Growth parameters of endangered freshwater pearl mussel (Margaritifera laevis, Unionoida)

We observed periodic increments of the annuli to verify the annual growth-age relationship in two populations of a freshwater pearl mussel, Margaritifera laevis (Bivalvia, Unionoida), in northern Japan and confirmed that one annulus is added each year. The relationship between yearly shell growth and age was regarded as a uni-modal distribution. We fitted several growth models to the shell length-age relationships of these two populations, which had different densities and age distributions. The Gompertz function showed the best fit, in terms of both RSS (residual sum of squares) and the difference between the observed maximum shell length and the asymptotic shell length. The life span of M. laevis was shorter than that reported for Margaritifera margaritifera populations at higher latitudes (>= 50 degrees N), which have been widely recognized as threatened. Margaritiferid mussels living at lower latitudes tend to have shorter life spans and smaller maximum size. These results suggest that margaritiferid mussels in southern regions such as M. laevis in Japan and M. margaritifera in Spain may be particularly vulnerable when conditions are unsuitable for juvenile mussels for prolonged periods.