Yellow Umbrella Slug

Image: Pablo Bou
Tylodina perversa

Someone needs to tell this chap it's too late for an umbrella once you live underwater.

The Yellow Umbrella Slug is a gastropod from the Mediterranean Sea and nearby parts of the Atlantic Ocean. It usually reaches about 4 cm (1.6 in) long.

Their weird umbrella-shell is reminiscent of the kind of thing limpets use to brave the crashing waves as they cling fast to rocks on the shore. Umbrella Slugs aren't what you might call "true limpets", though; they're just one of a whole host of snails who took to wearing their shell like one of those cool, Asian hats.

Image: Géry Parent

In the case of Umbrella Slugs, the shell is about as thin and flimsy as an actual umbrella. There simply isn't much of the calcium carbonate which other snails use to make their own shells strong. I just hope it won't turn inside out at the slightest gust of wind.

As it turns out, we needn't worry for the Yellow Umbrella Slug. They have a few tricks up their sleeve. And under their hat.

For one, it may be surprising to know that their vibrant yellow colour is great camouflage!

Yellow Umbrella Slugs are almost always found on one particular - extremely yellow - sponge called Aplysina aerophoba. They eat it...

Image: Philippe Guillaume

Sometimes they eat it so much they end up constructing a foxhole to hide in!

And where do you think the slug's yellow colour came from in the first place? It's from a pigment they "borrow" from the sponge.

But that's not all they get from their food. They also incorporate alkaloids which help deter predators! Granted the sponge was using it to deter its own predators before it got preyed upon, but still...

Forget umbrellas. Real security is a sponge.