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Bivalvia is a class of mollusks that all have two shells. It includes the clams, oysters, scallops and mussels. There are over 20,000 species of bivalves around the world, and probably thousands not yet discovered. Nearly all of them live in salt water, but a few groups have evolved to live in freshwater.

Bivalves have two shells that they can close together tightly for protection, with their whole body inside. Usually they open their shell just a small amount, so they can pull water in and out of their body for oxygen and food. Nearly all bivalves feed by filtering tiny organisms and other bits of food from the water they live in. They have a soft but muscular "foot" that they extend out of the shell to pull themselves along, or dig down in sand or mud.

Most bivalves live on the bottom in shallow water and bury themselves in sand or mud, with just the edge of their shell showing. Some of them, like oysters and ocean mussels, glue themselves to rocks. A few, like scallops, don't bury themselves, and move around. They can move quickly by slamming their shell shut fast.

Bivalves lay eggs. Some of them release their eggs into the water, and their young hatch out as swimming larvae. These drift in the water and feed, and eventually sink down to the bottom and transform into the adult clam shape. Some bivalves keep their eggs inside their shell for protection, and give the young a chance to develop and grow their shell before they leave their mother.

Michigan has three groups of bivalves. One is the freshwater mussels , a very diverse group of native species that live in rivers and lakes. They are unique to North America. Many species are endangered, and some of have gone extinct. Another is the peaclams or fingernail clams . These are very small clams that live in many different freshwater habitats. Finally there are two species from the family Dreissenidae , the zebra mussel and the quagga mussel that were accidentally introduced from Europe. These two are invasive species that are having very strong effects on freshwater habitats in Michigan, and in the whole Great Lakes ecosystem.

University of Michigan Museum of ZoologyNational Science Foundation

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. "Bivalvia" (On-line), Animal Diversity Web. Accessed April 24, 2024 at

BioKIDS is sponsored in part by the Interagency Education Research Initiative. It is a partnership of the University of Michigan School of Education, University of Michigan Museum of Zoology, and the Detroit Public Schools. This material is based upon work supported by the National Science Foundation under Grant DRL-0628151.
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