Are fish warm or cold blooded?

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.

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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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One thought on “Are fish warm or cold blooded?

  1. Thanks for answering this and explaining more. My friend was adamant fish were warm-blooded, but I was pretty sure they were cold-blooded 😉

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