Shape: Obovate to trapezoidal. Slightly laterally compressed. The shell has an angular ridge that runs from the beak to the basal part of the posterior margin; this ridge may be less angular in specimens living in slow-moving water. The ventral margin is usually straight. The shell is heavier than that of all other native species.
Periostracum: Color yellowish-brown to brown or black. No shell rays or sculpturing on the shell.
Lateral Teeth: Absent.
Pseudocardinal Teeth: The right valve has one small tooth and the left valve has either one small tooth or none at all. The teeth are small and compressed, sometimes hard to distinguish.
Nacre: Usually white, but sometimes salmon-colored in fresh specimens and pale blue toward the posterior margin and beak cavity.
Little is known about the life history of this species. Gravid females have been found during the months of April through July. Glochidia are released individually in watery mucous, or within light-colored leaf-like conglutinates. Fish host species are unknown. It is a relatively slow-growing, long-lived species; some authors have suggested a lifespan of 20-30 years based on counting its growth rings, but this method can be an unreliable way to age mussels25
Habitat descriptions are provided in survey reports. G. angulata occur in streams of all sizes and less frequently in lakes. They are found mainly in low to mid-elevation watersheds. They often share habitat with M. falcata throughout much of the Pacific Northwest, but their range rarely extends into high elevation headwater streams where M. falcata may occur. Like other stream-dwelling species, they are more common in stable stream reaches and tend to avoid areas with shifting sediments or areas prone to scour or frequent dewatering. However, G. angulata seem to be more tolerant of fine sediments than M. falcata and may occupy depositional habitats and banks. For example, in the Salmon River, Idaho, where both species co-occur, G. angulata dominated in sand and gravel bars, comprising 97.1 percent of overall mussel density, but in more stable, boulder-dominated reaches nearby, M. falcata comprised 94.9 percent of overall mussel density. Thus, distinct habitat preferences allow for habitat partitioning in streams where M. falcata and G. angulata co-occur. Unlike M. falcata, G. angulata also occurs in impoundments and natural lakes, including nutrient-rich waterbodies and in soft substrates to water depths of ten feet.
Also known as the Rocky Mountain ridged mussel, this species is widely distributed west of the Continental Divide from California to British Columbia. It is found east to Idaho and Nevada, and in the northern part of its range it is mainly distributed east of the Cascades but also occurs on the west side. There is a historical record from the Columbia River in western Montana, although this record is problematic because the Columbia River is not in Montana. Some researchers reason that the record was probably from the Clark Fork River or Kootenai River in the Columbia River headwaters, but these species are not now known to occur in these waters and they may have been extirpated following the construction of two impoundments or due to metal contamination. Its strongholds are in large tributaries of the Snake River and Columbia River in Washington, Idaho, and Oregon.
Distribution in British Columbia
This species is known from the Okanagan Basin and 1 record from Kootenay River (Gelling 2008).
G. angulata have disappeared from their original range in California, particularly in southern California and the Central Valley, as well as from many sites in the Snake and Columbia River basins, presumably due to environmental degradation. However, the magnitude and geographic extent of the declines are not fully known. There is a critical need to understand the life history, reproduction, distribution and ecology of G.. angulata in order to effectively conserve and manage them. In the absence of further research, it is prudent to take measures to protect the aquatic ecosystems where they currently exist.
The Unionidae are distributed throughout the world, but are most diverse in eastern and central North America, and in China and Southeast Asia.
Recommended citation: Author, Date. Page title. In Klinkenberg, Brian. (Editor) 2014. E-Fauna BC:
Electronic Atlas of the Fauna of British Columbia [efauna.bc.ca]. Lab
for Advanced Spatial Analysis, Department of Geography, University of British
Columbia, Vancouver. [Accessed:
25/05/2016 1:35:21 PM]
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