Large Arion Slugs Arion ater/rufus/vulgaris/flagellus/hortensis/distinctus/etc

There are several large slugs in the Arion genus prevalent in the UK, many introduced from elsewhere in Europe and invasive elsewhere globally. Among the most common are the Large Black (A. ater), the Large Red A. rufus and the Spanish vulgaris. All three can be black, dark brown, red, orange, yellow-brown, grey and even white with a red, brown or body-coloured, striped fringe to the footplate and they cannot be reliably differentiated by visible characteristics. Moreover, ranges overlap and hybrids exist.

The Large Black, also called European Black is native to Europe and invasive in Australia, Canada and USA. Adults are generally black but can be brown, red or grey; young are brown or white, turning grey. Foot fringe is usually black. The Large Red, also called Chocolate, is often red or orange but can be yellowish, brown, grey or, rarely, black. Its foot fringe is striped and usually a contrasting colour to the body. Its tentacles are usually darker than the body. Juveniles can have lateral bands. It is prevalent in much of temperate Western Europe including the southern UK, but the Large Black is more common in northern UK and Scandinavia. The two can hybridise.

They can reach some 10-15cm (4-6 inches) long when fully extended. The Large Red is often among the longest. The Spanish Slug is typically dark brown but can be other colours. It is native to southern France and northern Spain but is now prevalent in much of Europe, including the UK, and hybridises with the other large Arion slugs. It has also displaced the Large Red from some areas of the UK. The Durham Slug A. flagellus is native to the northern Iberian peninsula but has also been introduced to the UK and elsewhere.

Like other slugs, they are hermaphrodite (both male and female) but rarely self-fertilise. However, they can mutually fertilise so that two mating partners each can lay 20-150 eggs every 1-3 weeks a season. Eggs are white, opaque (due to calcium) and spherical, up to 5mm diameter. Despite having different-shaped genitalia (one of the distinguishing features together with DNA), the Arion species can and do hybridise.

Their high reproductive ability is in part because they have an important job in recycling decaying matter. They are mainly nocturnal. Diet is omnivorous, including fungi, (mainly decaying) plant matter, carrion, faeces and worms. Predators to keep their population in check include some birds, hedgehogs, badgers, foxes, toads, snakes and slow worms.