The weirdly disgusting hagfish

Like us they are vertebrates, but unlike us they do not actually have bony vertebrae in their backs: they are literally spineless. They have several hearts, and at least twice as much blood in their bodies as other fish. On top of that, they have only half a jaw, yet they can still tear through tough flesh. They lack scales, they can absorb some of their food straight through their skin – bypassing their half-jawed mouths altogether – and they have an almost unrivalled ability to turn seawater into thick gloopy slime.

"They're wandering around the ocean with only a set of upper teeth," says Uyeno. "They don't have an opposable lower set. So how do they create a forceful bite?"

Hagfish, they say, bite through tough flesh by tying themselves in knots. The hagfish begins to tie the knot at its tail, says William "Austin" Haney, Uyeno's graduate student. When complete, the knot lies slightly more than halfway along the body towards the head. "Once it tightens, the knot is basically at the head," he says. "It's this poor hotdog-shaped animal's very best attempt to create an ad hoc lower jaw," says Uyeno. The "lower jaw" might not have any teeth, but it gets the job done.

The idea suggests that body knotting is no mere hagfish party trick. Instead, the ability to tie itself in knots is a vital component of the hagfish's feeding behaviour. "When you really think about it, all these other features could be seen as adaptations to improve the hagfish's ability to knot their bodies," he says. "I thought: why has nobody looked into this idea before?"

Tying a knot in an animal involves stretching some parts of the body to form the knot's tight loops. In the same way that tight-fitting jeans might split at the seat when their wearer sits down, a tight-fitting skin might tear during knotting. "The hagfish skin fits like a pair of slacks," says Clark. It has just the right degree of bagginess to allow for tear-free knotting.

What's more, if the hagfish has a loose skin, it needs some sort of fluid to fill the cavity between this skin and the body. Blood is as good a fluid as any for this job, even though having so much blood requires a bit of extra power to pump it around. This could explain why hagfish have extra hearts.

Even features like the absence of scales and the ability to make mucus can be seen through the prism of knotting behaviour. A knot must be able to slide up the body easily, and if the hagfish body is smooth and slimy, knots find it that little bit easier to slip along.
http://www.bbc.com/earth/story/20160905-the-strangest-fish-in-the-sea