Video Shows How Sharks 'Sleep' in Large Groups

Sharks have long been seen as voracious predators and misunderstood sea creatures, but one role they're rarely cast in is as peaceful sleepers.

Video taken in Revillagigedo, Mexico, in the waters off the country's Pacific coast, just might prove otherwise. During a dive organized by Quebec-based company Scubacadémie, divers captured footage of whitetip reef sharks huddled together on the ocean floor near an underwater cave. Some appear to fidget about while others lie completely motionless. The large pile of sharks appears to be roughly 20 strong and unbothered by nearby human presence.

While the scene may appear odd to the untrained marine enthusiast, the arrangement is quite common.

National Geographic Explorer-in-Residence Enric Sala has spent quite a bit of time in marine ecosystems around the world. In an emailed statement, he noted that sharks can often be seen piled up like this, not only in Mexico, but also all across the Pacific. He's personally seen them in Revillagigedo, doing the exact same thing.

Unlike human beings that divide their waking and sleeping hours evenly from day to night, sharks oscillate between wakeful and restful periods.

Jay Bradley is the curator of the Blue Wonders exhibit at the National Aquarium and notes that sharks, and fish in general, don't really "sleep" in the traditional way the word is defined.

"They don't go into an unconscious state," said Bradley. "We still don't fully understand what they do during rest periods."

One commonly told myth about sharks is that they must continuously swim to avoid drowning. Some sharks, such as great whites, hammerheads, and megamouths, move while in these restful states in order to keep oxygen-rich water flowing through their gills. Some studies have suggested, however, that the spinal cord, and not the brain, allows the sharks to swim, meaning they may experience periods of mental inactivity while moving along the sea floor.

Other shark species, such as whitetips, stay completely immobile to rest. Spiracles, or small holes, force water across their gills.

Despite the large gathering of sharks seen in the video, whitetip sharks aren't particularly social.

They're known to be loners, but are rarely territorial, meaning they willingly congregate in spots that are generally appealing. Caves and crevices where sharks can find shelter from larger predators are common gathering spots.

"These sharks rest in groups during the day, often close together under ledges or in small caves, then scour the reef in packs at night to hunt," said Caitlin Scully, a spokesperson for the Birch Aquarium at the Scripps Institution of Oceanography. "Though they travel together, they are usually seen hunting individually rather than cooperatively."

Sharks will also rest in parts of the ocean floor with strong currents that allow water to easily flow over their gills.

While these populations in the northern hemisphere produce offspring in early summer months with litters as large as 15, it's also unlikely that these sharks were gathered with offspring their.

According to Bradley, "They pretty much just pup them and go," meaning sharks aren't known to be particularly attentive parents.

To film such intimate footage of these sharks sleeping doesn't present a dangerous task. Unlike oceanic whitetip sharks, reef whitetip sharks are relatively harmless to people thanks to their more docile disposition and small teeth.

(Read more about how oceanic whitetip shark populations have been decimated by fishing and the shark fin trade.)