The female draws in sperm to fertilize her eggs. Unless males and females are near each other, the odds for fertilization are slim. The female broods the fertilized eggs inside her, then releases thousands of tiny larvae called glochidia. She must time the release so the tiny baby mussels can hitch a ride on a fish. She senses the presence of a fish, then shoots out glochidia.
Many species release their glochidia in clumps shaped like small worms or fish food.
Fish feed on them and then expel them through their gills, where the glochidia of many species settle. When a nest-building fish stirs up the river bottom, some glochidia in the river bottom may attach and hitch a ride. When they touch the tissue of a fish, the microscopic glochidia clamp or hook themselves onto the fish's scales, fins, or gills. Some glochidia need a certain species of fish to grow on. A layer of skin grows over the hitchhiker, forming a cyst.