Bivalves reproduce by releasing sperm into the water, where they either meet eggs that have also been released or are sucked in to fertilize eggs within another bivalve. Bivalves can have both sexes in one individual or they can have separate sexes, depending on the species.
Bivalves include over 10,000 species, such as clams, oysters, mussels and scallops among others. In species which externally fertilize, the fertilized eggs go through their larval stages floating in the water column. In species that take in sperm, the larvae partially develop either in the gills or a cavity of their mother. Some still release them as larvae, while others keep them until they reach the juvenile stage. The larvae of freshwater mussels attach to fish and parasitize them to develop.
Bivalves are named for their two shells, which in most species are flattened top to bottom and hinged at the back. In some species, however, shells are reduced or even absent. They live a wide range of lifestyles, from burrowing under sediment to attaching permanently to rocks or other fixtures in the water. Most bivalves use their gills to feed, filtering water through them and capturing food particles. They lack any head or jaws.