Most remain fairly stationary unless some habitat stress occurs. Since unionids are not very mobile, finding a member of the opposite sex can be difficult. Sexes are usually separate, but if a clam is isolated from a member of the opposite sex, it can turn into a hermaphrodite, both male and female, and fertilize its own eggs. Female mussels brood their young from the egg to the larval stage in their gills. In the spring or summer, depending on species, they expel the larvae, called glochidia, into the water where they attach to the gills or fins of an appropriate fish host in order to become encysted and complete their metamorphosis to the juvenile stage.
Some female clams have a unique way of attracting the right type of fish.
These females have developed a fake fishing lure that looks like an item the fish would like to eat, and the clam wiggles this lure so that it looks alive. When a fish tries to eat the lure, the female unionid releases glochidia that attach to the fish. After a period of attachment to the fish ranging from 1 to 25 weeks, depending on the type of unionid, the juvenile mussel detaches from its host and falls to the substrate to complete its development into a free-living adult. Some species may successfully use a variety of fishes, but the majority are thought to be host-specific.