Freshwater Mussels of Florida

Genus Megalonaias

281 Genus Megalonaias Genus Megalonaias Utterback 1915 There are currently two recognized species of Megalonaias in North America, M. nervosa and M. nickliniana. Megalonaias nervosa is the more widespread species, occurring in the Mississippi River basin and Gulf Coast basins from northeast Mexico east to the Ochlockonee River in Florida. Fossil records from the Pleistocene extend its prehistoric range eastward into the Suwannee basin and Tampa Bay area of peninsular Florida. Megalonaias nickliniana occurs in Mexico and Central America. Megalonaias boykiniana was formerly recognized from the Apalachicola and Ochlockonee River basins until it was placed in synonymy of M. nervosa by Mulvey et al. (1997) based on genetic data. Turgeon et al. (1998) recognized only M. nervosa from the United States. Megalonaias nervosa attains a length of 280 mm and is the largest mussel in North America. type Species Unio heros Say 1829 = Megalonaias nervosa (Rafinesque 1820) Diagnosis Shell thick; sculptured with various plications and wrinkles; outline oval to trapezoidal ; low posteriodorsal wing may be present; umbo sculpture thick, double-looped ridges, nodulous near posterior ridge; periostracum dull to clothlike; pseudocardinal teeth massive, 2 in left valve, 1 in right valve; lateral teeth long, straight, 2 in left valve, 1 in right valve; umbo cavity wide, deep. Excurrent aperture almost smooth; mantle bridge separating excurrent and supraanal apertures short; inner lamellae of inner gills usually completely connected to visceral mass; all 4 gills marsupial; glochidia held throughout gill; marsupium thickened, not extended beyond original gill margin when gravid; glochidial outline subelliptical, without styliform hooks (Utterback 1915). Accounts for Genera and Species 282 Megalonaias nervosa (Rafinesque 1820) Washboard Megalonaias nervosa – Upper image: length 127 mm, UF 243972. Ochlockonee River at State Highway 20 bridge, 12 miles west of Tallahassee, Leon County, Florida, 12 July 1980. © Richard T. Bryant. Middle image: length 99 mm, UF 269752. Escambia County, Florida. © Richard T. Bryant. Lower image: length 29, UMMZ 197242. Ochlockonee River, 11 miles northwest of Tallahassee, Leon County, Florida. © Richard T. Bryant. Description Size: length to 280 mm, in Florida to 216 mm. Shell: thick; with 3–5 large, oblique, parallel plications from near umbo to posterioventral margin, umbo with wrinkled sculpture that may extend onto disk, wrinkles occasionally present across entire disk; moderately compressed to moderately inflated, width 30%–50% of length; outline trapezoidal to oval; anterior margin rounded ; posterior margin obliquely truncate to rounded, posterior edge of shell usually 283 Genus Megalonaias undulating; dorsal margin straight to slightly rounded, often obliquely angled as part of low dorsal wing; ventral margin straight to convex, occasional large individuals concave ; posterior ridge indistinct, obscured by plications; posterior slope low, typically with arcuate corrugations extending from posterior ridge to posteriodorsal margin, often with smaller, irregular ridges; umbo broad, slightly to moderately inflated, elevated slightly above hinge line; umbo sculpture thick, double-looped ridges, nodulous near posterior ridge; umbo cavity moderately wide, deep. teeth: pseudocardinal teeth very large, triangular, 2 divergent teeth in left valve, 1 tooth in right valve, occasionally with accessory denticle anteriorly and/or posteriorly; lateral teeth moderately long, thick, straight to slightly curved, 2 in left valve, 1 in right valve; interdentum short to long, moderately wide. Nacre: white to bluish white, large individuals occasionally with purple tint, iridescent . Periostracum: dull to clothlike; dark olive brown to black. Glochidium Description Outline subelliptical; length 235–280 µm; height 310–380 µm; dorsal margin 135– 179 µm; ventral margin with internal irregularly perpendicular rows of micropoints (Surber 1912, 1915; Utterback 1915; Hoggarth 1999; Kennedy and Haag 2005). Similar Sympatric Species Megalonaias nervosa most closely resembles Amblema neislerii and Amblema plicata but differs from those species in having wrinkled sculpture present on the umbo. Megalonaias nervosa may also resemble Elliptoideus sloatianus, but the posterior ridge of Elliptoideus is typically more prominent and usually ends in a blunt point posterioventrally . The umbo cavity of M. nervosa is deeper and the nacre is white, usually bluish white to purplish in E. sloatianus. In large M. nervosa the posterior edge of the shell is usually undulating compared to the relatively straight edge in Elliptoideus. Megalonaias nervosa also resembles Plectomerus dombeyanus but usually has a more rounded posterior margin and less pronounced posterior ridge. Megalonaias nervosa has a deeper umbo cavity and white nacre. Distribution in Florida Megalonaias nervosa occurs in Escambia, Apalachicola, and Ochlockonee River basins (Figure 10.67). ecology and Biology Megalonaias nervosa inhabits large creeks to rivers in slow to moderate current in sand...