The cold doesn't agree with everyone. Humans are largely specialized to live in a temperate environment, and even then, we need clothes to keep us warm.
For some species, though, the cold is their forte, and many are so adapted that to take them out of their chilly environment would be torture. Beyond even those species are creatures so dependent upon the cold, so specialized to the most frigid, high-pressure places on earth, that you'd hardly even recognize them as being from this planet.
1. Antarctic krill
At first glance these tiny crustaceans, which form the bulk of the diet for many larger animals in the region, appear to be much the same as krill found anywhere else in the world. But these tiny animals (under two grams each—the same weight as roughly 10 grains of rice) are able to mass together in such huge quantities (over 500,000,000 tonnes—twice that of every human in the world) that their daily migrations to and from the surface actually change the currents in the water. They sink carbon from the atmosphere into the depths of the sea, and in some way or another, provide nutrients for nearly every single sea-dwelling creature around Antarctica.
2. Antarctic Salps
If the Antarctic krill had an arch-enemy that they didn’t even know they were fighting, it would be the salps. Salps are colonial tunicates that look like tiny jellyfish, but are actually much more closely related to vertebrates. While krill are responsive to extreme blooms in phytoplankton (their food source), salps can reproduce far more quickly, and can bud off in less than a tenth of the time that it takes for krill to procreate. The little barrel-shaped tunicates expend very little energy moving (unlike krill), and can spend that saved energy reproducing. Salps can strip a phytoplankton bloom before krill even have a chance to reach their second generation, and their slimy little bodies can clog the zooplankton schools. Where krill are nutritionally dense, salps can be up to 97 percent water, provide nearly no nutrients, and when they’re the predominant zooplankton in an area, zooplankton-dependent species leave or die out. Once considered relatively uncommon in the Southern Ocean, their rising presence threatens many fish and bird species who rely upon krill for their food.
3. Bowhead Whale
On the other side of the world, the massive bowhead whales make their way around the Arctic, filtering the ocean for some of the tiniest animals out there: copepods. Unlike their cousins the rorquals (including the blue whale and the fin whale), bowhead whales do not feed by gulping prey-laden water and then expelling it, catching food on the plates as the water is ejected. Instead, they swim through shoals of the smallest zooplankton, mouths open wide, continuously filtering the water as they move forward. This behavior is much more similar to the basking shark than to most baleen whales. Bowheads have the thickest blubber of any animal—up to 20 inches thick—so that they can weather the frigid Arctic seas. The cold waters slow the bowhead whales to the point that their life is extended, possibly up to 250 years—though due to extensive whaling in the last two centuries, it is difficult to prove this extreme lifespan.
One of the truly unique Arctic animals is the narwhal. A member of the toothed whale family, narwhals are actually nearly toothless. Its “horn” is actually an extremely overgrown left canine tooth, present only in males. This tusk was passed off for centuries as a unicorn horn, and Vikings who were lucky enough to find or harvest one could sell them for many times their weight in gold. Today we know that their tooth is highly-innervated and in addition to being a secondary sex characteristic (as it seems to be used in courting and is only found in the male), it is also hypothesized to be used to stir up sediment on the seafloor, unearthing their flatfish prey. Because of their toothlessness, these whales consume the bottom-dwelling fish by sucking them into their mouths.
5. Greenland Shark
A deep-sea neighbor of the narwhal, Greenland sharks are a member of the “sleeper shark” family—and the family name fits them well. Slow moving, slow living, and slow breathing, these sharks can still, slowly, end up reaching the size of a great white (21 feet long, and weighing over a ton).
Because of the extreme depths of their environment (up to 7200 feet below the surface) and the cold, ice-packed waters that they live in, information on live Greenland sharks is hard to come by. One thing we do know is that while they’re an apex predator of their habitat, they’re also big consumers of carrion. Specimens that have been caught have had horses, polar bears, and entire reindeer in their stomachs, and given their top movement speed of 1.6 mph (well, that and the fact that horses and reindeer aren’t exactly Arctic water swimmers), it’s unlikely that they killed any of those animals.
Greenland sharks are extreme survivors, despite their slow, dark lives. It’s thought that they can reach over 200 years of age. Despite the fact that their flesh is toxic due to compounds produced to compensate for the frigid depths of the sea, people still eat it. After aging it for several months to destroy the neurotoxins (but not the ammonia compounds), hákarl is considered a delicacy in Iceland, though many of the younger generation no longer consume it.
6. Giant Scale Worms
One species of giant Antarctic scale worm (Eulagisca gigantea, above) made the “nightmare fuel” internet rounds last year, often cited as a new species (they’ve been known since the 1920s), but mostly gawked at for their comparatively large mouths. Maybe the fear was how they evert their pharynx (the top of their throat) to expose their chompers?
No matter how much the internet hates them, Antarctic scale worms are extreme survivors, eating any food in front of them, from detritus and carrion to other invertebrates and small fish. Scale worms exist around the world, but only the Antarctic species become “giants.” You probably wouldn’t want to touch the tiny ones any more than the giants, though. Just like most caterpillars with “fur” or bristles, the bristles of the scale worms are usually very irritating to the skin.
7. Antarctic toothfish
A relative of the Patagonian toothfish (otherwise known as “Chilean sea bass” for marketing purposes), the Antarctic toothfish lives in deeper, colder water than its cousin. At over six feet long and 250 lbs when fully grown, the Antarctic toothfish is more than twice as large as any other fish found in the Southern Ocean. The adults of this species eat any smaller fish that they come across, regardless of species, and they will cannibalize their own young if they’re within range. Unlike the Patagonian toothfish, which is otherwise extremely similar to the Antarctic, these guys have antifreeze glycoproteins in their blood, meaning that they can survive and thrive in the coldest water on earth. These fish are some of the most important food sources for the giants of Antarctica - colossal squid and sperm whales are especially dependent upon the huge adults of this species.
8. Antarctic proboscis worm
Did someone forget their intestines during a dive? Oh, nevermind, that’s just the Antarctic proboscis worm. Proboscis (or ribbon) worms are found throughout the land and sea, and are generally less than 8 inches long. Antarctic proboscis worms, on the other hand, can be up to 7 feet in length. Despite this, they're very lightweight, not known to reach above 5 ounces (less than many cell phones). They have a tiny, pressure-loaded “dagger” proboscis at the front of their head, which is barbed and sticky. They use this to capture their prey, though they also eat a significant amount of carrion. Because their bodies are rather acidic (with a pH of just 3.5—around that of red wine or lemonade), and they’re mostly slime with not much substance, this species is rarely intentionally eaten by others.
9. Crocodile Icefish
Where some fish just evolve antifreeze proteins to survive the deep and dark waters of the Antarctic, other fish give up “normal” blood altogether. The crocodile icefish do not transport oxygen to their tissues like other vertebrates. In fact, the crocodile icefish family (a scant 16 species in all) are the only vertebrates in existence that have evolved a circulatory system that does not use hemoglobin. They once had it—the coding is still partially present in their genome—but along the way, ancestors lost the ability to make red blood cells (RBCs).
Oddly, this is not actually a beneficial adaptation for the fish (shown in the larval stage above). It just so happened that during a species crash in the Tertiary period—when there were few predators to pick off the weak—the well-mixed, highly-oxygenated Antarctic waters allowed animals with no hemoglobin to survive, using inefficient direct oxygenation. Millenia later, they still exist, survive well in their environment, and have radically adapted their bodies to live with no RBCs: their blood vessels are huge, they have twice as much blood volume, and their heart output is more than 5 times higher than fish of comparable size. Their body is still inefficient and a lack of hemoglobin is still not a benefit to them.
10. Antarctic Sea Spiders
Though sea spiders (no relation to true spiders) live throughout the world, most are tiny, so small that their muscles are often only one cell long. In the Antarctic, however, sea spiders can reach up to 10 cm long, with legs sometimes 40 cm across. This is another instance of Antarctic gigantism, which isn’t fully understood. Sea spiders do not have a respiratory system, even when they get to sizes that would seem to require one for oxygenation. They eat primarily soft-bodied invertebrates, such as sea anemones, using a pointed proboscis to suck out part of the fleshy body. Unlike most invertebrates, the male is the sole caretaker of sea spider eggs. After mating, the female deposits her eggs and leaves, and the male will carry them next to his body until they hatch.
11. Colossal Squid
What would the Southern Ocean be without the colossal squid? Well, probably less terrifying, but these guys don’t like the surface waters, and can only thrive under massive amounts of pressure. While colossal squid have been found as far north as the coasts of Patagonia and the southern edge of New Zealand, they’re primarily denizens of the deep ocean surrounding Antarctica. While their tentacles are actually shorter than the giant squid, their mantle (body) is over twice as long, and at least three times as massive as their giant cousins. With eyeballs nearly a foot across—the largest in the animal kingdom—these ambush predators swim slowly about the abyss, primarily consuming toothfish, and primarily being consumed by sperm whales. In fact, they provide nearly 75 percent of the biomass consumed by the southern population of sperm whales. The long, rotating hooks on the suckers of the colossal tentacles have left inch-deep scars on the bodies of the whales who prey on them.