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You may have come across eel-like hagfishes from the few times they have gone viral on the internet, but they might be even more unusual than the videos and photos suggest.
While some might describe a hagfish as 'repulsive,' 'disgusting' or, more poetically, 'Lovecraftian', we would have to strongly disagree.
Hagfishes are excellent: they are part of the ocean's clean-up crew, have changed relatively little in 300 million years and can produce extraordinary quantities of slime.
Hagfishes (Myxini) are sometimes referred to as slime eels and occasionally as snot snakes, but they are neither eel nor reptile.
They are fishes, but belong to an exclusive group called cyclostomes, also known as jawless fishes. They are joined in this group by the equally wonderfully weird lampreys.
There are 76 species of hagfish around the world. All are found in salt water but range in preferred depth from the shallows to about 1,700 metres deep. The largest species, Eptatretus goliath, has been recorded at over a metre long, whereas some smaller species can be just a few centimetres.
They are eyeless animals, though they have eyespots that can detect light. Instead of sight, they find their way around and locate food using pairs of barbels - ultrasensitive, fleshy, whisker-like protrusions around their mouths - and by using their incredible sense of smell. Most of a hagfish's brain is devoted to scent detection.
Hagfishes are counted as vertebrates, but they do cause some problems for taxonomists. A vertebrate is typically defined as an animal that has a spinal cord surrounded by vertebrae. Hagfishes don't have a bony skeleton, however - there are no vertebrae in these vertebrates. They have a notochord (a flexible cartilage-like structure that runs along their back) and a cartilaginous skull, and are the only animals known to have this unique combination of features.
Hagfishes have been described by some scientists as 'ancient vertebrates', and they have been suggested to be primitive ancestors of vertebrates, though this idea has been contested.
A hagfish fossil found in Illinois, USA, that dates to about 330 million years ago looks quite a lot like today's hagfishes, though, suggesting that these animals may have long since found an ideal body form, all but halting their evolution.
Hagfishes provide an important ecological service as part of the ocean's clean-up crew. Feasting on dead and dying animals that fall to rest on the ocean floor, they are known foremost as scavengers. There have, however, been instances recorded of hagfishes preying on live fish and invertebrates.
Hagfishes tend to burrow into their food face first, eating it from the inside out. While they don't have traditional jaws, hagfishes have two tooth-like plates made of keratin that can pinch together to tear off food.
But tooth-like rasps are not the only way to eat. Hagfishes can survive for several months without a meal and are able to absorb nutrients through their skin. As they are thought to be scavengers for the most part, the ability to absorb nutrients in the surrounding water may help them through lean times when no food has fallen to the ocean floor.
Hagfishes are unusual animals before slime even comes into the equation. But it is this snot-like substance that they are probably best known for.
They can produce a bucketful of slime almost instantaneously, without the need for a constant giant tank of slime in their body ready to deploy. Their way around this is that they don't hold the slime in their bodies in its final form.
Hagfish slime has three main components: seawater, mucins and slime threads. Data shows that hagfish slime is 99.996% seawater, 0.0015% mucin and 0.002% threads. Around 3-4% of a hagfish's body mass is its slime. For a 60-gram hagfish, that would mean that about 2.2 grams is slime - 73 milligrams of this is equal parts mucin and slime thread, and the rest is water.
When this slime precursor is exuded from the fish, it rapidly mixes with the surrounding water and expands.
The slime threads are tiny but vital to this process and unique to hagfish. Each is 10-17 centimetres long, but 1% the width of a human hair. Inside the hagfish, the threads are tightly wound up in miniature ellipsoid-shaped skeins, but when they are released they quickly unravel. A 0.015-centimetre bundle can expand to 15 centimetres in a fraction of a second - that's a growth to 10,000 times their original size.
The threads spread out in the water and tangle together, creating a network that works together with the mucin to trap water, forming the slime. Its structure acts more like a fine sieve that slows water down rather than a coherent gel.
Hagfish slime is much more dilute than mucus secretions of other animals. If the mucins in hagfish slime were as concentrated as they are in mammalian mucus, hagfishes would only be able to produce 12 millilitres of it. But low mucin concentration means that in total a hagfish can make an amount of slime equal to 400 times their own volume - for example, a Pacific hagfish (Eptatretus stoutii) could make about 24 litres.
A potential downside to being an animal that pumps out slime is the risk of getting well and truly enveloped in a bubble of it. Looking at a hagfish you might wonder how this soft, limbless, finless animal deals with its slimy problem.
Two more things to note: first, hagfish slime isn't sticky - it is incredibly soft, once described as being up to 100,000 times softer than jelly. Second, hagfishes can tie themselves in knots.
The hagfish will create a knot with its tail by wrapping it around its body. The fish then slides the knot from head to tail, wiping off the slime as it goes. It's a nifty and potentially important trick as hagfishes have apparently been known to die when left in their own slime.
Hagfishes can also use this knot tying ability to help them feed. When the fish bites into flesh, it will form a knot that it slides up and over its head. This pushes the hagfish away from the carcass with a lovely mouthful of meat.
Hagfishes slime when they are stressed.
In fact, one now-internet-famous image of a slime drenched car was the result of an overturned truck filled with the eel-like animals. This incident left a lot of suddenly very stressed hagfish strewn across a road in Oregon, USA.
Generally, though, it isn't car accidents that give hagfishes a reason to produce slime. It seems that hagfishes use their sliming abilities to protect themselves from predators.
As a very squishy and scaleless creature, a hagfish would likely seem an easy meal to a larger animal. If that predator happens to have gills, however, a hagfish would be a very poor choice.
It has long been thought that hagfish slime clogs the gills of fish, causing them to choke. There is now video evidence that appears to back this up, with a hagfish seen fending off sharks and other large fishes as it tries to concentrate on munching on the bait dropped by researchers.
Hagfishes have about 100 slime glands that funnel their slime precursor through the 90-200 pores that run along the animals' sides. Hagfishes will jet their slime from the part of their body that is under attack. If a shark tries to take a bite, the hagfish sends slime straight into the mouth and gills of the predator, forcing them to let go.
It isn't known what the ultimate fate of the predator is, whether the slime suffocates them or it just dissolves away. The hagfish, however, is free to carry on with its day seemingly unscathed.
But hagfishes only slime once they perceive that they are under attack, prompting scientists to question how these soft animals could possibly survive a shark bite.
The answer is in how squishy they are. While most fishes' skin fits tightly 'like spandex,' hagfish skin fits around the animal like a very loose sock and is only attached in a couple of places. Scientists have found that their barely attached, flaccid skin allows for the important parts of their body to simply squish out of the way of any penetrating teeth, preventing serious injury.
Their flexible body plan also allows them to squeeze through tiny gaps. They can get inside carcasses, burrows and small spaces to hide from predators with ease by rapidly redistributing their blood, which they have a lot of. Hagfishes have one of the highest ratios of blood-to-body mass of any vertebrate.
With how roomy it is inside these animals' bodies, you could fit about 46% more hagfish in before their skin would begin to stretch.
Hagfishes aren't impervious to all predators, only those with gills. They are known prey to marine mammals, sea birds and octopuses, but they don't seem to be an important part of their diets.
They are also in demand in on land, particularly in Korea, where approximately 2.2 million kilograms (5 million lbs) of hagfish is consumed by humans each year. Hagfish skin is also commonly used to produce 'eel leather'.
However, because of overexploitation and some detrimental fishing practices, there are major threats to several hagfish species. Of the 76 species, 12% are now thought to be at a heightened risk of extinction.
It's thought that hagfish slime may have potential for human use as hydrogels, materials that are used in a variety of products from moisturisers to contact lenses.
The United States Navy is also innovating with the slime, synthesising it using E. coli bacteria. The application would be to stop suspicious enemy boats by firing the slime at their propellors and clogging them, to replace current, less reliable and less safe methods.
Scientists are also eyeing up a more everyday use for hagfish slime, as their slime threads may have the potential to be developed into fabric that works as a natural alternative to Lycra, which is currently derived from oil. The strong, stretchy fibres of these underappreciated, slime-producing masters may one day feature in wardrobes around the world.