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Round Hickorynut (Obovaria Subrotunda)

COSEWIC Status Report
on the
Round Hickorynut
Obovaria subrotunda
in Canada
2003

Species Information

Name and Classification

Scientific name:
Obovaria subrotunda (Rafinesque, 1820)
English common name:
round hickorynut
French common name:
Obovarie


The recognized authority for the classification of aquatic molluscs in the United States and Canada is Turgeon et al. (1998). The currently accepted classification of this species is as follows:

Phylum:
Mollusca
Class:
Bivalvia
Subclass:
Palaeoheterodonta
Order:
Unionoida
Superfamily:
Unionacea
Family:
Unionidae
Subfamily:
Lampsilinae
Genus:
Obovaria
Species:
Obovaria subrotunda


Parmalee and Bogan (1998) provide a complete list of synonyms for this species. Specimens from Lake Erie were once considered by some to be a separate species, O. leibii, due to their smaller size, more regular and distinct growth rests, and lighter colour (Ortmann 1919).


Description

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 "l’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 (Ortmann 1919). The species shows considerable ecophenotypic variation in shell inflation among specimens from large rivers, small rivers, and lakes.

The round hickorynut may grow up to 65 mm in length but seldom exceeds 60 mm in Tennessee (Parmalee and Bogan 1998) or 50 mm in New York (Strayer and Jirka 1997). According to Clarke (1981), O. subrotunda rarely exceeds 40 mm in Canada. However, the authors have observed shells (and one live individual) up to about 60 mm long in southern Ontario rivers, and live specimens up to 51 mm long in Lake St. Clair. Figure 1A shows the external features of the shell and internal features of the left valve (hinge teeth), and Figure 1B is a photograph of a live specimen collected from the Sydenham River near Alvinston, Ontario on 21 June 2001.

Figure 1A: Line Drawings of the External Features of the Shell and Internal Features of the Left Valve (hinge teeth) of Obovaria subrotunda

Figure 1A: Line drawings of the external features of the shell and internal features of the left valve (hinge teeth) of Obovaria subrotunda.

Reproduced from Burch (1973).


Figure 1B: Photograph of a Live Specimen from the Sydenham River, Ontario

Figure 1B: Photograph of a live specimen from the Sydenham River, Ontario.

Photo credit: D.T. Zanatta, NWRI.