Find out more about our Species at Risk

Threaded Vertigo

Threaded Vertigo – Nearctula spp.

Species: Threaded Vertigo – Nearctula spp.

BC Status: Blue

COSEWIC Status: Special Concern

Key Information:

This tiny little snail can be found in moist mixed forest stands, particularly on bigleaf maple trees in the crevices of the bark. The threaded vertigo does hibernate in winter and can go into a state of torpor (inactivity) during the hot dry spells in the summer. During the autumn is the most likely time to find these little snails.

The threaded vertigo is about 2.5 mm in size and their food requirements are on the micro scale as well. They typically feed on microorganisms found in the bark of trees. Epiphytes, such as mosses, lichens, liverworts, etc, provide habitat for their food sources as well as for them! They also can be found feeding upon decaying plant matter and fungi while they are in the leaf litter.

These little snails do have predators, songbirds that feed on trees and shrubs can be found feeding on these little snails. There are certain snail-eating beetles that scale the trees at night for their prey. Being a fairly sedentary species (typically living on one tree), they have very few defense mechanisms aside from their camouflage and finding an elusive spot to live and hide out.

During surveys, up to twelve snails have been found on one tree up to 2 m from the base, but field recordings and studies are very limited at this stage.  There have been an increase in sightings, but that is only in recent years as scientists and naturalists alike have done more research into the threaded vertigo, and there is still very much to learn. Cortes Island is the most northern extent of its range and the distribution here is likely much smaller than down south. This could be due to climate and/or available tree species.

Loss in habitat for this little snail has been the largest cause of its decline in the population size.  Human activities such as logging, housing development, and removal of wood for things like firewood are all threats to the threaded vertigo. Their host trees are typically maples and although they are not sourced for lumber, the felling of these trees in forests and their lack of replanting are detrimental to the snails’ future.

On top of tree removal, any hydrological changes are also threatening to the threaded vertigo. These snails and other moist-forest creatures need a particular moisture content to thrive. Introduced species such as carabid beetles are another threat, as predators to these little snails.

Identification – what to look for:

  • Tiny snail with a cylindrical shell between 2.5 mm to 3.3 mm
  • Dull to dark brown shell in colour with parallel lines across it
  • Dark grey body

Where are they found?

  • Lowland areas around the Straight of Georgia, in southwestern BC down into California
  • Moist, mature mixed forests with an elevation below 200 m
  • In the bark of bigleaf maples, on lichens/mosses on trees, amongst the leaf litter, or on the undersides of ferns

Cool facts!

  • The threaded vertigo can live for up to 2 years!
  • They are hermaphroditic and they can cross fertilize with other threaded vertigo snails
  • Eggs are laid singly
  • These snails can be transported via windstorms and water surges

Useful links: 

COSEWIC Assessment and Status Report–Threaded Vertigo (pdf file)
Species At Risk Fact Sheet – Nearctula sp. 1 (pdf file)
Salt Spring Island Conservancy–Threaded Vertigo
Management Plan for the Threaded Vertigo (Nearctula sp.) in Canada (pdf file)
Government of Canada – Species Profile – Threaded Vertigo

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Friends of Cortes Island Society (FOCI) is a charitable organization that has been active for over 25 years. Our organization exists to monitor and preserve the health of local ecosystems, and to provide educational programs that foster a greater understanding of the natural environment. Through all of our projects, we work to promote environmental integrity through community responsibility.