Sharks are the apex predators of the oceans. They dominate the seas with their powerful jaws and high-speed pursuits.
These powerful creatures have few predators or threats to their lives and yet have some biological peculiarities that leave them vulnerable to drowning, even though they spend their entire lives in the ocean.
But do sharks need oxygen to breathe? And if they do, can a shark drown?
Is it possible for a shark to drown?
Despite being ocean dwellers, most sharks need oxygen to breathe. They don’t have lungs but instead absorb oxygen from the water using their gills. If there’s not enough oxygen available in the water, sharks can’t breathe and can easily drown. This isn’t the only reason sharks drown.
Many people believe that if a shark stops swimming, it will instantly drown. While this is true for some species, it depends on the breathing techniques the shark employs.
We tend to think of great whites whenever anyone mentions sharks. Like the tiger and mako sharks, the great whites are obligate ram breathers. This means they must force water over the gills in order to breathe. They can do this either by swimming or finding a fast-moving current to linger in.
Reading Suggestion: Do Sharks Have Tongues?
Other species, including the ragged-tooth and salmon shark, use buccal pumping as well as ram ventilation to breathe. These sharks have a bony plate, known as an operculum, covering their gills.
The shark closes this when it draws water into its mouth or buccal cavity. It then closes its mouth and opens the operculum, forcing water over its gills as it does so. This enables the shark to remain completely stationary for long periods without the risk of drowning.
Numerous species of shark, including the ragged-tooth and salmon shark, use buccal pumping as well as ram ventilation to breathe.
Benthic shark species spend most of their time languishing on the ocean floor. Nurse sharks aren’t called the “couch potatoes of the ocean” for nothing! They spend much of the day hiding in crevices and under coral shelves. At night, they feed, expanding their “muscular pharyngeal cavity” to suck in their prey.
Thanks to the specialized holes, or spiracles, situated behind their eyes, these sharks can breathe easily even while remaining completely motionless. Spiracles act a little like straws and enable the shark to draw water over the gills and out again even when the shark is submerged under the ocean floor.
Can You Drown a Shark?
You’d be hard-pressed to drown a 10ft nurse shark, but you could, theoretically, drown a great white or any other species of pelagic shark.
Unlike benthic sharks, pelagic sharks rely on ram ventilation exclusively. That means they can’t move backward without flooding their gills with water. This will cause them to suffocate or drown.
If you wanted to drown a pelagic shark, all you’d need to do would be to get a good grip on its tail and pull it back through the water. This reverses its ventilation process, preventing the shark from getting the oxygen he needs.
This won’t work with all species of shark, however. The curious epaulet shark
Anything that prevents a pelagic shark from swimming will cause it to suffocate. Some unethical shark fishing operations only bother to harvest the fins of the shark as these are the most valuable. The rest of the shark is thrown back into the sea, still alive but without the ability to swim or, subsequently, breathe.
This illegal practice is, according to the UK’s Animal Welfare Minister, Lord Goldsmith, “indescribably cruel and … unforgivably wasteful.”
One other way to drown a shark would be to damage its gills. Some types of fishing equipment and hooks can injure the gills, making it difficult for the shark to breathe and causing his eventual demise.
How do Sharks Drown?
Sharks drown if they can’t get enough oxygen. Any damage to the gills will make it difficult for a shark to breathe unless it belongs to the benthic family, which relies on buccal pumping.
Humans sometimes damage a shark while fishing, but bacteria and other pathogens can also cause the gills to stop working. For sharks, this feels a little like a bad case of pneumonia would to a human.
Sharks are immune to many diseases but, like any species, are susceptible to parasites, disabilities, and even cancer.
Reading Suggestion: Do Sharks Jump Out of the Water?
Intravascular nematode larvae can cause “interstitial inflammation in the gills,” making it difficult for the shark to breathe. Similarly, flatworm infections affecting the gills are often “associated with significant morbidity due to associated proliferative and inflammatory gill lesions.”
Sudden or extreme changes in water temperature or pH levels increase the chances of sharks contracting diseases and picking up parasitic infections, as do the introduction of pollutants and chemicals to the ocean.
Since the 1950s, the world’s oceans have lost around 2% of their dissolved oxygen. Scientists predict that this could rise to between 3 to 4% by 2100.
Climate-driven deoxygenation has already affected many sharks, with studies showing that the blue shark has, due to “vertical habitat compression,” become more vulnerable to surface fishermen.
Deoxygenation causes the ocean’s hypoxic zones to expand, forcing pelagic sharks to utilize the oxygenated surface layers and avoid the lower, hypoxic zones.
Sharks cope with mild hypoxia by changing their “circulatory and/or ventilatory responses” but these strategies may not be effective enough for the sharks to “endure moderate, progressive or prolonged hypoxia or anoxia.”
As the oxygen levels in the ocean continue to drop, so more pelagic sharks, in particular, will become susceptible to drowning.
How do Sharks Breathe?
A shark’s body (respiratory system) is very different from a human’s. Like most fish, they don’t have lungs and instead use gills to extract oxygen from water. As the water flows over the gills, tiny blood vessels or capillaries extract the oxygen. The gills then release the carbon dioxide waste.
Active, pelagic sharks need more oxygen than the more sedentary benthic species and, therefore, tend to swim closer to the surface where the water is most heavily oxygenated. Others, like the epaulet shark, can more easily tolerate low-oxygen zones as they can lower their energy demands, and therefore use less oxygen.
As we mentioned earlier, sharks use two different breathing techniques:
#1 Buccal Pumping
The oldest method, which was also used by the ancestors of our modern sharks, is buccal pumping. Buccal, or cheek, muscles pull water into the mouth and over the gills.
This way, the shark doesn’t have to swim to keep water moving over the gills. Many shark species still use buccal pumping, such as nurse, angel, and carpet sharks.
Reading Suggestion: Why Do Sharks Migrate?
Skates and rays also use buccal breathing. Those species of shark, skate, and ray that are flattened when they lay on the ocean floor, however, can’t pull water in through their mouth.
These species have a more prominent spiracle, a tube behind the eyes that pulls water into the gills and out of the gill slits. Insects also have spiracles on their external skeleton that they use for breathing.
#2 Ram Ventilation
As sharks evolved, they became more active and developed a more energy-efficient way of breathing. Species like the great white, blue, and mako shark need to keep moving all the time. As they move, oxygen rich water is literally rammed into their mouths. From there, it flows through the gill slits, where oxygen is extracted.
Not all sharks rely on 1 or the other, some sharks like the sand tiger shark, swap between buccal pumping and ram ventilation, depending on how fast they’re swimming. This means they can transfer the work of breathing from the buccal and opercular pumps to the swimming muscles to make themselves more energy-efficient.
The sand tiger shark is the only species known to take gulps of air from the surface. This isn’t a breathing mechanism, however. Instead, this air is stored in the shark’s stomach, boosting its buoyancy so it can float above the ocean floor to look for prey.
Do Sharks Drown if They Stop Swimming?
Some sharks have completely lost the ability to breathe by buccal pumping, and these are the sharks that will indeed drown if they stop swimming and ramming water.
These sharks are known as obligate ram ventilators. The great white shark, the mako shark, the salmon shark, and the whale shark are all obligate ram ventilators.
What Happens When a Shark Stops Swimming?
If a shark stops swimming, will it die? Sharks that use both buccal pumping and ram ventilation techniques will be fine if they stop swimming. Species like the nurse shark can lie around all day without any unpleasant consequences.
Some of the other more iconic species, like the great white, whale, hammerhead, and mako sharks will all suffocate if they stop swimming and there is no current to ram water over the gills.
Why do Sharks not Drown?
Even though sharks need oxygen to breathe, they don’t easily drown. Their gills enable them to extract oxygen from the surrounding water. Using buccal pumping and/or ram ventilation, sharks manage to avoid drowning most of the time.
Can a Shark Drown Upside Down?
Sharks spend most of their time the right way up but do, occasionally, turn over and lie on their backs. When they do so, they don’t drown but do enter a position known as tonic immobility.
This is a trance-like state in which the shark’s muscles relax and its breathing deepens.
Researchers often use tonic immobility to subdue smaller sharks and make them easier to handle. If turned upside down, a shark will usually enter tonic immobility in less than a minute and can remain in this state for up to 15 minutes.
Reading Suggestion: Do Sharks Sleep?
In most species, tonic immobility is believed to be a defense mechanism designed to As the shark is an apex predator, it doesn’t have much use for such adaptations. Instead, some scientists believe tonic immobility in sharks may be linked to mating, as “female sharks seem more responsive than males.”
Orcas have also been known to induce tonic immobility to subdue their prey. In 1997, an orca attacked a great white off the coast of California.
The orca held the shark upside down in the water for 15 minutes, causing the shark to enter tonic immobility. It’s not clear whether the orca’s actions were intentional or not, but the result was the same, and the defenseless shark drowned.
Can Sharks Hold their Breath?
Yes. A study of scalloped hammerhead sharks off the coast of Hawaii shows that these “cold-blooded species… may maintain their body temperature during the deep dives by holding their breath.”
These sharks hold their breath by shutting their mouths or clamping their gills shut. Mark Royer, a marine biologist at the University of Hawaii Manoa, said, “Either one of those mechanisms means there is no longer gas exchange at the gills, so the shark is basically holding its breath.”
Hammerheads need to hold their breaths, Royer explained, because, if their “body temperature gets too low, they lose muscle function, visual acuity, and their metabolism slows down. If a shark gets too cold, it can’t keep itself moving and breathe.”
Do Sharks Sleep while Swimming?
Pelagic shark species have to keep moving to stay alive, but studies suggest they may alternate between active and restful periods.
Although it’s difficult to say whether a pelagic shark sleeps, research suggests that they can rest, to some extent. As the spinal cord coordinates the shark’s swimming movements, a shark can rest its brain while continuing to swim.
Pelagic sharks species are entirely dependent on ram ventilation to breathe. If there is no water flowing over the gills, these species of shark will be unable to breathe and subsequently drown.
Other diseases and parasitic infections that affect the gills can also cause drowning, as can shark finning. Those species that use buccal pumping to breathe are more resilient and less likely to drown.