Sphaerium occidentale is endemic to North America (Mackie, 2007). The species is discontinuously distributed from Newfoundland and New Brunswick to British Columbia in regions containing calcareous deposits. Its southern distribution extends to Florida, Utah, and Colorado (Clarke, 1981; Williams et al, 2014).
This species was first found in British Columbia in 1890 in what is now Yoho National Park (Canadian Museum of Nature Collections). It was not found again until 2012, when it was discovered in the Flathead River Valley by a joint team from the Royal BC Museum, Sierra Club BC, and Wildsight.
This species has a preference for, or requires, a habitat that dries up for part of the year (Herrington, 1962). Typical sites are ditches, swamps, and small shallow ponds (Clarke, 1981). It is found among grass and leaves (Herrington, 1962), and is sometimes found living among damp leaves entirely out of water (Clarke, 1973).
Note Author: Ian Gardiner
