AIM

Bivalvia (Pelecypoda)

Introduction

Clams and mussels. The second largest group of mollusks. All species are aquatic, most are marine. About 264 species occur in the freshwater of North America. The list of Montana species can be found here.

Recognition

Laterally compressed mollusks with a bivalved shell with a dorsal hinge. The head is poorly developed. The mantle cavity is capacious, the gills are large and serve also in feeding.

Life Cycle

Marine forms normally have external fertilization and free-swimming trochophore and veliger larvae, freshwater forms usually shed the sperm into the water, but the eggs are fertilized and brooded within the gill cavity. The freshwater fingernail clams (Sphaeriidae) are hermaphroditic and self-fertile, and they have direct development with young clams shed from the gills. Most species complete the live cycle in a month to a year. Freshwater mussels (Unionidae) are mostly dioecious. Their eggs are brooded until the glochidia larval stage is achieved. The glochidia are parasitic on fish or aquatic amphibians. The glochidia transforms to a young mussel in a few days to few weeks and drops to the bottom to begin feeding as an adult. Breeding may be limited to the spring or throughout most of the year. Eggs may be brooded in all 4 gills (Ambleminae) or only the 2 outer gills (Unioninae). Adults may hibernate or remain active throughout the winter. It may take several years to reach maturity. Adults may live for many years.

Trophic Roles

Uniformly collectors using the large gills to filter organic particles from the water. Under many conditions, most of the food is obtained from the substrate. They are able to take very small particles, and use dissolved organic compounds. They are sensitive to siltation, and they may bio-accumulate toxins.

Communities

Relatively simple in our area in both lentic and lotic waters. Some occur in deep water. Some Sphaeriidae may have very large populations. The species are separated primarily by habitat (depth, current, and substrate) and temperature. More diverse communities occur in non-impacted waters of the Southeast. The least diverse occur at the headwaters of the continent.

Economic Importance

Important in some food chains involving fish and wildlife, many important marine fisheries, but freshwater species are rarely eaten by humans. Formerly important in the button industry, but now used mostly for cultured pearl nuclei, many populations are over-harvested. Montana has no populations of economic importance. The introduced Corbicula and Dreissena may be a serious nuisance in water systems. Either or both of these might establish in Montana, but neither is likely to invade the suitable habitat within the state directly. Dreissena is also a macrofouling organism, that may have tremendous impact on other organisms and water quality. Glochidial infections may cause mortality of fish in hatcheries, but cause little damage in nature. Sphaeriidae may be good acid rain indicators.

Zoogeography

The introduced Corbicula and Dreissena can have a free-swimming larva unknown among our native bivalves, both species have spread rapidly since first introduced. Sphaeriids are distributed by habitat and generally widespread. Unionids are distributed by drainage, often with interesting longitudinal patterns with some species very local. The glochidia can be distributed by migrating or transplanted fishes. Altering fish communities directly alters mussel communities.

Conservation

Much of the diverse freshwater mussel fauna of North America is threatened in several ways. Dams, channel alterations, pollution, alteration of fish communities, and direct overharvest have all had affects, but now the zebra mussel is the biggest threat. These may attach to the native mussels in large numbers and even seal the shell closed or force it open by growing on the inside edges. Over 40 species of unionids are currently listed as endangered, many more are proposed for listing, 13 species are thought already extinct.

Collecting

Sphaeriids are found by dredging up the surface layers of the sediment, and then separated from the sediment by sieving and panning. Unionids are usually picked up by hand, working when the water is low and clear, diving may be necessary. A variety of drags and dredges can be used in deep or turbid water.

Preservation

A good pelecypod collection consists of two parts- soft parts and shells.

  1. Soft parts- Large specimens are best killed by freezing or heating and then carefully removed from the shell by cutting the two adductor muscles by sliding a thin knife in against the shell. The animals are then fixed in Kahle's or Bouin's fluid, or 10% formalin. Small specimens can be narcotized within the shell by keeping them alive in a clean container using the water of origin, and adding a few crystals of menthol. After several hours, when the animals do not respond to touching, the water may be carefully removed and replaced with Kahle's or Bouin's fluid. These acidic fluids will damage or destroy the shell. The animals are stored in 70% ethanol.
  2. Shells- The shells are best preserved dry. If they are large and alive the animal should be removed as above. Small whole shells can be soaked in neutral 10% formalin or 70% ethanol for a day or week. The shells should be cleaned with ammonia water if necessary, rinsed with fresh water and allowed to throughly dry. Many unionid shells are then dipped in an organic solvent containing dissolved paraffin to help prevent cracking.

Taxonomy

Early attention produced a sever overabundance of names based on small variations in the shells. Some groups remain difficult. Soft parts are used to define genera and higher taxa, and shell characteristics are used to define species and subspecies. Most shells can be identified to species with experience, but they cannot be gotten directly through most keys. Sexual dimorphism in the shells can be confusing.

Some Key References

  • Burch, J.B. 1975. Freshwater Unionacean Clams (Mollusca: Pelecypoda) of North America. Malacological Publications, P.O. Box 4115, Ann Arbor, Michigan. 365pp.
  • Burch, J.B. 1975. Freshwater Sphaeriacean Clams (Mollusca: Pelecypoda) of North America. Malacological Publications, P.O. Box 4115, Ann Arbor, Michigan. 365pp.
  • Turgeon, D.D. et al. 1988. Common and scientific names of aquatic invertebrates from the United States and Canada: Mollusks. American Fisheries Society Special Publication 16.

Synopsis Of Nearctic Families

  • Unionoidea- freshwater mussels or naiads, hinge ligament external, pseudocardinal teeth at the anterior end of the hinge (sometimes lacking), lateral teeth occur on the posterior half of the hinge (sometimes lacking), shell with a nacreous layer.
    • Unionidae- the 2 subfamilies are often treated as families.
      • Margaritiferinae- mantle without supra-anal openings or a thickened area between the excurrent and incurrent chambers, shells elongate and compressed, 2 genera and four species.
      • Unioninae- mantle with supra-anal openings and a thickened area between the excurrent and incurrent chambers, shells diverse, 44 genera and 223 species.
  • Corbiculoidea- (Sphaeroidea) freshwater clams, hinge ligament external, cardinal teeth at the center of the hinge (usually with 2 on the left and 1 on the right), lateral teeth occur on both sides of the cardinal teeth (single on the left valve and double on the right valve), shell without nacreous layer.
    • Sphaeriidae- fingernail clams, pea clams, or pill clams, lateral teeth smooth, 2-20 mm, 4 genera with 33 native and 4 introduced species.
    • Corbiculidae- lateral teeth serrated, up to 60 mm, 1 introduced species.
  • Dreissenoidea- marine, brackish and freshwater mussels, mytiliform (attached with a byssus), hinge ligament internal, hinge without distinct teeth, shell without nacreous layer.
    • Dreisseniidae- most species occur in brackish water. One introduced species, the zebra mussel Dreissena polymorpha (Pallas) in the Great Lakes and surrounding areas.

Other sites

17 DEC 1995, updated on 08 FEB 1997 D.L. Gustafson


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dlg@rivers.oscs.montana.edu