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. However, among smaller marine animals generally assumed to be the same in Antarctica and the Arctic, more detailed studies of each population have often—but not always—revealed differences, showing that they are closely related cryptic species rather than a single bipolar species. Antarctic animals have adapted to reduce heat loss, with mammals 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 meter (10.8 sq ft). As the seafloor environment is very similar all around the Antarctic, hundreds of species can be found around the mainland, which is a uniquely wide distribution for such a large community. Polar and deep-sea gigantism, where invertebrates are considerably larger than their warmer-water relatives, is common in this habitat. These two similar types of gigantism are believed to be related to the cold water, which can contain high levels of oxygen, combined with the low metabolic rates ("slow life") of animals living in such cold environments.