The wildlife of Antarctica are extremophiles, having to adapt to the dryness, high temperatures, and high exposure common in Antartica. The extreme weather of the interior contrasts to the relatively mild conditions on the Antarctic Peninsula and the subantarctic islands, which have warmer temperatures and more liquid water.
At least 235 marine species are found in both Antarctica and the Arctic, ranging in size from whales and birds to small marine snails, sea cucumbers, and mud-dwelling worms. The large animals often migrate between the two, and smaller animals are expected to be able to spread via underwater currents. Antarctic animals have adapted to reduce heat loss, often developing warm windproof coats and layers of blubber.
Antarctica's cold deserts have some of the least diverse fauna in the world. Terrestrial vertebrates are limited to sub-antarctic islands, and even then they are limited in number. Antarctica, including the subantarctic islands, has no natural fully terrestrial mammals, reptiles, or amphibians. Human activity has however led to the introduction in some areas of foreign species, such as rats, mice, chickens, rabbits, cats, pigs, sheep, cattle, reindeer, and various fish. Invertebrates, such as beetle species, have also been introduced.
The benthic communities of the seafloor are diverse and dense, with up to 155,000 animals found in 1 square metre (10.8 sq ft). As the seafloor environment is very similar all around the Antarctic, hundreds of species can be found all the way around the mainland, which is a uniquely wide distribution for such a large community. Deep-sea gigantism is common among these animals.
The rocky shores of mainland Antarctica and its offshore islands provide nesting space for over 100 million birds every spring. These nesters include species of albatrosses, petrels, skuas, gulls and terns.[The insectivorous South Georgia Pipit is endemic to South Georgia and some smaller surrounding islands. Freshwater ducks inhabit South Georgia and Kerguelen.
There are very few species of fish in the Southern Ocean. The Channichthyidae family, also known as white-blooded fish, are only found in the Southern Ocean. They lack haemoglobin in their blood, resulting in their blood being colourless. One Channichthyidae species, the mackerel icefish (Champsocephalus gunnari), was once the most common fish in coastal waters less than 400 metres (1,312 ft) deep, but was overfished in the 1970s and 1980s. Schools of icefish spend the day at the seafloor and the night higher in the water column eating plankton and smaller fish.
The flightless penguins are all located in the Southern Hemisphere, with the greatest concentration located on and around Antartica. Four of the 18 penguin species live and breed on the mainland and its close offshore islands. Another four species live on the subantarctic islands. Emperor penguins have four overlapping layers of feathers, keeping them warm. They are the only Antarctic animal to breed during the winter.
Seven pinniped species inhabitat Antartica. The largest, the elephant seal (Mirounga leonina), can reach up to 4,000 kilograms (8,818 lb), while females of the smallest, the Antarctic fur seal (Arctocephalus gazella), reach only 150 kilograms (331 lb). These two species live north of the sea ice, and breed in harems on beaches. The other four species can live on the sea ice. Crabeater seals (Lobodon carcinophagus) and Weddell seals (Leptonychotes weddellii) form breeding colonies, whereas leopard seals (Hydrurga leptonyx) and Ross seals (Ommatophoca rossii) live solitary lives. Although these species hunt underwater, they breed on land or ice and spend a great deal of time there, as they have no terrestrial predators.