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The shell morphology and population dynamics of the five British Unionidae are compared within a sympatric population. Pseudanodonta complanata is distinguished from Anodonta anatina and A. cygnea by the hinge length-shell length relationship; this morphological distinction may serve as a useful tool in the identification of this threatened species. The shell length at a given annulus was remarkably similar for all five species, although the symptotic length is reached most quickly in P. complanata and Unio pictorum. P. complanata is relatively short-lived and attains the lowest maximum length, while A. cyanea lives more than twice as long and attains almost double the length of P. complanata. Unio spp. have a short gravid season over the summer, while Anodonta spp. have a long gravid period, lasting from Autumn through to Spring. Unlike other members of the Anodontinae, P. complanata has a short breeding season, overlapping with that of the Unio spp. [ABSTRACT FROM PUBLISHER]