Prophysaon coeruleum, (the blue-grey taildropper), is a species of slug belonging to the genus Prophysaon, a genus known for the autotomy of its tail. It is native to western North America, from southwestern British Columbia to northwestern California, with a more isolated population in Idaho. There are conservation efforts in place to protect the blue-grey taildropper, and it is on multiple lists of species at risk.Environment and Climate Change Canada. (2018). Recovery Strategy for the Blue-grey Taildropper (Prophysaon coeruleum) in Canada. Environment and Climate Change Canada.
Description Prophysaon coeruleum is a small, narrow slug which is notable for its typical blue-grey colour. It varies in length, adults ranging from , with nearly one-third of that length being the mantle.COSEWIC. (2016). COSEWIC Assessment and Status Report on the Blue-grey Taildropper Prophysaon coeruleum in Canada. Committee on the Status of Endangered Wildlife in Canada. Its tail, which can be self-amputated, has grooves along the length of it and a line at the point of amputation, although it does not have a dorsal keel. The blue-grey taildropper has a narrow margin surrounding its foot, which is typically light grey or white in colour, and it produces a clear mucus. The breathing pore, called a pneumostome, is found on the right front side of the body, on the mantle.
The blue-grey taildropper, despite its name, has been described in several colours, including blue, blue-gray, dark gray, brown and black, all observed with opalescent spots. The bluer individuals have typically been found in the middle to northern part of this species' range, whereas the darker individuals are typically observed in the south of its range. It has also been observed in light gray or white, the individuals of which are found throughout its range. The variation in colour is thought to be due to differences in the pigment melanin, although the reason for the variation in pigment between slugs is unknown. However, it may be due to differences in elevation: higher elevations expose the slugs to increased levels of UV-radiation, from which the blue-grey taildropper can be protected by having more melanin.
Taxonomy and phylogeny The blue-grey taildropper belongs to the class Gastropoda, the order Stylommatophora, and the family Ariolimacidae.MolluscaBase eds. (2024). Prophysaon Bland & W. G. Binney, 1873. MolluscaBase. https://www.molluscabase.org/aphia.php?p=taxdetails&id=995729 It is a member of the genus Prophysaon, the species of which can be found in the Pacific Northwest, from California to southern Alaska, and are known for the autotomy of their tail. There are ten known species in the genus Prophysaon according to Molluscabase: Prophysaon andersonii (J.G. Cooper, 1872), Prophysaon boreale (Pilsbry, 1948), Prophysaon coeruleum (T. D. A. Cockerell, 1890), Prophysaon dubium (Cockerell, 1890), Prophysaon fasciatum (Cockerell, 1890) Prophysaon foliolatum (A. Gould, 1851), Prophysaon humile (Cockrell, 1890), Prophysaon obscurum (Cockerell, 1890), Prophysaon pardalis (Pilsbry, 1948), and Prophysaon vanattae (Pilsbry, 1948).
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. 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.
History There is genetic variation within this species, along with evidence to suggest that it is composed of three major groups, as well as eight subgroups. This genetic variation, mostly observed in its southern range (southern Oregon and northern California), likely occurred 2.6 to 5.9 million years ago, during the Pliocene or late Miocene. This variation may have been caused by geological events, which would have caused fragmentation and genetic drift within the species. There is also evidence showing that disturbances from less than 2 million years ago, during the Pleistocene, also led to some variation. There is also the possibility that this species may actually be a cryptic species complex, as opposed to one singular species.
Canada { "type": "FeatureCollection", "features": [
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] }The blue-grey taildropper was first observed in Canada in 2002, though, as of 2016 there are 15 known subpopulations of this species in British Columbia, Canada. Most subpopulations of Prophysaon coeruleum observed in British Columbia were found in the Capital Regional District, with two subpopulations being instead located in the North Cowichan District, slightly further north on Vancouver Island (see map). However, there may be more subpopulations which have not yet been observed. Prophysaon coeruleum has an extent of occurrence of 658 km2 (254.1 mi2), however the fragmented nature of its distribution means that its index of area of occupancy is measured at 56 km2 (21.6 mi2) as of December 2015. The northern portion of this species' range is fragmented, which may be due to human activity leading to the fragmentation of their habitat, as well as the slow dispersal of these slugs. This low dispersal ability mixed with their specific habitat needs (moist and cool forest floor litter), means that if this species is extirpated from an area, it may be difficult to recolonize. The population size of the blue-grey taildropper in Canada has been estimated at fewer than 10 000 individuals, although the accuracy of this estimate is unclear.{ "type": "FeatureCollection", "features": [
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United States In the United States, the blue-grey taildropper can be found in western Oregon and Washington State, northern California, and northwestern Idaho (see map). More specifically, populations have been observed in the Coast Range, in the Cascade Range of Oregon and Washington, the Klamath mountains in southwestern Oregon and northern California, and the Puget Trough. The population observed in Idaho seems to be isolated from the rest, and also does not seem to be connected to the population in British Columbia.
Habitat The habitat of the blue-grey taildropper includes older forests or forests with similar features, which include 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 found 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 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 above sea level).
Diet The main components of the diet of Prophysaon coeruleum are fungal spores and hyphae. They come largely from mycorrhizal fungi, which are fungi that form symbiotic relationships with various vascular plants. The blue-grey taildropper 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 ability to self-amputate its tail is a behaviour that is triggered in response to disturbances, and likely serves as a defense against predators such as beetles. 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 (eg. 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. The blue-grey taildropper seems to reproduce within the same year it hatches, 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, and 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, as well as 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. Because a large part of its diet includes mycorrhizal fungi, this species may play a role in spreading the fungal spores of these fungi.
Threats There are several threats which may affect the blue-grey taildropper, although these vary according to the organisation in question. In the 2016 report published by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC), it lists the threats to the Canadian population of Prophysaon coeruleum, as determined using the International Union for Conservation of Nature's (IUCN) threats calculator. They cite the three most severe threats to the blue-grey taildropper. First, there are the changes to this species' natural habitats: for example, the introduction of exotic species of plants can inhibit native plant growth and affect habitat quality. Next, introduced invertebrates affect not only the habitat, but may also directly compete with or predate on the blue-grey taildropper. Third, climate change can have effects on these slugs, as well as severe weather, which can have various effects such as habitat loss and direct mortality, as well as long-term modifications of habitat. For example, droughts affect the moist habitat that this species needs, and can also limit food resources. Other potential threats which may affect the success of the blue-grey taildropper include the development of commercial and residential buildings, as well as transportation and human recreational activities.