Hammerhead sharks “hold their breath” to conserve body temperature when hunting in cold, deep waters. This strategy may be common among other deep-sea marine creatures. Hammerhead sharks “hold their breath” to conserve body temperature when hunting in deep, cold waters, according to a new study. A previously unobserved and unexpected phenomenon that bears great resemblance to the breath-holding thermoregulation strategies used by cetaceans may be common among other deep-sea sharks and fish.

Like most fish, sharks are completely ectothermic and their body temperature is largely regulated by their immediate environment. This can pose a physiological problem for large predatory fish that need to maintain a certain body temperature to function but also dive into colder, deeper waters in search of prey.

For example, scalloped hammerhead sharks (sphyrna lewiniLiving in the warm surface waters of temperate and tropical coastal regions, they regularly dive deeper than 800 meters to hunt their prey, with water temperatures as low as 4 degrees Celsius. Given the lack of morphological and vascular adaptations to actively maintain body temperature, it is unknown how these sharks are able to maintain their body temperature during deep, cold dives.

To investigate this mystery, Mark Royer and his colleagues developed state-of-the-art remote biorecorders that are inserted inside adult sharks and measure depth, ambient water temperature, activity level, body movements and body temperature. The authors found that the sharks maintained a high body temperature (up to 20 degrees Celsius above ambient temperature) during the deepest part of each dive, and only lost heat rapidly after returning to the surface.

Royer et al. He suggests that sharks maintain their body temperature by effectively “holding their breath” while diving. Sharks can minimize body heat loss through cold water “breathing” by tightly closing their mouths and/or gill slits to reduce the flow of cold water through their gills. The rapid loss of body heat during ascent to warmer waters likely reflects reopening of the gill slits and convective heat transfer.

The authors note that more work is needed to confirm the thermoregulation strategy.

“Given the strong selective forces that shape respiratory efficiency and behavioral and physiological thermoregulation, this strategy may be common among other epipelagic and bony fish,” write Mark Mikan and Adrian Glace from a similar perspective.