Description
SHELL: Shell round or circular, moderately thick, and inflated. Anterior and posterior ends rounded in males, somewhat truncated in females. Umbos low and centrally placed, slightly elevated above the hinge line. Beak sculpture consists of a few indistinct, concentric ridges, usually evident only on very young shells. Shell smooth, growth lines dark but often indistinct. Periostracum yellowish brown to dark chestnut brown or blackish, and rayless. Adult size to 3 inches. Pseudocardinal teeth moderately small, serrated, and divergent. Lateral teeth slightly curved, fairly short, and moderately thin. Interdentum narrow or absent. Beak cavity fairly deep. Nacre white, iridescent posteriorly (Cummings and Mayer, 1992).
ANIMAL: "Structure of soft parts essentially the same as in O. retusa. In the sterile female, the ovisacs are slightly narrower than the regular water-tubes. Number of ovisacs up to thirty and more. As in the preceding species the edge of the mantle in front of the branchial is in the female slightly lamellar and crenulated, but has only a brown (not blackish) mark along it." (Ortmann, 1912:323).
From COSEWIC (2003): The round hickorynut, Obovaria subrotunda (Rafinesque, 1820) is easily recognized by its circular shape, centrally located beaks, unsculptured and unrayed periostracum, and relatively small size. It may be occasionally confused with smooth specimens of Quadrula pustulosa, which, however, have a bright golden-yellow periostracum. The type locality is "Ohio" (the Ohio River). The following description of the species was adapted from Clarke (1981), Strayer and Jirka (1997) and Parmalee and Bogan (1998). The shell is circular to subcircular and thick. The surface is smooth except for prominent growth rests. The periostracum is generally dark brown or olive-brown and without rays except in some very young specimens. The posterior slope is distinctly lighter than the remainder of the shell. Beaks are centrally placed, curved inward, and elevated well above the hinge line. Beak sculpture is fine and consists of 4-6 short, slightly sinuous bars. Hinge teeth are rather heavy and strong. The left valve has two thick, roughened, triangular pseudocardinal teeth and two short, strong, slightly curved lateral teeth. The right valve has one large, triangular serrated pseudocardinal tooth, usually with two small, compressed teeth on either side, and one short, thick, roughened lateral tooth, often with a secondary inner low, incomplete lateral tooth. The interdentum is narrow or absent. Adductor scars are deeply impressed. The nacre is silvery white, with a tinge of blue or pink in some specimens. Sexual differences in the shell are obscure; in females, the posterior margin of the shell may be truncated. However, there is a distinct difference in size, with females being considerably smaller than males. The species shows considerable ecophenotypic variation in shell inflation among specimens from large rivers, small rivers, and lakes.
Habitat
This species is found in small streams to large rivers, and lakes, with sand, gravel, and cobble substrates and moderate flow (Cummings and Mayer 1992; Parmalee and Bogan 1998; COSEWIC 2003; USFWS 2019).