Synonyms

Megalonaias gigantea

  Basis for Listing

The washboard is a large river species historically found in the Minnesota and St. Croix rivers and in the Mississippi River below St. Anthony Falls (Grier 1922; van der Schalie and van der Schalie 1950). It was likely always rare in the Minnesota River, where surveys by Bright et al. (1990) found only 2 dead shells and no live individuals. The washboard is now very rare in the Mississippi (Thiel 1981; M. Davis, Minnesota DNR, pers. comm.) and St. Croix rivers (Heath 1990), comprising only 1% of the mussels identified by Hornbach et al. (1995). The washboard was listed as a threatened species in Minnesota in 1996.

  Description

The shell of the washboard can reach up to 20 cm (8 in.) long, and it is rectangular with valves that are thick and heavy. The posterior wing often has numerous ridges, especially in younger individuals, and the beak sculpture is comprised of well-developed, double-looped ridges. The entire surface of the shell is heavily sculptured with fine ridges and folds in the first few years of growth. The ridges become more pronounced with age. The outside of the shell is brown or black, and rayless. The pseudocardinal and lateral teeth are heavy, and the inside of the shell is white. The beak sculpture distinguishes the washboard from the rock pocketbook (Arcidens confragosus) and the threeridge (Amblema plicata). Its thick shell and heavy teeth also help distinguish it from the rock pocketbook.

  Habitat

The washboard is typically a large river species, inhabiting the main channel areas of a stream. Suitable habitat consists of slow current areas with substrates composed of sand, gravel, or mud.

  Biology / Life History

Mussels are long-lived animals. Members of many species may live for several decades and in some instances, a century or more. The washboard may live to be more than 30 years old, and in a study in the upper Mississippi River, most washboards were found to be mature at 8 years of age (Woody and Holland-Bartels 1993).

Mussels spend most of their lives buried in the bottom sediments of permanent water bodies, and often live in multi-species communities called mussel beds. Since washboards are primarily a riverine species, they exist in clusters of populations and are usually found in mussel beds containing other species. Mussels are primarily sedentary, but they can move around with the use of their foot, which is a hatchet shaped muscle that can be extended out between the valves (shells). A mussel will burrow its foot into the sediment and then contract it to pull itself slowly along the bottom of its aquatic habitat (Sietman 2003).

Mussels eat by filtering bacteria, protozoans, algae, and other organic matter out of the water. They draw water into their body through their incurrent siphon, remove food and oxygen with their gills, and then expel the filtered water through their excurrent siphon. Food particles are carried to the mussel's mouth by tiny hairlike cilia located on the gills. Waste is expelled through the excurrent siphon (Sietman 2003).

Mussels have a complex and distinctive reproductive cycle. Males release sperm into the water, which are drawn in by females through their incurrent siphon. Fertilized eggs are brooded in the female's gills, where they develop into tiny larvae called glochidia. Studies in the upper Mississippi River suggest that the washboard is tachytictic, with females brooding their young short-term before they are released as glochidia in October. Once the glochidia are expelled from the female's gills, they attach to fish gills or fins by clamping onto them with their valves. The glochidia live as parasites on the host fish until they develop into juvenile mussels, at which point they detach from the fish and fall to the streambed as free-living mussels. Host fishes for the glochidia of the washboard have been verified as the green sunfish (Lepomis cyanellus), black bullhead (Ameiurus melas), and the channel catfish (Ictalurus punctatus) (Woody and Holland-Bartells 1993).

  Conservation / Management

Threats to the viability of remaining populations of washboard mussels found in Minnesota include the continuing decline in habitat conditions on the Mississippi River associated with its management as a navigation canal, and non-point and point source water and sediment pollution. Dams, channelization, and dredging increase siltation, physically alter habitat conditions, and block the movement of fish hosts. The washboard is also being impacted by the infestation of non-native zebra mussels (Dreissena polymorpha) in the Mississippi and St. Croix rivers and their tributaries. Zebra mussels can attach themselves in large numbers to the shells of native mussels, eventually causing death by suffocation. Additional impacts come from harvesting due to the high commercial value of washboard shells to the cultured pearl industry. If the effects of these factors cannot be mitigated, the washboard may become endangered in the near future.

  Conservation Efforts in Minnesota

A 10-year statewide mussel survey initiated by the Minnesota DNR in 1999 resulted in a better understanding of the washboard's ecology and current status in Minnesota. Additionally, over 600 washboard mussels were collected from zebra mussel infested habitats in the Mississippi River in 2000 and translocated into areas of the Mississippi River south of the Twin Cities, where habitats were devoid of zebra mussels.

  References and Additional Information

Bright, R. C., C. Gatenby, D. Olson, and E. Plummer. 1990. A survey of the mussels of the Minnesota River, 1989. Final report submitted to the Natural Heritage and Nongame Research Program, Minnesota Department of Natural Resources. 106 pp.

Grier, N. M. 1922. Final report on the study and appraisal of mussel resources in selected areas of the Upper Mississippi River. American Midland Naturalist 8:1-33.

Heath, D. J. 1990. Identification of distribution, abundance, and critical habitat for Lampsilis higginsi and Category 2 species of mussels - performance report - Ocober 1, 1987 to September 30, 1988. Wisconsin Endangered Resources Report #65. 11 pp. + tables and figures.

Hornbach, D. J., P. Baker, and T. Deneka. 1995. Abundance and distribution of the endangered mussel Lampsilis higginsi in the lower St. Croix River, Minnesota and Wisconsin. Final report submitted to the U.S. Fish and Wildlife Service, Minneapolis, Minnesota. 68 pp.

Sietman, B. E. 2003. Field guide to the freshwater mussels of Minnesota. Minnesota Department of Natural Resources, St. Paul, Minnesota. 144 pp.

Thiel, P. 1981. A survey of unionid mussels in the upper Mississippi River (pools 3-11). Technical Bulletin 124. Wisconsin Department of Natural Resources, Madison, Wisconsin. 24 pp.

van der Schalie, H., and A. van der Schalie. 1950. The mussels of the Mississippi River. American Midland Naturalist 44:448-464.

Woody, C. A., and L. Holland-Bartells. 1993. Reproductive characteristics of a population of the washboard mussel Megalonaias nervosa (Rafinesque 1829) in the upper Mississippi River. Journal of Freshwater Ecology 8:57-66.