The zebra mussel’s clam-like shape (which was characteristic of the larval stage) is replaced by a more triangular or mussel-like shape as the mussel enters the juvenile stage. Mussels are considered adults when they become sexually mature. Adult mussels range from approximately
6 to 45 mm and generally live to be 2-3 years old in temperate climates.
Zebra mussels are diecious (i.e., separate sexes) and are almost always capable of reproducing within their initial 12 months of life. Mussels settling in late spring or early summer typically grow and mature quickly during the warm summer months. Adults with shell lengths exceeding 8 to
9 mm can start to spawn (release gametes into the water column) as early as May of the following year. Fertilization occurs in the water column, and adults have been known to annually produce over 1 million eggs or 10 billion sperm (Sprung 1991).
Recently settled juveniles may be attached to any hard substrate, including other zebra mussels. Here they appear as white grains of sand.