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Paul McFarlane
Title: Warm-Blooded Fish
The body temperature of most fishes is the same as that of their surrounding water.  But Tunas and Mackerel Sharks have the ability to raise their body temperatures, which gives them several advantages.
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Date first published: January 1999
Publication: Monthly Bulletin, Hamilton and District Aquarium Society (Ontario, Canada)
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March 2005: translated into Hebrew language on of Tel Aviv, at:
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Warm-Blooded Fish

by Paul McFarlane
From the Monthly Bulletin of the Hamilton and District Aquarium Society, January 1999

As most aquarists know, fish are cold blooded animals. This simply means that they have no built-in biological means of controlling their own body temperature and therefore assume the temperature of the water in which they find themselves. Most fishes can only survive at a relatively small range of body temperatures; it is for this reason that they die or become seriously weakened and prone to disease if tank temperatures are allowed to fluctuate by more than a few degrees, especially if the fluctuation is rapid.

The fact that fishes are unable to maintain their own body temperatures can be partially explained as follows. Land animals, and some aquatic ones, obtain oxygen by breathing air. Air is both rich in oxygen and has a low capacity for absorbing heat; water on the other hand, from which most fish must obtain their oxygen, contains only about 2.5% as much oxygen and absorbs heat at 3000 time the capacity of air. This means that although a small amount of heat (about enough to raise the body temperature one degree) can be generated by the utilization of oxygen in the fish's body, when the blood is returned to the gills for a further supply this heat is very rapidly lost to the surrounding water. The overall effect is that the fish's body temperature remains the same as the surrounding water.

Certain types of fishes, notably the Tunas and one family of sharks (the Mackerel Sharks) are able, seemingly in defiance of nature to maintain their body temperatures up to 20F or so above the water temperature. This phenomenon was noticed as early as 1835 but has only recently been fully investigated. Such fishes contain organs, near their muscles, which are called "rete mirable". These consist of a series of very small parallel veins and arteries that supply and drain the muscles. As the warmer blood in the veins is being returned to the gills for a fresh supply of oxygen it comes into close contact with cold, newly oxygenated blood in the arteries. The system thus acts as a heat exchanger and the heat from the blood in the veins is given up to the colder arterial blood rather than being lost at the gills. The net effect is an increase in temperature. Fish from warmer water only elevate their temperature a few degrees whereas those from cold water may raise it as much as 20F.

A fish with the ability to raise and maintain body temperature has several definite advantages over its less fortunate brethren. Such a fish need not be selective in its range because of different water temperatures. Nor would it be as affected by geological or climactic changes in its environment. The additional heat supply to the muscles is also distinctly advantageous because of the resulting extra power and speed. Yellowfin and Wahoo tuna for example, are noted for their speed; they have been clocked in excess of 40 mph during 10 to 20 second sprints. Such fast swimming predatory fishes would find an abundance of food in the form of squid, herring, mackerel, etc., that slower predators cannot capture.

As a last note, it is interesting that the two groups, the Tunas and the Mackerel Sharks, which have developed this mechanism, are not related; the sharks are a much older group. It would seem to be a classic example of parallel adaptations in which two unrelated groups independently evolved the same means of elevating body temperature in order to exploit a previously unavailable source of food.