These beautiful (yes, beautiful) black slugs are native to western and central Europe, but have been introduced to south-eastern Australia and North America; mainly in Newfoundland, British Columbia, pacific north-west USA and some parts of Alaska.
I find myself drawn to their slow serene movements and a fascinating textured top-side. Being boneless, these slugs can often extend up to 20cm in length, but 10-15cm is more normal. And as the name suggests, most of these slugs are black but they can also vary to be chestnut-brown, orange, or pale grey or cream.
The black slug has two sets of tentacles; the larger, upper set are sensitive to light, but cannot differentiate colour. The smaller, lower set sense smell, and both sets retract when the slug detects danger or it’s disturbed. You can see in my video that the slug had retracted both tentacles as I had clearly disturbed them.. but they then sensed I wasn’t too threatening so they appeared again. It was super neat to see!
Below the tentacles, their mouth is found on the underside of the their head. They have a toothed tongue known as a radula, which has up to 27,000 teeth on it! Kind of reminds me of how a cow’s tongue feels when you feed them hay or some kind of snack and their tongue rasps on your hand roughly. I’m guessing that’s how the slug’s tongue feel but on a much smaller level!
Slime assists the slugs to move smoothly over leaves and debris on the forest floor. This sticky mucus also helps keep the slug moist, which is important to keep the slug breathing properly. And when the slug is threatened, it will produce more slime which tastes very unpleasant and makes it more difficult for the predator to want to get near it. What a cool trick!
At the base of their tail, you’ll sometimes notice the black slugs have a few pieces of forest debris stuck to it. I at first thought this was the slug’s being ‘sloppy’ and accidentally picking up junk on their way across the ground, but I recently learnt they actually use this part of their body to sometimes collect ‘snacks’ for later on. Definitely a cool feature, indeed.
I was seeing a lot of these slugs in the Fall here in North Vancouver, but now that Winter is nearly underway I barely see them. When I read up it, it seems like they mostly hide deep in the soil when the temperature drops under 5 degrees celsius. On the opposite side of the spectrum, in the warmth of spring and summer they will also hide to keep themselves as moist as possible.
I hope you learn a few things today about this beautiful and odd-looking creatures. Although they’re invasive, I still find them fun to photograph and see what they’re up to.
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