Synonyms

Plagiola lineolata

  Basis for Listing

Based on the presence of old shells, the butterfly was once present in the Minnesota, St. Croix, and Mississippi rivers, but was comparatively rare, even in the 1900s (Coker 1919; Bright et al. 1990). The butterfly mussel is now uncommon in the St. Croix River, where it was found by Doolittle (1988) at only 2 sampling points. Hornbach et al. (1995) quantified it as less than 1% of all the mussel fauna in the St. Croix River. The butterfly is also rare in the lower reaches of the Mississippi River in southeastern Minnesota (Fuller 1985). In a 1995 survey of Mississippi River Pool 7, only 1 live butterfly mussel was found out of the more than 2,000 specimens examined (Davis and Hart 1995). The butterfly has apparently been extirpated from the Minnesota River (Bright et al. 1990). Given its low numbers and the vulnerability of its habitat, the butterfly mussel was listed as a threatened species in Minnesota in 1996.

  Description

The shell of the butterfly mussel is somewhat triangular, with rounded dorsal, anterior, and ventral margins. The shells of male butterfly mussels are compressed, while those of females are modestly inflated. The shell of both sexes is thick and up to 12.7 cm (5 in.) long. The outside of the shell has numerous interrupted brown rays and is generally yellowish, but can be brown in old specimens. The hinge ligament is sometimes dark green. The beak is turned forward and the beak sculpture consists of a few fine, double-looped lines, which are usually obscure. The pseudocardinal and lateral teeth are well developed and the inside of the shell is white. The butterfly does not closely resemble any other Minnesota mussel species.

  Habitat

The butterfly mussel usually inhabits areas of large rivers with swift currents in sand or gravel substrates. However, it appears that the butterfly has adapted to life in reservoirs in some southern states, where it is found in water depths up to 6 m (20 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).

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 butterfly mussel is bradytictic, with females brooding their young long-term from August through July 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. Known fish hosts for the glochidia of the butterfly mussel include sunfish (Lepomis spp.), sauger (Stizostedion canadense), and drum (Aplodinotus grunnieus) (Fuller 1978).

  Conservation / Management

The viability of remaining butterfly mussel populations in the Mississippi River is jeopardized by the continuing decline in habitat conditions associated with the river's management as a navigation canal, and with 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 butterfly 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 in large numbers to the shells of native mussels, eventually causing death by suffocation. Historically the butterfly mussel was harvested for use in the pearl button industry (Baker 1928), and today it is harvested in some portions of its North American range for use in the cultured pearl industry (Oesch 1984). If observed trends cannot be reversed, the butterfly mussel may become endangered within Minnesota in the 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 butterfly mussel's ecology and current status in Minnesota. Additionally, over 500 butterfly mussels were collected from zebra mussel infested habitats in the Mississippi River in 2000 and translocated into areas devoid of zebra mussels south of the Twin Cities.

  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.

Coker, R. E. 1919. Fresh-water mussels and mussel industries of the United States. Bulletin of the U.S. Bureau of Fisheries 36:13-89.

Davis, M. and R. Hart. 1995. Mussel habitat in the Richmond Island/Lock and Dam 6 tailwater area of Pool 7, Mississippi River, and its importance for recovery of the federally endangered mussel, Lampsilis higginsi. Final report submitted by the Minnesota Department of Natural Resources. 34 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.

Fuller, S. L. 1978. Fresh-water mussels (Mollusca: Bivalvia: Unionidae) of the Upper Mississippi River: observations of selected sites within the 9-foot channel navigation project on behalf of the United States Army Corps of Engineers (USACE). Report submitted to the USACE, No. 78-33. Academy of Natural Sciences of Philadelphia, Division of Limnology and Ecology, Philadelphia, Pennsylvania. 401 pp.

Fuller, S. L. 1985. Freshwater mussels of the Upper Mississippi River. I. Brynildson, editor. Wisconsin Department of Natural Resources, Madison, Wisconsin. 63 pp.

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.

Oesch, R. D. 1984. Missouri naiades: a guide to the mussels of Missouri. Missouri Department of Conservation. Jefferson City, Missouri. 270 pp.

Parmalee, P. W., and A. E. Bogan. 1998. The freshwater mussels of Tennessee. The University of Tennessee Press, Knoxville, Tennessee. 328 pp.

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