Dusky slug (Arion subfuscus/fuscus) is common, exotic, terrestrial slug. It is native to northern Europe and was introduced into North America in the vicinity of Boston in 1842. By 1940 it was widespread across North America. It is found in moist or wet areas in deciduous and coniferous woodlands, in meadows on rocks, and in old fields and waste places. It is often encountered in areas of human activity, including in roadsides, gardens, campgrounds, wood piles, and window wells. In natural areas it is sometimes more abundant than native snail and slug species. It can a pest of agricultural crops, forest replantings, and gardens.
Two species of dusky slug are often treated as a single species complex, a group of species so similar that the boundaries are unclear. Aside from their geographic distribution, the two species can only be distinguished by the size and color of the genitalia of dissected individuals, or by analysis of their alloenzymes. Both species have been introduced into North America.
Adults are long and slender when extended, short and bell-shaped when contracted. The body is covered with rows of pale bumps, giving it a finely granular appearance. It is variable in color, but populations generally fit into one of four color groups: blackish-brown, yellowish-brown, orange, and reddish-brown. The orange or yellowish-orange color is mostly – or completely – due to a covering of mucus. When handled, the mucus will stain the handler’s fingers. There is usually a brown stripe on each side.
Great pond snail (Lymnaea stagnalis) is a very large, air-breathing, freshwater snail. It is commonly sold as an aquarium pet. The body contains both male and female reproductive organs (hermaphroditic). During copulation either the male role or the female role can be performed, though not both. This makes it the ideal subject for recent scientific research into handedness (chirality).
Great pond snail occurs in Europe, Asia, North America, and southern Australia. It occurs throughout the United States and Canada, but it is uncommon south of the 40th parallel. It is common in Minnesota. It is found in permanent, slow or still waters, usually with dense vegetation, including creeks, streams, rivers, marshes, swamps, and reservoirs, and at the edges of lakes and large ponds.
The shell is thin and has 4½ to 6 whorls. The last whorl is the body whorl and is greatly inflated. The remaining whorls are elevated forming a sharply pointed spire. When seen with the tip at the top and the opening facing up, the opening is on the right side. There is no door-like structure covering the opening of the shell. The shell is variable in color, tan to dark brown, and it has no obvious markings. A cavity within the shell has an air bubble that is refreshed every time the snail rises to the water surface to breathe.
Gray field slug (Deroceras reticulatum), also known as milky slug, is a common, exotic, terrestrial, smooth land slug. It is native to northern Europe, North Africa, and the Atlantic islands. It was introduced into North America and now occurs across the continent. It is most common in southern Canada and northern United States. It can be a serious crop pest, but is not listed as invasive nationally or in Minnesota. It is usually found above ground but under stones or leaf litter in open areas, especially cultivated areas.
Gray field slug is stout, 1⅜″ to 2″ long and white, cream, gray, or tan. When at rest, the body is contracted and the tentacles are retracted. When traveling, the body is stretched out, the tentacles are extended, and it exudes a clear mucus. When disturbed, it exudes white mucus over its entire body, leading to one of its common names, milky slug.