Blue-grey taildropper slug (Prophysaon coeruleum) COSEWIC assessment and status report: chapter 5

COSEWIC Status Report
on the
Blue-grey Taildropper Slug
Prophysaon coeruleum
in Canada
2006

Species Information

Name and Classification

Prophysaon coeruleum Cockerell, 1890 (Blue-grey Taildropper slug), is a member of the large, cosmopolitan family Arionidae. The genus Prophysaon is endemic to western North America and consists of nine described species in two subgenera: Mimetarion, which includes P. vanattae, P. obscurum, P. fasciatum, and P. humile; and Prophysaon, which includes P. andersoni, P. boreale, P. coeruleum, P. dubium, and P. foliolatum. Characteristics that separate this genus from other North American genera include a greatly reduced penis, specialized epiphallus, and ability to self-amputate the tail (Pilsbry 1948). Further genital characteristics separate subgenera.

Prophysaon coeruleum was described based on material from Washington State with the type locality in Olympia. The classification of the species is as follows: Phylum Mollusca, Class Gastropoda, Subclass Orthogastropoda, Order Pulmonata, Suborder Eupulmonata, Infraorder Stylommatophora, Superfamily Arionoidea,Family Arionidae, Genus Prophysaon, Subgenus Prophysaon, Species P. coeruleum.


Morphological Description

Prophysaon coeruleum is a small to medium-sized slug with the length of adults up to about 45 mm when extended in movement (Kelley et al. 1999). The body is slender and tapering, and the tail lacks a dorsal keel. The mantle is large (almost 1/3 of the total length of the animal), and the pneumostome (or breathing pore) is located slightly to the anterior of the mid-point of the margin of the mantle on the right side. Distinguishing external characteristics include solid blue-grey colour without dorsal or lateral stripes and distinct, parallel grooves and ridges on the back and sides of the foot (Figure 1; Pilsbry 1948, Kelley et al. 1999). The margin of the foot is narrow and has a distinct border. Specimens found in British Columbia were mid-tone blue-grey, speckled with scattered, lighter flecking; two specimens found in late autumn lacked speckling. In specimens from the United States, the colour of the back and sides ranges from very pale, occasionally white, to very dark blue or blue-grey. The sole of the foot is light grey or white, and the mucus is clear. A thin, oblique constriction or impressed line is usually visible at the site where tail autotomy (self-amputation) takes place (Figure 1, bottom).


Figure 1: The Blue-grey Taildropper Slug, Prophysaon coeruleum, from Vancouver Island

The Blue-grey Taildropper slug, Prophysaon coeruleum, from Vancouver Island.

Specimen at top is from Colwood; specimen at bottom is from Rocky Point peninsula. Both slugs measured about 25 mm fully extended. Photographs by K. Ovaska.

As for most slugs, characteristics of the reproductive anatomy of P. coeruleum are useful for identification. In this species, the epiphallus consists of a distal bulbous, muscular portion and a proximal long, slender portion, which is in a tangled coil immediately anterior to the muscular structure. The ovotestis is large, with numerous lobules. The size of the penis appears to be variable and can be almost as large as the muscular portion of the epiphallus in some specimens (Ovaska et al. 2004). The vagina and the entire common duct are slender and long. This species differs from all species of the subgenus Mimetarion by having the distal portion of the epiphallus highly muscularized and abruptly enlarged (Pilsbry 1948). It differs from the larger P. andersoni and P. foliolatum by having a shorter, almost spherical distal portion of the epiphallus and a longer penis (Pilsbry 1948). Diagrams of the reproductive anatomy of P. coeruleum can be found in Pilsbry (1948) and Ovaska et al. (2004).


Genetic Description

Wilke and Duncan (2004) examined genetic structure of P. coeruleum throughout the species’ range using markers from two mitochondrial genes. The study included two specimens from Rocky Point, Vancouver Island, British Columbia. Within the overall range, the analysis showed a complex genetic structure and revealed three major clades, presumed to be about 2.6 – 5.9 million years old. Superimposed on this deep phylogenetic structure is a shallow structure with a total of eight minor clades. This shallow structure is less than 2 million years old and probably reflects perturbations associated with Pleistocene glaciations. The genetic distances among the three major clades are similar to or greater than those between other species of Prophysaon, and it is possible that the form currently known as “Prophysaon coeruleum” represents a complex of cryptic species (Wilke and Duncan 2004).

Most of the genetic fragmentation is found within the southern portion of the species’ range, especially in the Klamath region in southern Oregon (Wilke and Duncan 2004). The geographic distribution of the large major clade that includes British Columbia extends from southern Vancouver Island south through the Cascade Mountains in Washington State to south-central Oregon; a disjunct population in Idaho is also included within this major clade. This major clade, in turn, consists of three minor clades – a northern contingent, a small clade in south-central Oregon, and a curious clade with representatives in both southern Oregon and northern Idaho. The British Columbia specimens are a part of the large northern minor clade, with representatives also in Washington State and Oregon. Therefore, the Vancouver Island population does not represent a genetically distinct population.

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2018-01-02