The world's fastest shark is no match for a sack of flaccid hagfish skin

What’s like a loose, slippery, yet surprisingly tough old sock? You guessed it: hagfish skin. These eel-like sea monsters are arguably some of the grossest creatures alive, and are best known for producing vast quantities of thick mucus when attacked. Scientists always thought they used that goo to escape being eaten, but a new study suggests that it’s more about the way their skin (or, more specifically, their skin sacks) can squish out of the way of predators’ teeth.

Mucus seems like a great way to keep someone from chomping down on you. If your predator's breathing apparatus is suddenly clogged with slime, they're liable to stop worrying about dinner and start finding ways to clear the gunk from their gills. But there’s something of a timeline issue with this theory, as hagfish only secrete their goo after they’ve been attacked. It does help them escape, but they have to survive a shark bite first.

It turns out their skin is the key to escaping that first round unscathed. To figure this out, biologists at several universities collaborated to make a shark-tooth guillotine, the details of which they published alongside their results in Journal of the Royal Society Interface. Not many animals will even try to attack a hagfish, and mako sharks are some of the proud few, so the researchers gathered mako teeth and glued them to the edge of a metal plate. This plate could be pulled back into the top of the guillotine, compressing a spring as it went, such that it shot down when released. Your bog-standard dropping guillotine wasn’t going to provide enough force to mimic a real shark attack, but the spring-loaded mechanism did the trick.

Once they had assembled their death machine, the biologists got a bunch of Pacific and Atlantic hagfish. And since a live hagfish would never consent to sitting still under a teeth-laden guillotine, they were euthanized first. Sorry, hagfish lovers. When the hagfish were unaltered, the shark teeth punctured skin almost every time—but the muscle underneath didn’t get damaged even once. So it seemed like hagfish skin, once thought to be bite-resistant, wasn’t especially tough after all. But skin is much easier to heal than muscle, you can’t expect to come out of an encounter with a shark totally unscathed.