Black slug, Babosa común

15 cm long. Maturity is reached at about 2.5 cm.

The colour of the black slug is generally black, but the colouration is very variable and this slug can even be white. 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 (Red Slug). Young specimens of black slug do have a brown colour, which is later lost if and when the slug changes colour 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 (breathing hole) 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.

The black slug is mainly nocturnal and avoids exposure to sunlight. It is omnivorous, eating carrion, fungi, and vegetation (living and decaying). 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.