Variations in shell morphology of freshwater mussels Anodonta anatina and Unio tumidus (Bivalvia: Unionidae) from Lake Polvijärvi, Finland

Freshwater mussels play an important role in aquatic ecosystems and have been the subject of numerous ecological and taxonomical studies. The present study investigated the shell morphology of A. anatina and U. tumidus with focusing on the width-length, height-length, and width-height relationships in Lake Polvijärvi, Finland. Both species revealed distinct external shell morphologies, where A. anatina exhibited an elongated shell shape with a slightly curved dorsal margin and a smooth surface, while U. tumidus had a shorter and flat shell with a more swollen umbo region and a rough surface. The correlation analysis demonstrated strong positive relationships between width-length, height-length, and width-height variables for both species (R2 ≥ 0.82). Linear regression models accurately described these relationships indicating that shell width and height increased consistently as shell length increased. Both of A. anatina and U. tumidus exhibited positive slopes in the width-length relationship suggesting a widening of shells with increasing length, however, A. anatina had a higher intercept than U. tumidus indicating a wider shell at a given length in comparison to U. tumidus. In the height-length relationship, both species showed positive relationships, but U. tumidus exhibited a steeper slope indicating a faster rate of increasing shell height with increasing length compared to A. anatina. These findings contribute to understand the patterns of shell morphology for both species; and the observed species-specific dependence have implications for their growth, proportions, ecological adaptations, and shedding light on their ecological niches and evolutionary dynamics.