Scallop hammerhead sharks close their gills to keep their body temperature down during deep-sea diving.
Deron Verbeck
The scalloped hammerhead shark appears to hold its breath as it dives into the cold, deep water. By closing their blood-rich gills, they may be able to keep warm while hunting prey, effectively avoiding their own cold-blooded biology.
Researchers have already identified the scalloped hammerhead shark (Sfirna Lewini) said Mark Royer of the University of Hawaii at Manoa, who made repeated short dives at night. The dramatic descent may be for hunting, as a squid beak from the deep waters it visited appeared in the shark’s stomach.
However, it was not clear how tropical hammerhead sharks could survive such deep-sea frigid temperatures, typically around 5°C. Other fish, such as great white sharks and tuna, have circulatory systems that recycle heat generated by muscle flexion, allowing active predators to stay warm in cold water. Not so with Hammerhead, Royer says. He and his colleagues captured scallop hammerhead sharks in Hawaii’s Kaneohe Bay, simply strapped them to the side of a boat and attached a package of instruments to the base of each shark’s dorsal fin. These instruments measured water temperature and muscle temperature, as well as the shark’s tail heartbeat and omnidirectional acceleration. After weeks of collecting data on the three sharks, the package detached and floated to the surface for the team to retrieve.
Researchers found that during a dive, the hammerhead shark suddenly leaped toward the bottom at an 80-degree angle, lashing its tail violently.
“You should expect their temperature to drop quickly,” Royer says. “But what is happening is not.”
Instead, body temperature remains above ambient during a few minutes of rapid descent to about 800 meters. The shark then zooms towards the surface. Then the temperature will drop.
Sharks lose some heat through their body walls to cooler surroundings, and the researchers suspected these puzzling effects might be related to shark gills. Sharks breathe by absorbing oxygen dissolved in water through the blood vessels in their gills. In other words, there is a lot of blood in the part of the shark’s body that is exposed to cold water.
“The largest percentage of heat loss in gill-breathing animals is through the gills,” Royer says. “It’s basically like having a giant radiator strapped to your head.”
If the shark had been breathing heavily during the harsh dive, its body temperature would have dropped rapidly. The researchers speculated that the diving hammerhead shark avoided this by tightening its gill slits.
In fact, video footage of a scalloped hammerhead shark swimming a kilometer downstream showed that its gill slits were tightly closed.
José Emilio Trujillo of the University of Otago in New Zealand wonders how hammerhead sharks can be so athletic in the absence of oxygen. Diving mammals have adaptations to deal with very low oxygen in their tissues, so probably hammerhead sharks as well, he added.
The breath-holding hypothesis is intriguing and deserves further investigation, says Philip Morrison of Canada’s University of Vancouver Island. However, without further research, such as analysis of body temperature at different levels of gill heat loss, we are not convinced that it is the ‘only mechanism’.
“If [the researchers] That’s right. I think that’s one of the best physiological traits he has in sharks,” Morrison says.
Other species of sharks may also use this freediving strategy. Mr. Royer said that marine whitetip sharks (Medicahinus longimanus) repeat a similar steep dive.
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