Hammerhead sharks hold their breath in deeper, colder waters

Tracking hammerhead swimmers at frigid depths with electronic tags and sensors: How do they survive an ocean-level dive?

The work, they wrote, is another example of how new electronic tags and sensors are helping to explain “the extraordinary persistence of these animals across 400 million years of changing ocean environments.”

The hammerhead doesn’t have any of the advantages, yet it has been tracked doing rapid dives to the depths of around 800 metres.

The reason is that sharks are cold-blooded, and that their body temperature matches the waters they swim in. To do deep dives, they have to conserve their body heat, and the best way to do that is to close their gill slits.

The body temperature does not drop, which is really wild. It stays the same until they return from the deep. Royer quickly realized what was going on: “They were closing their gill slits and preventing the water from flowing across their gills that would cool their body down,” he says.

He says that this is similar to putting a tracker on a shark. The sharky Fitbit would release itself from the fin after several weeks under water.

Royer set out to study the diving habits of a species of sharks called scalloped hammerheads. They can dive to more than 800 meters below the surface, but earlier studies showed they could swim in coastal waters with warmer temperatures. At those frigid depths, the water temperature drops to as low as 41 degrees Fahrenheit (5 degrees Celsius).

Some other large fish have specialized anatomy that allows them to stay warmer at great depths, but scalloped hammerheads do not, Royer says. “How can a warm tropical species go down into these deep depths and survive?” was the obvious question.

Climbing to the top: How sharks shut down oxygen in their bodies when they jump into a warm ice bath in a tropical environment

It’d be a shock to anyone’s system, Royer says: “Imagine you’re on a warm sunny beach, and then you hop out of the warm water and immediately plunge into an ice bath.”

It all makes sense, but the idea of a fish holding its breath underwater “still shocks and baffles me,” says Mark Royer, a researcher at the University of Hawaii at Manoa and the Hawaii Institute of Marine Biology, who led the study.

Some fish, such as the whale shark (Rhincodon typus), are able to conserve their body heat when diving through sheer size. The mako shark and great white shark are part of the family that also includes tuna and marlin so they have developed specialized heat exchange systems at their gills.

Marine biologist Colin Simpfendorfer, at James Cook University in Townsville, Australia, says the study shows how sharks are well adapted to the limits of their environment. It is a fairly extreme move to dive to a few degrees centigrade from tropical temperatures at the surface.

The function of shutting down oxygen in this way is yet to be discovered, but if it is, it suggests the hammerhead must be able to deal with diving low blood-oxygen levels. “What they could be doing is slowing the heart muscle, slowing the pumping of blood around the body,” Meekan says. The shark might have evolved to hold more oxygen in its tissues and blood, or perhaps be able to deal with the by-products of anaerobic respiration, which can be toxic, similar to the adaptation people who live at altitude have.