Fawnsfoot (Truncilla donaciformis) COSEWIC assessment and status report: chapter 2

Executive Summary

Fawnsfoot
Truncilla Donaciformis

Species information

The Fawnsfoot is a small freshwater mussel with a typical adult length in Ontario of approximately 35 mm and a reported maximum length of 45 mm. The shell is moderately thick, oval to triangular, rounded on the anterior end and bluntly pointed on the posterior. The prominent posterior ridge is rounded and flattened dorsally. The beaks (the oldest part of the shell) are full, central and slightly elevated above the hinge line. The beaks are marked with 3-8 fine bars; the first bar is concentric and the others are weakly double-looped. The shell is smooth, yellow to greenish with numerous dark green rays often broken into v-shaped or chevron markings.

Distribution

The global range of the Fawnsfoot is limited to central North America where it is widely distributed, occurring in 23 American states and one Canadian province. In Canada, the Fawnsfoot is known only from the Great Lakes area where it historically occurred in lakes Huron, St. Clair and Erie as well as their connecting channels and the lower reaches of some tributaries. The current distribution is restricted to a single site in the St. Clair delta, a single site in Muskrat Creek of the Saugeen River drainage, a single site in the lower Sydenham River, the lower Thames River below London and the lower Grand River between Dunnville and Port Maitland.

Habitat

The Fawnsfoot is known to occur in areas of moderate to low flows in medium and large rivers at depths ranging from less than 1 m to greater than 5 m, although they can adapt to low flow environments such as lakes and reservoirs. The Fawnsfoot is usually associated with substrates of sand or mud but can be found in areas with coarser substrate. Remaining populations in Canada are usually found in the lower portions of larger rivers on soft sand or gravel substrates.

Biology

The Fawnsfoot has separate sexes but males and females do not appear outwardly different. Glochidia (immature juveniles) are obligate parasites on a fish host. Although not yet confirmed for Canadian populations, the host fishes are believed to be the freshwater drum and sauger, with freshwater drum assumed to be the primary host. The Fawnsfoot is a long-term brooder that reproduces in the spring and maintains glochidia in a pouch in the female’s gill overwinter for release the following spring. Juvenile mussels remain burrowed in the substrate for the first 3-5 years of life feeding on bacteria, detritus, and algae obtained from interstitial pore water. Adults can be found at the substrate surface and feed primarily by filtering bacteria, plankton and algae from the water column.

Population sizes and trends

The Fawnsfoot has always been a small component of the overall mussel community (< 5%) wherever it occurs, and recent records indicate that this continues to be the case. Of the five extant occurrences, two are represented by single specimens (Lake St. Clair, Muskrat Creek,) while another two (Sydenham River, Grand River) represent multiple individuals but from only a single site. Only the Thames River occurrence represents multiple animals collected at more than one site. The Great Lakes and connecting channel populations, excluding one in the St. Clair delta, have been lost. Populations in the Sydenham and Grand rivers still occupy the known historical range. No assessment can be made of the Muskrat Creek and Thames River populations as no historical information exists.

Limiting factors and threats

The major factors shaping the current distribution of the Fawnsfoot in Canada are the establishment of zebra mussels, which has resulted in large portions of historical habitat being rendered unsuitable, and a limited availability of riverine habitat primarily restricted by the distribution of its presumed glochidial host, the freshwater drum. Remaining riverine populations are limited to relatively small sections of the lower reaches of rivers where they are subjected to declining water quality resulting from agricultural and urban influences in the upper portions of the watersheds.

Special significance of the species

Freshwater mussels are sensitive indicators of overall ecosystem health and play important roles in structuring aquatic communities. Mussels provide food and habitat for other animals ranging from microscopic bacteria and plankton to large aquatic and terrestrial vertebrates. The presence of mussel beds provides physical stability of substrates and the filtering capacity of the mussels can have profound influences on water quality. The Fawnsfoot is one of only two members of the genus Truncilla that occurs in Canada.

Existing protection or other status designations

The federal Fisheries Act represents the single most important piece of legislation currently protecting the Fawnsfoot in Canada. The collection of freshwater mussels requires a permit issued under the authority of the Fisheries Act. The last remaining lake population is located in the territorial waters of the Walpole Island First Nation (WIFN). These waters are relatively low-impact areas used primarily for hunting and fishing, and access is regulated through user permits issued by WIFN. Areas where the Fawnsfoot occurs overlap with the distributions of several species at risk protected under the Species at Risk Act. The Fawnsfoot may benefit indirectly from protection afforded to these species or actions implemented under the direction of existing recovery strategies.

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 (2008)

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) Footnote c

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.

Page details

Date modified: