Size-specific growth patterns and estimated longevity of the unionid mussel (Pronodularia japanensis)

Despite the dwindling populations and an urgent need for conservation of unionid freshwater mussels in Japan, there are gaps in our understating of their fundamental ecology. This study examined size-dependent annual growth rates, elucidated size-specific intra-annual growth patterns, and estimated age and longevity of P. japanensis individuals for two locally isolated populations in agricultural drainage channels. Annual growth rates of P. japanensis were strongly size-dependent, with growth rates being exponentially decelerated with increasing shell length. Irrespective of sizes, individuals ceased to grow in winter when water temperature fell below 10A degrees C. Intra-annual growth patterns were weakly explained by the changes in water temperature and differed among size classes; juveniles (< 25 mm) maximized growth rate in May whereas the growth rates were the highest in June or July for larger individuals. Only adult individuals exhibited growth cessation in the July-August period, suggesting that energy investment was directed towards reproductive activities. Adults also showed negative growth rates (shrinkage of individuals) in winter, suggesting possible dissolution of shell margins. Age estimation based on two 1-year periods suggests that large numbers of P. japanensis individuals were > 10 years old, and the oldest individuals were > 20 years old for both study populations. Our findings suggest that anthropogenic activities conducted in spring may have strong influences on juveniles and population dynamics of P. japanensis and underscore the need for accurately determining age and longevity of remaining populations of unionid mussels.