Identification of ecophenotypic trends within three European freshwater mussel species (Bivalvia: Unionoida) using traditional and modern morphometric techniques ALEXANDRA ZIERITZ* and DAVID C. ALDRIDGE Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, UK Received 4 May 2009; accepted for publication 6 July 2009 Most species of freshwater mussels (Unionoida) show a wide variability in shell form and size but an understanding of which factors determine unionoid morphology is poor. We identified ecophenotypic trends in shell and internal characters within three unionoid species from two habitat types (marinas and river) of the River Thames, UK, using traditional and modern morphometric techniques. In marinas, all species grew to larger maximum sizes than in the river, which might be a result of higher temperatures and phytoplankton densities in marinas. Unio pictorum in marinas was more elongated than in the river and Fourier shape analysis revealed a trend from dorsally arched river specimens to straight dorsal and pointed posterior margins in marina individuals. The degree of shell elongation and shape of dorso-posterior margin were not associated with sediment composition, but were associated with the different hydrological characters of the two habitat types. Relative shell width was a poor indicator of collection site and influenced by allometric growth. Unlike U. pictorum, a difference in shell elongation of marina and river mussels could not be detected in Unio tumidus and Anodonta anatina. However, all three species showed the same trends regarding the shape of the dorso-posterior shell margin. This shell character may thus have broad ecological significance and could have considerable utility to palaeontologists, taxonomists, and conservation biologists. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 98, 814–825. ADDITIONAL KEYWORDS: fourier shape analysis – intraspecific variability – palaeontological reconstruc- tion – Unionidae. INTRODUCTION Freshwater mussels of the order Unionoida display a wide range of intraspecific morphological variability, which has in the past led to considerable synonymy (Küster, 1848; Lea, 1870; Locard, 1890). Besides the obvious value for taxonomic research, identifying habitat characteristics associated with given pheno- types of a species can provide valuable information about its ecology, assist in the management of endan- gered species (Bogan, 1993), and enable the re- construction of palaeoenvironments. In addition to those concerning a mussel’s habitat, other factors, such as its sex or ontogenetic stage, might influence the morphology of a given specimen (Seed, 1980; Tevesz & Carter, 1980). However, the importance of the respective nonhabitat and habitat factors on par- ticular morphological trends in freshwater mussel species and the mechanisms involved remains poorly understood. The relationship between morphotype and habitat has been studied in unionoids for over 100 years (Hazay, 1881; Buchner, 1910; Israel, 1910; Haas & Schwarz, 1913; Ortmann, 1920; Grier & Mueller, 1926; Bloomer, 1938). However, such descriptions, usually based on measurements of the three shell dimensions (length, height, and width), are often con- tradictory. For example, in their study on 18 unionoid *Corresponding author. E-mail: alexandra.zieritz@gmx.net Biological Journal of the Linnean Society, 2009, 98, 814–825. With 8 figures © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 98, 814–825 814