Black slug

Appearance

The size of the slug varies from 10 to 15 cm. Maturity is reached at about 2.5 cm.

The color of the black slug is generally black, but the colouration is very variable and this slug can even be white. The general trend is for a darker pigmentation the farther north the species is found. The classification of brown-coloured and rust-coloured "black" slugs is somewhat disputed; the brown variation is sometimes considered to be a separate species, ''Arion rufus'' . Young specimens of black slug do have a brown colour, which is later lost if and when the slug changes color to the mature state.

The slug covers itself in a thick foul-tasting mucus which serves as both protection against predators as well as a measure to keep moist. It is somewhat difficult to wash off.

Like other members of the family Arionidae, the black slug has a pneumostome on the right side of its mantle through which it breathes. This mantle is the part which in snails secretes a shell, and in this species of slug the mantle contains a resilient protective structure of calcareous granules.

Distribution

This species is found in northern Europe and the Pacific Northwest.

⤷  Great Britain
⤷  Ireland
⤷  Sweden
⤷  Germany...hieroglyph snipped...
⤷  Newfoundland...hieroglyph snipped...
⤷  Scandinavia
⤷  British Columbia, Canada

This species is well-established in Washington and Oregon, and it is considered to represent a potentially serious threat as a pest, an invasive species which could negatively affect agriculture, natural ecosystems, human health or commerce. Therefore it has been suggested that this species be given top national quarantine significance in the USA.

Behavior

The black slug is mainly nocturnal and avoids exposure to sunlight. It is omnivorous, eating carrion, fungi, and vegetation . The slug prefers moist conditions, such as lawns, making it an unwelcome sight for homeowners. This moist environment is essential for the terrestrial locomotion of the slug to function.

Like all other species of the Arionidae family, the black slug is a hermaphrodite, meaning it can fertilize itself if needed, although a mate is preferred. After mating, the slug lays eggs about 5 mm in diameter. The favoured location for eggs is a dark, cold, damp place such as a compost heap.

In the past two decades, its mating with the non-native pest species ''Arion vulgaris'' has resulted in a more resilient hybrid exhibiting increased tolerance to cold.

The mucus of the slug is highly distasteful to many animals. However, this slug does have some natural predators, including the hedgehog, badger, shrew, mole, mouse, frog, toad, snake, carnivorous beetle, and some birds. When picked up or touched, the black slug will contract to a hemispherical shape and begin to rock from side to side. This defensive behaviour confuses predators, and is unique in the Arionidae family.