Growth characteristics of the endangered thick‐shelled river mussel (Unio crassus) near the northern limit of its natural range

The thick-shelled river mussel, Unio crassus (Bivalvia: Unionoida), is one of Europe's most-threatened mussels. Finnish populations of U. crassus lie close to the northern limit of its natural distribution. Extirpation of these populations will reduce the range of this endangered species. Growth characteristics of U. crassus were measured in a river running through the Helsinki metropolitan area. Shell dimensions (size-at-age data) and annual shell growth increments were used to reconstruct growth rate and its variation during the lifespan of individual mussels and to investigate the relationship between growth rate and longevity (age-at-death). Reconstructed growth rates compared well with size-at-age data conventionally used to study individual growth in natural populations, and fitted von Bertalanffy growth functions very well. Based on the same function, reconstruction and size-at-age methods resulted in similar estimates of growth rate. Shell weight explained the body size more reliably than age, suggesting that growth rate varied significantly among individuals. Comparison of individual growth histories revealed a negative correlation between age-at-death and growth rate, i.e. slow-growing mussels lived longer, and vice versa. In comparison with populations from central and southern Europe, U. crassus populations in the north of its range grew more slowly but lived longer, a phenomenon most likely explained by latitudinal changes in ambient temperature. Although northern populations are expected to benefit from a relatively high number of reproductive periods and lower juvenile mortality, the individuals studied here died earlier and suffered higher mortality than expected and an elevated conservative status is identified. Any management programme should take into account the life-history traits essential to the recognition of management units of U. crassus. Copyright © 2016 John Wiley & Sons, Ltd.