A dramatic aspect the shark holds its breath for about 17 minutes to stay warm on cold-water hunting dives, scientists have found.
Scalloped hammerhead sharks (Sphyrna lewini) are found in warm, near-surface ocean waters around the world, especially along coastlines, and can reach lengths of about a dozen feet. But these sharks venture into deep, cold waters to hunt. This is potentially problematic because scalloped hammerheads, like most the Sharks, are cold-blooded, which means that their body temperature matches that of their immediate surroundings. “When you go into cold water, if your body gets cold, you can’t hunt as effectively,” says Mark Royer, a shark biologist at the Hawaii Institute of Marine Biology. “To be an effective hunter, it’s important to warm up as much as possible.”
Some fish like bluefin tuna, great white sharks (Carcharodon carcharias) and swordfish have special muscles and blood vessels that allow them to keep their brains, eyes, and swimming muscles warm even in cold waters, but scalloped hammerhead sharks do not. Yet as Royer and his colleagues analyzed 106 deep dives made by six individual scalloped hammerhead sharks, they found that the animals mysteriously stayed warm during these excursions, according to sensors that recorded muscle temperatures, orientations and depths of shark diving and ambient temperature. temperatures of the water around them.
“We looked at body temperature data, [and] right from the start, we could see that these sharks were doing something very interesting and very different from any other fish,” says Royer. “It’s obviously not just the thermal inertia that these sharks rely on to maintain their body temperature during these deep dives. There is something much more complex.
The researchers found that the sharks made steep, rapid descents to spend a few minutes feasting in deep waters, then accelerated most of the way to the surface. The animals stayed warm until the point where their ascent slowed, an average of 17 minutes, Royer says. The team’s findings were published on May 11 in Science.
Researchers believe the mystery lies in the gills of sharks. These organs act like lungs, drawing oxygen from seawater. But gills have a downside because large amounts of water pass through them. “The gills are like giant radiators strapped to your head,” says Royer. “Anytime a shark or a fish goes into colder waters, it’s going to very quickly lose its body heat through its gills because the gills have all this big surface area.”
The team could not directly monitor the gills of the sharks, although Royer says that in the future he would like to equip the sharks with cameras with pectoral fins pointing at the gills. But for now, scientists believe the sharks are closing their gills – holding their breath, essentially – during their daring dives. (Royer notes that other researchers have images of a scalloped hammerhead shark diving with its gills closed.)
“It is certainly something that has not, to my knowledge, been put forward for an ectothermy [cold-blooded] fish but which I think will create some really interesting avenues of research,” says Ashley Stoehr, a marine biologist at Sacred Heart University who specializes in fish and was not involved in the new research.
Scalloped hammerhead sharks may not be the only ones to use this technique for deep dives, Royer says. Oceanic sharks (Carcharhinus longimanus) don’t make these dives as often as scalloped hammerhead sharks, he says, but they’re still known for their spectacular dives. And scientists know that these sharks also lack the specialized muscles and blood vessels that can keep fish warm. Still, it seems the scalloped hammerhead shark apnea technique is a rare approach to daring cold waters. “It’s incredible and unexpected behavior from a remarkable species,” says Royer.