Down in the frigid depths, these sharks hold their breaths.

Hammerhead sharks like it warm, but for a good meal they're willing to feel cold. The flat-headed predators dive more than 2,600 feet [800 meters] from tropical surface waters into the ocean's frigid depths multiple times every night to hunt for fish and squid, tolerating a 68-degree Fahrenheit plunge in temperature to dine.

How do these coldblooded chondrichthyans tolerate these temperatures without turning into frozen fish?

A new study shows how one species, Sphyrna lewini scalloped hammerhead sharks, stay warm during nightly dives : They skip the frills and close their gills, essentially holding their breath.

This strategy for regulating a coldblooded fish's temperature has never been observed before and distinguishes them from high-performance fish [ yes, that's the scientific term ] like great white sharks or Atlantic bluefin tuna that use vastly different strategies to tolerate extreme cold.

Mark Royer, a shark biologist in Hawaii, was inspired to investigate the scalloped hammerhead's secret heating technique after noticing how deep they were diving. He attached a package of sensors near the dorsal fins of six hammerheads near Hawaii.

He and his team found that hammerheads lose body heat when they start their descent, but quickly return to the same temperatures they were at the surface as they swam deeper.

Even when the water was as cold as 39 degrees fahrenheit [4 degrees Celsius], the sharks had body temperatures around 75 degrees [ 24 degrees Celsius ] during hourlong dives. [ Darren Incorvala ]


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