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Ash-black slug (Limax cinereoniger)

This massive mollusc is one of the world’s largest land slugs, and it’s found right here, in the UK’s ancient woodlands.

Ash-black slugs live in ancient woods, feeding on fungi and lichens under cover of darkness.

Credit: Fabrice Cahez / naturepl.com

Quick facts

Common name: ash-black slug

Scientific name: Limax cinereoniger

Family: Limacidae

Habitat: ancient woodland

Diet: fungi, lichens and algae

Predators: birds, mammals and predatory invertebrates

Origin: native

What do ash-black slugs look like?

Ash-black slugs measure in at between 10cm and 20cm-long, though larger ones have been recorded. This makes them one of the world’s largest land slugs.

Adults: are usually black or dark grey in colour, with a whitish or yellowish ridge, called a keel, running at least half the length of their back. The sole of their foot is dark grey with a white stripe down the middle, and their tentacles (eye stalks) are grey and spotted.

Juveniles: are often a warm toffee-brown colour and have a completely white foot.

What do ash-black slugs eat?

Ash-black slugs are nocturnal, emerging at night to feed on fungi, lichens and algae on the woodland floor, on tree stumps or even up trees. They’re excellent climbers.

Credit: Blickwinkel / Alamy Stock Photo

How do ash-black slugs breed?

Ash-black slugs have some of the most bizarre mating habits in the natural world. Being hermaphrodites, they have both female and male sex organs, but it still takes two slugs to tango.

To attract a mate, ash-black slugs take to the treetops, leaving a trail of scented slime in their wake. Once paired up, they twist together and hang, suspended, from a tree branch, secured only with sticky mucus. Each slug then extends its penis – which is as long as its own body! – and entwines it with its mate’s. They each pass a package of sperm to the other, store it in a special organ, and self-fertilise it.

Mating can take place in autumn or spring, with the eggs laid in damp soil or under leaf litter and other woodland debris. Juvenile ash-black slugs can appear between October and May, with most sightings between April and May.

Did you know?

Slugs and snails belong to a class of invertebrates called gastropods, which literally translates to ‘stomach foot’.

Where do ash-black slugs live?

Ash-black slugs are widespread across the UK, but uncommon. They live exclusively in ancient woodland which means that, with the loss of this habitat, these specialised slugs are now harder to find.

Credit: ImageBROKER / Alamy Stock Photo

Signs and spotting tips

Ash-black slugs aren’t easy to spot. They hide away under large pieces of dead wood during the day and emerge at night to feed. If you’re out and about after dark, you’ll likely spot them on the ground or tree stumps with fungi or lichens to feed on.

Did you know?

Slugs were once used to cure warts. The Pitt Rivers Museum in Oxford owns a glass jar containing a slug impaled on a thorn. The label reads:

'Charm for Warts, Oxfordshire. Go out alone and find a large black slug. Secretly rub the underside on the warts and impale the slug on the thorn. As the slug dies the warts will go.'

Threats and conservation

Ash-black slugs are picky about where they live, preferring very old or ancient woods. Sadly, the loss and degradation of these habitats means there are less places for these slugs to live, as old broadleaved trees are removed or replaced with conifers. As a result, the slugs are now very localised and scarce.

It’s also thought that air pollution is killing some of the lichen species that ash-black slugs feed on.

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