Shell morphology, growth and longevity of Unio tumidus (Bivalvia: Unionidae) from an archaeological site and contemporary population inhabiting the Oder estuary

Mussel shells are often found in archaeological excavations and can provide information useful for ecological reconstruction and assessment of anthropogenic impact on waters. In this study, two sample groups of swollen river mussel ( Unio tumidus ) which occurred during the Early Middle Ages (EMS) and currently (MS) in the Oder river estuary (Baltic basin) were compared. Allometric shell growth, morphological characteristics of the shell (length, width, height and thickness), age structure and growth of mussels were analysed using the von Bertalanffy equation. All three types of allometric growth (isometry and both positive and negative allometry) were observed in the studied mussels. In both groups, typical values of shell length, width, height and thickness were recorded. However, higher values of these morphological characteristics were recorded in the EMS group. Moreover, the EMS group, compared with the MS group of U. tumidus , was characterised by a higher longevity (12 and 10 years, respectively) and asymmetric length ( L ∞ ) (93.09 and 83.23 mm, respectively). Both groups of mussels had a similar growth rate ( k ). Larger shell sizes in the EMS group were probably caused by differential preservation and/or differential archaeological recovery, and resulted from differences in the age structure, especially higher mortality rate amongst individuals older than 6 years in the MS group.