Eastern pondmussel (Ligumia nasuta) COSEWIC assessment and status report: chapter 2

Executive Summary

Eastern Pondmussel
Ligumia Nasuta

Species information

The Eastern Pondmussel, Ligumia nasuta (Say, 1817), is a medium-sized freshwater mussel with an average length of about 70 mm. It has a compressed, elongate shell with a distinctive, bluntly pointed posterior end. The outside of the shell varies in colour from yellowish- or greenish-black in juveniles to dark brown or black in adults. Narrow green rays, concentrated at the posterior end of the shell, are often visible in juveniles and light-coloured adults. The nacre is usually silvery-white or bluish-white in specimens from the Great Lakes basin.

Distribution

The range of the Eastern Pondmussel is restricted to eastern North America where it extends from the lower Great Lakes east through New York to New Hampshire and south, in coastal rivers, to South Carolina. In Canada, Ligumia nasuta is known only from the Great Lakes region of Ontario where it historically occurred in Lakes St. Clair, Erie and Ontario, their connecting channels, and the lower reaches of some tributaries. The Eastern Pondmussel appears to have been lost from nearly all of its former range in Canada, but still occurs in the delta area of Lake St. Clair. Another population was recently discovered in Lyn Creek, a small tributary of the upper St. Lawrence River near the outlet of Lake Ontario.

Habitat

The Eastern Pondmussel occurs in sheltered areas of lakes, in slack-water areas of rivers and in canals, where it prefers substrates of fine sand and mud at depths ranging from 0.3 to 4.5 m. In Lake St. Clair, it is currently found on substrates composed of over 95% sand at the transition zone between the emergent wetlands and the open waters of the lake.

Biology

The Eastern Pondmussel has separate sexes, but males and females differ only slightly in shell shape and are often difficult to tell apart. The glochidia (larvae) of Ligumia nasuta, like those of most other freshwater mussels, are obligate parasites of fishes. Ligumia nasuta is a long-term brooder that spawns in late summer, broods its glochidia over the winter and releases them in the spring. Fish hosts for the Eastern Pondmussel are unknown, but the occurrence of this mussel in coastal rivers along the Atlantic seaboard suggests that at least one of the hosts is tolerant of brackish water. Adult L. nasuta feed on bacteria, algae and other organic particles that are filtered from the water column. Juveniles live completely buried in the substrate and feed on similar food items obtained directly from the substrate or interstitial water.

Population sizes and trends

The Eastern Pondmussel was one of the most common species of freshwater mussel in the lower Great Lakes, numbering in the billions, prior to the invasion of the zebra mussel (Dreissena polymorpha) in the late 1980s. It has likely been extirpated from nearly all previously inhabited areas in Canada due to the impacts of zebra mussels. A remnant population of Ligumia nasuta, with an estimated size of 22,000 – 44,000 individuals, currently occupies shallow, nearshore areas of the Lake St. Clair delta within the territory of the Walpole Island First Nation. A second population of unknown size was discovered in Lyn Creek, a tributary of the upper St. Lawrence River, in 2006.

Limiting factors and threats

Zebra mussels constitute the most significant threat to the continued existence of the Eastern Pondmussel in Canada. More than 90% of historical records for Ligumia nasuta are from areas now infested with these aquatic invasive organisms. According to several climate change models, climate warming is likely to cause a drop in water levels in the lower Great Lakes which in turn could cause the shallow St. Clair delta to dry up and further reduce the amount of habitat available to native mussel communities. 

Special significance of the species

The Eastern Pondmussel was a significant component of the Great Lakes mussel fauna historically, being the fourth most common species in the lower Great Lakes and connecting channels prior to 1990. It is reasonable to assume that this species contributed significantly to the function of freshwater mussel communities in the Great Lakes ecosystem up to that time.

Existing protection

Ligumia nasuta is ranked G4G5 globally, N4N5 in the United States and N2N3 in Canada. It is listed as Lower Risk-Near Threatened in North America on the IUCN Red List of Threatened Species. It has been designated as Endangered in Ohio and Delaware, Threatened in New Jersey and North Carolina and Special Concern in Massachusetts and Connecticut. The federal Fisheries Act protects freshwater mussels and their habitats in Canada because fishes are broadly defined under the Act to include shellfishes. The collection of live mussels in Ontario is considered “fishing” and falls under the Ontario Fishery Regulations made under the Fisheries Act. The population of the Eastern Pondmussel that inhabits the Canadian portion of the Lake St. Clair delta is somewhat protected from human disturbance as user permits are required to access Walpole Island First Nation territory.

COSEWIC History

The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) was created in 1977 as a result of a recommendation at the Federal-Provincial Wildlife Conference held in 1976. It arose from the need for a single, official, scientifically sound, national listing of wildlife species at risk. In 1978, COSEWIC designated its first species and produced its first list of Canadian species at risk. Species designated at meetings of the full committee are added to the list.  On June 5, 2003, the Species at Risk Act (SARA) was proclaimed. SARA establishes COSEWIC as an advisory body ensuring that species will continue to be assessed under a rigorous and independent scientific process.

COSEWIC Mandate

The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) assesses the national status of wild species, subspecies, varieties, or other designatable units that are considered to be at risk in Canada. Designations are made on native species for the following taxonomic groups: mammals, birds, reptiles, amphibians, fishes, arthropods, molluscs, vascular plants, mosses, and lichens.

COSEWIC Membership

COSEWIC comprises members from each provincial and territorial government wildlife agency, four federal entities (Canadian Wildlife Service, Parks Canada Agency, Department of Fisheries and Oceans, and the Federal Biodiversity Information Partnership, chaired by the Canadian Museum of Nature), three non-government science members and the co-chairs of the species specialist subcommittees and the Aboriginal Traditional Knowledge subcommittee. The Committee meets to consider status reports on candidate species. 

Definitions

Wildlife Species:
A species, subspecies, variety, or geographically or genetically distinct population of animal, plant or other organism, other than a bacterium or virus, that is wild by nature and is either native to Canada or has extended its range into Canada without human intervention and has been present in Canada for at least 50 years.

Extinct (X):
A wildlife species that no longer exists.

Extirpated (XT):
A wildlife species no longer existing in the wild in Canada, but occurring elsewhere.

Endangered (E):
A wildlife species facing imminent extirpation or extinction.

Threatened (T):
A wildlife species likely to become endangered if limiting factors are not reversed.

Special Concern (SC)Footnotea:
A wildlife species that may become a threatened or an endangered species because of a combination of biological characteristics and identified threats.

Not at Risk (NAR)Footnoteb:
A wildlife species that has been evaluated and found to be not at risk of extinction given the current circumstances.

Data Deficient (DD)Footnotec:
A category that applies when the available information is insufficient (a) to resolve a species' eligibility for assessment or (b) to permit an assessment of the species' risk of extinction.

 

Canadian Wildlife Service

The Canadian Wildlife Service, Environment Canada, provides full administrative and financial support to the COSEWIC Secretariat.

 

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