Unionidae

From Wikipedia, the free encyclopedia

Unionidae

Duck mussel, Anodonta anatina

Scientific classification

Kingdom:	Animalia
Phylum:	Mollusca
Class:	Bivalvia
Subclass:	Paleoheterodonta
Order:	Unionoida
Family:	Unionidae

Genera

See text for genera and species.

Unionidae is a family of freshwater mussels, found world-wide living buried in mud, silt or sand in rivers, streams and lakes. The shell is equivalve and nacreous and has a thick periostracum. The hinge mostly has two cardinals and two posterior lamellar teeth.

All Unionidae live half-buried in the sediment, their siphons sticking out. They pump water through the inflow and outflow siphons, obtaining oxygen and food.

Unionidae have a remarkably complex life history. All Unios are of separate sex. The sperm is ejected from the mantle cavity through the male’s excurrent siphon and taken into the female's mantle cavity through the incurrent siphon. Fertilized eggs move from the gonads to the gills (marsupia) where they further ripen and turn into the first larval stage, the glochidium. Mature glochidia are ejected by the female and then attach to the gills, fins or skin of a host fish. A cyst is quickly formed around the glochidia and they stay on the fish for several weeks or months before they fall off as juvenile mussels which bury themselves in the sediment.