Phytoplankton, krill, fish
Most of the Antarctic coastline is hidden beneath the ice flowing off the continent. Beneath the seemingly lifeless expanses of pack-ice and sea-ice lies a much more rich and varied world of animals than on land.
The factor which has the greatest effect on life in the Southern Ocean is not temperature but light. Without light plants cannot grow, and without plants there is no food for the animals at the bottom of the Antarctic food chain.
The food chain is based on phytoplankton, a varied group of tiny free floating plants. In spring and early summer their numbers increase rapidly, producing "blooms" like a think pea soup which can cover thousand of square kilometres of the ocean. These blooms provide a food source which is 300-400 times more concentrated than normal for a variety of zooplankton (tiny animals, especially copepods and krill, which in turn provide food for fish, seals, whales and penguins. Not surprisingly, zooplankton grow rapidly, increasing their body weight by around 5% a day over the short summer.
Krill spawn in summer, and mature females can do so twice a season. They lay 2000-3000 eggs each time and the eggs sink into deep water where they are carried southward on the currents to the edges of the continent. There they hatch into larvae, which go through several stages as they gradually rise, emerging on the surface as adults in two or three years.
Krill live a long time for plankton, up to seven years. In winter, they feed on algae on the underside of the pack ice. They shrink on this sparse diet, but make up for it the following summer when the phytoplankton blooms once more.
There polar fish have become well adapted to cold, their body fluids remain still liquid at temperatures below the freezing point. Their body contain different anti-freeze molecules which impede crystal growth and prevent ice form spreading through the body fluids. Exactly how this works is not yet fully understood.
The most unusual fish in Antarctic water are the ice fish. They are the only vertebrates whose bodies entirely lack haemoglobin, the red oxygen-carrying pigment in the blood. As they have no haemoglobin, they have only 10% of the normal oxygen-carrying capacity, so they have to compensate with other special adaptations suited to low temperatures. They need less energy to circulate the blood because the viscosity of the blood is lower without haemoglobin. The circulation is even quicken by a larger heart which bears faster. All this makes the transfer of oxygen from blood to tissues more efficient. They also use less energy to maintain their metabolism when resting than red-blooded fish.
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