Basis for Listing

The Fawnsfoot (Truncilla donaciformis) is a small, freshwater mussel that typically inhabits flowing waters of large rivers. This species historically occurred in the Mississippi, Minnesota, and St. Croix rivers (Sietman 2003) and was previously reported as one of the most abundant mussels in the Mississippi River (Fuller 1980). Based on results from an extensive, statewide survey initiated by the Minnesota DNR in 1999, the Fawnsfoot is currently uncommon in the Mississippi River, where its distribution has been shrinking since the 1970’s, and populations are greatly diminished in the Minnesota and St. Croix rivers (Sietman 2003). Threats to the Fawnsfoot include the continued loss and degradation of habitat, including dams that limit host movements and fragment rivers, non-point and point source pollution, siltation, and infestation of non-native Zebra Mussels (Dreissena polymorpha) in the Mississippi and lower St. Croix rivers. Given the substantial reduction in range and abundance of this species in Minnesota over the last several decades, in 2013, the Fawnsfoot was designated a threatened species. 

  Description

The Fawnsfoot is a small species with a stout, elliptical shell, moderately inflated, up to 5 cm (2 in.) long. Sexes are similar; however, males usually have a more sharply pointed posterior, whereas females have a more rounded or angled ventral margin. The outside surface of the shell is smooth; brown, yellowish, or greenish in color, and has V-shaped or zigzag markings with overlaying green rays.  Beak sculpture is inconspicuous, fine, double loops. The pseudocardinal and lateral teeth are well developed and the color of the shell interior (nacre) is white.  The Fawnsfoot is similar to the Deertoe (Truncilla truncata), but is less triangular, and the posterior ridge is not as sharply angled (Sietman 2003).

  Habitat

In Minnesota, the Fawnsfoot occurs in flowing areas of large rivers in soft or coarse substrate, and they have been found at depths up to 9 m[MD1]  (30 ft.).  The species’ range has expanded above St. Anthony Falls, a historical barrier on the Mississippi River that is now inundated by a lock and dam, northward to Coon Rapids dam.

  Biology / Life History

For most of their lives, mussels are sedentary filter feeders that live buried in the sediments of permanent rivers or lakes. Mussels burrow into the substrate and can move around slowly with the use of their foot, a hatchet shaped muscle that is extended out between the valves. A mussel will insert its foot into the sediment and then contract it to pull itself along the bottom of its aquatic habitat. Mussels frequently live in dense aggregations called “mussel beds” that contain 10-30 different species.

Mussels eat by filtering out small, organic particles including bacteria, algae, and fungus from the water. They draw in water through an incurrent siphon, remove food and oxygen with their gills, and expel the water and waste through their excurrent siphon. Food particles are carried to the mussel's mouth by tiny, hair-like cilia on the gills (Cummings and Graf 2014).

Like all native mussels, the Fawnsfoot has a complex and distinctive [BS1] life cycle [BS2] (Haag 2012). Fertilized eggs are brooded in the female's outer gills, where they develop into tiny clam-shaped larvae called “glochidia”. The Fawnsfoot is thought to be a long-term larval brooder (Williams et al. 2008; Haag 2012), but mature glochidia have only been encountered during early summer (Surber 1912; Utterback 1916a, b; Minnesota DNR unpublished data), suggesting it is in fact a short-term brooder. This is consistent with a Minnesota DNR study that found the closely related Deertoe to spawn in spring and hold glochidia for only about a month. To complete development, the glochidia must attach to the gills or fins of a suitable host fish.  While attached to the host, the glochidia undergo an anatomical metamorphosis where adult structures develop including paired adductor muscles, gills, a foot, and a digestive system. When juveniles are fully developed, they release from the fish, fall to the substrate, and begin life as free living mussels (Haag 2012).

Freshwater Drum (Aplodinotus grunniens) is presumed to be the exclusive host for Fawnsfoot (Haag 2012). The host infection strategy used by Fawnsfoot is unknown, but is probably similar to the Deertoe.  Gravid female Deertoe emerge from the substrate to varying degrees, often laying on their side, with their mantle edges extended to shield a prominent gape between the valves (Minnesota DNR, unpublished data). When this “lure” is touched, the female retracts the mantle to expose the gravid gills, which presumably occurs during an attack by the host fish. Furthermore, large drum are well known predators on freshwater mussels, and it has been suggested that drum become infested with glochidia by consuming gravid female Truncilla (Howard 1914; Barnhart et al. 2008; Haag 2012), a hypothesis known as “female sacrifice”.  Fawnsfoot is among the relatively few species with miniature glochidia (about 60 microns, or < 1/10th of a millimeter (< 4/1000th of an inch) that grow substantially while attached to the host, increasing by over 200% (Minnesota DNR, unpublished data)

The lifespan of mussels varies widely among species, with some attaining advanced ages of many decades to over a century. The Fawnsfoot is estimated to reach sexual maturity by age-two and maximum age reported is eight-years (Haag 2012).

  Conservation / Management

Because mussels are sedentary animals with a complex life cycle, they are sensitive to a variety of environmental disturbances. The glochidia and newly transformed juveniles are especially vulnerable. Pollution and habitat degradation throughout the species’ range are ongoing threats. Of particular concern is erosion and sediment pollution in large rivers that degrade mussel habitat, and dams that fragment populations by limiting host fish movements and create areas of unsuitable habitat, thus increasing the likelihood of local extinctions.

  Conservation Efforts in Minnesota

The MN DNR Mussel Program continues to survey streams in Minnesota to locate new populations of the Fawnsfoot, and monitor survival and recruitment in existing populations. 

The DNR has field guides available to help identify freshwater mussels occurring in Minnesota (Sietman 2003) and the Mississippi River (Tiemann et al., 2015). Additional information on the DNR’s mussel program is found here (http://dnr.state.mn.us/mussels/index.html). 

  Authors/Revisions

Bernard E. Sietman (MNDNR), 2018

(Note: all content ©MNDNR)

  References and Additional Information

Barnhart, M. C., W. R. Haag, and W. N. Roston. 2008. Adaptations to host infection and larval parasitism in the Unionoida. Journal of the North American Benthological Society 27(2):370-394.

Cummings, K. S., and D. L. Graf. 2014. Mollusca: Bivalvia. Pages 423-506 in J. Thorp and D. C. Rogers, editors. Ecology and General Biology: Thorp and Covich's Freshwater Invertebrates. Academic Press, Cambridge, Massachusettes.

Fuller, S. L. H. 1980. Fresh-water mussels (Mollusca: Bivalvia: Unionidae) of the Upper Mississippi River: observations at selected sites within the 9-foot navigation channel project for the St. Paul District, U.S. Army Corps of Engineers, 1977-1979. Report No. 79-24F. Academy of Natural Sciences of Philadelphia, Division of Limnology and Ecology, Philadelphia, Pennsylvania. 175 pp.

Haag, W. R. 2012. North American freshwater mussels: natural history, ecology, and conservation. Cambridge University Press, New York, New York. 538 pp.

Howard, A. D. 1914. Experiments in propagation of fresh-water mussels of the Quadrula group. Appendix IV to the Report of the U.S. Commissioner of Fisheries for 2013. Bureau of Fisheries Document No. 801, Washington, D.C. 51 pp. + VI plates.

NatureServe. 2009. NatureServe Explorer: an online encyclopedia of life [web application]. Version 7.1. NatureServe, Arlington, Virginia. <http://www.natureserve.org/explorer>. Accessed 12 June 2009.

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

Surber, T. 1912. Identification of the glochidia of freshwater mussels. U.S. Bureau of Fisheries Document No. 771. Washington, D.C. 10 pp. + III plates.

Tiemann, J., S. McMurray, B. Sietman, L. Kitchel, S. Gritters, and R. Lewis. 2015. Freshwater mussels of the Upper Mississippi River. Third edition. Upper Mississippi River Conservation Committee. 68 pp.

Utterback, W. I. 1916a. The naiades of Missouri. VI. American Midland Naturalist 4(9):387-400.

Utterback, W. I. 1916b. Breeding record of Missouri mussels. Nautilus 30(2):13-21.

Willams, J. D., A. E. Bogan, and J. T. Garner. 2008. Freshwater mussels of Alabama and the Mobile Basin in Georgia, Mississippi and Tennessee. Universiy of Alabama Press, Tuscaloosa. 960 pp.