Slimy, silent, and surprisingly elusive, Britain’s biggest slug is a creature of the night. The ash-black slug (Limax cinereoniger) is a remarkable nocturnal gastropod known not just for its impressive size but also for its climbing skills and secretive woodland lifestyle.
Despite being one of the world’s largest land slugs, it often goes unnoticed, slithering up trees and hiding beneath logs while the rest of the forest sleeps.
How big are ash-black slugs?
The ash-black slug (Limax cinereoniger) is one of the world’s largest land slugs, measuring between 10cm and 20cm in length – but have been known to reach up to 30cm.
What do they look like?
Adult ash-black slugs are darker in colour than their juvenile counterparts, which are a warm toffee-brown colour rather than the classic black or dark grey of the adult. Fully matured ash-black slugs will also have a white or yellowish ridge – called a keel, which runs at least half the length of their back. Their tentacles are spotted grey and they have a pale stripe along their foot as well.
Where are they found?
Ash-black slugs are found across the UK but are uncommon, restricted to old or ancient woodland. These habitats are under significant threat, so these slugs are becoming more difficult to find. Air pollution also threatens many of the lichen species the ash-black slug feeds on.
As these woodland animals are nocturnal and usually live under dead wood or leaf cover, ash-black slugs are tricky to spot.
What do ash-black slugs eat?
These woodland dwellers feed on fungi, lichens and algae – all found in their surroundings on the forest floor.
How do they breed?
Although they’re hermaphrodites (with both female and male sex organs) and can fertilise their own eggs if a partner is unavailable, they prefer to mate with another slug.
These excellent climbers will climb to the top of trees to find a mate – leaving a trail of scented slime in their wake to entice nearby slugs. When they find a lucky partner, they’ll twist together and hang from a tree branch, hanging from a sticky mucus.
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The slugs will both unfurl and extend their penises – which are as long as their bodies – and entwine them together. Sperm is exchanged and later self-fertilised in an organ within the slug’s body.
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Mating tends to take place in autumn or spring, with the eggs subsequently laid in damp soil or under leaves. Juveniles are born between October and May.
