Do Some Sharks Hold Their Breath Underwater?

Sharks are known to be one of the most fascinating creatures in the ocean, with their incredible hunting skills and mysterious behavior. But did you know that scalloped hammerhead sharks may have a new trick up their sleeve? According to a recent article published in Science, these sharks may be holding their breath when they dive deep into frigid waters, and it could be their secret weapon for regulating their temperature while they hunt.

Mark Royer, a shark biologist at the University of Hawaii who led the research, says this behavior is "completely unexpected" and has never been observed in any kind of deep-diving fish before. The revelation is raising questions about how widespread the practice of breath-holding may be among other species. Scalloped hammerhead sharks are critically endangered and rely on forward movement to force water across their gills, enabling them to extract needed oxygen to breathe. However, when they swim half a mile down to catch squid and other prey, the colder water can impact their metabolism, cardiac function, and eyesight—all factors that would reduce their hunting prowess.

The team placed tiny sensors, basically a "Fitbit for sharks," on adult scalloped hammerhead sharks to analyze detailed information about their swimming behavior, depth, and location. The probes recorded the animals' muscle temperatures during repeated nighttime dives, indicating that the sharks managed to largely maintain their body temperatures both when they were at surface waters around 80 degrees and when they plunged to more than 2,500 feet below, where temperatures dropped by roughly half, to around 41 degrees. Surprisingly, the sharks' body temperatures eventually dropped when they had ascended and reached slightly warmer waters about halfway back to the surface—apparently when they had opened their gills to obtain needed oxygen. That's not what you would expect with thermal inertia, according to Royer. Though they didn't observe the sharks actually closing their gills, his team suspects that's what's happening. To fully confirm their breath-holding hypothesis, they will next need to affix cameras to the pectoral fins of these hammerheads to observe the gills opening and closing as sharks dive.

It's still unclear how the 12-foot-long species may have acquired this breath-holding skill. "It's possible they learn it from social interactions from other hammerheads that are diving," says Royer. "Another possibility is they're following the example of other animals that dive and eat prey at the same depth." This unexpected behavior in scalloped hammerhead sharks raises intriguing questions about their survival tactics and how they adapt to changes in their environment. It's just one more reason to be fascinated by these incredible creatures of the deep.