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Freshwater Mussel Life History and Reproduction
Life in the Mud
Adult freshwater mussels don't lead a very exciting life. They stay in one spot, bury their back end in the soft river bottom, and leave their front end and two siphons exposed like snorkels. They continuously pump water through their bodies, in one siphon and out the other. They use their gills to filter oxygen and food from the water. Their food consists of plankton (microscopic plants and animals) and other organic matter suspended in the water. And as they eat, they are cleaning the water.
Reproduction
In the summer when the mussels are ready to reproduce, the males release sperm into the water, and the females catch what they can. The sperm is siphoned by the female and used to fertilize her eggs internally. 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. There they can get oxygen and have a place to brood until they develop into glochidia—the larval stage of mussels.
It is the larval stage that is more exciting! 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 mussel species favor salmon, so without salmon, mussels cannot successfully reproduce.
Once the larval mussels attach to the fish, the fish body reacts to form a cyst that covers them with cells. The glochidia remain in the cysts for two to five weeks (depending on the temperature). Hitchhiking on a fish is a baby mussel's only opportunity to travel, which 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. They bury themselves in the bottom and begin to live an independent life. Only one in a million survive to the adult stage, but to offset these low odds, mussels lead a very long reproductive life and produce millions of eggs per year!