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Rhode Island
Freshwater Clams and Mussels

Michael A. Rice

With the growing concern about the potential invasion of Rhode Island's freshwater bodies by the European zebra mussel (see Rhode Island Sea Grant Fact Sheet P1326 "Zebra Mussel: An Unwelcome Visitor"), there has been growing interest in the native clams and mussels that inhabit Rhode Island's freshwater lakes and streams.

There are two main reasons for this concern. First, public familiarity with the most common native freshwater clams and mussels may provide a basis for comparison that promotes vigilance for the appearance of any unusual mollusks. Accurate reports can then be made to responsible state officials, so timely notifications and appropriate remedial actions can be taken. Secondly, experience in the Great Lakes region - already infested by zebra mussels - has shown that the native clams and mussels can be harmed or killed by the exotic invader. Cases have been described in which large numbers of native clams were killed by the mussels' attaching themselves to the shells of the native clams and smothering them, or starving them by outcompeting them for available food in the water.

One threat of the zebra mussel invasion may be loss of some of the biodiversity in our freshwater fauna. The native freshwater clams and mussels of North America show the greatest species diversity of any freshwater bivalve group in the world. These bivalves fall under two main categories: the unionid clams that are commonly referred to as freshwater mussels, and the "fingernail clams," named for their small size and appearance.

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There are close to two hundred
species of freshwater clams
and mussels in North America.

Considerable work was done in the 19th and early 20th centuries to describe the numerous species of freshwater mussels in the United States. This effort resulted largely from the mussels' importance as a fishery product to supply the button industry prior to the introduction of plastics in the 1920s.

In southern New England, 12 species of unionid clams and 21 species of fingernail clams have been described. Many of the the species of freshwater mussels and clams found in New England are commonly found throughout the freshwater drainages of the eastern seaboard of the United States and Canada. Some representative species that can be found in Rhode Island are as follows:

Elliptio complanata--This is probably the most common freshwater mussel in the state. It grows to about 3 inches in length and is found in many streams and freshwater ponds. ec_fs.gif (2890 bytes)
pc1_fs.gif (3160 bytes) Pyganodon cataracta--This is a common freshwater mussel found in rivers and streams throughout Rhode Island. It often grows to greater than 10 centimeters (cm) in length. It prefers slow-moving waters and is often found in stream impoundments.
Anodonta implicata--This freshwater mussel is found in coastal
freshwater drainages along the south shore of Rhode Island. It is similar to P. cataracta in size and shape, but it has a distinctive shell thickening at the outer margin.
ai_fs.gif (1834 bytes)
ms_fs.gif (2624 bytes) Musculium securis--This is a common species of fingernail clam
that grows to no more than 1/4 inch and occurs in many different
aquatic habitats, including temporary "vernal" ponds.
Lampsilis radiata--This mussel prefers the larger rivers and ponds in the state, and is locally abundant in Watuppa Pond along the eastern Rhode Island border with Massachusetts. lr_fs.gif (3428 bytes)
ss_fs.gif (4191 bytes) Sphaerium simile--This fingernail clam grows to a maximum
size of about 3/4 inch, is found throughout the state, and can
be locally abundant in backwaters and slow streams.
Pisidium compressum--This fingernail clam very rarely exceeds
1/4 inch in length and is very common in permanent freshwater
ponds throughout Rhode Island.
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The native freshwater bivalves are found in the sediments of streams or lakes. Their shells can be partially exposed, as in the case of Elliptio or other freshwater mussels, or completely buried - allowing breathing through tiny siphons that extend to the sediment surface - as in the case of the fingernail clams.

cf_fs.gif (3503 bytes) One potential invader to Rhode Island's freshwaters that also inhabits sediments is Corbicula fluminea, that is native to the Far East. As of 1991, this Asian clam had established itself in the lower Connecticut River, causing concern to Connecticut's state conservation agency. The Asian clam has a heavier shell than most native bivalves, grows to about an inch in length, and has a black exterior coat (periostracum) on the shell.


zeb_fs.gif (4899 bytes) Zebra mussels, Dreissena polymorpha, are different from the native bivalves and the Asian clam in that they tend to settle on and encrust hard surfaces, such as rocks or man-made structures, by adhesive threads called byssal fibers. Zebra mussels can colonize sediment surfaces by initially attaching to a hard surface, such as an exposed rock, with subsequent generations attaching themselves to the shells of the older mussels. Zebra mussels rarely exceed one inch in length, and often have banded markings on their shells.


Further Information About Native Freshwater Bivalves

Baker, F.C. (1928). The fresh water mollusca of Wisconsin. Part II. Pelecypoda. Bulletin of the Wisconsin Geological and Natural History Survey 70:1-482.

Johnson, R.I. (1980). Zoogeography of North American Unionacea (Mollusca: Bivalvia) north of the maximum Pleistocene glaciation. Bulletin of the Museum of Comparative Zoology (Harvard University) 149:77-189.

Lefevre, G. and W.C. Curtis. (1910). Studies on the reproduction and artificial propagation of freshwater mussels. Bulletin of the United States Bureau of Commercial Fisheries 30:105-201.

Smith, D.G. (1991). Keys to the Freshwater Macroinvertebrates of Massachusetts. Department of Zoology, University of Massachusetts, Amherst. 236pp.

R.I. Department of Environmental Management Fish & Wildlife staff: http://www.dem.ri.gov/programs/bnatres/fishwild/staff.htm

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