Being a spineless bottom-dweller might sound like an insult, but the hagfish has enough unique talents to put humans to shame.
These long, eel-like fish have stellar defenses (blasting predators with slime), impressive offenses (tying themselves in knots), and some truly bizarre bodies (sightless eyes and multiple hearts).
Nearly 80 species of the tentacled deep-sea dwellers exist worldwide, most of which hunt small invertebrates and scavenge carcasses on the seafloor, says Douglas Fudge, a biologist at Chapman University in California.
Recently, fishermen in Bío Bío, Chile, were surprised to catch a hagfish, which they placed on a rock for a closer look.
"We’d never seen anything like it," Lissette Hermosilla, who was part of that group, told National Geographic in an email. "We just observed it until it returned to sea, dragging itself like a snake. I feel relieved that she returned to sea [to] survive in her habitat."
Here are seven reasons hagfish deserve our respect.
They're Super Slimers
The pudgy ghouls from the Ghostbusters movies have nothing on the hagfish, which, when threatened or disturbed, spew out slime at the stunning rate of four cups in a fraction of a second.
Their slime consists of mucus and thin fibers, which clog the gills of any predator that dares eat it, says Fudge.
They're Like Houdini
A hagfish—usually about a foot long—can squeeze through an incredibly small gap as narrow as half its body width, according to a study led by Calli Freedman and published recently in the Journal of Experimental Biology.
That's because, unlike other fish skin, hagfish skin is very loosely attached to the body, which means their skin can hold large volumes of blood.
"When blood gets squeezed backward as they fit through a hole, that extra volume can be accommodated without a rise in back pressure," Fudge says.
They Are Tenacious
In 2013, Daniela Silvia Pace and colleagues observed a bottlenose dolphin in the Tyrrhenian Sea off Italy with what looked like a parasitic lamprey stuck in its blowhole. Closer observation revealed it was a hagfish—the first-ever record of a hagfish anchored to a whale or dolphin, according to a study published in 2016 in the Journal of Mammalogy.
It's unknown how the hagfish got there, but it's possible that the dolphin encountered the hagfish while poking its head into the sand at the bottom of the ocean, a known hunting strategy, says Pace, president of the nonprofit Oceanomare Delphis Onlus and a biologist at Rome's Sapienza University.
The hagfish may have defended itself by grabbing onto or burrowing into whatever it could—in this case, the dolphin's blowhole. The primitive animals don't have true jaws, but they have tooth plates, which is likely how this hagfish then attached itself to the soft tissues inside the blowhole. (Read about five times evolution ran in reverse.)
The dolphin could still breathe—a totally obstructed blowhole would kill it—but the mammal was also showing signs of distress, staying at the surface and refusing to dive like its family members, Pace says. A month later, she saw the same dolphin healthy and hagfish-free.
"We were very happy to see this animal survived after that experience," Pace says.
They Tie Themselves in Knots
Hagfish are the only animals with a cranium but no spinal column. They do have a cartilaginous notochord—a skeletal rod that most vertebrates lose in utero—that gives them amazing flexibility.
For instance, if a predator gets a good hold on a hagfish, the animal can wriggle out of its grasp by tying itself into a knot. (Read about other crazy sea creature survival skills.)
Likewise, if hagfish are tearing a piece of flesh off a "big, yummy carcass," they'll use their body knot as leverage, Fudge says. And in the rare situation hagfish get stuck in their own slime, a knot also helps them escape. (Hagfish will also sneeze if their own nostrils clog with slime.)
They're Fashion Forward
Hagfish slime threads are almost as strong and light as spider silk, which researchers have been trying to adapt for use in clothing for years. (Related: “Twisting Everyday Fibers Could Make 'Smart Clothes' a Reality.”)
The genes that make a hagfish thread-producing cell are smaller than spider silk genes, so it should, in theory, be possible to implant those genes into a bacterium and produce reams and reams of hagfish slime silk for use anywhere we’d currently use nylon—think stockings or workout pants. And since nylon is ultimately derived from petroleum, a hagfish-slime substitute could actually be good for the planet.
They Sometimes Live Inside Dead Bodies
"When something dead falls to the bottom of the ocean, it's usually a hagfish that gets there first," Fudge says. "It’s what they do when they get there that sets them apart from other animals."
Namely, the slithery creatures burrow their entire bodies into the carcass. If one gorges itself so much it can't get out—Winnie the Pooh style—it may be forced to live there until it digests the food, he says.
Their Hearts Beat Without Oxygen
A rotting carcass is "not a hospitable place to be for an animal that needs oxygen," Fudge says, which is likely why the animals are able to survive extremely tough conditions. For instance, their three hearts can beat for hours with no oxygen supply, possibly powered instead by fats in the body, according to research by Todd Gillis.
There's much more to learn about the mysterious animals and their abilities—so much that Fudge has devoted his career to studying them.
"The original plan was to work on squid," Fudge says, "and hagfish kept rising to the top of my list."
Rachel Kaufman contributed reporting.