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Round Pigtoe (Pleurobema Sintoxia)

Habitat

Habitat Requirements

The Round Pigtoe is typically found in medium-sized to large rivers (van der Schalie 1938; Strayer 1983; Parmalee and Bogan 1998), but also occurs in Lake Erie and Lake St. Clair (Clarke 1981; Strayer and Jirka 1997). Ortmann (1919) reported collecting the river form of this species “going up far into the headwaters” of the upper Allegheny system, and found the lake form “in a few feet of water upon pure sand” in Presque Isle Bay, Lake Erie. In smaller rivers, the Round Pigtoe may often be found deeply buried in mixtures of gravel, cobble, and boulder substrates, in or below riffles with moderate flows (Ortmann 1919; Gordon and Layzer 1989; Parmalee and Bogan 1998). In larger rivers, it is found in mud, sand and gravel substrates at depths greater than 3 m, but may occur in shallows on sand or gravel bars (Gordon and Layzer 1989). In southeastern Michigan, it was found to be especially abundant in medium to large streams away from the lake plain (Strayer 1983). In Lake St. Clair, P. sintoxia currently occupies shallow (<1 m) nearshore areas with firm, sandy substrates (Zanatta et al. 2002).

The habitat preferences of juvenile mussels are believed to be different from those of adults, but there have been few studies on this topic (Gordon and Layzer 1989). The juvenile life stage is certainly more vulnerable than the adult stage, because juveniles have no control over the habitat into which they are released by their host and may die quickly in unsuitable habitats. The glochidial (larval) stage is the most vulnerable and specialized life stage, because the glochidia must successfully attach to an appropriate host in order to complete their metamorphosis to the juvenile stage.


Trends

Habitats for P. sintoxia and other unionids in Lake Erie, Lake St. Clair, and the Detroit and Niagara rivers have been largely destroyed by the Zebra Mussel. Native mussel communities were virtually extirpated from the offshore waters of western Lake Erie by 1990 (Schloesser and Nalepa 1994) and the offshore waters of Lake St. Clair and the Detroit River by 1994 (Nalepa et al. 1996; Schloesser et al. 1998). The mussel communities of Lake Erie were already in decline, probably due to a general decline in water quality over the past 40 years (Nalepa et al. 1991), but Lake St. Clair and the Detroit River still supported abundant and diverse mussel assemblages as recently as 1986 (Nalepa and Gauvin 1988) and 1992 (Schloesser et al. 1998), respectively. Unionids continue to survive in some nearshore areas with very shallow water, a high degree of connectivity to the lake (which ensures access to host fishes), and harsh conditions for Zebra Mussels (high water temperatures and considerable wave action in summer; ice scour in winter). However, such “refugia” are rare, and most of the unionid habitat in the Great Lakes has been permanently lost.

Agriculture is believed to be the main cause of the destruction of mussel habitat across North America (Strayer and Fetterman 1999). Since agricultural accounts for 75-85% of land use in the Grand, Thames and Sydenham River basins, it is likely that agricultural impacts (e.g., runoff of sediment, nutrients and pesticides, increased water temperatures due to loss of riparian vegetation, destruction of habitat by tractor crossings and cattle) have contributed to the deterioration of mussel habitat in these rivers. Municipal and industrial pollution may be responsible for the greater loss of mussel habitat in the heavily populated Grand and Thames River watersheds than in the primarily agricultural Sydenham River watershed (Metcalfe-Smith et al. 2003).


Protection/Ownership

Land ownership along the reaches of the Sydenham, Grand, and Thames rivers where P. sintoxia occurs is mainly private. Most of the lands are in agricultural use, including cash crops, pastures and woodlots. There are only two publicly owned properties in the Sydenham River watershed that are somewhat protected from development, i.e., 50 acres of forest owned by Mosa Township and 17 acres owned by the St. Clair Region Conservation Authority. In the Thames River watershed, there are 21 natural areas (Conservation Areas, E.S.A.’s, Provincial Nature Reserves, etc.) covering over 6200 ha; however, little of the area along the reach where this species occurs is protected (Thames River Background Study Research Team 1998). Protected areas along the Grand River are too small to have any significance for the protection of this species (Peter Mason, Grand River Conservation Authority, pers. comm. October 2002).

The population of P. sintoxia in the Canadian waters of the St. Clair delta, is located entirely within the territory of the Walpole Island First Nation. The area is largely undisturbed and is likely to remain so in the future. The Walpole Island Heritage Centre is aware of the presence of P. sintoxia within their territory, and of the national significance of the population.