The deep ocean around Antarctica is a unique place. The weight of the ice sheets has pushed the continental shelf to be deeper than most places. Usually, the continental shelf is found between 100 to 200m down but in the Antarctic waters it is at 600m. This speaks to the sheer weight of the ice sheets pressing from above.
The water temperature near the surface is not so different from the cold depths meaning creatures can come up to much shallower water. The waters around Antarctica are relatively stable varying from about 3°C to −2°C. This has led to a long evolutionary period of isolation for many of the organisms inhabiting its waters. As such, a world of wonder lies in wait. This is made possible by the freezing of Antarctic seawater in winter which leaves behind extremely cold, salty, oxygen-rich water that flows around the world giving life to a submerged barren landscape.
Creatures make the most of this phenomenon. Gigantic sea spiders tread the ocean floor with leg spans of up to 40cm. This is far larger than other sea spiders that grow to be a few millimeters. These giant sea spiders exhibit polar gigantism. The tendency of deep-sea invertebrates to grow larger.
Two-meter giant sea sponges cling to stones. Feather stars and cold water corals have colonized the region feeding on the abundant marine snow. Well adapted to a stable unchanging environment these plant-like species grow very slowly but to enormous sizes.
Surviving the cold
Freezing isn't as entirely straightforward as might be imagined however, often a nucleation point is required for the freezing to begin, without this, it is possible for a liquid to reach its freezing point or get even colder, but not turn to a solid and freeze. An ideal nucleation point for the growth of ice is an ice crystal itself.
Deep ocean animals such as fish in Antarctica never actually encounter ice as they live too deep in the sea. They can be caught and brought to the surface where they can freeze instantly on contact with ice which causes crystals to grow in their cold tissues. So one way of surviving very cold temperatures as a marine animal is to make sure you don't actually contact ice which means you can become supercooled (below freezing point without actually freezing) without fear.
Other marine animals lower the freezing point of body tissues to avoid freezing in the first place. Sea water freezes at -1.86°C though the exact temperature depends on the precise salinity (amount of salt dissolved in the water). Animals can use the same idea with chemicals dissolved in their body fluids to lower their freezing points to below this temperature, it doesn't need to be far, just a little below that of the surrounding water. As long as the concentration of dissolved chemicals in the body fluids is greater than the surrounding sea water the body tissues can escape damage by freezing.
The ice fish
The most obvious and peculiar thing about the icefish is that they completely lack hemoglobin, the pigment that makes blood red, so many of the species appear a ghostly white, a feature that is particularly noticeable in the gills. Whereas the gills of other fish are a deep wine-red colour, the gills of icefish are translucent white.
They don't have red blood, but they do have blood, lots of it. They get by without hemoglobin: by having a much greater volume of blood than other fish, by having a greater cardiac output (the heart pumps lots of blood), by having increased blood flow, by having low viscosity blood due to the lack of red blood cells, by having thin skin rich in capillaries across which oxygen can diffuse.
This works because oxygen dissolves far better in cold water than in warm water. The icefish take advantage of this fact and that coastal Antarctic waters are well oxygenated in order to make energy and resource savings from not having to make red blood cells filled with hemoglobin protein and requiring iron. They also have antifreeze proteins in their clear blood, as you might expect from something called an icefish!
Creatures of the Antarctic belong to a community that has been isolated from the rest of the planet for millions of years. The powerful Antarctic circumpolar current swirls around the frozen continent caging the deep sea organisms to an area 40km off the coast. There is still much to explore in this alien underwater world.
For more on deep sea gigantism and how some sea animals are able to grow to such enormous sizes read part 1 of the series.
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