The phylum Mollusca includes a diversity of species that inhabit oceanic, freshwater and terrestrial habitats – from octopus and squid to clams, mussels, snails and slugs. Two major classes of mollusks include the bivalves and the gastropods. Bivalvia means “two doors,” or in this case, two halves of a shell, and includes mussels and clams. They have a large, strong foot for locomotion, and two tubes with which they pull water into the body, filter out food, then push the used water back out of the body. Gastropoda means “stomach foot,” and includes the snails, slugs and limpets. These diverse and important mollusks slide along the ground scraping up food with a rasp-like mouthpart called a radula.

Estimates vary, but there are at least 70,000 species of mollusks (not including those known only from fossils), and about 600 new species are described every year. Unfortunately, mollusks, and especially those that live in freshwater or on land, also have the highest extinction rate of any group of species: 42% of animals that have gone extinct in the last several hundred years are mollusks (almost 300 species). North America is a hotspot for freshwater mollusk diversity, but approximately 74% of freshwater gastropods and 72% of freshwater mussels in North America are either extinct, imperiled, or vulnerable to extinction. These aquatic animals are most threatened by pollution changes made to their habitat. The Xerces Society is working to conserve freshwater mussels in western North America, and we also use the Endangered Species Act to advocate for the conservation of a number of snail, limpet and abalone species.


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Mussel bed composed of Anodonta sp. and Gonidea angulata, Middle Fork John Day River, Oregon. Copyright CTUIR freshwater mussel project.