Blue-grey taildropper slug (Prophysaon coeruleum) COSEWIC assessment and status report: chapter 2

Species Information

The Blue-grey Taildropper is one of nine described species of taildropper slugs (genus Prophysaon) endemic to western North America. It is a small- to medium-sized slug (up to 45 mm long when extended) with a slender body. Distinguishing external features include blue-grey colour and parallel grooves and ridges on the back and sides of the tail. Internally, the slugs are distinguished from related species by features of reproductive anatomy.

Distribution

The species ranges from southern British Columbia south to Puget Lowlands in Washington State and through the Cascade Range into Oregon and northern California. Much genetic variation exists in the southern portion of the range. An isolated population exists in northern Idaho. In Canada, the species is known only from five sites on southern Vancouver Island, British Columbia.

Habitat

The Blue-grey Taildropper inhabits moist, coniferous or mixed-wood forests of varying age classes. All records from British Columbia are from within the Coastal Douglas-fir biogeoclimatic zone. In Washington State, the species is often associated with older forests. Required microhabitat features include abundant coarse woody debris or other cover, a deep forest litter layer, and shaded, moist forest floor conditions.

Biology

This species appears to have an annual life cycle, maturing and reproducing within one year. In British Columbia, specimens (all adults) have been found in late autumn only. The Blue-grey Taildropper feeds extensively on fungi, including species that form symbiotic mycorrhizal associations with roots of conifers. Like other taildropper slugs, this species is capable of self-amputation of the tail, an adaptation that is an effective anti-predator mechanism against invertebrate predators, such as ground beetles and carnivorous snails.

Population Sizes and Trends

The Blue-grey Taildropper slug was first documented from British Columbia in 2002, and due to its recent discovery virtually nothing is known of its population sizes and trends. These slugs tend to be solitary or occur in small groups at scattered locations. Only a few specimens have ever been found, although hundreds of localities on Vancouver Island and in the Lower Fraser Valley on the mainland have been searched for terrestrial gastropods over the past decade. The extent to which the slugs’ secretive habits, highly seasonal activity, and scattered distribution pattern contribute to their apparent rarity is presently unknown.

Limiting Factors and Threats

This species exists at the northern extremity of its geographic range in southwestern British Columbia. Most likely, its present-day distribution reflects the glacial history of the region and resettlement from the south after glacial retreat.

Main immediate threats to populations include habitat loss, fragmentation, and degradation through human activities and land conversions. Additional threats include competition with exotic gastropods and predation by native and introduced animals. Forest habitats outside protected areas are shrinking at an alarming rate on southern and eastern Vancouver Island. Isolation of populations in remnant forest patches increases the probability of local extinctions. Population isolation is likely to be exacerbated by summer droughts, predicted for the region under global climate change scenarios.

Special Significance of the Species

At the localities where it occurs, the Blue-grey Taildropper might perform an important ecological role by dispersing spores of mycorrhizal fungi that form essential symbiotic associations with tree roots. Although it also feeds on green plant matter, it is unlikely to become a horticultural pest because of its low densities and association with forests rather than with open habitats. With its often-brilliant bluish colouration this species is attractive and could become an emblem of invertebrate conservation in British Columbia.

Existing Protection

The Blue-grey Taildropper was placed on the provincial Red List of species at risk in British Columbia in 2005. Species on the Red List are species that are extirpated, endangered, or threatened in British Columbia. While not currently listed as an Identified Wildlife Species in British Columbia, the species could become a candidate for inclusion in a future version of the provincial Identified Wildlife Management Strategy.