Two Black Slugs (Arion ater) mating.

Dear Readers, of the many things that you expect to see on a Bank Holiday morning as you gaze sleepily into the garden, two slugs mating is pretty low down the list, but here we are. These two are different colour forms of the Black Slug (Arion ater) – these are large but inoffensive detritivores, who clean up any dropped suet pellets from the bird table and also apparently have a taste for fungi. In the West Country I see more of the jet-black colour morph – I think that they’re rather beautiful, and look as if they’ve been chiselled out of a block of coal. The brown form is commoner in my garden, and has an  attractive tomato-red frill around the edge.

What on earth is going on here though? It looks as if the slugs are trying to trying to create a yin and yang pattern. The truth is even stranger.

Slugs are hermaphrodites, and have both male and female sexual organs. Black slugs can lay fertilised eggs all by themselves, but these would be clones, so they seem to prefer to find a partner to mate with (it’s good for genetic diversity, after all). The white ball in the middle is actually two packages of sperm, one produced by each slug.

Then, each slug will take its partner’s sperm into its body, and each will head off into the undergrowth to lay eggs.

I find all this rather amazing (though apologies to anyone who is currently eating their breakfast). Black Slugs will lay about 150 eggs each time they mate, several weeks after mating. The eggs will take between 4 and 6 weeks to hatch, depending on temperature, and the young slugs will be grey with a darker grey head. A Black Slug can live for up to two years, taking refuge in cracks and crevices when it’s too hot/dry/cold, and as mentioned previously, it is a relatively benign garden inhabitant, preferring to eat plants that are already sick and decomposing to nice fresh seedlings, though I suspect it is not immune to having a nibble if it happens to pass some freshly-planted greenery. As Black Slugs are a great favourite with hedgehogs I am inclined to turn a blind eye, but no doubt some of you will have a lower tolerance.

I am always quite taken by the way that Black Slugs compact themselves into little square blocks if threatened.

A threatened Black Slug (Photo by Saharima Roenisch from https://www.naturespot.org.uk/species/Arion_ater_ss

It’s interesting how slugs seem to have a much higher ‘ick’ factor than snails, even though a slug is literally a snail with either a small internal shell, or no shell at all. I suppose there’s something endearing about snails carrying their homes about on their backs and peering out with their eyes on the end of stalks. Both of these molluscs are slimy critters, for sure (to the extent that the Black Slug was once used to lubricate the axles of carts in Sweden and Germany), but this is mainly for protection from predators, and to enable them to move about. I am finding them fascinating, and for more information, let me refer you to the talk on slugs by Imogen Cavadino that I watched back in 2020. If she doesn’t convert you, no one will.