The sperm is siphoned by the female and used to fertilize her eggs internally. Obviously, if they aren't grouped fairly closely, reproduction is hard to achieve. After fertilization, the female then holds up to several thousand eggs at a time in her gills. In the late spring or early summer, the glochidia are expelled into the water where they have to fend for themselves. They need to attach themselves to the gills of a host fish within a couple days.
Most freshwater mussels team up with only one type of fish.
Our northwest species favor salmon and without salmon as hosts, mussels cannot successfully reproduce. Hitchhiking on a fish is a baby mussel's only opportunity to travel and experience more of the world, and traveling this way results in a free ride to a new home. After the mussels change from the larval form and begin to resemble adults, they break out of the cyst and fall to the bottom of the stream where they bury themselves in the bottom and begin to live an independent life. Although one would expect to see mussels in most of our streams and rivers, they often aren't there. They are only found in the cleanest streams and rivers with cool, clear water and bottoms that aren't muddy.