Basis for Listing

The Eastern Elliptio (Elliptio complanata) is an eastern North American species that reaches its western-most distribution in Minnesota. This species is ubiquitous and abundant in the Atlantic Slope region (Haag 2012) but is found only in portions of the Lake Superior watershed in Minnesota (Dawley 1947; Graf 1997a; Sietman 2003). Surveys have shown this mussel is abundant in the St. Louis-Lake Superior estuary but is less common and more sparsely distributed in nearby river habitat. Because the Eastern Elliptio appears to have immigrated to Minnesota recently (Baker 1928; Graf 1997b), they likely have the capacity to expand their range within the Lake Superior drainage unless barriers interrupt their dispersal. However, current populations are threatened by Zebra Mussel (Dreissena spp.) infestation in Lake Superior and the lower St. Louis River. Large numbers of Zebra Mussels can attach to native mussels, eventually causing death by suffocation and starvation. Because of its restricted range and vulnerability to Zebra Mussels, in 2013 the Eastern Elliptio was designated a species of special concern.

  Description

The Eastern Elliptio exhibits highly variable shell morphology and may represent a species complex (Williams et al. 2008). It has a stout, elongate and semi-rectangular shell up to 13 cm (5 in.) long. The shell is usually compressed (laterally flattened) and has a well-developed posterior ridge extending from the umbo to the posterior margin. The outside surface of the shell is smooth (lacks ridges or pustules), light to dark brown in color, with a cloth-like or matte finish; fine green rays are present in young individuals. Beak sculpture is 2 or 3 coarse angled ridges. The pseudocardinal and lateral teeth are well developed, the beak cavity is shallow, and the color of the shell interior (nacre) is whitish or purple. Co-occurring species that are somewhat similar in appearance to the Eastern Elliptio are the Fat Mucket (Lampsilis siliquoidea), Black Sandshell (Ligumia recta), and Giant Floater (Pyganodon grandis). The Eastern Elliptio is distinguished from these species by its cloth-like periostracum (outermost shell layer), well developed posterior ridge, and coarse angled beak sculpture.

  Habitat

The Eastern Elliptio is a habitat generalist; it occurs in both lakes and streams and can be found in a variety of substrate types (Matteson 1948; Balfour and Smock 1995). In Minnesota, it occurs only in the Lake Superior drainage; the lower St. Louis River, Lake Superior estuary, and Pigeon River, and probably in the littoral zone of Lake Superior. It is relatively abundant in the Lake Superior estuary, where it outnumbers other native species despite heavy Zebra Mussel infestation.

  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 frequently live in dense aggregations called “mussel beds” that contain 10-30 different species. 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. Balfour and Smock (1995) found that more than 90% of Eastern Elliptio burrowed below the substrate surface in December and January. The lifespan of mussels varies widely among species, with some attaining advanced ages of many decades to over a century (Haag 2012). The lifespan of the Eastern Elliptio may exceed 30 years (Kesler and Bailey 1993), though this is probably uncommon.

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 Eastern Elliptio has a fascinating and complex life cycle [BS2] (Haag 2012). Spawning occurs in late April to late May (Matteson 1948); fertilized eggs are brooded in the female's outer gills, where they develop into tiny clam-shaped larvae called “glochidia”. Since the Eastern Elliptio is a short-term brooder, glochidia are held in the gills for only about a month before they are released (Matteson 1948). To complete development, the glochidia must attach to the gills of a suitable host fish for a period of about 2 – 4 weeks. During this time, they 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).

Mussels employ a variety of strategies to transfer their glochidia to the host, many of which involve elaborate examples of food mimicry to attract a suitable host. Female Eastern Elliptio broadcast glochidia in a mucus strand or web (Lellis et al. 2013) that entangles passing fish and allows glochidia to attach to gills. Several fish species have been identified as hosts for the Eastern Elliptio in laboratory trials (reviewed by Lellis et al. 2013); American Eel (Anguilla rostrata) and Yellow Perch (Perca flavescens) appear to be important hosts. American Eel is apparently rare in the Lake Superior drainage so it is likely that Yellow Perch (in particular), sunfish (Lepomis spp.), black bass (Micropterus spp.), and Banded Killifish (Fundulus diaphanus) are hosts in Minnesota.

  Conservation / Management

Because mussels are sedentary animals with a complex life cycle, they are sensitive to a variety of environmental disturbances. Glochidia and newly transformed juveniles are especially vulnerable. The abundance of Zebra Mussels in the Lake Superior estuary and lower St. Louis River is a significant threat to the Eastern Elliptio, though it appears to be surviving and reproducing, possibly because winter burrowing can reduce infestation by Zebra Mussels. Pollution, habitat degradation, and dams are ongoing threats to the Eastern Elliptio and all mussel species in general. 

  Conservation Efforts in Minnesota

The DNR completed surveys of the Lake Superior watershed over 10 years ago, providing a baseline of the Eastern Elliptio’s distribution and abundance. Additional surveys in the future will evaluate distribution and population trends for this species.

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, MN DNR, 2018

(Note: all content ©MNDNR)

  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.

Balfour, D. L., and L. A. Smock. 1995. Distribution, age structure, and movements of the freshwater mussel Elliptio complanata (Mollusca: Unionidae) in a headwater stream. Journal of Freshwater Ecology 10(3):255-268.

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.

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

Graf, D. L. 1997b. Northern redistribution of freshwater pearly mussels (Bivalvia: Unionoidea) during Wisconsin deglaciation in the southern Glacial Lake Agassiz Region: a review. American Midland Naturalist 138(1):37-47.

Graf, D.L. 1997. Distribution of Unionoid (Bivalvia) faunas in Minnesota, USA. The Nautilus 110 (2): 45-54.

Graf, D. L., and J. C. Underhill. 1997. The western Lake Superior freshwater mussel (Bivalvia: Unionidae) community and its origin. Occasional Papers on Mullusks Vol. 5, No. 74:409-417. Department of Mollusks, Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts.

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

Kesler, D. H., and R. C. Bailey. 1993. Density and ecomorphology of a freshwater mussel (Elliptio complanata, Bivalvia: Unionidae) in a Rhode Island lake. Journal of the North American Benthological Society 12(3):259-264.

Lellis, W. A., B. St. John White, J. C. Cole, C. S. Johnson, J. L. Devers, E. van Snik Gray, and H.S. Galbraith. 2013. Newly documented host fishes for the Eastern Elliptio Mussel Elliptio complanata. Journal of Fish and Wildlife Management 4(1):75-85.

Matteson, M. R. 1948. Life history of Elliptio complanatus (Dillwyn, 1817). American Midland Naturalist 40(3):690-723.

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.

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.

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.