Lasmigona costata (Rafinesque, 1820)
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Basis for Listing
The fluted-shell is a relatively widespread but uncommon species in Minnesota, occurring in the Red, Minnesota, St. Croix and Mississippi (below St. Anthony Falls) river drainages. Grier (1922) reported low numbers from the Mississippi River, where it represented less than 1% of the mussel fauna. Bright et al. (1990) characterized the fluted-shell as once "sparingly established" in the Minnesota River (although they found only dead shells at 13 sampling stations), and on the verge of extirpation from the entire drainage. Cvancara (1970) found it present in the Red River drainage. There are recent reports of live specimens from the Cannon, Clearwater, Ottertail, Root, Zumbro, and Mississippi rivers (Bright et al. 1988, 1994; Hart 1995, 1999), but it is only common in the St. Croix River, where Doolittle (1988) found live specimens at many sampling points throughout the river. The species' perilously low numbers make it vulnerable to catastrophic events. For this reason, the fluted-shell was listed as a special concern species in Minnesota in 1996.
The shell of the fluted-shell mussel is elongate, moderately thick, compressed, and up to 18 cm (7 in.) long. The shell usually has numerous ridges (flutes) on the posterior slope (sometimes reduced or unnoticeable), and a beak sculpture of 2-4 coarse ridges that are drawn up slightly in the middle. The outside of the shell is greenish brown to dark brown, and sometimes has green rays. The pseudocardinal teeth are present, and the lateral teeth are poorly developed. The inner shell is white, sometimes cream or salmon in the beak cavity. The fluted-shell's posterior ridges and beak sculpture separate it from the creek heelsplitter (Lasmigona compressa) and the white heelsplitter (L. complanata).
The fluted-shell prefers habitats of medium to large rivers (Cummings and Mayer 1992), dominated by gravel substrates in areas with swift currents and water that is at least 0.6 m (2 ft.) deep (Hart 1995).
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
Degradation of mussel habitat in streams throughout the known range of the fluted-shell is a continuing threat to this species. Decline in habitat conditions is associated with management of the Mississippi River as a navigational canal, and with non-point source water pollution and sediment pollution. Dams, channelization, and dredging increase siltation, physically alter habitat conditions, and block the movement of fish hosts. Further survey work in medium sized rivers is needed to verify the fluted-shell's status in the rest of its former range.
Conservation Efforts in Minnesota
A 10-year statewide mussel survey initiated by the Minnesota DNR in 1999 resulted in a better understanding of the fluted-shell's ecology and current status in Minnesota. Hart's (1999) survival study of fluted-shell mussels is ongoing, and recovery of marked mussels is scheduled to continue so that accurate measurements of the long-term mean annual survival rate of this species can be determined.