Millennial-scale changes in size and growth of Unio pictorum (L. 1758) and U. tumidus (Philipsson, 1788), in the Oder river

Unio pictorum (L. 1758) and Unio tumidus (Philipsson, 1788) are common bivalve molluscs from the Unionidae family, with significant ecological importance in aquatic ecosystems. Their shells are essential for species identification and can also be used to assess changes in population structure, individual growth, and body form under varying environmental conditions. The aim of this study was to: (i) compare the individual size and shape of shells, (ii) assess length growth, and (iii) analyse population structures (based on length and age) of the bivalve molluscs Unio pictorum and Unio tumidus between specimens from the early medieval period (EMS) and those currently found (MS) in the Oder River near Szczecin, Poland. EMS shells were collected from archaeological excavations in cultural layers dated to the 10th and 11th centuries. They were located at three sites in Szczecin, approximately 500-700 m from the Oder River. MS specimens were collected using a bottom dredge between 12 and 21 May 2024 from sites within 1,000 m of the excavation area. For both groups, measurements included size, age, growth (using the von Bertalanffy model), and morphometric characteristics (length [L], height [H], width [W]), as well as the Elongation Index and Convexity Index. The shells of Unio pictorum and U. tumidus from the modern sample (MS) exhibited statistically significantly greater length, width, and height compared to those from the early medieval sample (EMS). At the same time, lower values of the Convexity Index and, in U. pictorum, the Elongation Index were recorded in the MS group. The length and age structure of both species in the MS group was dominated by larger and older individuals relative to the EMS group. The growth of contemporary Unio pictorum and U. tumidus can be considered moderate compared to other present-day populations, with maximum predicted ages of eight and nine years, respectively, and asymptotic lengths (L_∞) of 93.85 mm and 87.03 mm. In contrast, in the EMS group, the maximum predicted age and asymptotic length were 10 years and 70.12 mm for U. pictorum, and 6 years and 96.25 mm for U. tumidus, respectively. Higher values of the φ' and P growth indices in the MS group indicate that both species are currently growing at a faster rate than their early medieval counterparts (EMS). The observed differences in individual and population characteristics of Unio pictorum and U. tumidus between present-day specimens (MS) and those from the early Middle Ages (EMS) are attributed to long-term changes in temperature, nutrient availability, and water regime over the past 1,000 years.