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Blue-grey Taildropper
Conservation status
Vulnerable  (NatureServe)[1]
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.^[2] It is native to western North America, from southwestern British Columbia to northwestern California.^[3]

Prophysaon coeruleum is notable for its blue-gray colour. It has been described in several colours, including blue, glue-gray, dark great, brown and black, all observed with opalescent spots.^[4] It has also been observed in light gray or white.^[4] 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.^[4] 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.^[5] Its tail, which can be self-amputated, has grooves along the length of it and a line at the point of amputation.^[2][6]

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, USA.^[5] It is thought to only be able to disperse tens to hundreds of meters every generation, meaning it cannot move far in its lifetime.^[4] 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.^[5]

The habitat of the blue-grey taildropper includes older forests or forests with similar features, such as thick forest floor litter, duff, or woody debris, and are found in areas that are cool, moist, and shaded.^[3][4] These forests can include both coniferous and mixed-wood forests.^[6] The eggs of the slug also require moist habitats, as they are susceptible to desiccation if left without moisture.^[6]

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.^[6] 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.^[6] 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).^[6]

There is genetic variation within the different species, along with evidence to suggest that it's composed of three major groups and eight subgroups.^[4] This genetic variation, mostly observed in its southern range, likely occurred 2.6-5.9 million years ago due to geological events.^[4] There is also evidence showing that disturbances from less than 2 million years ago, during the Pleistocene, also led to some genetic variations.^[4]

The main components of the diet of Prophysaon coeruleum are fungal spores and hyphae.^[7] They come largely from mycorrhizal fungi, which form mutually beneficial (symbiotic) relationships with various vascular plants.^[5][7] It has also been recorded as eating vascular plant tissues, lichens, and imperfect fungi.^[7]

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.^[5] 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.^[5]

Much about the reproduction the 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.^[6] It most likely makes its nests in cool, moist environments, to keep its eggs from desiccating.^[6] Eggs from slugs of the genus Prophysaon are described as being white, opaque, and oval.^[6] 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.^[6][5] 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.^[5] 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.^[5]

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