Fish are ‘cold-blooded’ or poikilothermic: they do not alter their body temperature to compensate for the temperature fluctuations in their environment as birds and mammals do.
Although the range of temperatures encountered in bodies of water is smaller than the extremes experienced on land, some Antarctic fishes – the so called ‘ice-fishes’ – live in water that is below freezing point.
Crystallization of their body fluids would be fatal, but this is prevented by means of special ‘anti-freeze’ compounds in the blood. These fish lack the usual oxygen-carrier, haemoglobin, which gives blood its red colour, so their blood is almost transparent.
At the other extreme, some freshwater fish can live in the waters of hot springs at nearly 40 C (104 F). Perhaps the most rugged individuals, however, are those that live in shore pools in temperate seas: being exposed in sunlit pools at low tide, they have to survive a very wide range of temperatures.
Some active ocean swimmers, such as tuna, can maintain their body temperatures above that of the water by means of a heat-exchange mechanism between closely associated veins and arteries.
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