During the spawning season, mature males release prodigious quantities of sperm into the water. For fertilization to occur, these sperm must pass into the incurrent apertures of sexually mature females of the same species. The sperm travel through the aperture into the suprabranchial chamber where the eggs are held. The fertilized eggs are then transferred into the gill chambers. The gill chambers form a modified brood pouch called the marsupium. While in the marsupium, the fertilized eggs metamorphose from an embryo into the larval form known as the glochidium. Glochidia may be released after a few weeks, or mature glochidia may be held in the brood chamber for months, depending upon the species.

The glochidia of most freshwater mussels are obligate parasites of fish. Once the glochidia have matured within the gills of the female mussel, they must attach to the gills or fins of a suitable fish species to go through yet another metamorphosis into free-living mussels. Fish may become infested with glochidia in several ways. The glochidia of some mussel species are lightweight and float in the water column after they are released. These are "eaten" by fish but instead of passing into the fish’s digestive tract, the glochidia attach to its gills. Other glochidia are heavy, and once they are released, they sink to the bottom of the stream. These glochidia are more likely to attach to the fins of fish as they swim over them.

Females of several mussel species use mimicry to lure fish hosts to themselves before they release their glochidia. The mantle tissue of these species are modified in such a way that they look like prey fish or insects to the fish host. When the female waves these tissues, the fish host may attack the "minnow" and instead of a meal receives a mouthful of glochidia. The glochidia then attach to the gills of the fish. Other mussel species use a kind of fishing lure instead of their own tissues. These mussels produce a gelatinous matrix around the mature glochidia before they are released. This matrix and the glochidia together are called a conglutinate. Some mussel species completely release the conglutinate, and it drifts into the water column or onto the substrate, looking much like a worm or insect, where it is "eaten" by the fish host. Other mussel species release the conglutinate, but it remains tethered to the female mussel by a "fishing line" of the matrix material. The conglutinate waves about in the water column and is "eaten" by the fish host.

Once the glochidia have successfully attached to the fish host, they remain attached for a period of time that varies by species. While attached, the glochidia metamorphose into juvenile mussels, developing a true heart, liver, digestive tract, and muscular foot. When the metamorphosis is complete, the juvenile mussels excyst from the fish host and begin an independent life.