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Montana Field Guide

Montana Field Guides

Reticulate Taildropper - Prophysaon andersoni
Other Names:  Arion andersonii, Prophysaon hemphilli, Prophysaon pacificum, Prophysaon flavum

Species of Concern
Native Species

Global Rank: G5
State Rank: S1S2

Agency Status

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General Description
A moderately large slug of about 50 mm extended, but may reach 60 mm or slightly larger. Dorsal base color varies from dark grayish-brown to reddish-brown or yellowish, a diamond-mesh furrow pattern on the foot, sometimes highlighted with dark pigment. The mantle is 1/3 to 2/5 the body length, is sometimes distinctly paler, and appears granular, with a pair of dark lateral bands. Head color similar to foot, but antennae often darker. Pneumostome is near the middle of the mantle in the anterior half, and on the right side. The tail is not keeled, although there may be a lighter mid-dorsal stripe in some individuals. The sole is undivided (not tripartite) and pale, with a line of abscission near the posterior quarter (sometimes distinct only under magnification); the mucous is yellow to orange, especially in disturbed animals. As the common name indicates, taildroppers have a line of abscission where the tail (end of the foot) can be dropped (autotomized), much as in lizards, as a defense against predator attack (Hendricks 2012, Burke 2013). Internal anatomy is described by Pilsbry (1948) and Forsyth (2004).

Diagnostic Characteristics
Unlike other Montana slugs except the congeneric Smoky Taildropper. Dorsal surface of the Reticulate Taildropper has a distinct diamond-like mesh pattern of dark furrows. Smoky Taildropper has an indisctinct diamond-mesh pattern on the tail, but has a broad mid-dorsal stripe that is bounded by darker bands (lacking on the Reticulate Taildropper).

Species Range
Montana Range Range Descriptions


Range Comments
Pacific coastal region from Alaska to California; disjunct populations in northern Idaho and adjacent northwestern Montana west of the Continental Divide (Burke 2013). In Montana through 2009, 10 records in two counties: Flathead (7), Sanders (3). Elevation range is 674 to 1239 m (2210 to 4065 ft). Older reports of this species in northern Idaho are questioned by some authorities, but its presence in adjacent Montana indicates the records from Idaho are probably valid. First recorded from Montana in 2005. Up to 14 individuals were found at one Sanders County site in mid-October (Hendricks 2012).

Observations in Montana Natural Heritage Program Database
Number of Observations: 10

(Click on the following maps and charts to see full sized version) Map Help and Descriptions
Relative Density



(Observations spanning multiple months or years are excluded from time charts)

No information; relatively sedentary.

Mostly in mesic mixed conifer forest, often relatively close to water. Canopy species include western redcedar, western hemlock, grand fir, Douglas-fir, black cottonwood, paper birch, aspen, Engelmann spruce, western larch, western white pine, and lodgepole pine, with secondary canopy sometimes including alder, Pacific yew and mountain ash. Usually found under woody debris and leaf litter or in downed rotten wood, sometimes under rocks (Hendricks 2012).

Reproductive Characteristics
Hermaphroditic, based on internal anatomy (Pilsbry 1948 and Forsyth 2004).

Threats or Limiting Factors
Habitat occupied by Prophysaon andersoni (low to moderate elevation mixed conifer forest, often in valley bottoms near water) is threatened by logging, grazing, fire, and possibly weed control and rural home development. The impact of fire retardant on this and other terrestrial mollusks is not known. Little is known about this species, including its sensitivity to disturbance.

  • Literature Cited AboveLegend:   View Online Publication
    • Burke, T. E. 2013. Land snails and slugs of the Pacific Northwest. Corvallis, OR: Oregon State University Press. 344 p.
    • Forsyth, R.G. 2004. Land snails of British Columbia. Royal British Columbia Museum: Victoria, British Columbia, Canada. 188 pp.
    • Hendricks, P. 2012. A Guide to the Land Snails and Slugs of Montana. A report to the U.S. Forest Service - Region 1. Montana Natural Heritage Program, Helena, MT. vii + 187 pp. plus appendices.
    • Pilsbry, H.A. 1948. Land Mollusca of North America (north of Mexico), Volume II Part 2. The Academy of Natural Sciences of Philadelphia Monograph Number 2(2): 521-1113.
  • Additional ReferencesLegend:   View Online Publication
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    • Frest, T.J. and E.J. Johannes. 2001. An annotated checklist of Idaho land and freshwater mollusks. Journal of the Idaho Academy of Science 36(2):1-51.
    • Grimm, F.W., R.G. Forsyth, F.W. Schueler, and A. Karstad. 2009. Identifying land snails and slugs in Canada: introduced species and native genera. Canadian Food Inspection Agency, Ottawa, ON. 168 pp.
    • Hendricks, P., B.A. Maxell, and S. Lenard. 2006. Land mollusk surveys on USFS Northern Region lands. A report to the USDA Forest Service, Northern Region. Montana Natural Heritage Program, Helena, Montana. 11 pp. plus appendices.
    • Hendricks, P., B.A. Maxell, S. Lenard, and C. Currier. 2007. Land mollusk surveys on USFS Northern Region lands: 2006. A report to the USDA Forest Service, Northern Region. Montana Natural Heritage Program, Helena, Montana. 11 pp. plus appendices.
    • Smith, A.G. 1943. Mollusks of the Clearwater Mountains, Idaho. Proceedings of the California Academy of Sciences, fourth series, 23:537-554.
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Citation for data on this website:
Reticulate Taildropper — Prophysaon andersoni.  Montana Field Guide.  .  Retrieved on , from