Freshwater Molluscan Shells

Bivalvia = Pelecypoda

Order Paleoheterodonta, Superfamily Unionoida or Unionacea:
(pearly freshwater mussels, the following seven families)

Margaritifera Northern Hemisphere, discontinuous
(North American) (European) (Asian)
Hyriidae (
Australian) (South American)
Mycetopodidae (
South American)
Mutelidae (
"river oysters", Africa
"river oysters", South America, India

Order Veneroida
Sphaeriidae: pill clams and fingernail clams. world wide.
Corbiculidae: basket clams, mostly Asiatic, also South America. Asiatic species are invasive in North America South America, and Europe.
Dreissenidae: false mussels and zebra mussels, invasive in North America and Europe.

All bivalves are two-shelled aquatic filter feeders that lack a head or radula, and most live partly or completely buried in sand or mud or attached to some firm surface. There are on the order of 20,000 total species, mostly marine, including the oysters, mussels, and scallops, and a great variety of creatures referred to as clams.

Bogan (2007) tabulates a total of 1026 freshwater bivalve species in 206 genera from 19 families in his treatment of worldwide species diversity. Roughly 800 of those species and 180 genera comprise the seven unionacean families, making them by far the most successful and diverse group of freshwater mollusks. All unionaceans are commonly referred to as "freshwater mussels" collectively, even though "clam" would not be incorrect. In addition to the unionaceans and the three additional primarily freshwater families above, a few species adapted to freshwater occur in the generally marine families Arcidae, Mytilidae, Cardiidae, Solenidae, Donacidae, Navaculidae, Trapziidae, Corbulidae, Erodonidae, Teridinidae, and Lyonsiidae. Some of these are included on the page with the Dreissenidae linked above.

Family tree showing relationships between major freshwater lineages of the Pelecypoda.
Arrangement is from Brusca and Brusca, 1990. Filibranchia includes the ark clams, marine mussels, scallops,
true oysters, pearl oysters, thorny oysters, etc, mostly marine. Paleoheterodonta also includes the nearly
extinct marine superfamily Trigoniacea (not shown), while Veneroida includes most other "clams". For another
perspective on Unionacean systematics, see the dissertation by Graf,
Chapter 7b (pdf file).


Stereo pair of Fusconaia ebena, right valve interior.
Picture arrangement is non-standard: sit well back from screen and cross your eyes slightly.

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