Observations on the Growth of Some Populations of the Freshwater Bivalve Aspatharia sinuata (Unionacea, Mutelidae) in Nigeria
Abstract
The growth habits of Aspatharia sinuata in 2 reservoir systems and 3 lotic habitats in the ‘middle belt’ of Nigeria were studied by the interpretation of growth interruption marks on shells, monthly length frequency distributions, and direct growth observations. Fastest growth occurred in Oyun Reservoir and Agbuur River while slowest growth rates were recorded in Asa Reservoir and River Oyun. Analysis of the data by Walford plots shows that the maximum theoretical length (L∞) of the bivalves in Asa Reservoir, Oyun Reservoir, River Oyun, Odo-Otin River and Agbuur river were 7. 39, 9.64, 6.75, 7.66 and 9. 86 cm, respectively. Generally, growth was fast during the first 2 years of life but this slowed down thereafter. The absence of significant shifting modes in the length distributions of the reservoir populations is, presumably, a consequence of the preponderance of adults with reduced growth rates in samples. Mark-release-recapture data from Asa Reservoir tends to suggest that one major growth ring is formed on the shells annually. It would also appear that the annual draw-down of water levels with the resultant aestivation of some bivalver, and reduced transparencies are the major factors causing a growth check in A. sinuata
