Epipelagic, usually oceanic but seasonally coming very close to shore. Bluefin are found in moderately warm seas but are more tolerant of cold water than are most of their relatives. Offshore in the northwest Atlantic, large bluefin are taken at surface temperatures of 43.5-83.8°F (6.4-28.8°C). Tunas have evolved elaborate rete mirabilia, "wonder nets," of capillaries that act as countercurrent heat exchangers. These heat exchangers form thermal barriers that prevent metabolic heat loss and enable bluefin to maintain a high internal temperature, as high as 83.8°F (28.8°C) for a bluefin taken in 45.1°F (7.3°C) water.
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