The team discovered this unexpected phenomenon after fitting deep-sea hammerhead sharks with sensors that measure muscle temperature, depth, body alignment and activity. Despite the cold water in the deep sea, the muscles remained warm and did not cool until the sharks approached the surface again. Computer simulations led the researchers to conclude that in order to stay warm while hunting at this depth, hammerhead sharks must prevent heat loss through their gills.
Video footage of a hammerhead shark swimming from more than 1,000 meters deep on the ocean floor showed its gill slits tightly closed. In contrast, it was always wide open near the surface. The sudden drop in muscle temperature when they surfaced, the researchers said, indicated that they had reopened their gill slits to breathe again, even though they were still in cooler water.
“While holding your breath keeps the hammerhead warm, it also cuts off its oxygen supply,” says Royer. On average, the animals stop breathing for 17 minutes, but spend an average of only four minutes at extreme depth before quickly returning to warmer surface waters, where breathing resumes.
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