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COSEWIC assessment and status report on the Rocky mountain ridged mussel Gonidea angulata in Canada

Executive Summary

Rocky Mountain Ridged Mussel

(Gonidea Angulata)


Species information

The freshwater mussel Gonidea angulata, commonly known as the Rocky Mountain Ridged Mussel, was described by Lea in 1839. The shell is up to 125 mm long, 65 mm high, 40 mm wide, and with shell wall up to about 5 mm thick at mid-anterior; variable in form but typically rather thin, trapezoidal in shape, with posterior margin obliquely flattened and relatively broad, and with a sharp and prominent posterior ridge running from the umbo to the angular basal posterior margin of each valve. Juveniles of this species may be greenish/tan in appearance while adults are typically darker, becoming bluish-black.



The Rocky Mountain Ridged Mussel occurs from southern British Columbia in the Columbia River system south in Washington, Oregon, Idaho, Nevada and south-central California. Recently (April 2002), a dead shell of Gonidea angulata was collected from Park Rill Creek, a tributary to the Okanagan River, in the vicinity of Oliver, B.C. Other recent collections (1991) of live specimens in B.C. are from Skaha Lake and Vaseux Lake. Other collections have been made from Osoyoos Lake, Okanagan Lake at Okanagan Falls, and from Okanagan Lake (Haynes Point Park). Collections from these locales have occurred between 1906 and 1991. It is not known if the shells were collected live or dead. Historically, this species occurred from southern British Columbia to south-central California; however, the current range is believed to be considerably smaller.



Habitat occupied by G. angulata is generally characterized as substrates of lakes, streams, and rivers that range in size from gravel to firm mud with the presence of at least some fine material (e.g. sand, silt or clay). Preferred sites generally have constant flow, rather shallow water (typically < 3 m in depth), and well-oxygenated substrates, especially when occurring in finer sediments.



There is relatively little specific information available regarding the biology of G. angulata. At present it is presumed to reproduce like other unionids, via internal fertilization, producing glochidia that are parasitic (commesalistic) on an undetermined fish host. Due in part to its broad historical range, glochidia of G. angulata are likely associated with more than one species of fish. Adults of this species are generally sessile and may move only if repeatedly disrupted. From counts of annual growth rings, it is believed that G. angulata can live up to about 30 years.


Population sizes and trends

Population sizes in British Columbia have not been determined; however, they are believed to be declining, which may be inferred from the relatively few specimens of G. angulata collected and preserved in Canadian collections and declines in habitat water quality. There appear to be two distinct, severely fragmented populations of G. angulata in the Okanagan River system. In the U.S., this species is patchily distributed and locally abundant. In general, U.S. populations of G. angulata are regarded as declining. Documented densities in some areas are as high 183/m2 and local populations can be in the tens of thousands.


Limiting factors and threats

In both British Columbia and the western United States this species is threatened by the continued loss or degradation of suitable habitat. In general, unionid mussels are very sensitive to environmental changes and consequently the order contains a high percentage of endangered species in North America. Other threats to this species are eutrophication, heavy metals, and transition elements. In the short-term, a proposed re-alignment of the Okanagan River could negatively impact existing G. angulata populations, which would presumably benefit over the long-term.


Special significance of the species

This species is the only known living taxon in the genus. The monospecific Gonidea, however, has an extensive fossil record in the western portions of the U.S., dating at least to the Miocene. The genus is taxonomically isolated and not closely related to any of the numerous eastern North American forms. There may be one additional living species from Korea, thus providing one of the most significant examples of the Asian affinities of the western North American freshwater mollusc fauna.


Existing protection

No explicit protection exists for this species. Globally this species is considered to be vulnerable. In B.C., the exact status of G. angulata is unknown; however, some currently believe the species to be at least threatened. Gonidea angulata occurs on British Columbia’s Red-list, which includes any indigenous species or subspecies that have, or are candidates for, Extirpated, Endangered, or Threatened status in British Columbia.



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 5th 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.



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



(After May 2003)

Species: Any indigenous species, subspecies, variety, or geographically or genetically distinct population of wild fauna and flora.

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

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

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

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

Special Concern (SC)*: A species of special concern because of characteristics that make it particularly sensitive to human activities or natural events.

Not at Risk (NAR)**: A species that has been evaluated and found to be not at risk.

Data Deficient (DD)***: A species for which there is insufficient scientific information to support status designation.

*      Formerly described as "Vulnerable" from 1990 to 1999, or "Rare" prior to 1990.
**    Formerly described as "Not In Any Category", or "No Designation Required."
***  Formerly described as "Indeterminate" from 1994 to 1999 or "ISIBD" (insufficient scientific information on which to base a designation) prior to 1994.

Environment                           Environnement
Canada                                  Canada

Canadian Wildlife                   Service canadien
Service                                  de la faune

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