Age structure and growth patterns of the endangered freshwater mussel Unio crassus Philipsson, 1788 under different environmental conditions

Understanding the age structure and growth patterns of freshwater bivalve species is crucial for their conservation, and these data are still lacking for many species and locations. The main objective of this study was to gain insight into the age structure and growth patterns of the endangered mussel Unio crassus at the unpolluted and polluted sites on the Mrežnica River (Croatia), as well as construct the first-ever chronology for this species. Bivalve sampling was carried out in July 2020. The age and growth dynamics were determined by analysing internal growth lines in acetate peel replicas of shell cross sections. The length range of shells from the unpolluted station was 4.5–5.6 cm, with an estimated age range of 10 to 29 years, whereas shell length at the polluted site was 5.2–6.1 cm, with an estimated age range of 10 to 22 years. Our results indicate that industrial pollution may influence life strategies in U. crassus populations, with mussels growing faster and living shorter at polluted site. Compared to other parts of Europe, the estimated ages obtained in our study were the most similar to those in central Europe. According to growth curves, mussels grow the fastest during their first 10 years, after which their growth significantly slows down. Growth chronologies were constructed by measuring growth increments in acetate peels of shell cross-section, and some correlations between shell growth and environmental parameters have been observed, indicating the potential for further sclerochronology research on this species.