Skip booklet index and go to page content

Round Pigtoe (Pleurobema Sintoxia)

Limiting Factors and Threats

The introduction and spread of the exotic Zebra Mussel (Dreissena polymorpha) throughout the Great Lakes drainage has led to dramatic declines of native freshwater mussels in colonized areas. Zebra Mussels have infested 63% of sites where P. sintoxia was known to occur prior to 1990 (Metcalfe-Smith et al. 1998b). The Zebra Mussel invasion of Lake St. Clair, Lake Erie and the Detroit and Niagara rivers has led to the reduction or elimination of P. sintoxia and other native mussel species from these waters (e.g., Schloesser and Nalepa 1994, Nalepa et al. 1996, Schloesser et al. 1998). Zebra Mussels may threaten the population of Round Pigtoes that still survives in the delta area of Lake St. Clair, as it is not known if the unionid community is stable or if the process of extirpation by Zebra Mussels is just slower in this area (Zanatta et al. 2002). Zebra Mussels are unlikely to endanger the other significant population of P. sintoxia in Ontario, i.e., the population in the Sydenham River, because the river is not navigable by boats and has few impoundments that could support a permanent colony. Nevertheless, the reservoirs at Coldstream and Strathroy in the headwaters of the East Sydenham River are of some concern. Potential colonization of the Grand and Thames rivers with Zebra Mussels continues to be a major worry, because large sections of these rivers are impounded. In fact, Zebra Mussels have recently been found in the Fanshawe and Springbank reservoirs on the Thames River (S. Hohn, Upper Thames River Conservation Authority, September 2003).

Anthropogenic stressors such as high loadings of sediment, nutrients and toxic compounds originating from urban and agricultural sources are potential problems in southwestern Ontario where P. sintoxia occurs. Siltation resulting from intensive agriculture has fouled many of the sand and gravel riffles in rivers inhabited by this species. Tile drains, cattle access to streams, and the reduction or elimination of riparian buffer strips have all contributed to this problem. Nutrient loadings through the application of fertilizers and the discharge of municipal sewage can have detrimental effects on rare fauna. Pesticides from farms and chlorides from winter road salting can also impact the benthic fauna (Jacques Whitford Environment Limited 2001).

Pleurobema sintoxia is a commercially valuable species. It was historically used in the pearl button industry and it may now be taken for the cultured pearl industry (Oesch 1995). According to Busby and Horak (1993), P. sintoxia is one of 12 commercially valuable species in Kansas. Baker (1993) notes that there has recently been a shift in market demand from the large washboards, Megalonaias nervosa, to the Mapleleaf (Quadrula quadrula), Threeridge (Amblema plicata) and pigtoes (Fusconaia and Pleurobema spp.). Overharvesting has seriously depleted mussel stocks in the U.S.A., and the commercial harvest been closed in many states (e.g., Anderson et al. 1993 for Indiana). There was a short-lived mussel fishery on the Grand River in the early 1900s (Detweiler 1918), but there is no commercial harvesting of mussels in Canada at the present time. A request was made in the 1990s for a license to commercially harvest mussels from the Thames River, but the request was withdrawn before it could be considered (A. Dextrase, Ontario Parks, Ontario Ministry of Natural Resources, pers. comm. August 2003). Poaching is a potential threat if there are further closures, stricter regulations and/or stiffer fines associated with commercial musseling in the U.S.

The most significant natural controls on the size and distribution of mussel populations are the distribution and abundance of their host fishes, and predation. Unionids can not complete their life cycle without access to their appropriate glochidial host. If host fish populations disappear, or decline in abundance to levels below that which can sustain a mussel population, recruitment will no longer occur and the mussel species may become functionally extinct (Bogan 1993). As noted earlier (section on Biology), several fishes known to be glochidial hosts for the Round Pigtoe in the United States also occur throughout the mussel’s range in Canada (Bluegill, Spotfin Shiner, Bluntnose Minnow and Northern Redbelly Dace) and are therefore likely hosts in Ontario. Laboratory testing and field confirmation is required to identify the functional host(s) with certainty.

Freshwater mussels are known to be food sources for a variety of mammals and fish (Fuller 1974). Predation by muskrats (Ondatra zibenthicus), in particular, may be a limiting factor for some mussel species. Tyrrell and Hornbach (1998) and others have shown that muskrats are both size- and species-selective in their foraging, and can therefore significantly affect both the size structure and species composition of mussel communities. However, heavy-shelled mussels like P. sintoxia may escape predation because they are too difficult to open. During their study of muskrat predation on 34 species of mussels in the St. Croix and Mississippi rivers in Minnesota and Wisconsin, Tyrell and Hornbach (1998) observed that the Round Pigtoe was one of the least preferred prey species. Watters (1993-94) compared the composition of the mussel community at two sites in the lower Muskingum River in Ohio with the composition of shells in nearby muskrat middens. The Round Pigtoe was too rare to be considered in the study, but the closely related Ohio Pigtoe (P. cordatum) was one of two dominant species that were underrepresented in muskrat middens. It appears that muskrats are unlikely to be a significant limiting factor for the Round Pigtoe in Ontario. Raccoons (Procyon lotor) are another potential predator. Although we are not aware of any studies on raccoon predation, we have observed raccoons feeding on mussels in the field and there is anecdotal information from the farming community in the Sydenham River watershed that the recent adoption of conservation tillage practices has led to an explosion in the raccoon population.