Most Fish Are Cold-Blooded

Most Fish Are Cold-Blooded

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Like the amphibians and reptiles to which they're distantly related, the vast majority of fish are ectothermic, or cold-blooded: they rely on the ambient temperature of the water to fuel their internal metabolisms. Surprisingly, though, barracudas, tunas, mackerels and swordfish—which belong to the fish suborder Scombroidei—all have warm-blooded metabolisms, albeit using a system quite different from that of mammals and birds; a tuna can maintain an internal body temperature of 90 degrees Fahrenheit even when swimming in 45-degree water! Mako sharks are also endothermic, an adaptation that endows them with added energy when pursuing prey.