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.

  Description

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).

  Habitat

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).

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. The fluted-shell is bradytictic, with females brooding their young long-term, from August through May, before they are released as glochidia (Baker 1928). 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. Several species of fish serve as suitable fish hosts for the glochidia of the fluted-shell, including bowfin (Amia calva), northern pike (Esox lucius), bluegill (Lepomis macrochirus), largemouth bass (Micropterus salmoides), yellow perch (Perca flavescens), walleye (Stizostedion vitreum), gizzard shad (Dorosoma cepedianum), and river redhorse (Moxostoma cartinatum) (Weis and Layzer 1995). Hart (1999) has reported that like other long-lived organisms, the fluted-shell has high mean annual survival rates (>99%).

  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.

  References and Additional Information

Baker, F. C. 1928. The fresh water mollusca of Wisconsin: part II: Pelecypoda. Wisconsin Geological and Natural History Survey Bulletin No. 70, Part II. University of Wisconsin, Madison. 495 pp.

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.

Bright, R. C., E. Plummer, and D. Olson. 1988. A survey of the mussels of the Zumbro River drainage, southeastern Minnesota. Report submitted to Minnesota Department of Natural Resources. 27 pp. + tables, figures, and appendices.

Bright, R. C., T. Atkinson, and C. Gatenby. 1994. Survey of the mussels of the Otter Tail and Pelican rivers, Minnesota: the data. Report submitted to the Minnesota Department of Natural Resources. 191 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.

Cvancara, A. M. 1970. Mussels (Unionidae) of the Red River Valley in North Dakota and Minnesota. Malacologia 10(1):57-92.

Davis, M. 1987. Freshwater mussels (Mollusca: Bivalvia: Unionidae) of the Cannon River drainage in southeastern Minnesota. Final report submitted to the Nongame Wildlife Program, Minnesota Department of Natural Resources. 21 pp. + figures and original data sheets.

Dawley, C. 1947. Distribution of aquatic mollusks in Minnesota. American Midland Naturalist 38:671-697.

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.

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.

Hart, R. A. 1995. Mussel (Bivalvia: Unionidae) habitat suitability criteria for the Otter Tail River, Minnesota. Thesis, North Dakota State University, Fargo, North Dakota. 60 pp.

Hart, R. A. 1999. Population dynamics of unionid mussels in Lake Pepin, Upper Mississippi River, Minnesota and Wisconsin. Dissertation, North Dakota State University, Fargo, North Dakota. 162 pp.

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

Weiss, J. L., and J. B. Layzer. 1995. Infestations of glochidia on fishes in the Barren River, Kentucky. American Malacological Bulletin 11(2):153-159.