Warm-Blooded Fish
by Paul McFarlane
From the Monthly Bulletin of the Hamilton and District Aquarium Society, January 1999
Aquarticles
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.
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