Shells of Unio tumidus (Bivalvia, Unionidae) from an archeological site and contemporary population inhabiting the same lake

Mollusc shells from archaeological excavations are often well preserved and allow for species identification and measurements. Large shell assemblages can provide information for environmental reconstruction and investigation of anthropogenic impacts. In this study we compare freshwater mussel shells from a shell midden excavated on a lake island in northern Poland, dated 2,700–2,600 BP, with contemporary populations from that lake. No changes in species composition occurred, with only Unio tumidus and Anodonta sp. present in the prehistoric sample, and Unio tumidus and Anodonta anatina in the contemporary one. In both the archaeological and contemporary samples, shells of Unio tumidus were relatively small (mean shell length 49.3 ± 5.2 mm, max 64 mm, and 46.0 ±5.9 mm, max 61 mm, respectively). However, the values of all measured characteristics (length, height, mass, and thickness) were significantly lower in the contemporary compared to prehistoric shells. In modern times, in the region of the study largescale forest clearing and conversion of land into agricultural areas occurred; presently areas of intensive agriculture directly adjoin the lake. Dwarfing of the shells likely results from anthropogenic deterioration of mussel habitat.