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SF Fig. 4.6. (A) Yellowfin tuna elevate their whole body temperature
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Image courtesy of International Game Fish Association
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Compare-Contrast-Connect: Warm-bodied and Hot-headed Fish
NGSS Crosscutting Concepts
(B) Blue marlin keep their heads hotter than the surrounding water. Image courtesy of Richard Theiss
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SF Fig. 4.6. (A) Yellowfin tuna elevate their whole body temperature (B) Blue marlin keep their heads hotter than the surrounding water.
Most fishes are poikilothermic, meaning their internal body temperatures are largely determined by the ambient temperature of their surrounding environment. Some fishes, however, have evolved the ability to elevate their body temperature above ambient conditions. For example, bluefin tuna can elevate their bodies as much as 21 degrees Celsius above the water temperature.
Tuna have a special system of countercurrent exchange that helps to retain body heat (see SF Fig. 4.7). However, tuna cannot keep its body at a fixed temperature the way mammals can. Thus, a tuna’s temperature elevation is different than temperature regulation because the tuna’s body does not maintain a constant temperature.
The extensive network of countercurrent exchange in tunas is often called a “rete mirabile” or wonderful net. It results in a concentration of blood vessels and elevation of body temperature that can help predators catch prey and help prey avoid being caught. This is because chemical reactions proceed faster at higher temperatures, so warmer fish can see, think, and move faster than colder fish.
However, the ability to elevate body temperatures requires significant energy input. Thus, this warm-bodied ability is usually found in large, open water predators that have the swimming muscles needed to generate body heat. Tuna meat is well known for its red color (compared to the white color of many other fish). The red color implies lots of heat and oxygen carrying red blood cells that help the tuna to swim continuously (see SF Fig. 4.8).
SF Fig. 4.8. Tuna steaks with dark red color. Image courtesy of Toru Hanai/Reuters
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Countercurrent systems are also useful for oxygen exchange. The concentration of oxygen dissolved in water is much lower than the concentration of oxygen in air. Fish have to be very efficient about maximizing the oxygen they can take up in their gills. The blood vessels in the gills used for gas exchange also run on a countercurrent system, so that the concentration of oxygen in blood leaving the body is always lower than that entering the body, creating a gradient that maximizes oxygen uptake.
Question Set
- Why do you think it might be advantageous for a fish to have an elevated body temperature (hint: think about the speed of a reptile on a cold day verses the speed of a reptile on a hot day)?
- Why do you think it might be disadvantageous for a fish to have an elevated body temperature?
- Billfishes are sometimes called “hot-heads”. They are related to tunas, but they only have the ability to elevate their head temperature. What structures are contained in the head that might benefit from the increased reaction rates associated with higher temperatures?
- Some sharks, the lamniformes (including the great white, mako and salmon sharks) also elevate their body temperature above the surrounding water level. What do these have in common with the tunas described as warmer-blooded? What does this tell you about the needs of some kinds of fishes?
- Arctic birds use a similar counter current exchange to keep from losing their body heat as they stand on the ice. How is this use of countercurrent exchange similar and different to the tuna and the swordfish?
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Image captionSF Fig. 4.9. Countercurrent exchange in gills.
Image copyright and sourceImage courtesy of Cruithne9
- Use the diagram in SF Fig. 4.7 (and the information above) to describe how the countercurrent exchange system in the gills is similar to the countercurrent exchange system that allows tunas to keep their body temperature warmer than the surrounding waters.
