ALLOMETRY OF SHELL GROWTH OF CAGED AND UNCAGED ZEBRA MUSSELS (DREISSENA-POLYMORPHA) IN LAKE ST CLAIR

Growth and shell morphometrics of zebra mussels, Dreissena polymorpha (Pallas, 1771), were studied using caged and uncaged animals in Lake St. Clair. During unrestricted growth, mussels increased in length at about twice the rate as in height or width. Growth of mussels in cages was virtually isometric regarding shell length and width, but not so for length and height as well as for height and width. During growth from 4-20 mm, the L:W ratio was unchanged, the L:H ratio increased by 14%, and the H:W ratio decreased by 13%. In contrast, uncaged mussels, from either low or high density locations, showed strong allometry in terms of L:W ratio. L:W declined by approximately 25% during growth from 4-30 mm; L:H was isometric and H:W was allometric. Zebra mussels reared in the smallest cage compartments had reduced shell dimensions, as well as weight, compared to individuals reared in large compartments. Weight was most strongly reduced (30.2%) followed by length (17.5%), width (15.7%), and height (11.4%). Crowding by up to ten individuals recruited from the outside did not significantly reduce the weight gain of caged mussels. Frequent sampling (12 times/yr) of caged zebra mussels did not affect final length, but significantly reduced weight gain and increased mortality, compared to infrequently sampled mussels (two times/yr). The use of cages with individual compartments for in situ zebra mussel studies offers certain advantages, i.e. individuals are free of competition, predation, and other field complications due to crowding. However shell morphometrics are altered by such treatment. In general, final weights and lengths of zebra mussels in cages were comparable to both those achieved in uncaged animals and reported from European cage studies.