Terrestrial slugs produce two other forms of mucus that facilitate locomotion and prevent death from drying.

[4] Native to Europe, the black slug is an invasive species in Australia, Canada (British Columbia, Newfoundland, Quebec), and the United States (Pacific Northwest, Alaska).

Young specimens tend to be brown or ivory whitish, turning to grey before becoming characteristically black at maturity.

[6] Rust-brown individuals are arguably classified as a separate species Arion rufus (Red Slug).

[8] The black slug is omnivorous, and its diet includes fungi, carrion, earthworms, leaves, stems, dead plant material and dung.

After mating, the black slug seeks a dark, moist environment such as beneath mosses—occasionally within topsoil—to lay its eggs about 5 mm (0.20 in) in diameter.

[8] The ovotestis (gonad or hermaphrodite gland) produces gametes at the proximal end of the reproductive system.

All three forms of mucus help protect the slug from dehydration, as well as to locate and identify other slugs to mate with, or even prey upon.This species is found in northern Europe (including Great Britain and Ireland), Canada, the Pacific Northwest,[6] and most recently in Australia and some Caribbean countries: The black slug is mainly nocturnal and avoids exposure to sunlight, although in introduced areas, it has been observed to be active all parts of the day and night.

It is omnivorous,[17] eating carrion, fungi, animal feces, algae, lichen, and vegetation (living or decaying).

The study theorized these high assimilation rates might result from abundant enzymes in slug digestive systems: amylase, invertase, cellulase, xylanase, and chitinase.

A recent study further explored Arion ater’s gut microbiome and discovered the black slug to have far more bacteria within its gut than did neighboring, northern European insects, including members of the orders Coleoptera, Isoptera, Orthoptera, and Diptera.

This study learned the black slug’s gut microbiome to be functional at a wide range of temperatures and pH levels, and its results suggest that the mycobacterial culture may be uniform throughout the black slug’s gut and that Gammaproteobacteria composes most of this culture.

Most animals prefer not to prey upon the black slugs because of the taste of its mucus and because this mucus can make them slippery and consequently difficult to capture; however, this slug does have some natural predators, including the hedgehog, badger, shrew, mole, mouse, frog, toad, snake, carnivorous beetle, and some birds.

[22] Arion ater has been introduced to southeastern Australia and to North America, where it occurs in Newfoundland, southern British Columbia, the Pacific Northwest of the United States and some parts of Alaska.

In the past two decades, its mating with the non-native (at northern latitudes) pest species Arion vulgaris (or Spanish slug) has resulted in a more resilient hybrid exhibiting increased tolerance to cold.

Black slugs are of special concern in fragmented ecosystems and areas with high shrub and tree cover.

[18] Around the 1970s, the black slug came to Alaska via nursery plants, potting soil, or among shipments containing wood pallets.

They ranked the species based upon distribution, biological characteristics, ecological impacts, and feasibility of control.

Again, the black slug is a prolific, omnivorous consumer, capable of inhabiting a wide range of conditions with high fecundity.

[23] Its effect upon local ecology received a moderate score; while the black slug puts pressure on native systems, it does not appear to catalyze entire regime shifts within the habitats where it has invaded.

[7] Arion ater has spread in Alaska in the following places: Anchorage, Cordova, Yakutat, Gustavus, Juneau, Sitka, Tenakee Springs, Ketchikan, and Kodiak Island, and in some of these areas, the black slug threatens lilies and orchids.

[23] It is unlikely complete removal can be achieved given high fecundity, but fencing and educating the public to hand pick black slugs might mitigate associated invasion risks.

Additionally, this study could not determine whether high fungi-consumption resulted from dietary preference or from higher concentrations of alien slugs amidst Douglas fir forests, which promote fungal growth.

However, they concluded the black slug may eat more because of its shorter life span (one compared to several years), requiring faster consumption rates to reach sexual maturity.

Fences can be used to protect small areas and can be commercial (often electrical) or easily homemade with sawdust, crushed eggshells, ground oyster shells, soap, cinders, or diatomaceous earth.

[25] Black slugs are edible but rarely consumed by humans; they taste horrible, may bioaccumulate pesticides, and potentially carry French heartworm (Angiostrongylus vasorum).