Substratum patch selection in the lacustrine mussels Elliptio complanata and Pyganodon grandis grandis

1. Sediment selection was investigated under controlled conditions in two common lake-dwelling species of freshwater mussels (Bivalvia: Unionidae), Elliptio complanata and Pyganodon grandis grandis. 2. Sediment choice was determined in six independent experiments under controlled conditions by distributing mussels randomly or evenly in tanks containing patches of sand and mud, and following their movement among sediment patches in experiments lasting between 30 and 45 days. 3. In all experiments, both species were found most frequently in muddy sediment patches. Movement toward muddy patches occurred rapidly: an average of nearly 80% of Pyganodon grandis grandis were found in mud after 30 days. Elliptio complanata moved rapidly to patches of mud at the start of experiments, but occupation of muddy sediments appeared to decrease after about 30 days. 4. Our results contrast with many field studies that suggest populations of lake-dwelling freshwater mussels infrequently inhabit mud and silt. We therefore postulate that large-scale mussel distribution in lakes is influenced most strongly by factors other than sediment composition.