Fish are described as cold-blooded, meaning that their body temperature varies with the external temperature. Fish do, however, produce metabolic heat (that is, heat derived from the oxidation, or "burning," of food and from other processes), but much of this heat is lost to the outside at the gills. Blood passing through the gills loses heat to the water quite rapidly, so that a fish's body temperature is usually within a degree or so of the water temperature. Tunas and mackerel sharks, however, are warm-bodied fishes. They have evolved countercurrent circulatory networks that consist basically of paired ingoing and outgoing blood vessels. In this way the heat of the warm blood going to the gills is transferred to the cooled blood coming from the gills, and the heat is kept within the fish's body. By using these networks, yellowfin and skipjack tuna are able to keep their body temperature from about 5 deg to almost 12 deg C (9 deg to 21 deg F) above the water temperature. One skipjack taken in warm waters registered a body temperature of 37.8 deg C (100 deg F). The bluefin tuna does even better and might qualify as a warm-blooded (as opposed to warm-bodied) fish. It is able to maintain a fairly constant body temperature across different water temperatures, its body temperature varying only about 5 C degrees (9 F degrees) over a 20 C degree (36 F degree) range of water temperatures.
One of the advantages of warm-bodiedness is an increase in muscle power. Muscles contract more rapidly when warm without loss of force. If with a 10 C degree (18 F degree) rise in body temperature a muscle can contract three times as fast, then three times the power is available from that muscle. More muscle power means more speed in pursuing prey, escaping enemies, and shortening the time required for long-distance migration.
Distribution - Anatomy - Circulation -
Water Balance - Swimming - Gas Bladder
Lateral Line System - Evolution - Reproduction
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