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Dwarf Wedge Mussel

Walking along a small creek in Queen Anne's County, Maryland, you may notice an odd pattern in the sandy-mud bottom at the base of the stream bank. These singular, squiggly lines are not made by a tiny water snake or worm. These are the tracks of an exquisite and rare shellfish, known as the Dwarf Wedge Mussel (Alasmidonta heterodon).

History
Historically, the Dwarf Wedge Mussel has been found at 70 sites from New Brunswick to North Carolina. Today, it is confined to only 10 known sites, three of which are in Maryland. These three sites include two creeks in Queen Anne's County and one in St. Mary's County.

From the late 1800s to the beginning of World War II, many species of freshwater mussels were harvested for the pearl button industry. A hooked dredge was dragged along the bottom of a stream, and the hooks of the dredge would lodge between the mussels' open shells. The mussels were steamed to remove the meat, and the shells were cleaned and ground to make buttons. Thousands of tons of shells were processed. But in the mid-1940s, with the development of usable plastics, the pearl button industry ceased.

In the 1950s, another industry began to utilize freshwater mussels. The Japanese discovered that beads made from crushed mussel shells were excellent nuclei for the formation of cultured pearls. Freshwater mussels again began to be harvested commercially and the shells exported to Japan. The midwestern and southeastern U.S. continue to supply most of the mussel shells used by the Japanese cultured pearl industry today.

Life Cycle
Like other freshwater mussels, the Dwarf Wedge Mussel is one of the few species of mussel that reproduce sexually. Eggs, carried in the gills of the female, are fertilized as sperm-laden water passes through the gills. The female may carry as many as several thousand eggs at one time. Within the female's gills, these fertilized eggs develop into larvae, called glochidia, resembling miniature adults. The glochidia are released into the water and, in order to continue growth, must attach themselves to the tissue of the host fish, where they develop into juvenile mussels. At the end of the parasitic stage, the juvenile mussels detach themselves from their host, leaving the fish unharmed, and sink to the bottom of the stream where they continue to develop.

Chart: Dwarf Wedge Mussel Life Cycle

Reasons for Decline
The decline of the Dwarf Wedge Mussel is mainly due to degradation of its habitat. The Dwarf Wedge Mussel has very specific habitat requirements. It needs a stable, silt-free stream bed and well-oxygenated water, free of pollutants.

Pollutants such as chemical fertilizers and heavy metals from agriculture and industry threaten this species. Water runoff contains excess nutrients, which cause low oxygen levels. Sediment carried in the runoff may fill in the stream bed. These conditions produce a habitat that is unsuitable for the Dwarf Wedge Mussel.

The presence of the correct host fish is also crucial to the Dwarf Wedge Mussel. Many scientists believe the host is an anadromous fish, one that must migrate from the ocean into freshwater to spawn. Damming and channelization not only alter the stream bed, but may prevent the host fish from reaching the mussel larvae. Without this host fish, the Dwarf Wedge Mussel cannot complete its life cycle, and extinction is inevitable.

Why Is the Dwarf Wedge Mussel Important?
The Dwarf Wedge Mussel is an integral part of its habitat. It contributes to the biological diversity of our environment. As in all natural systems, the disappearance of one species can upset the balance of the whole system. Because of its extreme rarity, this species is classified as a State Endangered Species, one that is in jeopardy of extinction in Maryland. The other populations of Dwarf Wedge Mussel, located in New England and North Carolina, are also rapidly disappearing. The U.S. Fish and Wildlife Service has also listed the Dwarf Wedge Mussel as being Endangered.

There is another, more practical, and equally important reason why this species must be protected. The Dwarf Wedge Mussel is important as an "Indicator Organism." This is a species whose presence or absence is indicative of the relative health of a natural ecosystem. Coal miners employed this concept by lowering canaries, a species that is extremely sensitive to toxic gases, into mine shafts. If the bird was still alive when brought up, the miners knew the shaft was safe for humans. Likewise, the Dwarf Wedge Mussel is a gauge for water quality. The continuing existence of the Dwarf Wedge Mussel in Maryland creeks indicates that these watercourses are still relatively healthy and clean. By ensuring the continued survival of the Dwarf Wedge Mussel, other aquatic resources are also maintained in good health.