Utterbackiana suborbiculata Say, 1831
Basis for Listing
The Flat Floater (Utterbackiana suborbiculata) is distributed across parts of the central and eastern United States and is at the northern extent of its continental range in Minnesota. This species was not historically known from the state; it was first discovered in Minnesota in a backwater lake in Pool 8 of the Mississippi River by Havlik (1981). It has since been found at a handful of additional sites in Pools 9, 7, and 5A, and in Lake St. Croix of the St. Croix River near Stillwater, suggesting its range may be expanding in Minnesota as it has in other areas (Williams et al. 2008; Watters et al. 2009; Howells et al. 1996). Although these populations are apparently reproducing, fewer than 30 live individuals have been collected from the Minnesota portion of the Mississippi River, and only four live individuals have been collected from the St. Croix River (as of 2015). This species appears to be restricted mostly to backwater lakes with little or no incoming flow, a habitat that is limited in the Mississippi River due to sediment load filling. More surveys of backwater habitats in the Mississippi River are needed to establish the distribution, abundance, and ecology of this species in Minnesota. In 2013, the Flat Floater was designated a species of special concern.
The Flat Floater has a large and fragile sub-circular and laterally compressed (flat) shell, with a straight dorsal margin. The umbos (beaks) are flat and do not extend above the hinge line; they are sculptured, with two diverging rows of shallow tubercles or broken ridges resembling double loops. Teeth are absent, and the beak cavity is shallow. The outside of the shell is light brown or golden, smooth and glossy, sometimes with weak green rays. The inside of the shell (nacre) has a silvery, salmon, or orangish color. Length: up to 18 cm (7 in.), sexes are alike. The Giant Floater (Pyganodon grandis) is similar but is less round, more inflated, not glossy; umbos extend above the hinge line, and beak sculpture has more robust double loops. The Pink Papershell (Potamilus ohiensis) is also similar but usually has a more pronounced posterior and anterior wing, darker color, pinkish-purple nacre, and thin pseudocardinal and lateral teeth.
In Minnesota, the Flat Floater occurs primarily in silt or soft mud dominated substrates of backwater lakes with little or no incoming flow that are deep enough to prevent complete freezing in winter. The population in Lake St. Croix occurs in similar substrate at depths of up to 9 m (30 ft.). In other regions, the Flat Floater occurs in floodplain lakes, sloughs, oxbows, and reservoirs associated with large streams and rivers (Williams et al. 2008).
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, but isolated lentic (still, fresh water) habitats frequented by the Flat Floater are less diverse and contain fewer than 10 different species (Haag 2012). 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.
The Flat Floater, like other freshwater mussel species, has a fascinating and complex 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”. In Shannon Lake, Kansas, Flat Floater spawned from mid-September through mid-October, and glochidia were released by the end of February (Barnhart et al. 1996). Utterback (1915) observed gravid individuals in Missouri holding mature glochidia from December through March. This variation could be regional or due to differences in seasonal temperature curves. The spawning and larval brooding cycle has not been examined in Minnesota. To complete development, the glochidia must attach to the gills or fins of a suitable host fish, where they undergo an anatomical metamorphosis and 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. Flat Floater mussels have hooked glochidia, barbed spikes on the ventral margin of the larval shell that are adapted for attaching to fins and other soft external tissues. These glochidia are released in a mucus strand or web that entangles on passing fish and allow glochidia to attach to fins (Roberts 1997). Several fishes have been identified as hosts for the Flat Floater in laboratory trials, including Warmouth (Lepomis gulosus), White Crappie (Pomoxis annularis), Largemouth Bass (Micropterus salmoides), Golden Shiner (Notemigonus crysoleucas), and Mosquitofish (Gambusia affinis). Other fish species have been found with attached glochidia in the wild: Gizzard Shad (Dorosoma cepedianum), White Crappie, Bluegill (Lepomis macrochirus), Largemouth Bass, Golden Shiner, Freshwater Drum (Aplodinotus grunniens), and Brook Silverside (Labidesthes sicculus). Since glochidia are released in winter, when the water temperature is cold, the period of attachment to the host is likely prolonged. Roberts (1997) found that it took about 43 days for glochidia to transform into juveniles at 10 °C (50°F), 20 days at 15 °C (59°F), and only 9 days at 20 °C (68°F).
The lifespan of mussels varies widely among species, with some attaining advanced ages of many decades to over a century (Haag 2012). Flat Floaters grow rapidly, reaching about 7 cm (3 in.) in their first growing season and sexual maturity by age one or two (Barnhart et al. 1996). The maximum age reported is six years (Cope 1983 in Barnhart et al. 1996).
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. Pollution and habitat degradation throughout the Flat Floater’s range are ongoing threats. Of particular concern is the sedimentation and filling of backwater lakes, the primary habitat for this species in Minnesota.
Conservation Efforts in Minnesota
The MN DNR’s Mussel Program continues to survey streams in Minnesota to locate new populations, including backwater habitats frequented by the Flat Floater, 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).
Bernard E. Sietman (MNDNR), 2018
(Note: all content ©MNDNR)