The continental shelf around Antarctica is relatively narrow, 60 to 240km wide. It ranges from very shallow areas of less than 50m near the coast to areas deeper than 800m deep. The average depth is 500m. Beyond the shelf the Antarctic continental slope descends to over 3000m and levels out on the abyssal plains at depths of 3700–5000m (5km) deep. Soft sediments (mud, sand and gravel) are the single largest habitat on the continental shelf, slope and abyssal plains in Antarctica, and probably cover over 90% of the seabed. The abyssal plains consist only of soft-sediments with occasional boulders.
The animals and plants that live on the seabed are known as benthic communities. Benthic simply means pertaining to the sea bed. Soft sediments are home to very diverse and abundant communities of animals. In some areas of Antarctica over 155 000 animals per square metre have been recorded. These consist of small crustaceans, worms, echinoderms, shellfish (molluscs), and many other colourful, weird and wonderful beasts.
Many of the animals in soft-sediment communities in Antarctica are unusual in comparison with the rest of the world in that they are very large. Gigantism is quite common in Antarctic benthic faunas. For example, the isopod Glyptonotus antarcticus is up to twenty centimeters in length and seventy grams in weight. In comparison, isopods in other parts of the world may reach a maximum size of several centimeters. Glyptonotus antarcticus is found throughout Antarctica, the Antarctic peninsula and subantarctic Islands from the intertidal to 790m depth.
Many crustaceans such as Glyptonotus antarcticus have a non-seasonal breeding cycle and their young are incubated and raised in a brood pouch (marsupium) and released throughout the year. This is thought to be an adaptation to slow development in such cold and adverse conditions.
Many animals in soft-sediments burrow into the sediment. Some form burrows and tubes in the sediment to live in and filter food from the water column overhead. These include many worms, crustaceans and molluscs such the bivalve Laternula eliptica. Others actually burrow through and eat the sediment and absorb nutrients that are found in the sediment, such as the burrowing sea urchin Abatus.
Many animals simply live on top of the sediment surface, where they can move around in search of food. These include starfish, sea cucumbers, worms, crustaceans and bivalves such as the Antarctic scallop Adamussium colbecki.
Giant underwater relatives of spiders, which are known as pycnogonids, are quite common in Antarctica. They are very slow moving and quite harmless, feeding on small corals, sponges and bryozoans (lace corals). Some of them grow to a size bigger than a human hand.
Small crustaceans known as amphipods are one of the most common and abundant animals found in soft-sediments. There are many different species, some of them prey on other small animals, some of them graze on algae, and some of them eat whatever they can find.
Many Antarctic benthic animals are found all the way around Antarctica; their distribution is said to be circum-Antarctic. This is also quite unique, as it is very rare for a species to be found all the way around a continent the size of Antarctica, let alone many hundreds of species. One reason for this is that the climate and habitat is very similar all around the coast of Antarctica.
Benthic animals are a source of food for many larger animals including fish, penguins and seals. Studies of the stomach contents of fish and penguins often find many amphipods and other shrimp-like crustaceans.