Basis for Listing

The creek heelsplitter was once widespread and abundant in the Mississippi drainage north of St. Anthony Falls in Minnesota (Dawley 1947). While it still persists in a number of rivers in the state, it is present in low numbers. Doolittle (1988) reported the creek heelsplitter from only 1 sampling site on the St. Croix River, and Bright et al. (1995) found live individuals at only 5 sampling sites on the Chippewa River and 1 sampling site on the Pomme de Terre River in 1990. Bright et al. (1995) concluded from the distribution of live and dead shells in these rivers that the species was once more widely distributed than it is at present. Additionally, no recruitment was evident at any of their survey sites. The creek heelsplitter also persists in low numbers in the Cannon, Zumbro, and Ottertail rivers (Bright et al. 1988, 1994b), as well as in several northern rivers including the Mississippi River upstream of Little Falls, Minnesota (Bright et al. 1994a; Hove et al. 1995), and the St. Louis River system. Many of the streams from which Dawley (1947) reported the creek heelsplitter have not been recently surveyed. Given its decline and the degradation of its habitat, the creek heelsplitter was listed as a special concern species in Minnesota in 1996.

  Description

The shell of the creek heelsplitter is somewhat elongate or oblong, squared off at the posterior tip, moderately thin to stout, compressed, and up to 13 cm (5 in.) long. The outside of the shell is greenish or brown, and usually has green rays. The beak sculpture consists of 5-8 distinct double-looped ridges. The pseudocardinal teeth are present but somewhat reduced, and the lateral teeth are somewhat developed. The inside of the shell is usually white, but it can be cream or salmon colored. The creek heelsplitter resembles the fluted-shell (Lasmigona costata) and the white heelsplitter (L. complanata), but does not possess the flutes on the posterior slope of the shell like the fluted-shell, and is not rounded like the white heelsplitter.

  Habitat

The creek heelsplitter typically occurs in creeks, small rivers, and the upstream portions of large rivers. Its preferred substrates are sand, fine gravel, and mud (Clarke 1985). Baker (1928) noted that the creek heelsplitter most often colonizes areas downstream of riffles in small pools, and described the habitats used as characterized by swift currents and water depths ranging from 0.3-0.9 m (1-3 ft.) deep.

  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. Generally, males release sperm into the water, which are drawn in by females through their incurrent siphon, but the creek heelsplitter has been reported to be hermaphroditic (having both female and male reproductive organs) (van der Schalie 1970). Fertilized eggs are brooded in the mussel's gills, where they develop into tiny larvae called glochidia. Clarke (1985) and Baker (1928) described the gravid period of the creek heelsplitter as lasting from August to June. Once the glochidia are expelled, 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. Host fish species for the glochidia of the creek heelsplitter are yellow perch (Perca flavescens), black crappie (Pomoxis nigromaculatus), slimy sculpin (Cottus cognatus), and the spotfin shiner (Cyprinella spiloptera).

  Conservation / Management

Degradation of mussel habitat in streams throughout the creek heelsplitter's known range is a continuing threat to this species. Like most other freshwater mussels, this species is vulnerable by virtue of its dependence on fish hosts for the metamorphism of its larvae (Neves 1993). Williams et al. (1993) considered the creation of dams and the ensuing siltation, dredging, and change in water flow to be the most important threat leading to the extinction and extirpation of freshwater mussels. Impoundments change stream flows, which can in turn increase sedimentation rates and affect the physiological processes of mussels. Dams may also eliminate a mussel's requisite host fish species. Competition from other fish species may also locally reduce or eliminate the required host fish species from the environment. As a consequence, if the host species disappears from the river or stream, the mussel species will soon be extirpated from the habitat as well. To prevent the extirpation of mussel species, both the mussel and the host fish species must be preserved.

  Conservation Efforts in Minnesota

A 10-year statewide mussel survey initiated by the Minnesota DNR in 1999 resulted in a better understanding of the creek heelsplitter's ecology and current status in Minnesota.

  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, R. Heisler, E. Plummer, K. Stramer, and W. Ostlie. 1995. A survey of the mussels of the Pomme de Terre and Chippewa rivers, Minnesota, 1990. Final report submitted to the Natural Heritage and Nongame Research Program, Minnesota Department of Natural Resources. 131 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., R. Heisler, S. Breidenbach, and D. Rocha. 1994. Survey of the mussels of the Camp Ripley portions of the Mississippi and Crow Wing rivers, Minnesota: the data. Report submitted to the Minnesota Department of Natural Resources. Unpaged.

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.

Clarke, A. H. 1985. The tribe Alasmidontini (Unionidae: Anodontinae), Part II: Lasmigona and Simpsonais. Smithsonian Contributions to Zoology No. 399. 75 pp.

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.

Hove, M., R. Engelking, and C. Freiburger. 1995. Qualitative bivalve survey of the Sandy River drainage, Minnesota. Report submitted to the Natural Heritage and Nongame Research Program, Minnesota Department of Natural Resources. 39 pp.

Neves, R. J. 1993. A state-of-the-unionids address. Pages 1-10 in K. S. Cummings, A. C. Buchanan, and L. M. Koch, editors. Conservation and management of freshwater mussels. Proceedings of a UMRCC Symposium, 12-14 October 1992, St. Louis, Missouri. Upper Mississippi River Conservation Committee, Rock Island, Illinois. 189 pp.

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

van der Schalie, H. 1970. Hermaphroditism among North American freshwater mussels. Malacologia 10(1):93-112.

Williams, J. D., M. L. Warren, Jr., K. S. Cummings, J. L. Harris, and R. J. Neves. 1993. Conservation status of freshwater mussels of the United States and Canada. Fisheries 18(9):6-22.