So, I don’t know a whole lot about fish. I can identify a few, and I know about cool stuff like that warm-blooded fish, or the fact that fish aren’t real, but I don’t know, for example, how their gills actually function. I did not know, for example, that it’s apparently possible for sharks to hold their breath?

This was a complete surprise!” said Mark Royer, lead author and researcher with the Shark Research Group at the Hawaiʻi Institute of Marine Biology (HIMB) in the UH Mānoa School of Ocean and Earth Science and Technology. “It was unexpected for sharks to hold their breath to hunt like a diving marine mammal. It is an extraordinary behavior from an incredible animal.”

Shark gills are natural radiators that would rapidly cool the blood, muscles and organs if scalloped hammerhead sharks did not close their gill slits during deep dives into cold water. These sharks are warm water animals but feed at depths where seawater temperatures are similar to those found in Kodiak Alaska (around 5°C/40°F), yet they need to keep their bodies warm in order to hunt effectively.

“Although it is obvious that air-breathing marine mammals hold their breath while diving, we did not expect to see sharks exhibiting similar behavior,” said Royer. “This previously unobserved behavior reveals that scalloped hammerhead sharks have feeding strategies that are broadly similar to those of some marine mammals, like pilot whales. Both have evolved to exploit deep dwelling prey and do so by holding their breath to access these physically challenging environments for short periods.”

Marine mammals hold their breath because they can’t breath water. The sharks have no problem breathing, but the “air” is so cold down there that they’re better off holding their breath. I’d always had the vague impression that fish breathing was even more reflexive than human breathing, since gills don’t “hold” water, but I suppose it makes sense that with so much musculature going on, they’d have control over their gills. I’m now wondering if the hammerheads living around the Sharkcano do the same thing to keep from overheating or burning their gills sometimes.

Beyond the fun of learning something new about hammerhead sharks, I also appreciate this article for giving us a window into how scientists are able to figure this sort of thing out:

The research team discovered this unexpected phenomenon by equipping deep-diving scalloped hammerhead sharks with devices that simultaneously measured their muscle temperature, depth, body orientation and activity levels. They saw that their muscles stayed warm throughout their dive into deep cold water but suddenly cooled as the sharks approached the surface toward the end of each dive. Computer modeling suggested that hammerhead sharks must be preventing heat loss from their gills to keep their bodies warm during these deep-dives into cold water.

Additionally, video of a scalloped hammerhead shark swimming along the seabed at a depth of 1,044 meters (more than 3,400 feet) showed its gill slits tightly closed, whereas similar images from surface waters show these sharks swimming with their gill slits wide open. A sudden cooling in muscle temperature as scalloped hammerhead sharks approach the surface at the end of each dive suggests that they opened their gill slits to resume breathing while still in relatively cool water.

“Holding their breath keeps scalloped hammerhead sharks warm but also shuts off their oxygen supply,” said Royer. “So, although these sharks hold their breath for an average of 17 minutes, they only spend an average of four minutes at the bottom of their dives at extreme depths before quickly returning to warmer, well-oxygenated surface waters where breathing resumes.”

Thermal regulation by cold-blooded animals has always fascinated me. When I was a volunteer at the New England Aquarium, I saw sea turtles that had ended up in too-cold water, and gotten internal frostbite, which is one horror I’m glad I don’t have to worry about. I’ve always though that being unable to internally regulate temperature was limiting, and in some ways it definitely is. Simply having food allows us to comfortably function in a pretty wide temperature range, and by adding clothes (or thicker fur/feathers if you’re not human), you can expand that range pretty cheaply. It seems, however, that I’ve been underestimating our room-temperature brethren. I would imagine this sort of thing is easier with a larger body, but I’m now curious what other tricks there might be for accessing places that “ought” to be too hot or too cold.

The image shows a school of scalloped hammerhead sharks, photographed from below. The sharks seem to be mostly silhouetted, but you can see the sunlight, filtered blue by the water, reflecting off their sides. Photo uploaded to Wikimedia Commons by Ryan Espanto