Rocky Mountain Capshell
The Rocky Mountain capshell is a native mollusk that is believed to have been in Colorado since the ice age.
About This Species
The rocky mountain capshell is believed to be a relict population from the last ice age. It is the only member of Family Acroloxidae that occurs in North America.
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Physical Characteristics
The shell of the rocky mountain capshell is delicate and limpet-like, with a slightly greater width at the anterior end. The apex has a spiny appearance and leans slightly to the left toward the posterior end. Its shell measures 4 to 7 mm in length and 2 to 3 ½ mm in width. The shell color is yellowish brown with variegated spots. Growth lines and radial striae are also visible.
Range
The rocky mountain capshell is very rare across its range and is designated as an "imperiled" species. In Montana this species is considered "critically imperiled". No occurrences have been noted in any other adjacent states, although fossil records suggest the rocky mountain capshell once occurred in Nebraska, Kansas, and Oklahoma. Due to the capshell's specific habitat requirements, the primary threats to this species include timber harvesting, pesticide application, fisheries management, and some recreational activities. One population decline in Colorado was observed as a result of lowered water levels. In Peterson Lake, Colorado two individual studies were conducted that suggest significant population decline, from 72 individuals per m2 in 1970 to only 3 individuals identified in 1992. In one study tiles were added to the substrate, resulting in a population increase. It is unknown if the population reached a stable level or crashed in the years following. The best method of conservation for the rocky mountain capshell is to maintain healthy aquatic ecosystems.
Habitat
Originally the rocky mountain capshell was believed to be restricted to high mountain lakes in the Rockies, but a recent discovery occurred in a very slow-moving portion of Beaver River in Alberta, Canada. It has been found in six lakes in Colorado, located in: Boulder County, Rocky Mountain National Park, and both Routt and Roosevelt National Forests. It is also present in one site in Montana and isolated populations in Canada. Due to its minute size, it is difficult to find and may exist in other habitats. The Rocky Mountain capshell clings to vegetation, rocks, submerged wood, or other larger mollusks. This species is found in water bodies that are low in elevation, high in calcium content, and high in conductivity. They also seem to require rocky substrate that is not too muddy or silty. The temperature requirements range from 35°F to 77°F, but they do persist in one pond in Colorado that ranges from 19°F-78°F. In three of the water bodies where the rocky mountain capshell was observed, abnormally high calcium levels and conductivities occur; it is speculated that because these sites are adjacent to roadways, the high levels may be caused by the salts, dust, and/or emissions from the roads.