Prophysaon coeruleum

Blue-grey Taildropper
Conservation status
Vulnerable  (NatureServe)
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Mollusca
Class:	Gastropoda
Order:	Stylommatophora
Family:	Ariolimacidae
Genus:	Prophysaon
Species:	P. coeruleum
Binomial name
Prophysaon coeruleum Cockerell, 1890

Prophysaon coeruleum, (the blue-grey taildropper), is a small species of slug belonging to the genus Prophysaon, a genus known for autotomy. It is native to western North America, from southwestern British Columbia to northwestern California.

Description

Prophysaon coeruleum is notable for its blue-gray colour. It has been described in several colours, including blue, blue-gray, dark gray, brown and black, all observed with opalescent spots. It has also been observed in light gray or white. The variation in colour is though to be due to differences in the pigment melanin, although the reason for the variation in these slugs is unknown. It varies in length, adults ranging from 20 to 40 mm (0.8 to 1.6 in), with nearly one-third of that length being the mantle. Its tail, which can be self-amputated, has grooves along the length of it and a line at the point of amputation.

Distribution and dispersal

Prophysaon coeruleum can be found in western North America, from the southern tip of Vancouver Island, British Columbia, Canada, at its most northern point, down towards northern California, United States. It is thought to only be able to disperse tens to hundreds of meters every generation, meaning it cannot move far in its lifetime. Surveys suggest that the populations in its northern range are made up of several isolated populations, although the exact reasons for this isolation are not currently known.

Habitat

The habitat of the blue-grey taildropper includes older forests or forests with similar features, such as thick forest floor litter or coarse woody debris, and are found in areas that are cool, moist, and shaded. These forests can include both coniferous and mixed-wood forests. The eggs of the slug also require moist habitats, as they are susceptible to desiccation if left without moisture.

The populations of Prophysaon coeruleum observed in British Columbia has been observed in either the Coastal Douglas-fir biogeoclimatic zone, or the Coastal Western Hemlock biogeoclimatic zone. It was found in second-growth forests, with mixed-wood (coniferous and deciduous), and at elevations lower than 250 m (820 ft) above sea level. Observations from the United States, however, are more varied and include both coniferous and mixed-wood forests, and have been found at higher elevations (up to 1,650 m (5,410 ft) above sea level).

History

There is genetic variation within the different species, along with evidence to suggest that it's composed of three major groups and eight subgroups. This genetic variation, mostly observed in its southern range, likely occurred 2.6-5.9 million years ago due to geological events. There is also evidence showing that disturbances from less than 2 million years ago, during the Pleistocene, also led to some genetic variations.

Diet

The main components of the diet of Prophysaon coeruleum are fungal spores and hyphae. They come largely from mycorrhizal fungi, which form symbiotic relationships with various vascular plants. It has also been recorded as eating vascular plant tissues, lichens, and imperfect fungi.

Behaviour

Much is unknown about the behaviour of Prophysaon coeruleum, as it is largely unstudied. Its tail self-amputation is a behaviour that is triggered in response to disturbances, and likely serves as a defense against predators. During field surveys, this slug has been found both alone or with several individuals near each other, although it is unclear whether groupings occur due to the presence of preferred habitat or if is socially-motivated, such as for mating.

Reproduction

Much about the reproduction of the blue-grey taildropper is unknown. It is known to be oviparous, though its eggs and nests have only been observed in the United States. It most likely makes its nests in cool, moist environments, to keep its eggs from desiccating. Eggs from slugs of the genus Prophysaon are described as being white, opaque, and oval. P. coeruleum seems to reproduce within the same year they hatch, and may only survive for one year, as adults are rarely found in early spring. This would mean that they either spend the winter as eggs and then hatch in the spring, or that adults will hibernate through the winter and then lay their eggs in the early spring. This slug is also simultaneously hermaphroditic, meaning that a single slug will have two sets of reproductive organs (male and female) at one time, although self-fertilization of eggs has not been observed, which suggests that they likely mate with other individuals.

Ecology and interspecific interactions

Prophysaon coeruleum consumes both living and dead organic matter, such as plants and fungi, meaning they act as decomposers of organic matter. It is also a prey species, although the exact species which act as their predators are largely unknown, it is likely that they share similar predators as other slugs in the area. These animals may include vertebrates such as various types of birds, amphibians, and small mammals, and invertebrates such as ground beetles and Lancetooth snails. There may be competition between the blue-grey taildropper and other invertebrates, including introduced and invasive gastropods, which may have a negative effect on Prophysaon coeruleum populations. As a large part of its diet includes mycorrhizal fungi, they may play a role in spreading the fungal spores of these fungi.

Conservation

The blue-grey taildropper is listed on several lists of species at risk, both at the national level and the regional levels, and is subject to more than one conservation plan.

NatureServe

NatureServe evaluated Prophysaon coeruleum at the global, national, provincial and state levels. In 2010, it was given a global rank of G3G4 by NatureServe, meaning that it is between the statuses "Vulnerable" (G3) and "Apparently secure" (G4), although the rounded status is G3. This status is listed as needing a review.

In Canada, the blue-grey taildropper is only found in British Columbia, and the national and provincial NatureServe statuses are the same, at N2N3 nationally and S2S3 provincially, meaning they are between "Imperiled" (N2) and "Vulnerable" (N3). However, its national ranking in the United States falls on N3N4, falling between "Vulnerable" (N3) and "Apparently Secure" (N4). Additionally, there is a status for each state in which Prophysaon coeruleum can be found: "Unranked" in California, S1 ("Critically Imperiled") in Idaho, S3 ("Vulnerable") in Oregon, and S1 ("Critically Imperiled") in Washington state.

Canada

The blue-grey taildropper has a status of Threatened on Canada's Species at Risk Act (SARA) Schedule 1 as of 2019. Prior to this, from 2007 to 2019, they had a status of Endangered, but their status was changed in response to the change in evaluated status from Endangered to Threatened by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) in 2016.

COSEWIC determined the status of Prophysaon Coeruleum to be Threatened in 2016. Prior to that status, they had been listed as Endangered in 2006, but it was changed due to a re-evaluation.

Environment and Climate Change Canada (ECCC) published a recovery plan for the blue-taildropper in 2018, which was adapted from the Government of British Columbia's recovery plan. The recovery plan as published by the Government of British Columbia contains three recovery objectives, including the protection of habitats where they have been observed, the mitigation of threats to this species, and the continuation of research on the blue-grey taildropper.

United States

The blue-grey taildropper is not listed on the U.S. Endangered Species Act.

Idaho Fish and Game lists Prophysaon coeruleum's state conservation rank as being S1Q, meaning that they are "Critically Imperiled" (S1), but that there is some "uncertainty about taxonomic status" (Q). They are also listed as a Species of Greatest Conservation Need Tier 1 in the Idaho State Wildlife Action Plan.

Further reading

• Cockerell, T. D. A. (1890). The American Mollusca. Hardwicke's Science Gossip. 26: 114
• InvertEBase. (2018). Authority files of U.S. and Canadian land and freshwater mollusks developed for the InvertEBase (InvertEBase.org) project

External links

• Cockerell, T.D.A. (1890). "Notes on slugs, chiefly in the collection at the British Museum". The Annals and Magazine of Natural History. 6 (6): 278.