Hammerhead sharks clamp their gills shut to remain heat on deep dives

Scalloped hammerheads seem to carry their breath once they dive into chilly, deep water. By shuttering their blood-rich gills, they could preserve heat whereas looking prey, successfully sidestepping their very own cold-blooded biology.

Researchers already knew that scalloped hammerheads (Sphyrna lewini) made repeated, transient, plunging nighttime dives, says Mark Royer on the College of Hawaiʻi at Mānoa. The dramatic descents could also be for looking, because the beaks of squid from the visited depths have turned up within the sharks’ stomachs.

Nevertheless it wasn’t clear how the tropical hammerheads managed to tolerate the frigid temperatures in such deep water, generally round 5°C. Different fishes, equivalent to nice white sharks and tunas, have circulatory methods that recycle warmth produced by their flexing muscle tissue, letting the lively predators keep heat in chilly water. Hammerheads don’t, says Royer. He and his colleagues caught scalloped hammerheads in Kāneʻohe Bay, Hawaiʻi, briefly secured them in opposition to the facet of the boat and connected packages of devices to the bottom of every shark’s dorsal fin. These devices measured water and muscle temperature, and the shark’s tail beating and acceleration in all instructions. After just a few weeks of accumulating information on three sharks, the packages dislodged and floated to the floor for the workforce’s retrieval.

The researchers discovered that, when diving, hammerheads all of a sudden rocket in direction of the underside at an 80-degree angle, furiously beating their tails.

“You should expect to see their body temperature drop right away,” says Royer. “But that’s not what’s happening.”

As an alternative, their physique temperature holds excessive above ambient temperatures all through the several-minutes-long plunge to about 800 metres. The sharks then zoom in direction of the floor. Solely then does their physique temperature lastly drop.

Sharks lose some warmth to cooler environment by way of their physique wall, however the researchers suspected these puzzling outcomes needed to do with the sharks’ gills. Sharks breathe by absorbing oxygen dissolved in water by way of blood vessels of their gills, that means they’re part of the physique the place a lot of blood is uncovered to chilly water.

“The greatest rate of heat loss for any gill-breathing animal is through the gills,” says Royer. “It’s basically like having a giant radiator strapped to your head.”

If the sharks had been respiration closely throughout their demanding dives, their physique temperature ought to have quickly plummeted. The researchers suspected the diving hammerheads prevented this by clamping down their gill slits.

Certainly, video footage of scalloped hammerheads swimming a kilometre down has proven their gill slits tightly closed.

José Emilio Trujillo on the College of Otago in New Zealand wonders how the hammerheads handle to be so athletic with so little oxygen. Diving mammals have variations for coping with very low oxygen of their tissues, he provides, so maybe hammerheads do too.

The breath holding speculation is attention-grabbing and worthy of additional investigation, says Phillip Morrison at Vancouver Island College in Canada. Nonetheless, he isn’t satisfied it’s the “sole mechanism” with out extra analysis, equivalent to analyses of physique temperature beneath completely different ranges of gill warmth loss.

“If [the researchers] are correct, I think that this is one of the coolest physiological traits among sharks,” says Morrison.

Different shark species may additionally use this freediving technique. Royer factors out that oceanic whitetip sharks (Carcharhinus longimanus) make comparable repeated, steep dives.