Obovaria olivaria (Rafinesque, 1820)
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Basis for Former Listing
The hickorynut was historically found in the Minnesota and St. Croix rivers and the Mississippi River below St. Anthony Falls (van der Schalie and van der Schalie 1950), but it is now absent from the Minnesota River (Bright et al. 1990), rare in the St. Croix River (Doolittle 1988; Hornbach et al. 1995), and uncommon and scattered in the Mississippi River (Heath 1989). The range and abundance of this species and its host fish species have been reduced. For these reasons, the hickorynut was listed as a special concern species in Minnesota in 1996.
Basis for Delisting
The Minnesota DNR initiated a 10 year statewide mussel survey in 1999 which resulted in a better understanding of the current status and distribution of the hickorynut in Minnesota. Obovaria olivaria is common and widespread within the St. Croix River drainage. The Mississippi River from St. Anthony Falls down to Lake Pepin also supports a healthy population of hickorynut, which is attributed to improvements in water quality (Minnesota Department of Natural Resources 2012). The hickorynut could potentially be successfully reintroduced to the Minnesota River drainage if the habitat conditions and water quality improve. Because of the status of the current populations and the expansion of the distribution of the hickorynut, special concern status is no longer be necessary. The hickorynut was delisted in 2013.
The shell of the hickorynut is round or oval, thick, usually inflated, and up to 10 cm (4 in.) long. The beak is turned forward, and the beak sculpture consists of a few very fine, double-looped lines that are only visible on young mussels. The outside of the shell is greenish, tan, or brown, often with fine green rays. The pseudocardinal and lateral teeth are well developed. The left valve (shell) has 2 heavy, triangular-shaped pseudocardinal teeth, while the right valve has only 1 (Parmalee and Bogan 1998). The inside of the shell is white with a shallow beak cavity. Some individual hickorynuts may be confused with male Higgins eye (Lampsilis higginsii) specimens, but can be differentiated by the shape and alignment of the pseudocardinal teeth.
The hickorynut most often inhabits large rivers and is rarely found in smaller streams (Cummings and Mayer 1992). It is most often found in sand and gravel substrates in water depths that generally exceed 1.5-1.8 m (5-6 ft.) (Parmalee and Bogan 1998).
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. They spend most of their lives buried in the bottom sediments of permanent water bodies, and often live in multi-species communities called mussel beds (Sietman 2003).
Conservation / Management
The hickorynut in Minnesota is vulnerable to further declines from the hydrologic alteration of streams and their watersheds; 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 hickorynut is also being impacted by the infestation of non-native zebra mussels (Dreissena polymorpha) in the Mississippi River and its tributaries. Zebra mussels can attach themselves in large numbers to the shells of native mussels, eventually causing death by suffocation.
Conservation Efforts in Minnesota
A 10-year statewide mussel survey initiated by the Minnesota DNR in 1999 resulted in a better understanding of the hickorynut's ecology and current status in Minnesota. Additionally, 148 hickorynuts 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.
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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.
Cummings, K. S., and C. A. Mayer. 1992. Field guide to freshwater mussels of the Midwest. Illinois Natural History Survey Manual No. 5. 194 pp.
Doolittle, T. C. J. 1988. Distribution and relative abundance of freshwater mussels in the Saint Croix National Scenic Riverway. Final report submitted to the Natural Heritage and Nongame Research Program, Minnesota Department of Natural Resouces. Unpaged.
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