Depth distributions determined with SCUBA, and associated studies of the freshwater unionid clams Elliptio complanata and Anodonta grandis in Lake Bernard, Ontario

Underwater circular-plot sampling showed Elliptio complanata to be confined to the hard-sand, wave-swept shallows, on which the heavy shell and narrow ventral angle of this species permit it to retain a footing. The E. complanata population lay shoreward of 4 m, mostly at depths of 2.5 m or less. A shoreward, low-density extension of the Anodonta grandis population overlaps that of E. complanata, but the A. grandis population extends lakeward to depths of 13–14 m, where its lightweight shell and wide ventral angle permit it to sit on the soft, unconsolidated silt substrate. A blanket of 10 cm or more of this soft substrate has accumulated in Lake Bernard at depths of 6 m or more, beyond the disturbance of surface waves. Specimens taken from these greater depths were markedly stunted, but such stunting was shown to be a progressive phenomenon, increasing with greater depth, rather than a sudden threshold effect at any particular depth. Regression relations among length, height, obesity, and weight of A. grandis shells were the same from all depths. The speculation is advanced that A. grandis may have evolved primarily as a deep-water species.