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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. [ABSTRACT FROM AUTHOR]

This study aimed to elucidate the mechanism of glochidial release in Unio pictorum (L., 1758) and U. tumidus Philipsson, 1788, and thus interpret the confused reports of unionine reproduction in the literature. Both species release mature, hooked glochidia on mucous threads, resembling a pearl necklace. Individual threads can reach 15 cm in length and carry c. 500 glochidia. Under hypoxic conditions both species released conglutinate-like structures comprising c. 500 eggs and immature, unhooked glochidia. The conglutinate-like structures released by U. pictorum and U. tumidus are coloured yellow and white, respectively. Increased temperature did not induce the release of conglutinates. The conglutinate-like structures are unlikely to be involved in increasing reproductive success by attracting host fishes. Rather, the premature evacuation of the gill contents may relieve the respiratory burden on adult mussels under hypoxic stress. Ambient hypoxia during the reproductive season may help explain the irregular recruitment success of relatively long-lived mussels, such as U. pictorum and U. tumidus. [ABSTRACT FROM PUBLISHER]