Scalloped hammerheads appear to hold their breath when submerged in cold, deep water. By closing their blood-rich gills, they are able to keep warm while hunting for prey, effectively circumventing their own cold-blooded biology.
Researchers already knew that scalloped hammerheads (Sphyrna lewini) made repeated short, deep night dives, he says brand royer at the University of Hawaii at Manoa. The dramatic declines may be for hunting, as the beaks of squids from the deepest visited have appeared in the stomachs of sharks.
But it was not clear how tropical hammerheads managed to tolerate the frigid temperatures in such deep waters, commonly around 5°C. Other fish, such as great white sharks and tunas, have circulatory systems that recycle heat produced by flexing their muscles, allowing active predators to stay warm in cold water. Hammerheads don’t, says Royer. He and his colleagues captured scalloped hammerheads in Kāneʻohe Bay, Hawaii, briefly held them against the side of the boat and attached instrument packs to the base of each shark’s dorsal fin. These instruments measured the temperature of the water and muscles, as well as the throbbing and acceleration of the shark’s tail in all directions. After a few weeks of collecting data on three sharks, the packages broke loose and floated to the surface for the team to retrieve.
The researchers found that while diving, hammerheads suddenly shoot toward the bottom at an 80-degree angle, furiously flapping their tails.
“You should expect to see your body temperature drop immediately,” says Royer. “But that’s not what’s happening.”
Instead, its body temperature remains well above ambient temperatures during the multi-minute drop to about 800 meters. The sharks then come closer to the surface. Only then does his 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 respire by absorbing dissolved oxygen in the water through the blood vessels in their gills, which means they are a part of the body where a lot 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 temperatures should have plummeted quickly. The researchers suspected that diving hammerheads prevented this by squeezing their gill slits.
In fact, video footage of scalloped hammerheads swimming a kilometer down has shown their gill slits tightly closed.
jose emilio trujillo at the University of Otago in New Zealand wonders how hammerheads manage to be so athletic with so little oxygen. Diving mammals have adaptations to deal with too little oxygen in their tissues, he adds, so perhaps hammerheads do as well.
The breath-holding hypothesis is interesting and deserves further investigation, he says. phil morrison at the University of Vancouver Island in Canada. However, he is not convinced that it is the “only mechanism” without further investigation, such as analysis of body temperature under different levels of gill heat loss.
“Yeah [the researchers] are correct, I think this is one of the best physiological traits among sharks,” says Morrison.
Other shark species can also use this freediving strategy. Royer notes that oceanic whitetip sharks (Carcharhinus longimanus) perform similar repeated steep dives.