Western Pearlshell rescued from future work site in Morrison Creek.  Photo by K Clouston

Western Pearlshell rescued from future work site in Morrison Creek. Photo by K Clouston

The Western Pearlshell seems to prefer cold clean creeks and rivers that support salmonid populations. They prefer sand, gravel, and cobble, especially in stable areas of the streambed. In areas where host fish are abundant, physical habitat is ideal, and human threats are minimal, the Western Pearlshell can attain very high densities, often covering the stream bottom.

Western Pearlshell rescued from future work site in Morrison Creek.  Photo by K Clouston

Western Pearlshell rescued from future work site in Morrison Creek. Photo by K Clouston

This is one of 4 North American species that may be hermaphroditic, meaning that individuals may have both male and female reproductive traits. However, this is rare in Western Pearlshell populations, and most have separate sexes. Fertilization is thought to occur in the spring and gravid females can be found from late summer to early spring. Host fish for Western Pearlshell are thought to include native and non-native trout and salmon.  Their average life spans are 60 to 70 years, with some living to more than 100.

Conservation Concerns
Recent conservation concerns about the Western Pearlshell follow the decline of Pacific salmon. Both need clean cold streams and rivers, and Western Pearlshells need salmon and trout hosts. Climate change has been implicated in the decline of the closely related Margaritifera margaritifera in North America and Europe and it is likely that Western Pearlshell will be similarly affected. Invasive species that compete with native fish may also affect it. Western Pearlshell have been extirpated throughout much of the mainstem Snake and Columbia Rivers of Oregon and Washington and have dramati­cally declined in abundance in part of the Truckee River in California. It has probably been extirpated from Utah and its range is contracting in Montana. Many historic sites have been lost and some populations show little evidence of recruitment. The fate of this species throughout much of its native range remains uncertain.