Antarctic Biology

The Southern Ocean Ecosystem

The Southern Ocean, which is entirely surrounded by sea, is a mass of water and ice covering approximately 36 x 106 km2 which represents about 10% of the world's oceans. The northern limit of the Southern Ocean is set by the Antarctic Polar Front (Antarctic Convergence), identifiable as a 40 km wide ocean band where a 2-3° C change in temperature of the sea surface occurs. This relatively abrupt change in water temperature occurs where dense, cold polar water meets and flows under less dense warmer water from the tropics. The Antarctic Polar Front separates the Southern Ocean from the Atlantic, Pacific and Indian Oceans which border it to the north. Water in the Southern Ocean moves eastwards (clockwise) around Antarctica carried by the Antarctic Circumpolar Current, the largest ocean current in the world. A counter-current, the Antarctic Coastal Current, flows westward closer to the continent.

The Southern Ocean Food Web

The Southern Ocean food web describes the flow of energy within the Southern Ocean ecosystem from microbes to the top predators. Marine phytoplankton (the primary producers) use energy from the sun to make simple organic molecules by photosynthesis. The phytoplankton fix about 50 tonnes of carbon (from CO2) per 100 km2 of sea surface each year. Because of their dependence on light, phytoplankton growth is restricted to surface waters where there is little sea ice (which would otherwise limit light penetration), and it is influenced by seasonal variations in sunshine. A small amount (<10%) of the organic molecules produced by photosynthesis are recycled by other microorganisms which may drop to the sea floor where they fuel bottom-dwelling organisms. The major proportion passes through the food web at the hub of which is the Antarctic krill (a key herbivore) and a variety of zooplankton. Predators of krill such as the baleen whales, crab eater seals, penguins, fish and squid are the primary predators of the food web. Higher order predators include the killer whale, leopard seal and birds such as the wonderful skua.



The Antarctic krill, of which there are several species, is a shrimp-like crustacean growing to about 5 cm in body length. Krill (the name is derived from a Norwegian whaling term meaning "small fry") are widely distributed around the Antarctic continent. Euphausia superba dominates north of the pack ice, while the smaller E. crystallorophias is the principle krill species under the ice. E. superba form dense swarms, typically hundreds of metres across and 20 or so meters deep. The largest swarms can spread over several square kilometres and may extend to a depth of 200 metres. Swarms of such dimensions may contain up to 10 x 103 tonnes of krill which makes them an extremely abundant food supply. It has been estimated that there are over 6 x 1011 individual krill with a biomass of around 250-600 x 106 tonnes but, because of their patchy distribution, these figures may not be particularly reliable. Although labelled as the dominant herbivore in the Southern Ocean food web, krill are actually omnivores feeding on the zooplankton as well. The life history of E. crystallorophias is not well known and further study of this species remains a great challenge to scientists.
The Antarctic krill - Euphausia superba.

For more information about krill visit Uve Kils' website from where this image is sourced.


Krill display a number of interesting adaptations, particularly behavioural ones such as swarming and movement in the water column which may be related primarily to their feeding habits. Perhaps their most profound adaptation relates to their survival during winter when food supplies are relatively scarce. Instead of building up large fat reserves, krill reduce their size capitalising on their own body proteins as a source of energy. This reduction in size presents a special problem for animals which have exoskeletons. The problem is overcome in krill by retaining the ability to moult into adulthood. The ability to increase or decrease size depending upon food supply enables the krill to survive during the winter. This is further enhanced by their omnivorous rather then strictly herbivorous character thus maximising available food over the winter months.


The modern Antarctic fish fauna is exclusively marine, with 203 bottom-dwelling species and 75 mesopelagic species being recognised (reviewed in Eastman, 1993). This represents about 1% of all known fish species. The fauna is highly endemic (about 88% of the species are restricted to Antarctica) and of the bottom-dwelling species, 104 (51%) belong to a single suborder, the Notothenioidei. The fossil fish fauna shows that Antarctic fish have undergone a considerable reduction in diversity since the early Tertiary, but the reasons for this are not clear, particularly since there are no notothenioids in the fossil record to date. The dominance of notothenioids presumably reflects the presence at some early stage in their evolution of an ancestral species which overcame the difficulties of survival at low temperatures. Subsequent speciation and evolution of this ancestral species resulted in the current notothenioid fauna (monophyletic radiation). The suborder Notothenioidei comprises six families: the Nototheniidae (Antarctic cod), the Harpagiferidae (spiny plunderfishes), the Bathydraconidae (dragonfishes), the Channichthyidae (icefishes), the Bovichthyidae (thornfishes), and the Artedidraconidae (plunderfishes).

The nototheniid - Trematomus bernacchii.


After whom was this fish named?

How was he involved in early exploration of the Antarctic?



During their evolution the Antarctic fishes have adjusted to cope with the Antarctic environment (reviewed in Macdonald and Montgomery, 1990). None possess a gas-filled swimbladder (used for buoyancy) which suggests that the ancestral species was probably a bottom dweller. Some of the modern species such as Pleurogramma antarcticum have become secondarily pelagic, decreasing their densities by incorporating lipid into their tissues and developing a cartilaginous skeleton with hollow vertebrae. Many notothenioids have pelagic larvae which can feed directly on plankton. Since these are low in the food chain, the larvae benefit from a larger, potentially more available food source.

Although obviously cold (-1.86° C at high latitudes to +5° C near the Antarctic Polar Front), the waters of Antarctica are relatively stable (seasonal variation is typically < 0.2° C) and local fish species have evolved to become intolerant of temperature changes. The concentration of salts in sea water depresses its freezing point to about –1.9° C, while that in fish blood depresses it to about –0.8° C. To survive at near freezing temperatures fish contain various types of antifreeze in their blood, reaching a concentration of about 35 mg/ml. Fish of the species Pagothenia borchgrevinki contain 8 glycopeptide antifreezes of similar structure of different molecular weight (2,600-33,700 daltons). Each has a repeating tripeptide (3 amino acid) backbone (threonine-alanine-alanine) with a disaccharide extending from the threonine residue. The antifreeze glycopeptides act by binding to small ice crystals in the fish to prevent further growth. Although present in most body fluids, there is no anti-freeze present in the urine. This is a result of specialisation of the kidneys which do not contain glomeruli. Filtration of the blood takes place by active secretion through the walls of the kidney tubules, thus preventing loss of anti-freeze glycoproteins.

The Nototheniid - Pagothenia borchgrevinki.


After whom is this fish named?

What was his contribution to Antarctic exploration?

The viscosity of blood increases at lower temperatures, increasing the energy required to pump it around the body. Many Antarctic fish are adapted to this situation by having fewer red blood cells (the predominant blood cell type), thus effectively thinning the blood. In temperate fish the corresponding reduction of red blood cell haemoglobin would significantly lower the amount of oxygen carried to the tissues, but the increased solubility of gases at low temperature allows Antarctic fish to carry more oxygen dissolved directly in the blood. The solubility of oxygen in sea water at 0° C, for example, is 0.83 vol. % per atmosphere of air, while its solubility at 20° C is 0.53 vol. %.

The channichthyids (icefish) are unique among vertebrates in that they have a deleted gene for haemoglobin and thus they are totally dependent on the increased solubility of gases at low temperatures to transport oxygen in their blood plasma. The oxygen carrying capacity of icefish blood is about 0.7 vol. %, which is only 10 or so of the amount carried by the blood of haemoglobin-containing Antarctic fish. Transport of oxygen in icefish is facilitated by a number of anatomical and physiological adaptations including a large heart (similar in weight to that of a small mammal) with a large stroke volume (6-15 x that of other teleosts) increasing cardiac output at low heart rate and low ventral aortic pressure. They also have a high blood volume (2-4 x that of other teleosts), large diameter blood vessels to lower resistance, low blood viscosity, well vascularised gills and a scaleless skin to maximise the extraction of oxygen from the sea water, and a relatively low average metabolic rate minimising the demand for oxygen. The icefish are so well adapted they appear not to be compromised by their lack of haemoglobin and they can lead active, predatory lives.


There are a considerable number of avian species which visit Antarctica, but relatively few breed on the continent. Of those that do, discussion will be restricted to the skuas and the penguins.


There are five currently recognised species of skua. The two which dominate on the Antarctic continent are the South Polar skua (Catharacta maccormicki) and the brown skua (C. lonnbergi). The South polar skua is widely distributed over the mainland, whereas the brown skua is restricted to the Antarctic peninsula. Interbreeding is possible where the distributions of the two species overlap.

Lieutenant Evans (who went south with Scott) wrote that their greatest value to the early explorers was their tasty and nourishing flesh, reminding him somewhat of wild duck. He was either possessed of a wonderful imagination or extraordinary digestive powers.

An adult south polar skua (Catharacta maccormicki ) with chick.

There are 17 species of penguins distributed over six genera, all of which are found in the family Spheniscidae. They are all flightless, pelagic seabirds and vary in size from the Little Penguin (Eudyptula minor) which weighs just over 1 kg and is around 40 cm tall to the Emperor (Aptenodytes forsteri) which is a hefty 30 kg and reaches 115 cm in height. Only two penguins are restricted to the Antarctic, the Adelie (Pygoscelis adeliae) and the Emperor.


Like all animals living in Antarctic, birds are adapted to cope with the extremes of the environment. This is demonstrated clearly by the penguins which have a thick thermoprotective layer of subdermal fat to maintain body heat while also streamlining the body for swimming. Their short, overlapping feathers trap insulating air when diving and promote insulation even under windy conditions. Heat loss through the legs and feet is minimised by keeping them tucked up against the warmth of the body and by having heat exchangers in which heat is transferred between intertwined arteries and veins. Many penguins such as the emperors also huddle together in extreme conditions to retain body heat. Their efficiency in heat retention is such that they have evolved special adaptations for those occasional warm sunny days that do occur in Antarctica. These adaptations take the form of blood vessels which come close to surface on the almost featherless inside surface of their flippers. Thus in sunny conditions, or when otherwise hot, penguins can radiate metabolic heat by raising their flippers to expose the undersides.


The seals (Order Pinnipedia) are distributed over three families: the families Phocidae (true seals) and Otariidae (the eared seals) are found in Antarctica, whereas members of the family Odobenidae (the walruses) are not. There are six species of Antarctic seals. These include the Antarctic fur seal (Arctocephalus gazella) which represents the Family Otariidae, and five phocids represented by the elephant seal (Mirounga leonina), the Weddell seal (Leptonychotes weddelli), the Ross seal (Ommatophoca rossi), the leopard seal (Hydrurga leptonyx) and the crabeater seal (Lobodon carcinophagus).

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