Seasonal and Life-long Variation of Sr/Ca Ratio in Shells of Mya arenaria from Grays Harbor (Washington)—an Ancillary Criterion in Demographic Studies

Mya arenaria was introduced to Grays Harbor (Washington) during the 1880s. Maximum size of members of the extant population is much smaller than that of clams dead in situ in preceding decades and nowadays found forming extensive ʹdeath assemblagesʹ. This could reflect differences in growth rate and/or maximum age (i.e. survival rate). Major ʹgrowth marksʹ (seen in shell sections as translucent bands) are formed in the shell of extant clams during winter, when temperature and salinity are at their minima. Interpretation of the optical pattern observed in shells from the extinct population is difficult with criteria developed for the extant population, and cannot be validated experimentally. To assess the possible use of strontium/calcium ratios as an objective criterion for age estimation, profiles of Sr/Ca ratio and luminance were measured across thin sections of shell chondrophores of clams from the extinct (N=4) and extant (N=3) populations. Peaks in the Sr/Ca ratio always corresponded with translucent bands in shells from the two populations. Cross-correlation between vectors of slopes for two consecutive measurements of Sr/Ca and luminance was positive and significant for five of the seven clams. Cycles of high and low Sr/Ca ratios matched seasonality in the environment and are most probably governed by temperature, allowing validation of ages estimated for shells from the extinct population. There was a positive, significant correlation between mean Sr/Ca ratio and estimated age at the time of shell formation. This ontogenetic trend could be related to age, growth rate or both. Profiles of Sr/Ca constitute a useful ancillary criterion for age validation. Results support the hypothesis that the observed difference in maximum size is due to higher longevity in the extinct population.