The continental shelf around Antarctica 60 km to 240 km wide. This is relatively narrow. It ranges from very shallow areas of less than 50 m near the coast to areas deeper than 800 m. The average depth is 500 m.
Beyond the shelf, the Antarctic continental slope descends to over 3 km and levels out on the abyssal plains at depths of 3.7 km to 5 km.
Soft sediments (mud, sand and gravel) are the single largest habitat on the continental shelf, slope and abyssal plains. This kind of habitat probably covers over 90% of the seabed.
The animals and plants that live on the seabed are known as benthic communities. Benthic means pertaining to the seabed.
Soft sediments are home to diverse and abundant communities of animals. In some areas of Antarctica over 155,000 animals per square metre have been recorded.
Weird and wonderful animals
Many of the animals in soft-sediment communities in Antarctica are unusual in that they are very large. Gigantism is common in Antarctic benthic faunas.
The isopod Glyptonotus antarcticus grows up to 20 cm in length and 70 g in weight. In comparison, isopods in other parts of the world reach a maximum size of several centimetres. Glyptonotus antarcticus is found throughout Antarctica, the Antarctic peninsula and sub-Antarctic Islands from the intertidal to 790 m deep.
Many crustaceans (including Glyptonotus antarcticus) have a non-seasonal breeding cycle. They incubate and raise their young in a brood pouch (marsupium). They release the young throughout the year. This is thought to be an adaptation to slow development in such cold conditions.
Many animals burrow into the soft sediment. Some form burrows and tubes to live in, and filter food from the water column overhead. These include many worms, crustaceans and molluscs (such the bivalve Laternula eliptica). Others burrow through the sediment and eat it to absorb nutrients (such as the burrowing sea urchin Abatus).
Many animals live on top of the sediment, 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. They feed on small corals, sponges and bryozoans (lace corals). Some of them grow 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 of amphipods. Some prey on other small animals, some graze on algae and some eat whatever they can find.
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.
Many Antarctic benthic animals can be found all the way around Antarctica. Their distribution is said to be circum-Antarctic. 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.