Where to begin when describing the extraordinary biology of 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.
What's more, hagfish have skin so floppy that it should seriously compromise their swimming. 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.
Put simply, hagfish are like nothing else in the animal kingdom. But it is possible that many of their unusual traits can be explained by one final feature. These fish can tie their bodies into tight knots.
The eel-like hagfish sit right at the bottom of the family tree of vertebrates, near to another group of odd, eel-like fish called the lampreys.
Some biologists have even suggested hagfish are so unusual in lacking a backbone that they do not really belong in the vertebrate family tree at all. However, a DNA analysis published in 2010 rejected this idea.
"The most current research I've seen suggests hagfish are bona fide vertebrates that later lost spinal characteristics," says Theodore Uyeno at Valdosta State University in Georgia, US.
If hagfish did once have a primitive spine-like structure, they must have lost it a very long time ago. In 1991 a palaeontologist described a 300-million-year-old fossil found in Illinois, which looks remarkably like a living hagfish. As far as external appearances go, hagfish have hardly changed through that vast expanse of time.
Today, many hagfish hang around at the bottom of the ocean.
Some people call hagfish the "vultures of the sea", because they seem to get most of their food by scavenging.
"They feed on some really big things," says Uyeno. "They're often seen feeding on dead whale carcasses. They chew through the side of these animals and tear off big chunks of food."
It is the question of exactly how they tear chunks from these carcasses that most interests Uyeno and his colleague Andrew Clark at the College of Charleston in South Carolina.
We know how most animals would do it. First the upper jaw and lower jaw would clamp around part of the carcass. Then the jaw muscles would squeeze the upper and lower teeth together to grip and slice through the flesh.
"More than 99% of living vertebrates possess opposable jaws," says Clark. But hagfish do not.
"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?"
After plenty of careful observation, Uyeno, Clark and their colleagues think they can explain how hagfish get the job done. What's more, their idea might go a long way towards explaining many of the hagfish's other unusual biological features.
Hagfish, they say, bite through tough flesh by tying themselves in knots.
Biologists have known for decades that hagfish will sometimes tie their long rope-like body into a knot. An illustration of a knotted hagfish even made it onto the cover of Scientific American in 1966.
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.
This is a key observation, says Uyeno.
It means a hagfish can tie itself so that there is a broad flat knot in its body, exactly where most animals would have a lower jaw. This creates a solid surface that can work with the upper jaw to grip and squeeze food.
"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.
That realisation encouraged Uyeno to reassess what drove the evolution of the other unusual features of hagfish biology, in a paper published in June 2016.
"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?"
For instance, a backbone instantly makes an animal's body more rigid. But instead of a backbone, hagfish have a ridge of cartilage called a "notochord". This makes their bodies more flexible and better able to tie into relatively tight knots.
Their loose skin has obvious drawbacks. "It's horrible for swimming," says Clark, like trying to do breast stroke in a wedding dress rather than a wetsuit. "We've been perplexed by that loose skin."
But a loose skin has more "give" to it. 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.
"Hagfish are literally the bloodiest vertebrates on the planet," says Uyeno. "They have an incredibly high blood-to-volume ratio."
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.
The idea that some features of hagfish biology are best viewed through the point of view of hagfish feeding makes sense to Chris Glover at Athabasca University in Alberta, Canada.
"You can argue quite strongly that hagfish are 'designed to dine'," says Glover. "I think that Uyeno and Clark have provided robust evidence that the loose skin of hagfish certainly does appear to be a critical component facilitating the knotting behaviour."
This is particularly likely because, of the few other fish that can knot their bodies, most have loose skin, he says.
But Glover says it is less clear whether there are clear links between body knotting and some of the other strange features of hagfish biology.
In 2011, Glover and his colleagues showed that hagfish can absorb nutrients directly through their skin. This means they sometimes feed without using their mouth.
"The lack of scales could well be related to this passive form of feeding," he says. It would remove a barrier to the easy absorption of nutrients through the skin.
Similarly, hagfish slime is not necessarily all about lubrication during knotting. There is evidence that hagfish sometimes actively hunt other fish, and in this case the slime helps the hagfish clog the gills of their prey and kill it.
Meanwhile, the lack of a backbone might make knotting easier, but it is not necessarily a prerequisite for the behaviour. Moray eels can tie themselves in loose knots despite possessing a spine.
In 2015, Shanta Barley at the University of Western Australia in Perth and her colleagues studied wild honeycomb morays. They watched as a moray slipped a knot down its body towards the head, where it used it as leverage to pull food from a bait bag.
Other pressures might have led hagfish to lose their backbone-like structures, says Barley. "Predation springs to mind. To escape predators, hagfish have become adept burrowers and display unusual anti-predator 'coiling' behaviour."
"There is always the chicken-and-egg scenario at play," says Glover. "Did the need to knot drive the development of these features, or was it that these features that already existed then facilitated knotting?"
Uyeno and Clark still have some way to go to convince other biologists that the unusual features of hagfish biology are mostly adaptations for body knotting. But they are not giving up.
The work could even lead to technological spin-offs. "We're always looking to turn our findings into practical things for people to use," says Uyeno.
"Can you imagine a rappelling [or abseiling] rope that could automatically tie itself into a lifesaving knot at the end?" he says. With a full understanding of hagfish body knotting and the musculature that controls the ability, it might eventually be possible to design ropes with such features built in.
That is speculative for now. But if it ever does come to pass, one day a hagfish could save your life.