only element showing variation on the growth lines. High Sr/Ca ratios in the hinge occurred near annual growth lines identified in the acetate peel, suggesting that Sr/Ca ratios may be used to identify annual growth lines and estimate age in L. balthica. Furthermore, counting external lines (on the valve) always lead to an overestimation of age and thus an underestimation of annual growth rates. Therefore, an analysis of the cross-section of the valve seems necessary to accurately estimate age and growth rate in this species. This aspect should be taken into account in future studies of growth and population dynamics on other bivalve species.