Eastern pondmussel (Ligumia nasuta) COSEWIC assessment and status report: chapter 4

Distribution

Global range

The Eastern Pondmussel is restricted to eastern North America, having been recorded from 14 states and the Province of Ontario. It ranges from the lower Great Lakes east through New York to New Hampshire and south, in coastal rivers, to South Carolina. In the United States, Ligumia nasuta occurs in Michigan, Ohio, Pennsylvania, New York, New Hampshire, Massachusetts, Connecticut, New Jersey, Delaware, Maryland, District of Columbia, Virginia, North Carolina and South Carolina (Figure 2). NatureServe (2005) also lists the species as occurring in Vermont and Rhode Island, but this is believed to be erroneous (Nedeau pers. comm. 2005). Ligumia nasuta originated on the Atlantic coast of North America and reached the lower Great Lakes following the last period of glaciation (Nedeau et al. 2000). The post-glacial dispersal route for this species is believed to be unique among Great Lakes unionids. Most species reinvaded the Great Lakes from the Mississippian Refugium to the south, whereas several others originated in the Atlantic Refugium and entered the region from the northeast via the Champlain Sea/St. Lawrence River. Although L. nasuta is an Atlantic Slope species, it does not appear to have followed an east to west migration route. Stansbery (1961) proposed instead that the Eastern Pondmussel entered the Great Lakes via an eastward-flowing outlet of Lake Erie meltwater that would have provided access to host fishes from the Mohawk or Hudson Rivers to the east. Glacial geology has established the existence of this Mohawk-Hudson or Susquehanna Outlet (Hough (1950) cited in Stansbery 1961).

The current distribution of the Eastern Pondmussel is similar to its historical distribution, but the species is declining in many places, particularly the Great Lakes (NatureServe 2005). Populations of freshwater mussels in the Canadian and U.S. waters of the lower Great Lakes and connecting channels have been nearly extirpated due to the impacts of zebra mussels (Dreissena polymorpha). Zebra mussels attach to the shells of native freshwater mussels in the hundreds or even thousands, causing the native mussels to suffocate or die from lack of food. Small, isolated populations of native mussels can still be found in some nearshore areas where densities of zebra mussels have remained low; such locations have been termed “refuge sites.” There are four known refuge sites for unionids in U.S. waters along the south shore of Lake Erie. Ligumia nasuta was not among the species found alive in the western basin refuge in 1993 (Schloesser et al. 1997), in Metzger Marsh in 1996 (Nichols and Amberg 1999), or in Crane Creek Marsh in 2001 (Bowers and de Szalay 2004); however, it was one of the most abundant of nine species occupying the Thomson Bay refuge near Erie, PA in 2000 (Masteller pers. comm. 2002).

Figure 2. North American distribution (shaded area) of the Eastern Pondmussel, Ligumia nasuta, based on records from the Lower Great Lakes Unionid Database and data obtained from www.natureserve.organd jurisdictional authorities.

Figure 2. North American distribution (shaded area) of theEastern Pondmussel, Ligumia nasuta, based on records from the Lower Great Lakes Unionid Database and data obtained from www.natureserve.organd jurisdictional authorities.

Canadian range

In Canada, Ligumia nasuta is known only from the lower Great Lakes region of southern Ontario. There are no records of this mussel from Quebec (Gagnon pers. comm. 2005). The National Water Research Institute’s Lower Great Lakes Unionid Database was used to identify occurrence records for L. nasuta in Ontario. For a detailed description of the database and its data sources, see Collections examined. In the Lake Ontario drainage basin, L. nasuta was historically collected from the Bay of Quinte and its watershed including the Moira River, Consecon Lake and East Lake, and at scattered locations along the north shore of Lake Ontario to Hamilton Harbour. In the Lake Erie drainage, it was found in the Niagara and Welland Rivers, at various locations along the north shore of the eastern basin including the mouth of the Grand River, at Long Point and Rondeau Bay in the central basin, and at numerous locations in the shallow western basin, including Point Pelee, Pelee Island, Middle Sister Island and East Sister Island. It also occurred in the Detroit River and Lake St. Clair. The earliest well-documented records of this species in Canada were collected in the 1890s by J. Macoun, who found specimens in the Detroit River near Windsor and in Lake Erie near Rondeau Provincial Park (specimens held by the Canadian Museum of Nature). Figure 3 shows the historical distribution of the Eastern Pondmussel in Ontario based on 149 records collected between 1860 and 1996. Only about 15% of these records are for known live occurrences; the rest are for shells that, in many cases, could have washed up on the shore from deeper water. The current distribution of the species is shown in Figure 4 and is based on 64 records (live animals and shells) collected between 1997 and 2006. Live specimens were most recently collected from Lyn Creek in the upper St. Lawrence River drainage in August, 2006. It should be noted that Figure 4 shows the locations of all 531 sites that were surveyed for unionids between 1997 and 2006. Although many of these sites are outside the historical range of L. nasuta, it is useful to show them because they give an indication of recent survey effort throughout the region. Over 90% of the sites were surveyed using intensive semi-quantitative (timed search) or quantitative sampling methods.

Lake St. Clair and the Detroit River have been intensively surveyed for unionids in recent years. Zanatta et al. (2002) surveyed 95 sites in nearshore areas around Lake St. Clair between 1999 and 2001 and found live mussels at 33 sites, most of which were in the Canadian waters of the St. Clair delta. The Eastern Pondmussel was found at 16 (48%) of these sites. Metcalfe-Smith et al. (2004, 2005b) surveyed 15 sites in the Canadian waters of the delta in 2003 and 2005 and found live Eastern Pondmussels at 6 (40%) of these sites, although 59 of the 82 live animals were found in a single sheltered bay approximately 0.37 km² in size. Five live animals were also collected from 4 of 17 sites surveyed in the U.S. waters of the delta. Ligumia nasuta was collected from the offshore waters of Lake St. Clair prior to the dreissenid invasion (1986-1990), but by 1994 it had been eliminated (Nalepa et al. 1996). Similarly, L. nasuta was found alive in the Detroit River between 1987 and 1992 (Schloesser et al. 1998), but no specimens were found during follow-up surveys in 1997-98 (Schloesser et al. 2006).

Figure 3. Historical distribution (1860-1996) of Ligumia nasuta in Ontario, based on records from the Lower Great Lakes Unionid Database. Only 15% of the records are for known live occurrences; the rest are for shells (see text).

Figure 3. Historical distribution (1860-1996) of Ligumia nasuta in Ontario, based on records from the Lower Great Lakes Unionid Database. Only 15% of the records are for known live occurrences; the rest are for shells (see text).

Figure 4. Current distribution (1997-2006) of Ligumia nasuta in Ontario, based on records from the Lower Great Lakes Unionid Database. The locations of all 531 sites surveyed for unionids during this time period are shown.

Figure 4. Current distribution (1997-2006) of Ligumia nasuta in Ontario, based on records from the Lower Great Lakes Unionid Database. The locations of all 531 sites surveyed for unionids during this time period are shown.

Many of the historical sites for Ligumia nasuta in Lake Erie have been revisited in recent years and no live specimens were found. Metcalfe-Smith et al. (2000b) surveyed 94 sites throughout the Grand River watershed in 1995 and 1997-98 and found a single weathered valve of L. nasuta in MacKenzie Creek, a tributary to the lower reach of the river. Zanatta and Woolnough surveyed 6 sites in Rondeau Bay in 2001 while working for J.L. Metcalfe-Smith and found many weathered shells and valves of L. nasuta but no live specimens. The New York Power Authority commissioned Riveredge Associates to survey 15 sites around Grand Island in the Niagara River for rare, threatened and endangered mussels in 2001 and 2002. Only 46 mussels of six species were found alive at two sites in U.S. waters (New York Power Authority 2003). Thirty-one species are known from the river historically. The Eastern Pondmussel was not found alive during these surveys, but shells were observed at many of the sites. The authors of the present report surveyed 17 historical sites in the western basin of Lake Erie, including sites around Point Pelee and Pelee Island, in July 2005 and did not find any live unionids. Four weathered valves of L. nasuta were found near the historical site of Holiday Beach Park in Essex County. Researchers at the University of Waterloo who are studying the effects of dreissenids on food webs and nutrient cycling in Lake Erie conducted benthic invertebrate surveys at about 60 sites along the north shore of the eastern basin of the lake in 2001 and 2002 using either a Ponar dredge or airlift sampler. The project was expanded to include the central and western basins in 2004, and 220 sites ranging in depth from 2 to 63 m were sampled. No live unionids were found in any of the samples (Barton pers. comm. 2005). Historical sites around Middle Sister Island and East Sister Island in the western basin have not been surveyed recently, but 33 sites in U.S. waters around the nearby Bass Islands were surveyed in 1998 and no live unionids were found (Ecological Specialists 1999).

The Eastern Pondmussel has not been reported alive in the Lake Ontario drainage since 1996, when it was collected from two inland lakes (Consecon Lake and East Lake) in Prince Edward County (Metcalfe-Smith et al. unpublished data). No zebra mussels were present in the lakes at the time. Ligumia nasuta was not present at any of seven sites surveyed in the Moira River, Moira Lake, Skootamatta River and Salmon River in the same year. Three fresh shells were collected from Beaver Lake near Erinsville, Ontario, in the Salmon River watershed in 1998 (Schueler pers. comm. 2005). When the site was revisited in 2006, the lake was found to be infested with zebra mussels (Schueler pers. comm. 2006). The authors of the present report surveyed 15 sites in Consecon Lake, East Lake, West Lake and the Bay of Quinte in 2005. All sites were heavily infested with zebra mussels and not a single live unionid of any species was found. Based on conversations with the owner of a resort on Consecon Lake and the president of the Consecon Lake Cottagers’ Association, zebra mussels began to appear in the lake around 1999 and native mussels, which were once very abundant, have now disappeared. In 2006, several live specimens of L. nasuta were found in Lyn Creek, a tributary of Jones Creek which flows into the upper St. Lawrence River near Brockville, Ontario (Schueler pers. comm. 2006). The Lyn Creek record represents the easternmost occurrence of the Eastern Pondmussel in Canada.

The available data suggest that Ligumia nasuta has likely been lost from ~ 93% of its former range (in terms of extent of occurrence) in Canada due to the impacts of zebra mussels. The current extent of occurrence (EO) is approximately 3,400 km² as compared with 50,500 km² historically. The current area of occupancy (AO) is approximately 45 km², which was calculated using scales based on the current IUCN guidelines (Standards and Petitions Working Group 2006) as recommended by COSEWIC’s Criteria Working Group (Boles pers. comm. 2006). A 2×2 km grid (the standard grid size) was applied for estimating the AO of the Lake St. Clair population, but a 1×1 km grid was applied for estimating the AO of the Lyn Creek population because the Criteria Working Group recognizes that a smaller grid might be more appropriate for linear habitats such as rivers. In both cases, only live occurrences of the species were considered in the calculation of AO.

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