Mussels generally have separate sexes. However, some mussels are hermaphrodites, in
which each individual has both male and female reproductive organs in its body. During spawning, males release sperm into the water. The sperm are drawn inside a female's shell, where they fertilize eggs in her body.

The fertilized eggs develop into larvae (glochidia) and are stored for a time in the female's gills (point out this third use of their gills). When the glochidia mature the female expels them into the water where they must attach themselves as parasites to gills or fins of fish. The fish act like swimming nurseries for baby mussels!