The Lake Winnipeg Physa

This freshwater snail was described as being endemic to Lake Winnipeg, Manitoba and assessed as Endangered by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) in 2002. Despite annual searches, the last observation of the taxon was in 2006. The taxonomic uncertainty and the lack of genetic material for further study have resulted in a revised designation by COSEWIC of Data Deficient. Following an evaluation, the federal government has also determined that the species will not be listed under the Species at Risk Act.

General Description

The Lake Winnipeg Physa (Physa sp.) is described as freshwater snail and member of the Physidae. It has the following characteristics and distinguishing features:

• Shell is small (usually less than 11 mm long) and fragile with a dull, often pitted surface
• Shell is globe-shaped with a depressed spire and thin lip
• Fresh shells are bluish grey in colour, while the animal itself is light grey and sparsely peppered with black spots
• Shell width-length ratio is greater than 0.70, and the aperture-shell length ratio exceeds 0.80

Distribution

This snail has only recently been formally described and, therefore, there is no published information on its distribution. However, it appears to occur only in Lake Winnipeg, despite extensive surveys in Manitoba, Ontario, Saskatchewan, North Dakota, Minnesota and in nearby lakes and tributaries of Lake Winnipeg. Within Lake Winnipeg, its distribution is discontinuous and patchy, being found at only five of the 90 sites surveyed in 2001. Its disappearance from two historical sites suggests that it is in decline.

Habitat and Life History

The Lake Winnipeg Physa is found in shallow water (less than 1 m deep) on medium to large algae-coated rocks in windswept areas close to shore. The lake bottom is typically gravel, sand and rocks. It occurs only in areas where other snails thrive. The biology of the Lake Winnipeg Physa is not known. However, other Physa snails in Lake Winnipeg reproduce in June and July, and adults overwinter. The lifespan is likely not more than one year.

Diet

The Lake Winnipeg Physa feeds on algae.

Threats

Two factors have been identified as possible threats to the Lake Winnipeg Physa: habitat degradation due to shoreline modification and development, and water pollution from agricultural, municipal and industrial sources.

Similar Species

There are three other described species of Physa in Lake Winnipeg, P. gyrina, P. integra, P. jennessi skinneri, all of which are thinner in proportion to the total shell length than the Lake Winnipeg Physa. Other globe-shaped snails, which superficially resemble the Lake Winnipeg Physa include P. globosa and P. utahensis, both found in the United States.

Text Sources: Pip 2002 (COSEWIC Status Report).

For more information, visit the SARA Registry Website at www.SARAregistry.gc.ca.