NEW ORLEANS, LOUISIANA—When people say they get all tied up in knots, they don’t mean it literally. Such is not the case for the hagfish, a jawless, eellike creature that ties itself into pretzellike twists to tear apart its dinner. Hagfish are best known for their slime, which gums up the gills of any predator that tries to eat them, causing the attacker to spit them out unharmed. But hagfish have other unusual attributes: They can squeeze through devilishly tight spaces and survive shark bites unscathed, researchers reported here this week at the annual meeting of the Society of Integrative and Comparative Biology.
Hagfish aren’t a typical fish—they have cartilage instead of bones and a primitive skeletal rod (called a notochord) instead of a backbone. For years, Douglas Fudge studied their fibrous slime, which they make in enormous quantities when stressed. Then in 2011, the marine biologist at Chapman University in Orange, California, saw a video of a shark chomping down on a hagfish, which escaped without a scratch. Had its slimy coating and tough skin saved it?
Watching the video several times, Fudge found that the slime wasn’t released until after the attack. And experiments with machine-driven needles showed that the skin was, in fact, not strong enough to withstand a shark bite. Instead, the shark did no harm because the hagfish’s skin is only loosely connected to the muscles and organs inside, he and his colleagues reported at the meeting. Just under the skin is a blood-filled cavity with plenty of room to spare: Fudge’s team found that it could increase the fluid inside by 35% before it was full. When they simulated a bite with a guillotinelike machine topped with a shark’s tooth, the skin just folded around the tooth, giving the organs ample room to move out of harm’s way. But when they glued the same skin directly to a dead hagfish’s muscles, the tooth readily pierced it.
Most fish wear their skin “tight like spandex,” says Andrew Clark, a biomechanist at the College of Charleston in South Carolina. “By being loose, [hagfish skin] is hard to grab.” He, along with fellow biomechanist Theodore Uyeno from Valdosta State University in Georgia and their students, have independently examined and described the loose skin of several of the 80 species of hagfish. Fudge’s work “jives exactly with what we are doing,” Clark notes. Uyeno has also found an important difference between an Atlantic and a Pacific hagfish: The latter have muscle fibers embedded in the skin, he and his colleagues reported in a separate presentation at the meeting.
Those muscles may explain a seeming paradox. All hagfish can form knots with their bodies, another feat likely enabled by loose skin, says William Haney, a biomechanist who works with Uyeno at Valdosta. “The knots make up for the lack of traditional jaws,” he explains. By twisting into a knot, the hagfish can tear flesh off dead and rotting carcasses. But even though the Atlantic hagfish is long and slender, whereas the Pacific hagfish is short and thick, the latter can form more complicated and tighter knots than its svelte cousin. Altogether, both species use four simple body movements to come up with all their different knots, Haney reports today at the meeting.
One more unusual behavior may be explained by the hagfish lifestyle: The slime "eels" live on the sea floor, burrowing into the mud and even into dead whale carcasses to scavenge for food. Like octopuses, hagfish can squeeze through very tiny spaces—including slits half their body width, Fudge and his students reported yesterday. They do this by bending their heads, angling them through the opening, and wriggling their bodies back and forth, after which they form a loop with the part already past the opening. This gives them the leverage to get the rest of their bodies through.
However, key to this success is the fluid-filled cavity, or sinus, under the loose skin, Fudge notes. Blood gets pushed toward the tail as the hagfish squeezes through the slit. This movement can lead to dramatic swelling of the tail, says Fudge, which finally gets pulled through. (When a hagfish goes through a slit tail first, then it winds up with a swollen head, he adds.) He thinks other animals that squeeze through tight spaces, like octopuses and rodents, may use a similar strategy. “The loose skin has a lot of functions,” Clark concludes.
These Houdini maneuvers could prove useful for engineers. “As disgusting as all of this is, the biomechanics of squeezing through constricted spaces could have applications in running wires and cables through existing buildings and in search and rescue,” says Frank Fish, a biomechanist at West Chester University of Pennsylvania, who was not involved with the work. Haney hopes that this new understanding of how small-brained animals perform such complex tasks—including knotting—will one day help engineer flexible robots that can do just that.