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Round Pigtoe (Pleurobema Sintoxia)

Population Sizes and Trends

United States

Ortmann (1919) gives an overview of the distribution and abundance of P. sintoxia across its range in the United States at the turn of the 20th century. The Round Pigtoe was widely distributed in Pennsylvania, being “especially abundant” in the upper Allegheny, but less so in the Monongahela drainage where some smaller creeks had become polluted. It was also found in a Lake Erie tributary in Pennsylvania. The species was said to be found “all over the state” of Ohio, mainly in smaller streams in both the Ohio River and Lake Erie drainages. It was abundant in the headwaters of the Monongahela River in West Virginia; it occurred in the Great Lakes drainage in southern Michigan; and it was common in Indiana in both the Ohio and Great Lakes drainages. In Kentucky, it occurred historically in the Ohio River and most of its major tributaries (lower Tennessee and Cumberland, Green, Salt, upper Cumberland, Kentucky and Licking rivers and Tygarts Creek; R. Cicerello, Kentucky State Nature Preserves Commission, pers. comm. October 2002). In Tennessee, the Round Pigtoe was historically found in the Cumberland, Duck, Holston, and Tennessee rivers (Parmalee and Bogan 1998). Ortmann (1919) also mentions records from the western part of P. sintoxia’s range in Iowa, Missouri, Kansas, and northern Arkansas – possibly extending into Oklahoma. This information indicates that P. sintoxia was widely distributed and common in many parts of its range in the past.

Recent information suggests that the current distribution and abundance of the Round Pigtoe in the U.S. is generally the same as it was historically, although declines seem evident in a number of areas. In Pennsylvania, it is still known from 12 locations in the Allegheny River basin, 14 locations in the French Creek basin, and two locations in the Lake Erie drainage (R. Evans, Pennsylvania Natural Diversity Inventory, pers. comm. September 2002). In New York, it remains widespread in the Allegheny River basin, but populations in the Niagara River basin are likely gone (D. Strayer, Institute of Ecosystem Studies, pers. comm. September 2002). There are several records throughout West Virginia (although nowhere in any abundance) in the Monongahela River and Little Kanawha River drainages, the Elk River, Kanawha River, Ohio River, and Middle Island Creek (J. Clayton, West Virginia Department of Natural Resources, pers. comm. September 2002). In Michigan, P. sintoxia is fairly widespread throughout the state in both the Great Lakes and Ohio River drainages, although it is seldom common where it occurs (P. Marangelo, The Nature Conservancy – Michigan Chapter, pers. comm. September 2002). Pleurobema sintoxia is not a common species in Indiana and Illinois, but it remains widespread throughout these states in the Mississippi and Ohio River drainages (K. Cummings, Illinois Natural History Survey, pers. comm. September 2002). In Kentucky, P. sintoxia still inhabits the Ohio River where it is neither widespread nor abundant, the upper Green River and its major tributaries, the Rolling Fork River (a Salt River tributary), the upper Cumberland River’s major tributaries, and the Big Sandy River. Most of these populations are isolated by impoundments or polluted river segments, and evidence of recent recruitment is lacking for some streams (R. Cicerello, Kentucky State Nature Preserves Commission, pers. comm. October 2002). In Tennessee, the Round Pigtoe is now apparently restricted to the Cumberland, Big South Fork Cumberland, and Stones rivers (Parmalee and Bogan 1998). The Round Pigtoe was found in the Tennessee River in the Muscle Shoals region (Tennessee and Alabama) in the early 20th century and up to the late 1970s, but not during surveys in the late 1990s (Garner and McGregor 2001). It was collected live again in this area in 2001 (J. Garner, Alabama Division of Wildlife and Freshwater Fisheries, pers. comm. September 2002). Pleurobema sintoxia is relatively common in Missouri, where it is found in 13 major drainages; however, there is no evidence of recruitment in many basins. The Missouri Department of Conservation has 359 records of P. sintoxia in its database of collections made from 1977-1999, 79% of which were for live or fresh dead specimens (S. Bruenderman, Missouri Department of Conservation, pers. comm. September 2002). A small population of the Round Rigtoe exists in the Red River drainage of Oklahoma (Vaughn et al. 1997). Pleurobema sintoxia has not been seen in recent extensive surveys of rivers in Iowa, although it was reported from the state historically (D. Woolnough, Iowa State University, pers. comm. October 2002).


Great Lakes and Connecting Channels

As mentioned earlier, Ortmann (1919) recognized both a river form and a Great Lakes form of P. sintoxia. He reported the lake form as being rare in Presque Isle Bay, Lake Erie. The Round Pigtoe has not been found alive in recent years in Presque Isle Bay or in Thompson Bay (the outer harbour of Presque Isle Bay) where other unionids have persisted despite the Zebra Mussel invasion (Schloesser and Masteller 1999, E. Masteller, Penn State University at Erie Pennsylvania, pers. comm. July 2002). Pleurobema sintoxia was found during surveys of Lake Erie between 1913 and 1960, and 40 live specimens were collected from Put-in-Bay, Ohio in 1970 (Kokai 1976). More recently, the Round Pigtoe was represented by only weathered and fresh dead shells at 4 of 33 sites surveyed along the southwest shore and around the Bass Islands in 1998 (Ecological Specialists 1999). Sixteen species of unionids were collected from the western basin of Lake Erie between 1930 and 1982 (Nalepa et al. 1991). Pleurobema sintoxia was present in 1951-52, but not in 1961, 1972, 1973, or 1982. By 1991, the entire unionid community had been virtually eliminated by the Zebra Mussel – only four specimens of two species were found alive (Schloesser and Nalepa 1994). Although the species was recorded historically from Rondeau Bay on the north shore of Lake Erie, only old weathered shells were found during a 2001 survey, along with evidence of heavy Zebra Mussel infestation (Zanatta and Woolnough unpublished data). The only location where P. sintoxia appears to be extant in Lake Erie is Metzger Marsh, near Toledo, Ohio. The Round Pigtoe was one of 20 species found alive in the marsh in 1996 (Nichols and Amberg 1999). Surveys of seven other marshes near Metzger Marsh in 2000 produced few live mussels, none of which were P. sintoxia (Ecological Specialists 2001).

Pleurobema sintoxia was recorded from the Detroit River in the early 20th century by La Rocque and Oughton (1937) and through museum specimens collected by Bryant Walker in 1934 that are currently housed at the University of Michigan Museum of Zoology. More recently, 14 live P. sintoxia were collected from three stations in the river in 1982-83 (Schloesser et al. 1998). Following the Zebra Mussel invasion, Schloesser et al. (1998) collected 11 live P. sintoxia from six stations in 1992 and two live specimens from just one station in 1994. Based on the results of surveys conducted in 1997-98, it is now believed that P. sintoxia has been extirpated from the Detroit River (Schloesser et al. unpublished data).

There are nine historical records for P. sintoxia from the Niagara River. Nothing is known about the historical abundance of this species in the river. A survey was conducted in the Niagara River in the summer of 2001 for the New York Power Authority. Many recently spent shells of P. sintoxia were found, but no living specimens. According to the consultant responsible for the survey, “There were Zebra Mussels everywhere” (K. Schneider, Stuyvesant Falls, NY, pers. comm. November 2002). No further details about the survey could be disclosed, but it seems likely that P. sintoxia has been extirpated from the Niagara River.

The Round Pigtoe still persists in Lake St. Clair. Zebra Mussels are believed to have invaded the lake in 1986 (Hebert et al. 1989). Twenty-nine sites in the offshore waters of Lake St. Clair were surveyed for unionids in 1986, 1990, 1992, and 1994 (Nalepa and Gauvin 1988). One live P. sintoxia was recorded in 1986 (0.36% of the unionid community); two specimens were found in 1990 (0.81% of the community); and none were found in either 1992 or 1994. However, 42 Round Pigtoes measuring 31‑95 mm in shell length (Figure 6) were found alive during recent surveys of nearshore areas around the lake. Zanatta et al. (2002) surveyed 95 nearshore sites and found live unionids at 33 sites, all but two of which were in shallow areas (< 1m) with sandy substrates within an 8 km2 area near the St. Clair delta. The Round Pigtoe represented only 1.8% of the overall community by abundance (42 of 2356 live unionids). It was found at only three sites off Squirrel Island where it was relatively abundant, representing 70% of 20 unionids (6 spp.) at one site, 48% of 29 unionids (7 spp.) at the second site, and 8% of 36 mussels (8 spp.) at the third site. Quantitative sampling in 2001 yielded density estimates of 0.005, 0.022 and 0.022 Round Pigtoes/m2 at these sites. Sampling was repeated in 2003 and densities were found to have declined at all three sites, i.e., 0, 0.009 and 0.017 mussels/m2, respectively (Metcalfe-Smith et al., unpublished data) The St. Clair delta has been identified as a possible refuge for unionids from impacts of the Zebra Mussel (Zanatta et al. 2002).


Canadian Rivers

Pleurobema sintoxia has been reported from the Grand, Thames, and Sydenham rivers in Ontario. Metcalfe-Smith et al. (1998b, 1999) surveyed 74 sites on the Grand, Thames, Sydenham, Ausable, and Maitland rivers in 1997, 1998 and 2002 to determine the conservation status of rare species of freshwater mussels in southwestern Ontario. They used the timed-search technique, which they have shown to be the most efficient method for detecting rare species (Metcalfe-Smith et al. 2000a), with an intensive sampling effort of 4.5 person-hours (p-h)/site. Sites that were known to support rare species (including P. sintoxia) in the past were targeted. Results of these and other recent surveys were compared with the historical data to determine population trends for the Round Pigtoe. This species was not found in the Ausable or Maitland rivers in the lower Lake Huron drainage, nor did it occur there historically. 

Pleurobema sintoxia was first collected from the Grand River in 1885 near Caledonia by J. Townsend and it was collected again in 1890 near Cayuga by J. Macoun; both specimens are held by the Canadian Museum of Nature in Aylmer, Quebec. The species has been encountered regularly in the lower reaches of the river (downstream of Brantford) since that time, e.g., by Oughton in 1934-35 (ROM collection), Stansbery and Stein in 1963 (Ohio State University Museum of Zoology collection), and Kidd (1973). Metcalfe-Smith et al. (2000b) surveyed 95 sites in the river between 1995 and 1998, and found only one live P. sintoxia at each of three sites. Fresh shells were collected from an additional three sites, all in the lower mainstem of the river below Brantford. The Round Pigtoe accounted for 0.18% of the 1688 unionids collected during 4.5 p-h timed search surveys at 24 of these 95 sites. A site near York where one live P. sintoxia was found in 1998 was resurveyed in 2002 using 4.0 p-h of search effort, and 10 more live specimens were found (Metcalfe-Smith and Zanatta, unpublished data). Shell lengths of live P. sintoxia collected from the Grand River during the most recent surveys range from 55-101 mm (Figure 6). The small number of specimens found, and especially the lack of small specimens, suggests that reproduction rates may be declining.

There are six records of the Round Pigtoe from the Thames River between 1934 and 1995. Specimens or shells (no information available for 4 of the records) were collected at widely separated sites from Woodstock in the upper reaches to Chatham near the mouth to the Middle Thames River near Thamesford. This information suggests that the species was once broadly distributed, although rare, in the Thames River. Only two live specimens of P. sintoxia were found during timed search surveys of 16 sites in the Thames River drainage in 1997-98. Both specimens were found at a site on the Middle Thames River upstream of Thamesford and both were very large (95 and 115 mm) by Clarke’s (1981) standards (max. shell length of 100 mm reported for Canadian waters). The Round Pigtoe accounted for 0.11% of the community by abundance (a total of 1890 mussels were collected). It is likely that these specimens are remnants of a dying population. Weathered shells were found at an additional eight sites on the river between Thamesford and Chatham (~ 150 km reach), lending further support to the premise that the population was larger and more widespread in this river historically.


Figure 6: Size Frequency Distribution for Live Specimens of Pleurobema sintoxia Found in Lake St. Clair, the Sydenham River, and the Grand River Between 1997 and 2002

Figure 6: Size frequency distribution for live specimens of Pleurobema sintoxia found in Lake St. Clair, the SydenhamRiver, and the Grand River between 1997 and 2002.

The Round Pigtoe appears to have always been a rare member of the unionid community of the Sydenham River. It was first collected from the river in 1929 by R. Cain at a site near Alvinston (specimen held by the ROM). Stein and associates (Ohio State University) collected 2 live specimens and 16 fresh whole shells from a site near Florence in 1965 and 4 live specimens and 4 fresh whole shells from a site below Alvinston in 1967. Clarke (1973) reported it alive at three of 11 sites he surveyed in 1971 but did not provide information on abundance. Clarke (1992) also surveyed 16 sites in the river in 1991 and found a total of 5 live P. sintoxia at two sites. Metcalfe-Smith et al. (1998b, 1999) conducted 4.5 p-h timed search surveys at 17 sites on the river in 1997-1998, and an 18th site was surveyed in 2002 (Metcalfe-Smith and Zanatta, unpublished data). Twenty-two Round Pigtoes were found alive at six of these sites, representing 0.93% of the 2359 unionids collected from the river. Only 1-2 specimens/site were observed in the East Sydenham River, but 14 live animals were collected from a site near Warwick on Bear Creek. Relative abundance in terms of catch-per-unit-effort (CPUE) was similar to that reported in the past, i.e., 0.22-0.44 specimens/p-h (except for 3.0/p-h at the site in Bear Creek) vs. 0.20-0.66 specimens/p‑h between 1965 and 1991. Quantitative (quadrat) surveys were also conducted at 10 sites on the Sydenham River between 1999 and 2002, and 23 P. sintoxia were found alive at six of these sites (Metcalfe-Smith and Zanatta 2003). Density estimates ranged from 0.01 to 0.13 per m2. Shell lengths of the 45 live Round Pigtoes observed at 7 different sites in a ~75 km reach of the East Sydenham River between Rokeby and Dawn Mills and one site in the north branch of the river ranged from 30 to 133 mm (Figure 6), suggesting that recruitment is occurring. However, quantitative sampling at an additional five sites in 2003 yielded only 3 more animals and a closer examination of the shell length data showed that the Round Pigtoe is successfully reproducing at only one or two sites on the river (Metcalfe-Smith and Zanatta, unpublished data).

Table 1 summarizes the available information on frequency of occurrence and relative abundance of P. sintoxia in various locations in Canada and the United States. The species was found at about 25% of sites surveyed (range 2-60%), representing 1.5% on average (range 0.1-5.1%) of the total number of mussels collected. The Round Pigtoe could be described as a broadly distributed but uncommon species that is seldom, if ever, abundant.

Table 1: Frequency of Occurrence and Relative Abundance of Pleurobema sintoxia in Various Locations in the United States and Ontario
River/LakeState/ ProvinceFrequency of occurrence, as % of sites surveyed
(# sites)
 Relative abundance, as % of communityYear of survey
Red River drainageOK-2.7%; 1.8%1997a
Green RiverKY-1.8%1990-91b
Barren RiverKY-0.6%1990-91b
St. Joseph RiverOH35% (40)-1938-75c
Muskingum RiverOH 0.1%; 0.2%1992d
Clinton RiverMI15.8% (76)5.1%1977-78e
Clinton, Huron, and Raisin RiversMI34.7% (75)-1980f
upper Blanchard RiverOH45.5% (11)2.2%1994-96g
Black and Pine RiversMI21.7% (23)-1982-83h
Lake St. ClairON, MI3.4% (29)0.4%1986i
Lake St. ClairON, MI9.1% (33)1.8%1999-2001j
Sydenham River (timed search  method)ON35.3% (18)0.9%1997-98, 2002k, l
Sydenham River (quadrat search method)ON60% (10)0.6%1999-2002l
Thames RiverON6.3% (16)0.1%1997-98k
Grand RiverON12.5% (24)0.2%1997-1998k
Lake Erie (Western Basin)OH, MI, ON-0.3%1951-52m
Lake Erie (Put-in-Bay)OH-5.2%1970n
Detroit RiverON, MI17.6% (17)1.1%1982-83o
Detroit RiverON, MI41.2% (17)0.7%1992o
Detroit RiverON, MI11.1% (9)3.4%1994o
a Vaughn et al. (1997)
f Strayer (1983)
k Metcalfe-Smith et al. (1998, 1999)
b Cochran and Layzer (1993)
g Hoggarth et al. (2000)
l Metcalfe-Smith et al. (unpublished data)
c Clark (1977)
h Hoeh and Trdan (1985)
m Nalepa et al. (1991)
d Watters (1993-1994)
i Nalepa and Gauvin (1988)
n Kokai (1976)
e Strayer (1980)
j Zanatta et al. (2002)
o Schloesser et al. (1998)