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Round Pigtoe (Pleurobema Sintoxia)

COSEWIC Status Report
on the
Round Pigtoe
Pleurobema sintoxia
in Canada
2004

Species Information

Name and Classification

Scientific name:
Pleurobema sintoxia (Rafinesque, 1820)
English common name:
Round Pigtoe
French common name:
Pleurobème écarlate


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:
Ambleminae
Genus:
Pleurobema
Species:
Pleurobema sintoxia


Parmalee and Bogan (1998) provide a complete list of synonyms for this species. Pleurobema sintoxia had been known as P. coccineum until recently. Pleurobema sintoxia was reintroduced as the scientific name of the Round Pigtoe when it was discovered that the name sintoxia was first proposed for the species by Rafinesque in 1820. The reintroduction of sintoxia is in the interest of proper nomenclature (Oesch 1995). Ortmann (1919) recognized two subspecies of the Round Pigtoe – a river form (form coccineum) and a Great Lakes form (form pauperculum).

The taxonomy of the genus Pleurobema remains contentious. Strayer and Jirka (1997) consider P. sintoxia to be part of the P. cordatum (Ohio Pigtoe) species complex of closely related species or ecophenotypes – also including P. plenum (Rough Pigtoe) and P. rubrum (Pyramid Pigtoe) – that are found throughout the Ohio River drainage and in parts of the Mississippi and Great Lakes basins. Other taxonomists consider P. sintoxia to be a unique species. It is generally agreed that rigorous genetic, anatomic, and conchological studies are required to resolve the status of these taxa (A. Bogan pers. comm. Sept 2002).


Description

The Round Pigtoe (Figures figure1a and figure1b) is a medium to large sized freshwater mussel that is highly variable in morphology depending on habitat type. The river form is compressed, flattened, solid, and usually somewhat rectangular, but often oval or elongated. Pleurobema sintoxia was first described by Rafinesque in 1820. The type locality is the Mahoning River near Pittsburgh, Pennsylvania. The following description of the species was adapted from Clarke (1981), Oesch (1995), Strayer and Jirka (1997), and Parmalee and Bogan (1998). The beaks are compressed, slightly elevated, turned forward, and slightly extending beyond the hinge line. Sculpture consists of a few coarse, irregular ridges curving upward along the beak. The anterior end is rounded; the posterior end is squarely truncated; and the posterior ridge is rounded, ending in a blunt point. There are two stout, rectangular, serrated pseudocardinal teeth in the left valve and one in the right valve – the latter having a low, roughened linear tooth anteriorly and dorsally. The lateral teeth – two in the left valve and one in the right – are straight, moderately high, and finely serrated. The interdentum is wide; the beak cavity is very shallow; and muscle scars are deep. The surface is roughened with concentric rest lines. The periostracum in juveniles is dull tan in colour with distinct green rays that fade as the surface darkens to a deep reddish brown or black with age. The nacre is white or various shades of pink.


Figure 1A: Line drawing of the external features of the shell of Pleurobema sintoxia

Figure 1A: Line drawing of the external features of the shell of Pleurobema sintoxia. Reproduced with permission from Burch (1975).

Reproduced with permission from Burch (1975).


Figure 1B: Photograph of live specimens collected from the SydenhamRiver near Dawn Mills, Ontario in 1999

Figure 1B: Photograph of live specimens collected from the SydenhamRiver near Dawn Mills, Ontario in 1999 (Photo credit: J.L. Metcalfe-Smith, NWRI).

Photo credit: J.L. Metcalfe-Smith, NWRI.

According to Parmalee and Bogan (1998), P. sintoxia reaches a maximum length of 110 - 120 mm in medium-sized rivers. Clarke (1981) reports a maximum length of 90 mm in Canada; however, the authors have regularly collected animals >100 mm in length from rivers in southwestern Ontario. The Great Lakes form of P. sintoxia is generally smaller, rarely exceeding 75 mm in length (Strayer and Jirka 1997). The lake form is also more inflated and the beaks are full, elevated, and project forward well beyond the hinge line.

In Canada the Round Pigtoe may be confused with the Wabash Pigtoe (Fusconaia flava) (Figure 2). The main features distinguishing F. flava from P. sintoxia are a lower and more centrally located beak, deeper lateral sulcus, and deeper beak cavity in F. flava (Clarke 1981, Oesch 1995, Strayer and Jirka 1997). The lake form of P. sintoxia is somewhat similar to Obovaria olivaria, but has flatter valves, a duller periostracum, and is less likely to have coloured rays (Strayer and Jirka 1997).


Figure 2: Pleurobema sintoxia (lower left) with Fusconaia flava (upper right) for Comparison

Figure 2: Pleurobema sintoxia with Fusconaia flava for comparison. Note the deeper sulcus on F. flava.

Note the deeper sulcus on F. flava. Both specimens collected from the Sydenham River near Croton, Ontario in 2002 (photo credit: D. Zanatta, NWRI).