Megalonaias nervosa (Rafinesque, 1820)
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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.
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.
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).
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.