At the bottom of the ocean, hydrothermal vents spew out boiling-hot water into the ice-cold sea above. Within a few feet, the temperature goes from scalding to bone-chilling. On top of that, the pressure is crushingly intense and there is no light.

This unimaginable environment is home to "yeti crabs", clawed crustaceans first discovered in 2005. Their hairy arms make them look like the mythical abominable snowman.

Since the initial discovery, just five species of yeti crab have been found. But they have turned up all over the southern hemisphere. These strange little creatures have found a way to live in some of the most extreme environments in the world.

Back in 2005, Robert Vrijenhoek of the Monterey Bay Aquarium Research Institute in California was leading a team exploring deep-sea trenches in the south Pacific. With their ship anchored 930 miles off the coast of Easter Island, the scientists were piloting a submarine called DSV Alvin along the Pacific-Antarctic Ridge, 2,200m below sea level. On one dive, marine biologist Michel Segonzac, who is now at the National Museum of Natural History in Paris, France, spotted a huge blonde hairy crab-like creature on the seafloor.

I immediately realized these hairy white crabs were something new to science

On subsequent dives, many more "crabs" were seen, usually hiding under rocks with just the tips of their arms poking up. When the researchers brought a specimen back to shore, they found that it was not only a brand-new species, but it belonged to a new family: the Kiwaidae.

The tough crustacean was named Kiwa hirsuta. "Kiwa" is the name of a Polynesian deity associated with the ocean, and "hirsuta" means "hairy" in Latin. However, it was quickly nicknamed the "yeti crab", because the long shaggy white hair on its claws bears an uncanny resemblance to the abominable snowman. Despite the name, yeti crabs are not true crabs, which all belong to a related group.

"When first observed, I immediately realized these hairy white crabs were something new to science," says Vrijenhoek. "Though we obtained video of many individuals, my colleagues and I were able to sample only one living specimen from this very remote location in the Southern Ocean. Now residing at the Paris Museum of Natural History, it remains the only specimen of K. hirsuta."

The yeti crabs were seen living around vents that emanated from small cracks in basalt rock, and at the base of some smoker chimneys. Although the environment is extreme, with crushing pressures typical of the deep ocean, Vrijenhoek says the waters around the vent are a "balmy 32C, compared to the 2C typically found at the bottom of the ocean."

They were waving their claws up and down in the shimmering vent water

"They probably never get into the hot water, because we did not see them on the tops of black smokers or other chimneys," says Vrijenhoek. "They are mobile, so they can escape very hot water if they want."

The scientists also noticed that the yeti crabs were performing an odd behaviour. They seemed to drape their furry claws over plumes of hot water escaping from the hydrothermal vents. On closer inspection, the researchers saw that the hair on the claws was covered with thousands of bacteria.

"When I first saw them, I noticed they were waving their claws up and down in the shimmering vent water," says Vrijenhoek. "I speculated that they might be signalling, as male fiddler crabs do in shallow settings. Then one of my colleagues reminded me that it was perfectly dark down there."

Instead, the idea was put forward that the yeti crabs might be "farming" the bacteria as a source of food.

The picture became clearer when, the following year, a second species of yeti crab was discovered living on the ocean floor near Costa Rica. A team led by Andrew Thurber, a marine ecologist at Oregon State University in Corvallis, was studying methane seeps: habitats similar to hydrothermal vents that release methane and hydrogen sulphide gas. Unlike the vents, the water released from seeps is not hot, but the same temperature as the surrounding ocean.

The scientists became convinced that they were actively farming the microbes

During a dive in the submersible, pilot Gavin Eppard spotted a number of yeti crabs holding their claws over the seep, just like the yeti crab. He collected one and took it back to the lab. The new species was named K. puravida, after a Costa Rican saying that means "pure life".

Just like the first species, the hairs on the claws of K. puravida were coated in bacteria. To prove that the bacteria, rather than plankton, were the yeti crab's main food source, the scientists analysed the carbon and fatty acids in its body. They closely matched those found in the bacteria.

What's more, the carbon was in a specific form, which is only found in organisms that get their energy without using the Sun's light. In other words, the carbon cannot have come from the plankton, which photosynthesise. It must have come from the bacteria, which use a process called chemosynthesis that harvests energy from the gases released from the seeps.

The new yeti crabs were not just sitting there, passively allowing bacteria to grow on them. The scientists became convinced that they were actively farming the microbes, by performing a sort of dance and waving their claws through the water. The motion provides the bacteria with a constant flow of oxygen and sulfide gas, which could help them grow.

"K. puravida waves its arms back and forth in the seeping fluid to farm bacteria on its arms and body," says Thurber. "They swing back and forth rhythmically to make sure their microbial crops can grow as fast as possible. Sometimes they harvest these bacteria by using their mouth arms to scrape the bacteria off their bodies and arms, and putting it in their mouth. They have special hairs on these arms that are shaped like combs to allow them to do this."

Although life at the bottom of the ocean would be inhospitable to us, the yeti crabs take it all in their stride.

"They are blind, so the lack of light doesn't matter, and as for heat, salinity and water pressure, all of that is what they are used to and they are adapted for," says Thurber. "The temperature has been constant in that region for thousands of years. We are warming it now, but otherwise they live in one of the most stable environments on the planet."

Its hairy chest and muscular appearance reminded someone of the actor David Hasselhoff

The greatest challenge the yeti crabs seem to have is maintaining access to the fluid being released from the seafloor. It only comes out in certain areas. The yeti crabs crowd around those areas and wave their arms back and forth, to get as much access to the sulfide. They even appear to bat shrimp out of the way: these smaller animals may also be trying to get access to the fluid.

Fast forward to 2010 and a third species of yeti crab was discovered. This one was found on the hydrothermal vents of East Scotia Ridge, in the ice-cold waters off Antarctica, 2,600m underwater.

Named K. tyleri, it was nicknamed "the Hoff" because its hairy chest and muscular appearance reminded someone of the actor David Hasselhoff. It is the hardiest and toughest yeti crab discovered to date, as the conditions it has to cope with are so extreme.

That is because, although the gushing volcanic water can be as hot as 400C, just feet away from the vent the water is almost 0C. The Hoff crab may be the only animal that lives in both extreme hot and cold environments. Despite the conditions, the vent is teeming with yeti crabs, which range in size from half a foot to just under an inch.

The cold takes its toll on their bodies, so they only reproduce once before death

The yeti crabs are confined to a very small liveable area: if they get too close to the vent they would be boiled alive, and if they get too far away they could get hypothermia. So they pile on top of one another, filling every available space. The scientists counted 700 yeti crabs per square metre.

Perhaps to aid them when they wrestle for a coveted spot, K. tyleri yeti crabs are much more beefy and compact than their relatives. This helps them cling onto the vertical surfaces of vents.

However, their way of life does pose some logistical problems. The habitable zone around the thermal vents is too warm for yeti crab larvae, which need colder temperatures to develop. This forces the females to leave the safe haven of home and go out into the colder water to breed. The cold takes its toll on their bodies, so they only reproduce once before death.

A fourth species followed a year later.

In 2011, scientists led by Jon Copley were on an expedition examining hydrothermal vents about 2,000km south-east of Madagascar in the Indian Ocean. They spotted yeti crabs on hydrothermal vents around a place called Lonqi, or "Dragon's Breath", 2,800m underwater. In a study published in 2016, they described seeing the species for the first time.

The temperature where crabs like these live is no warmer than shallow tropical seas

The yeti crabs were found on the side of mineral spires known as "vent chimneys", which rise above the seabed and spew hot volcanic water. They are rich in minerals like copper and gold, making them ideal targets for seafloor-mining companies.

The area is full of life. Alongside the yeti crabs, researchers found two new species of snail, a new species of limpet, a new species of scaleworm, and another new species of deep-sea worm.

The new yeti crabs, which are yet to be named, are somewhat unlike their cousins. They do not have the same long claws or "chelae" as K. hirsuta and K. puravida. Also, their short claws do not have hairs on them, so they do not collect bacteria on their claws as those species do.

Instead, these yeti crabs have hairy chests, making them similar to the Antarctic "Hoff" yeti crabs. The hairs on their chests are typically covered in bacteria, which are either swept up from the vent chimney or "farmed" on the hairs to eat later.

While the temperature of the water gushing out of the chimneys is at least 300C, the temperature where the yeti crabs actually live is probably no warmer than 25C.

All the yeti crabs discovered so far have been in the southern hemisphere

"Apart from possibly coping with chemical conditions that may be challenging to most other marine life, such as higher-than-usual levels of hydrogen sulfide and occasionally lower-than-usual levels of oxygen where the warm fluids are diffusing away from the vents, the environment where the crabs live is not really 'extreme'," says Copley. "The temperature where crabs like these live is no warmer than shallow tropical seas, where plenty of marine life thrives. The depth of 2.8 km is only about a quarter of the depth, and therefore pressure, of the deepest point in the ocean, where animal life still persists, and other crab-like animals live much deeper than these vents."

The scientists analysed the genes of the yeti crabs and found they were closely related to the "Hoff" yeti crabs found off Antarctica. The two species last shared a common ancestor around 1.5 million years ago, which is not that long ago on the scale of evolution.

The fifth and (so far) final species of yeti crab was discovered in 2013. Named K. araonae, they were living next to hydrothermal vents about 2,000m down along the Australian-Antarctic Ridge. They have long claws like the "original" yeti crab K. hirsuta, but are not particularly hairy.

Now that a few species have been found, we can make an educated guess about where yeti crabs come from.

It is likely that yeti crabs have been about for quite some time. A 2013 analysis of their genes suggests the family evolved about 30 million years ago. That is about the same time that their ancestors are believed to have first colonised hydrothermal vents and cold seeps.

All the yeti crabs discovered so far have been in the southern hemisphere, most of them in the oceans bordering Antarctica. That suggests they originally evolved there.

The first one wasn't found until 2005 and we just keep finding more and more

However, a study published in 2000 described a fossil of a 100-million-year-old relative of yeti crabs that was found in what is now Alaska. Pristinaspina gelasina belongs to a different family but may be an ancestor of modern yeti crabs. Its descendants may have gone on to colonise those same hydrothermal vents and cold seeps around 70 million years later.

It seems likely that there are more yeti and Hoff crabs out there. So far they have only been found in vents and seeps, but there are other areas of the deep sea that have similar conditions and where they might conceivably thrive. Rotting whale carcasses are one such place.

"They exist in these habitats because they can feed upon the bacteria that get energy [from] sulfide and methane, and vents and seeps (as well as rotting whales, algae and wood) are the main places they can find this," says Thurber. "That isn't to say that they can't survive other places, we just haven't found them."

"So far, only five yetis have been discovered. However, the first one wasn't found until 2005 and we just keep finding more and more," says Thurber. "There is so much to be discovered yet in the deep sea that I'm not surprised we keep finding new things. But so many yetis is really astounding."

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