Growth increment periodicity in the shell of the razor clam Ensis directus using stable isotopes as a method to validate age

To evaluate the role of the razor clam Ensis directus in Dutch coastal waters, assessment and understanding its population dynamics is important. As such, the age structure of the population forms a key parameter. Accurate age determination in bivalve shells is not always straightforward due to the difficulty of interpreting externally visible growth lines. In the present paper, we aimed at validating the seasonality in growth line formation using visual techniques in combination with stable oxygen and carbon isotope analyses (δ18O and δ13C). High δ18O values in the shell coincided with growth marks on the external surface of the valve and in acetate peels of the shell's cross section. Most shell δ18O samples were assigned to the months June to September. From November to March no samples were retrieved indicating that the shell did not grow. The lowest reconstructed temperature (6.1 °C) suggests that ~6 °C may be the threshold temperature for growth. Nevertheless, most of the reconstructed values fell above 14.5 °C indicating growth occurred mainly in the summer at relatively high temperatures. Shell δ13C profiles followed a more or less seasonal cycle but no direct relationship could be made between δ13C values and annual growth lines. Although counting external annual growth lines led to a correct estimation of age and consequently of growth rates, we recommend analysing acetate peels of cross-sections to support the distinction between annual lines and disturbance lines.