Scalloped hammerheads appear to hold their breath when they dive into cold, deep water. By shuttering their blood-rich gills, they may keep warm while hunting prey, effectively sidestepping their own cold-blooded biology.
Researchers already knew that scalloped hammerheads (Sphyrna lewini) made repeated, brief, plunging nighttime dives, says Mark Royer at the University of Hawaiʻi at Mānoa. The dramatic descents may be for hunting, since the beaks of squid from the visited depths have turned up in the sharks’ stomachs.
But it wasn’t clear how the tropical hammerheads managed to tolerate the frigid temperatures in such deep water, commonly around 5°C. Other fishes, such as great white sharks and tunas, have circulatory systems that recycle heat produced by their flexing muscles, letting the active predators stay warm in cold water. Hammerheads do not, says Royer. He and his colleagues caught scalloped hammerheads in Kāneʻohe Bay, Hawaiʻi, briefly secured them against the side of the boat and attached packages of instruments to the base of each shark’s dorsal fin. These instruments measured water and muscle temperature, and the shark’s tail beating and acceleration in all directions. After a few weeks of collecting data on three sharks, the packages dislodged and floated to the surface for the team’s retrieval.
The researchers found that, when diving, hammerheads suddenly rocket towards the bottom 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.”
Instead, their body temperature holds high above ambient temperatures throughout the several-minutes-long plunge to about 800 metres. The sharks then zoom towards the surface. Only then does their body temperature finally drop.
Sharks lose some heat to cooler surroundings through their body wall, but the researchers suspected these puzzling results had to do with the sharks’ gills. Sharks breathe by absorbing oxygen dissolved in water through blood vessels in their gills, meaning they are a part of the body where lots of blood is exposed to cold 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 were breathing heavily during their demanding dives, their body temperature should have rapidly plummeted. The researchers suspected the diving hammerheads avoided this by clamping down their gill slits.
Indeed, video footage of scalloped hammerheads swimming a kilometre down has shown their gill slits tightly closed.
José Emilio Trujillo at the University of Otago in New Zealand wonders how the hammerheads manage to be so athletic with so little oxygen. Diving mammals have adaptations for dealing with very low oxygen in their tissues, he adds, so perhaps hammerheads do too.
The breath holding hypothesis is interesting and worthy of further investigation, says Phillip Morrison at Vancouver Island University in Canada. However, he isn’t convinced it is the “sole mechanism” without more research, such as analyses of body temperature under different levels of gill heat loss.
“If [the researchers] are correct, I think that this is one of the coolest physiological traits among sharks,” says Morrison.
Other shark species may also use this freediving strategy. Royer points out that oceanic whitetip sharks (Carcharhinus longimanus) make similar repeated, steep dives.