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Tuna are
several species of ocean-dwelling fish in the family
Scombridae, mostly in the genus Thunnus. Tunas are
fast swimmers—they have been clocked at 70 km/h (45
mph)—and include several species that are
warm-blooded. Unlike most fish species, which have
white flesh, tuna have flesh that is pink to dark
red. The red coloring comes from tuna muscle
tissue's greater quantities of myoglobin, an
oxygen-binding molecule. Some of the larger tuna
species, such as the bluefin tuna, can raise their
blood temperature above that of the water through
muscular activity. This ability enables them to live
in cooler waters and to survive in a wider range of
circumstances. Some tuna species and fisheries have
been overfished and some tuna fisheries are at risk
of collapse.
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