Hello Peanut9674-ga, and thank you for an excellent question, one that
warms the heart (if I may be so endothermic) of a marine biologist
like myself.
One thing we biologists are always trying to do is put the living
world into neat little boxes. And one of one primary lesson learned
is that, no matter what categories we create, there are critters who
simply won't fit in. Such is the case with the sort-of-coldblooded,
sort-of-warm-blooded tuna fish.
The key to the tuna's half and half existence is a wonderful
bio-mechanical process known as counter-current heat exchange. The
tuna has large muscles and a lot of blood flow (relatively speaking),
and a special network of blood vessels that efficiently transfer heat
inward to the body core as blood warmed by exertion passes by cooler
blood (that's the "counter current" part).
As for which part of the fish is which -- who knows??? The tuna is
certainly cold-blooded in that it's body temperature experiences a
much broader swing than can be tolerated by a typical warm-blooded
animal (such as ourselves -- imagine how you'd be feeling if your
termperature rose or fell a mere 5 degrees Celcius?). However, it
also can be thought of as a pseudo-warm blooded creature, as it is
able to fairly successfully maintain an internal body temperature
elevated over the ambient water temperature.
There is no one piece of the fish that is clearly a warm-blooded
piece, or a cold blooded piece. The organism works as a coherent,
efficient whole to be what it is -- and our classification schemes be
damned.
Here are a few links to sites that discuss this phenomenon in more
detail:
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http://www.bartleby.com/65/bo/bodytemp.html
The Columbia Encyclopedia, Sixth Edition. 2001.
Article on Body Temperature
Reptiles and other poikilothermic animals bask in warm weather and
must hibernate in winter. The body temperature of fishes must remain
close to that of the surrounding water, because heat is lost directly
into the water during respiration; however, in some fishes, such as
the bluefin tuna, a special network of fine veins and arteries called
the rete mirabile provides a thermal barrier against loss of metabolic
heat.
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http://www.biology.lsa.umich.edu/~www/bio154/bio154f98lect14.html
University of Michigan: Homeostasis and Temperature Regulation
Tuna maintain inner muscle temperatures 10-15 oC warmer than ambient
seawater
Veins and arteries in tuna swimming muscles are arranged in parallel
arrays so that the warmed venous blood transfers its heat efficiently
to the incoming cooled arterial blood, thereby returning much of it to
the red muscle core.
Anti-parallel arrangement of blood vessels allows efficient transfer
of heat, maximizing heat retention in the muscle core.
This is an example of a COUNTER-CURRENT HEAT EXCHANGER
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http://www.newswise.com/articles/2002/8/TUNA.PH2.html
News from the American Physiological Society
All too often the term "cold-blooded" is taken too literally, with
members of Osteichthyes (fishes) assumed to have their body
automatically change to the surrounding temperature. This assumption
misses an important finding that nature knows how to ensure that
certain fish species have physiological qualities that allow them to
regulate body temperatures to survive and thrive in different
environments.
One such example of this phenomenon is the tuna fish. Tunas are
regional endotherms, maintaining elevated temperatures in deep red
swimming muscles by way of vascular counter-current heat exchangers
(retia mirabilia) that trap heat produced as a by-product of tissue
metabolism. The close contact of blood vessels in the retia allows
heat in the venous blood to be transferred to arterial blood that
returns to the tissues, preventing loss of heat to the surrounding
water as the venous blood passes through the gills. Some tuna species
also warm the viscera (internal organs) and brain through associated
vascular networks or retia. These retia keep tissues at temperatures
higher than the surrounding water, enhance metabolic capacity, and act
as effective insulators, which slow heat loss during the fish's
journeys into cooler waters.
One species of tuna that makes such as journey is the yellowfin
Thunnus albacares, a tropical-subtropical inhabitant of surface
waters, that periodically makes rapid dives into deeper waters in
search of prey, often encountering temperature gradients as much as 10
degrees C cooler. Previous studies have found that the Thunnus tuna
possess a carotid rete, or vascular network, in the blood supply to
the eye and brain that acts as a thermal barrier.
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There are many more sites that discuss this, some with detailed
scientific studies. You can access them by repeating the same Google
search that I did:
"counter current" tuna
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I hope this information meets your needs. Please post a Request for
Clarification if anything is not clear, or needs more elaboration. |
