Montana Field Guide

A small slug, animals may reach 30 mm or more, but are often smaller. The overall appearance is dark. Base color on the mantle and back is dark brown to dark purplish, with abundant bluish flecking on closer inspection, on the tail highlighting a fine pattern of reticulation. The head and tentacles appear uniformly darker than the mantle and tail on some individuals. The mantle covers less than half the length of the body. The posterior margin of the mantle is deeply notched, similar to Udosarx. The pneumostome present in the posterior half of the mantle margin on the right side, above or slightly anterior to the mantle cleft. The reticulation on the posterior half of the body (tail) is indistinct, predominately with longitudinal lines above and oblique on the sides. The back of the tail has a prominent keel especially noticeable in contracted animals. The sole is tripartite (having two longitudinal furrows); the mucous is clear (Hendricks 2012, Burke 2013). Internal anatomy is described by Pilsbry (1948). The original description was based on animals from Meadows, Adams County, Idaho.

A combination of size, coloration, mantle markings, and overall morphology readily distinguish this species from other slugs. The sole of the foot is tripartite (having two longitudinal furrows best seen when the animal crawls on a clear surface), a strongly keeled back (most prominent when the animal is more or less contracted), a small notch on the right posterior mantle margin, a pneumostome (breething pore) above or anterior to the mantle cleft. The similar lyre mantleslug slug (Udosarx lyrata) shares these traits, but Zacoleus is dark brown to dark purplish (instead of cream or bluish-gray) with abundant pale blue flecking on the tail highlighting a fine pattern of reticulation, the mantle is uniformly colored lacking any dark markings.

Sedentary.

Primarily in mesic mixed conifer forest, often near water such as stream-side riparian areas and seeps, but also in more xeric sites. Canopy species include western redcedar, western hemlock, grand fir, Engelmann spruce, Douglas-fir, western larch, ponderosa pine, lodgepole pine, black cottonwood, paper birch, with a secondary canopy of alder, willow, and dogwood. Usually found under woody debris and leaf litter or in downed rotten wood, sometimes under rocks (Hendricks 2012).

The species is herbivorous; food includes the epiphytic cryptogam Frullania (Pilsbry 1948).

The species is hermaphroditic, based on internal anatomy (Pilsbry 1948).

Threats include logging and grazing over most of the known and potential ranges. The species is restricted to rather moist sites, generally in exceptionally botanically diverse and intact forests. Logging of relatively intact moderate-elevation Douglas-fir forest, grazing of much of the logged terrain, highway construction and other river right-of-way impacts, and severe forest fires all pose threats. The species has lost most of its habitat and most historic sites, according to Frest and Johannes (1995).