Distribution and life history of the Unionidae (Bivalvia: Mollusca) in the Assiniboine River drainage in Manitoba, with special reference to Anodontoides ferussacianus

Abstract

This study examined the distribution of the freshwater mussels in the Assiniboine River and some of its tributaries in Manitoba, and the life history of the cylindrical floater mussel ('Anodontoides ferussacianus ') in a small prairie stream. Twelve species of freshwater mussels were found in the Assiniboine River drainage. Each species exhibited specific st eam size and substrate preferences. Examination of species occurrences resulted in the identification of four species categories based on stream size preferences (small stream, medium stream, large stream, and ubiquitous categories), but some overlap existed between one category and the next. Individuals of 'A. ferussacianus' in the Cypress River near St. Alfonse, Manitoba, displayed a bradytictic breeding pattern. Fertilization of ova occurred in late July and early August, and mature glochidia were released from marsupia in May and June. Individuals in this population matured at age 1, which is the lowest recorded for any species of freshwater mussel.The growth rate and density of 'A. ferussacianus' in the Cypress River is similar to other populations of freshwater mussels in lotic systems. Young individuals comprised a large proportion of the mussels collected. This age structure may be due to selective predation of older year classes. Small, one year-old mussels were found beneath the sediment surface throughout the year. Older individuals (Year 2+) occurred at the sediment surface in May and June, after which they burrowed into the sediments. The seasonal vertical migration displayed by older individuals of 'A. ferussacianus' in the Cypress River probably was not a behavioral response, but rather because of high water velocities.