Freshwater Mussels of Florida

Genus Toxolasma

337 Genus Toxolasma Genus Toxolasma Rafinesque 1831 Toxolasma was recognized as having eight species by Turgeon et al. (1998). An additional undescribed species, Toxolasma sp. cf. corvunculus, which occurs in Escambia, Yellow, and Choctawhatchee River basins in Alabama and Florida, was recognized by Williams et al. (2008). Genetic analysis using mtDNA suggests there is another distinct lineage in Choctawhatchee River and upper portion of Chipola River (N.A. Johnson unpublished data). Rafinesque (1831) introduced the genus name Toxolasma but did not designate a gender, and it was not clear from the species included in the genus. Valentine and Stansbery (1971) first recognized Toxolasma as the senior synonym of Carunculina, but they did not address the gender question. A recent review determined that the gender of Toxolasma is neuter. While three species—Toxolasma corvunculus, Toxolasma cylindrellus, and Toxolasma pullus—required no change from the current spelling, endings for four species were changed: Toxolasma lividus to lividum; Toxolasma parvus to parvum; Toxolasma paulus to paulum; and Toxolasma texasensis to texasense (Lee 2006). Toxolasma are among the smallest of North American unionids, some rarely exceeding 40 mm in length. The genus is characterized by caruncles on the mantle margin, which move like “twiddling thumbs” that attract fish to eat the “wriggling red worms” (Morrison 1973). Host fishes probably include sunfishes of the Centrarchidae (Stern and Felder 1978; Watters et al. 2009). Toxolasma occurs in Great Lakes and Mississippi River basins and Gulf Coast rivers from Texas to Florida. It also occurs in southern Atlantic Coast rivers from North Carolina to Florida. type Species Unio lividus Rafinesque 1831 = Toxolasma lividum (Rafinesque 1831) Diagnosis Shell thin to moderately thick; smooth; somewhat compressed to inflated; outline oval to elliptical; females usually sexually dimorphic, often more truncate posteriorly than males; umbo sculpture thick, single-looped ridges; periostracum dull to clothlike, yellowish green, dark olive, brown to black, with or without green rays. Mantle margin just ventral to incurrent aperture papillate; 1 caruncle per mantle, reduced or absent in males; inner lamellae of inner gills connected to visceral mass only anteriorly; outer gills marsupial; glochidia held in posterior portion of gill; marsupium extended beyond original gill margin when gravid; glochidium outline subelliptical, without styliform hooks (Ortmann 1912; Simpson 1914; Britton and Fuller 1980). Accounts for Genera and Species 338 toxolasma parvum (Barnes 1823) Lilliput Toxolasma parvum – Upper image: length 29 mm, UF 370407. Commercial sport and bait fish ponds near Elliotts Creek, about 2 miles southwest of Moundville, Hale County, Alabama, August 1997. © Richard T. Bryant. Lower image: length 19 mm, UF 449289. Apalachicola River at river mile 46.8, Gulf County, Florida, 7 August 2006. © Richard T. Bryant. Description Size: length to 40 mm. Shell: moderately thin; smooth; moderately inflated; width 35%–45% of length; outline oval to elliptical; anterior margin rounded; posterior margin rounded, large females obliquely truncate; dorsal margin straight to slightly convex; ventral margin straight to slightly convex; sexual dimorphism subtle, with females slightly more inflated posteriorly than males; posterior ridge rounded; posterior slope moderately steep, slightly concave; umbo broad, moderately inflated; umbo sculpture moderately thick, single-looped ridges; umbo cavity shallow. teeth: pseudocardinal teeth small, compressed, 2 teeth in left valve, crests almost parallel to shell margin, 1 tooth in right valve, often with accessory denticle anteriorly; lateral teeth thin, straight to slightly curved, 2 in left valve, 1 in right valve; interdentum short, very narrow to narrow. Nacre: white to bluish white, iridescent. Periostracum: typically clothlike, occasionally dull; small individuals greenish yellow to dark brown, large individuals black, typically without rays. Glochidium Description Outline subelliptical; length 170–180 µm; height 200 µm (Surber 1915; Utterback 1916a; Ortmann 1919). 339 Genus Toxolasma Similar Sympatric Species Toxolasma parvum most closely resembles other Toxolasma species and is difficult to distinguish based on shell characters alone. The best character to distinguish live female T. parvum from all native Florida Toxolasma is the white marsupial gill, compared to the dark gray to black margin in both gravid and nongravid females of the other species . Pseudocardinal teeth of T. parvum are typically thinner than those of Toxolasma paulum. Many populations of T. parvum are not sexually dimorphic or if they are it is far more subtle than in T. paulum. Toxolasma parvum resembles some small Villosa lienosa but has white nacre, while that of V . lienosa is usually purplish. Live female T. parvum have a well-developed caruncle , which is absent in V. lienosa. Caruncles are...