Inside the Strange World of Slug Sex and Slime

From their multi-tasking mucus to their sexual acrobatics, slugs are wilder than you think.

When a reader asked about a tiny, black slug from Croatia that allegedly emits a foul odor when crushed, we decided to get up to speed on slugs. The first thing we learned is that the gastropods can be surprisingly colorful. Take a look at eastern Europe’s Carpathian blue slug, for example, and the bright orange or red Arion rufus, native to Europe but also found in the northern U.S. and Canada. And get a load of this giant pink Australian specimen.

Scent of a Slug?

As for our stinky Croatian slug, Ben Rowson, senior curator of mollusks at the National Museum of Wales in Cardiff, says he is unaware of any slug fitting that exact description. He says it’s possible, however, that keel back slugs and slugs of the Milacidae family, which occur in Croatia, could put out a bad smell if they’d eaten a plant like stinking madder, which has a foul odor when crushed.

Rowson notes that there is one species native to North Africa, Drusia deshayesii, that has a "nauseating" smell, according to a 2012 paper. The odor likely comes from Putoria tenella, a nasty-smelling plant that the slug is thought to feed on.

“Animal feces are also part of the diet of some slug species,” he says, which probably adds a certain pungency to a slug’s bouquet.

One slug relative, the garlic snail, gets its name from the garlic-like odor it emits when disturbed, says Rory McDonnell, an entomologist at Oregon State University.

We have to wonder if this sparked the idea for escargot.

Multi-Tasking Mucus

“Slugs are snails without external shells,” says Chris Barnhart, a biologist at the University of Missouri. Though some, like leopard slugs, which are native to southern and western Europe, carry their shells internally.

Others just carry them strangely, like the long-tailed slug that lives only in the state of Sabah, in Borneo. This “semislug” has a small, partially visible shell in the middle of its body that is too small for the slug to hide inside.

With little or no protection, slugs need some defenses. Enter their mucus: It numbs the mouths of potential predators, keeping them at bay.

Slug slime is an unusual compound, neither liquid nor solid. It solidifies when slugs are at rest, but liquefies when pressure is applied—in other words, when the slug starts moving. Its clingy-but-flexible properties recently helped inspire the development of a new surgical adhesive.

Slug mucus even works as a kind of slimy GPS. Land slugs, says Barnhardt, “can find their way home over good distances by tracking their own slime trails.”

We’ll just call it “goo”-gle earth.

Amorous Acrobatics

Slug slime also spurs romance.

Slugs are all hermaphrodites and can fertilize themselves, but they can mate, too. By releasing pheromones into their slime, slugs indicate a readiness to mate—and some make quite a spectacle of it.

Leopard slugs have a sexy, Cirque du Soleil-style mating ritual. Two will hang upside down from a rope of mucus and entwine their bodies around each other. They evert their long, blue penises from behind their heads and entwine those as well, fanning them out and transferring spermatophores. Picture a very slimy chandelier. (Read about far less pleasant banana slug sex in: Slugs and Bugs: 6 Bizarre Animal Mascots in U.S. Sports)

And that long-tailed semislug we mentioned shoots sharp, hormone-filled calcium carbonate “love darts” from a dart sac in between its penis and vagina into potential mates, possibly boosting its shot at reproduction. (Related: Love Hurts: What Happens When Snails Stab Their Mates)

So that’s where Cupid got the idea.

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