Sharks have a special place in the human imagination as sleek efficient killers that are always swimming towards prey. Although they are carnivores, there are fewer shark to human attacks each year than media hype would have one believe. Another myth is that all sharks must constantly move to live. True for some species, there are many exceptions to this supposed rule.
Do Sharks Die If They Stop Swimming?
Some species of shark, such as great whites and hammerheads, do need to keep moving in order to breathe and thus live. These sharks are called obligate ram ventilators and need to constantly swim forward in order to ‘ram’ oxygen-containing water through their mouths and over their gills. However, according to textbook editor William J. Bennetta in his article “Deep Breathing” for The Textbook Letter, fewer than two-dozen of today’s 375 living species of sharks breathe using only this method.
How Sharks Breathe
Similar to all fish, sharks breathe through gills, which take dissolved oxygen from the water. Sharks have two ways to get water over their gills and many species often switch between the two methods. As mentioned, a shark can use ram ventilation, relying on its forward movement to force water over its gills. According to the Aquarium of the Pacific’s webpage “Sharks: An Overview” this method is generally used by larger and more active sharks. Most sharks however do not need to move in order to breathe, instead they use their muscles to pump water over their gills. The Aquarium of the Pacific describes how a motionless shark can pump in water through its mouth or an opening called a spiracle and over its gills.
Sharks Resting in the Open Ocean
Another reason that sharks must keep moving is that they would sink otherwise. Sharks have no swim bladder to help them keep buoyant. However, this buoyancy issue doesn’t bother all sharks and many of these creatures can often be found motionless, resting on the bottom of the ocean’s floor.
R. Aidan Martin, shark expert and zoologist, reports several species of shark resting on the ocean’s sandy bottom in his post “40 winks under the sea” at the ReefQuest Center for Shark Research. Some sharks such as the Nurse Shark and Angelshark have powerful gills which make lying still easy. However many sharks that lie still have weak gills which means lying still uses up more energy than ram ventilation. Sharks seen motionless in the ocean include the Whitetip Reef Sharks off of Costa Rica, Lemon Sharks near the Bahamas, as well as Grey Reef and Blackfin Reef sharks in the tropical western Pacific Ocean. Martin believes sharks ‘rest’ this way for social reasons, perhaps to attract mates. However the reason is still up to debate.
Sharks Resting in Caves
Martin also describes how some sharks have been seen resting in caves. In the 1970s diver and research Ramon Bravo discovered Carribean Reef Sharks near the Isla Mujeres resting in caves for hours at a time. Bravo believed this was due to the high oxygen content and reduced salinity of the cave water, making it easier for the sharks to breathe. Other sharks found in the Isla Mujeres caves since include the Lemon Shark, Tiger Shark, and Blue Shark. Also, Banded Houndsharks have been found in the caves of Japan's Izu Oceanic Park piled on top of each other.
Shark Research
Ultimately, the motives of sharks are still unknown and people can’t say for sure why sharks sometimes rest on bottom of the open ocean or in caves. However, the myth that all sharks must always be moving has been proven to be just that – a myth. Although some species of shark must keep moving, it has been well documented that other species will take a break every once in a while.
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