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Recovery Strategy for the Northern Riffleshell, Snuffbox, Round Pigtoe, Mudpuppy Mussel and Rayed Bean in Canada (Final)


3. Species Information – Round Pigtoe


Common Name: Round Pigtoe

Scientific Name: Pleurobema sintoxia

COSEWIC Status: Endangered

COSEWIC Reason for designation: The Round Pigtoe is limited to three watersheds and Lake St. Clair in southern Ontario and is considered to be lost from 65% of its historical Ontario range. There are continuous declines in habitat because of urban, industrial and agricultural development. There is an irreversible impact from zebra mussels in Lake St. Clair and possibly impoundments in the Sydenham River.

Occurrence: Ontario

COSEWIC Status history: designated endangered in 2004

The Round Pigtoe is a medium to large-sized freshwater mussel with a highly variable morphology depending on the habitat. In rivers, this mussel has a compressed, solid and somewhat rectangular shell, with a compressed beak that is slightly elevated and projects forward only slightly beyond the hinge line. The Great Lakes form has a smaller and more inflated shell, with a full beak that is elevated and projects forward, well beyond the hinge line (COSEWIC 2004). The anterior end is rounded and the posterior end is square and truncated. The posterior ridge is rounded, ending in a blunt point. The shell in juveniles is dull tan with distinct green rays that fade as the shell becomes larger. Adults have deep mahogany coloured shells with dark banding and may grow up to 13 cm. The surface is rough with concentric growth rests. There are two pseudocardinal teeth in the left valve that are stout, rectangular, and serrated. There is one pseudocardinal tooth in the right valve which is low and roughened. There are two lateral teeth in the left valve and one in the right that are straight, moderately high, and finely serrated.

Figure 7. The Round Pigtoe (Pleurobema sintoxia). Photo courtesy J.L. Metcalfe-Smith, Environment Canada
Figure 7. The Round Pigtoe (Pleurobema sintoxia). Photo courtesy J.L. Metcalfe-Smith, Environment Canada

It can be found in a wide range of habitats from small to large rivers and large lakes in sand, gravel, boulder and mud substrates. Historically, this species was known from throughout the Mississippi and Ohio drainages including Alabama, Arkansas, Illinois, Indiana, Iowa, Kansas, Kentucky, Michigan, Minnesota, Missouri, Nebraska, New York, Ohio, Oklahoma, Pennsylvania, South Dakota, Tennessee, West Virginia, and Wisconsin. In Canada, the Round Pigtoe was found in the western basin of Lake Erie, Lake St. Clair and the Detroit, Grand, Niagara, Sydenham and Thames rivers.

The Round Pigtoe is broadly distributed but uncommon and rarely, if ever, abundant (COSEWIC 2004). In the United States, current and historic ranges of the Round Pigtoe are similar although large river populations have mostly disappeared from the upper Midwest. Many populations still exist in the Mississippi and Ohio drainages. It is ranked as common (G4) in North America although it is listed as endangered in Iowa and Pennsylvania, threatened in Minnesota, special concern in Michigan and Wisconsin, and a species of special interest in Ohio. It is not currently listed under the U.S. Endangered Species Act.

In Ontario, the Round Pigtoe is assumed to be eradicated from the offshore waters of lakes Erie and St. Clair, and the Detroit and Niagara rivers. It has not been seen in the western basin of Lake Erie since the early 1950s, nor in the offshores of Lake St. Clair (outside the delta area) since 1990 (COSEWIC 2004). Small pockets of isolated populations may still be found in some nearshore areas although, to date, none have been found. A 2001 survey of the Niagara River found no live native mussel species. The Grand and Thames rivers have small, possibly relict, populations of the Round Pigtoe. The Sydenham River still has reproducing populations in several different localities in the east and north branches.

Distribution

Global Range

In the United States, the Round Pigtoe occurs in Alabama, Arkansas, Illinois, Indiana, Iowa, Kansas, Kentucky, Michigan, Minnesota, Missouri, Nebraska, New York, Ohio, Oklahoma, Pennsylvania, South Dakota, Tennessee, West Virginia and Wisconsin.

Canadian Range

The populations of Round Pigtoe that are still reproducing are found in the St. Clair delta and the Sydenham River. Remnant populations still exist in the nearshores of Lake Erie and Lake St. Clair and the Grand and Thames rivers.

Percent of Global Range in Canada

Less than 5% of the species' global distribution is currently found in Canada.

Distribution Trend

In the United States, the present range of the Round Pigtoe is similar to its historical range, although most large river populations have disappeared in the upper Midwest. Populations in tributaries of the Mississippi and Ohio rivers still survive. In Canada, it was known from the western basin of Lake Erie and the offshores of Lake St. Clair but these populations have been lost. The remaining population in Lake St. Clair is located entirely within the Walpole Island First Nation. The Round Pigtoe was widespread in the upper and lower Thames River, but is now restricted to a very small (possibly relict) population in the upper reaches of the Middle and South Thames rivers. In the Grand River the Round Pigtoe historically occurred in the lower reaches of the river, downstream of Brantford although shells have occasionally been found higher in the watershed (Metcalfe-Smith et al. 2000).

The Round Pigtoe is well distributed, although not common, throughout the Sydenham River.

Figure 8. Global distribution of the Round Pigtoe.
Figure 8. Global distribution of the Round Pigtoe.

 

Figure 9. Distribution of the Round Pigtoe in Canada.
Figure 9. Distribution of the Round Pigtoe in Canada.

Population Abundance

Global Range

In the United States, many populations of Round Pigtoe have declined and there is no evidence of recent recruitment in some areas (COSEWIC 2004).

Canadian Range

The Round Pigtoe has not been seen in the western basin of Lake Erie since 1951-52, nor in the offshores of Lake St. Clair since 1990 (COSEWIC 2004). However, surveys in 2002 reported 42 Round Pigtoes from three nearshore sites off Squirrel Island in the St. Clair delta. Ninety-two other nearshore sites surveyed had no evidence of live specimens. Results from recent surveys of the Niagara River and Detroit River indicate that the Round Pigtoe is extirpated from these rivers. In the Grand River, low numbers of live specimens, and a lack of small specimens, indicates that reproduction rates are likely declining. The Thames River population is restricted to a very small area in the upper reaches of the Middle and South Thames rivers between Thamesford and London. The Round Pigtoe has always been rare in the Sydenham River. Forty five specimens were observed at seven different sites on the East Sydenham River between Rokeby and Dawn Mills and one site in the north branch (COSEWIC 2004).

Percent of Global Abundance in Canada

Less than 5% of the species’ global abundance is currently found in Canada.

Population Trend

The current Canadian distribution of the Round Pigtoe is restricted to the St. Clair delta and three southwestern Ontario rivers. The St. Clair delta has been identified as a possible refuge for unionids from impacts of the zebra mussel (Zanatta et al. 2002). Surveys in 2002 reported the Round Pigtoe from three sites in the St Clair River delta, however, repeated sampling of the same sites in 2003 reported declines in all three sites. In the Grand River, low numbers of live specimens, and a lack of small specimens, indicates that reproduction rates are likely declining. The Thames River has a relict population (large individuals, no signs of reproduction) in the upper reaches of the Middle Thames as well as a population between Thamesford and its confluence with the South Thames. In the East Sydenham River the Round Pigtoe was observed at seven different sites and another site in the north branch. The size of the specimens sampled indicates recruitment is occurring. The population in the Sydenham River is considered to be the healthiest in Ontario.

Biological Limiting Factors

Reproductive Attributes

The reproductive biology of the Round Pigtoe follows the general reproductive biology of most mussels. During spawning, male mussels release sperm into the water and females living downstream filter it out of the water with their gills. Female mussels brood their young from the egg to the larval stage in specialized regions of their gills known as marsupia. Immature juveniles, known as glochidia, develop in the gill marsupia and are released by the female into the water column to undergo a period of parasitism on a suitable host fish species. Further development to the juvenile stage can not continue without a period of encystment on the host.

The glochidia are subovate and without hooks, measuring 150 µm in both height and width (Clarke 1981). The lack of hooks indicates they are gill parasites.

Known host fishes for the Round Pigtoe include the bluegill (Lepomis macrochirus), spotfin shiner (Cyprinella spiloptera), bluntnose minnow (Pimephales notatus), northern redbelly dace (Phoxinus eos) and the southern redbelly dace (Phoxinus erythrogaster) (Hove 1995). In Ontario, all of these species, except the southern redbelly dace, occur commonly with the Round Pigtoe and are assumed to serve as glochidial hosts although no potential hosts have yet been tested as gravid females have not been located.

Dispersal

Like most freshwater mussels, the Round Pigtoe has very limited dispersal abilities. The Round Pigtoe adults are essentially sessile with movement limited to only a few meters on the river/lake bottom. Although adult movement can be directed upstream or downstream, studies have found a net downstream movement through time (Balfour and Smock 1995; Villella et al. 2004). The primary means for large scale dispersal, upstream movement, and the invasion of new habitat or evasion of deteriorating habitat, is limited to the encysted glochidial stage on the host fish.