A Quantitative Study of Growth and Production of Unionid Mussels in the River Thames at Reading

Five species of unionid freshwater mussels are found in the River Thames at Reading; of these Anodonta anatina (L.) and Unio pictorum (L.) are the most abundant, and Anodonta minima Millet and Unio tumidus Philipsson are less numerous; Anodonta cygnea (L.) is rarely found. It has been suggested that the dark lines on the shell of a mussel are formed annually, and can therefore be used to determine age. The formation of a dark line during periods in which there is no growth is well established (Crowley 1957), and it has been shown for A. cygnea that there is no growth during the winter (see Chamberlain 1931; Weymouth & McMillin 1930). The lines on the shells of many marine bivalves have been proved to be winter rings (Pecten maximus Mason 1957, Scrobicularia plana Green 1957 and Macoma baltica Segerstrale 1960). Chamberlain (1931) justified this method of ageing freshwater mussels by the uniformity of the growth curves produced from a large number of specimens. Crowley suggested that the absence of regular fluctuations in the linear measurements between successive rings indicates that only one ring is formed in a year. However, both Crowley and Okland (1963) stated that considerable experience may be necessary to distinguish true annual rings from supernumerary rings, and Comfort (1957) stated that the deposition of one ring in a year must be checked. During this investigation the dark rings were shown to be formed annually, and were used to calculate growth rates, age structure and annual production in a typical reach of the River Thames. Growth curves for mussels from the neighbourhood of the heated effluent of Earley Power Station were also produced. The age structure of the population was of particular interest as Isely (1911), Lefevre & Curtis (1910) and Crowley (1957) noted that young unionids were rarely found. The embryos of Anodonta and Unio develop into glochidia larvae in the outer gillchamber of the female. The glochidia then pass through a stage of parasitism on the fins (Anodonta spp.) or on the gills (Unio spp.) of fish, during which time they metamorphose into juvenile mussels. Anodonta spp. have a long breeding period, carrying mature glochidia from late summer until the following spring when they are released. Unio spp. have a short breeding season, from April to mid-August, and the glochidia are released as soon as they are mature (Lefevre & Curtis 1910; Coker et al. 1921). The timetable of breeding activity has been worked out for the Thames population and compared with temperature data.