Sharks are no chilly gillies

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Richard Robinson

Fish are cold-blooded, meaning they rely on their surroundings to regulate their body temperature. So how do hammerhead sharks—which frequently dive deep to hunt—maintain a constant body temperature when they’re in the frigid depths?

Turns out scalloped hammerheads hold their breath. Scientists tracking seven of the sharks off Hawai’i found that the predators likely close their gill slits on deep dives to preserve body heat—the first time this behaviour has been observed in fish.

The discovery was “a complete surprise”, according to the study’s lead author, Mark Royer. “Although it is obvious that air-breathing marine mammals hold their breath while diving, we did not expect to see sharks exhibiting similar behaviour.”

Across more than 100 deep dives, ranging from 418 to 825 metres beneath the surface, a device similar to a Fitbit recorded what each shark was doing.

After a gradual descent, the sharks sharply and swiftly dive down. During that plunge, they can experience an ambient temperature drop of about 20 degrees Celsius. They remain at depth for about four minutes, hunting, before steeply ascending again. Throughout each dive, the shark’s temperature remains constant until part-way through the ascent.

Computer modelling suggests that the sharks manage this by closing their gill slits, which are a major source of heat loss—an explanation supported by video footage of a hammerhead swimming at about one kilometre deep with its gills shut.

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