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Genus Uniomerus
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349 Genus Uniomerus Genus Uniomerus Conrad 1853 Three species were recognized in Uniomerus by Turgeon et al. (1998). An additional species, Uniomerus columbensis, was elevated from synonymy based primarily on zoogeographic patterns and preliminary genetic data (Williams et al. 2008). A more comprehensive examination of mitochondrial markers in Uniomerus from southern Atlantic and eastern Gulf Coast basins further supports the decision to recognize U. columbensis (N.A. Johnson unpublished data). Uniomerus occurs in the Great Lakes, Mississippi and Ohio basins, and Gulf Coast basins from Texas to Florida. Uniomerus also occurs in Atlantic Coast basins from Virginia to Florida. The variable shell morphology and weak genetic structuring in Uniomerus have created considerable confusion concerning the delineation of distribution boundaries and taxonomy in the genus. Uniomerus in Florida have been variously recognized under several names—U. obesus, U. tetralasmus, U. declivis, U. carolinianus, and U. columbensis (Clench and Turner 1956; Johnson 1972; Heard 1979a; Davis 1983; Brim Box and Williams 2000; Williams et al. 2008). Recent preliminary genetic data suggests that three species of Uniomerus occur in Florida—U. carolinianus from Ochlockonee River to St. Johns River and peninsular Florida; U. columbensis in Choctawhatchee River, Econfina Creek, and Apalachicola River; and U. tetralasmus in Escambia and Yellow Rivers (N.A. Johnson unpublished data). More detailed analyses are needed to resolve the taxonomic status and distribution of species of Uniomerus in Florida and elsewhere. Species of Uniomerus are among the most drought-resistant mussels in North America. The ability of Uniomerus to withstand periods of time without water has been observed on numerous occasions (Bosc 1803; Simpson 1892b; Frierson 1903a). type Species Unio excultus Conrad 1838 = Uniomerus tetralasmus (Say 1831) Diagnosis Shell thin to thick; smooth; outline quadrate to elliptical or subrhomboidal; dorsal and ventral margins almost parallel; umbo even with hinge line or elevated slightly above; umbo sculpture single-looped ridges; periostracum typically clothlike, yellow to brown or black, typically without rays; hinge plate well developed, thin; pseudocardinal teeth compressed, 2 in left valve, 1 in right valve; lateral teeth straight to slightly curved, 2 in left valve, 1 in right valve; nacre bluish white, gray, or pale purple. Incurrent aperture with arborescent papillae; mantle bridge separating excurrent and supra-anal apertures long; inner lamellae of inner gills connected to visceral mass only anteriorly; outer gills marsupial; glochidia held across entire gill; glochidium outline subelliptical, without styliform hooks (Conrad 1853; Ortmann 1912; Simpson 1914; Britton and Fuller 1980). In Florida Simpson (1892b) reported “I found thousands of the Blandingianus form [= Uniomerus carolinianus] in a little drain in the piney woods near Braidenton [sic], which was always dry except during the rainy season, and its banks of slightly damp sand were full of dormant specimens. A number of these survived after lying out in the sun for months.” Accounts for Genera and Species 350 Uniomerus carolinianus (Bosc 1801) eastern Pondhorn Uniomerus carolinianus – Upper image: length 58 mm, USNM 85715. St. Johns River, [near St. Augustine,] Florida. Photograph by James D. Williams. Middle image: length 51 mm, USNM 85374. Sink of Noonan’s [Newnans] Lake, [Alachua County,] Florida. Photograph by James D. Williams. Lower image: length 21 mm, UF 449437. Santa Fe River at Worthington Springs, Highway 121 bridge, Alachua and Union counties, Florida, 9 June 2011. © Richard T. Bryant. Description Size: length to 125 mm. Shell: moderately thin to thick; smooth; moderately inflated, width 30%–45% of length; outline quadrate to subrhomboidal; anterior margin rounded; posterior margin obliquely truncate to rounded; dorsal margin straight to convex; ventral margin straight to convex, occasional large individuals concave just anterior to posterior ridge; posterior ridge rounded; posterior slope moderately steep, flat to slightly concave; umbo broad, slightly inflated, elevated slightly above hinge line; umbo sculpture single-looped ridges (Figure 10.94); umbo cavity wide, shallow. teeth: pseudocardinal teeth moderately thick, compressed, 2 teeth in left valve, crests almost parallel to shell margin, 1 triangular tooth in right valve; lateral teeth short, [52.55.214.236] Project MUSE (2024-03-28 22:03 GMT) 351 Genus Uniomerus moderately thin, straight to slightly curved, 2 in left valve, 1 in right valve; interdentum moderately long, narrow. Nacre: bluish white, gray, or pale purple. Periostracum: typically clothlike, occasionally dull; yellowish green, olive brown to black, small individuals occasionally with faint green rays. Figure 10.94. Umbo sculpture of Uniomerus carolinianus, length 23 mm, UF 441122. Canal off Withlacoochee River [southern], at the county boat ramp off Highway 39, Citrus County, Florida, 6 February 2008. Illustration by Susan Trammell...