If you’ve ever been thrown into a highly stressful situation and left to fend for yourself, you’ve likely been told that your options are to “sink or swim.” This idiom, which has existed since at least the 14th Century and has appeared in the works of Chaucer and Shakespeare, is thought to have originated either from the fact that throughout history few people have known how to swim or from the practice of ducking witches. In the Middle Ages, those accused of witchcraft were often bound and thrown into a river or pond blessed by a priest. If the water rejected them and they floated, they were declared a witch and executed. If they sank and drowned, they were innocent. Today, however, the phrase is widely associated with sharks, which must constantly keep swimming or else they will sink and die. Or do they? When it comes to animal-based metaphors, we humans have a rather poor track record. For example, bats aren’t blind, lemmings don’t commit mass suicide, ostriches don’t stick their heads in the sand, and goldfish actually have pretty average memories. So, do sharks actually have to sink or swim? Well, it depends what kind of shark you’re referring to.

There are two main reasons why certain sharks must constantly keep swimming: buoyancy and respiration. Most osteichthyes or bony fishes – the ones you’re most likely to encounter in your local supermarket – possess a special gas-filled organ called a swim bladder, which allows them to regulate their buoyancy. By adjusting the amount of gas in their swim bladders, fish can achieve a state of neutral buoyancy where they neither float nor sink. This helps conserve energy as the fish do not need to exert themselves in order to maintain their depth. Sharks, on the other hand, are members of the chondricthyes, or cartilaginous fishes, which in addition to having skeletons made of cartilage instead of bone do not possess swim bladders. Instead, sharks maintain their depth using their large pectoral fins, which generate lift like an airplane’s wing. They are also aided by fatty oils in their large livers, which being less dense than water generate additional buoyancy. While the lack of a swim bladder means sharks must continuously expend energy in order to maintain their depth, it also confers certain advantages. Bony fish are typically restricted to a very narrow layer of the water column; if they rise too far too quickly, the gas in their swim bladders rapidly expands and can kill them. Sharks, on the other hand, are able to rise and descend freely over a wide range of depths.

The second reason certain sharks must constantly keep swimming is to maintain the flow of water over their gills. If this flow is interrupted, the shark can no longer absorb oxygen from the water and it begins to suffocate. Such sharks are known as obligate ram ventilators, and include large pelagic or open-water species like the Great White, Mako, Blue, Tiger, and Bull Shark. Most other sharks, however, are able to breathe even when lying still on the ocean floor by opening and closing their pharynx or throat to force water over their gills – a process known as buccal ventilation. Buccal ventilators feature prominent openings called spiracles just behind the eyes. Water enters the spiracles, passes over the gills, then exits through the shark’s gill slits. This feature allows bottom-dwelling ambush hunters like carpet sharks, rays, and skates to breathe while buried under the sand, lying still for hours waiting for prey to swim by. The spiracles of obligate ram ventilators are small to non-existent, forcing these sharks to swim in order to force water through their mouths and over their gills. Scientists once believed that ram ventilation was a more “primitive” form of respiration, and that only more “advanced” bony fishes were capable of buccal ventilation. We now know, however, that the ancestors of both bony fish and sharks were capable of buccal ventilation, and that certain species later lost this ability and became dependent on ram ventilation. Indeed, several species of pelagic bony fish, such as tuna and mackerel, are also obligate ram ventilators.

The fact that pelagic sharks must constantly keep swimming in order to breathe raises an intriguing question: do such sharks ever sleep? For decades, most scientists believed that the answer was no. But then a surprising discovery off Mexico’s Yucatan peninsula caused many to question long-held assumptions regarding shark biology. In 1969, Mexican diver and underwater filmmaker Ramón Bravo learned from a local lobster fisherman named Carlos “Válvula” García of an underwater cave called Los Cuevones near Isla Mujeres off the coast of Cancun. According to García, large pelagic sharks would often come to this cave and lie still on the bottom as if sleeping. At first Bravo dismissed these claims, insisting that the sharks García had seen were common bottom-dwelling species like nurse sharks. But at García’s insistence Bravo finally relented and decided to investigate for himself. To his astonishment, Bravo found and filmed large open-water species like tiger, mako, whitetip, and bull sharks lying still on the floor of the cave as if asleep, just as García had described. The “Sleeping Sharks of the Yucatan” made headlines around the world, and were featured in a 1975 episode of the television series The Undersea World of Jacques Cousteau. Cousteau and his team discovered that a natural spring beneath Isla Mujeres causes fresh, oxygenated water to flow through the cave, creating a current that allows sharks to ventilate their gills while lying still. But whether the sharks are actually sleeping remains a matter of some debate, as many divers have noted that the sharks, while immobile, still follow moving objects with their eyes. Some scientists have theorized that the mixing of fresh and salt water in the cave somehow intoxicates the sharks, or that it generates a mild electromagnetic field. Sharks’ heads are covered in hundreds of tiny receptors called ampullae of lorenzini, which allow them to detect electric fields produced by their prey. According to this theory, the electromagnetic field in the cave effectively “hypnotizes” the sharks, placing them in a sleep-like state. Another theory posits that the reduced salinity in the cave causes parasites on the sharks’ skin to loosen their grip, making them easier for remoras and other fish to eat them. The sharks thus use the caves as a sort of car wash.

More recent research suggests that while sharks likely do not sleep deeply like mammals do, they may engage in periods of restful swimming when their brains are less active. Others have theorized that sharks’ swimming is coordinated by their spinal cord, allowing the brain to rest while the shark continues to swim. According to George H. Burgess of the Florida Program for Shark Research, certain shake engage in a behaviour called “yo-yo swimming” actively swimming to the surface then sinking passively to the bottom. Burgess theorizes that these sharks may use the downward leg of this cycle to briefly rest their brains. However, with little concrete evidence to support any of these theories, whether sharks sleep remains something of a scientific mystery.

In the end, the notion that sharks must constantly swim in order to stay alive turns out to be both true and false at the same time, proving once again that when it comes to nature, nothing is ever simple. So next time someone says it’s sink or swim, take it as a compliment, since they’re effectively comparing you to a Great White Shark. It other words: you’ve got this.

Bonus Facts

#1: In addition to lacking swim bladders and having skeletons made of cartilage, sharks, rays, and skates differ from bony fish in a number of fascinating ways, including in the way they regulate the water content of their bodies. As seawater is hypertonic or more concentrated with solutes than a fish’s body, fish constantly lose water to the surrounding ocean. Bony fish compensate for this by ingesting water, continuously replacing what is lost through their gills. Sharks, however, employ a completely different strategy. Rather than expelling metabolic wastes like urea and ammonia, sharks store these in their tissues, making them hypertonic compared to the surrounding ocean and causing water to flow into their bodies. This high concentration of metabolic wastes not only gives shark meat a distinctive smell and taste, but also makes it toxic unless properly prepared. Traditionally, Inuit hunters in the Arctic fed their sled dogs with meat from the Greenland shark, a species of sleeper shark native to the deep waters of the North Atlantic and Arctic Oceans. The flesh of the Greenland sharks contains high concentrations of trimethylamine oxide, a compound that prevents its proteins from deforming under high pressure. Trimethylamine oxide is toxic and in dogs can cause symptoms resembling alcohol intoxication, inspiring the Inuit expression “drunk as a dog.” In order to be eaten by humans, shark meat must be soaked in water to leach out the toxins, or fermented and cured as in Hákarl, the national dish of Iceland.

But whatever osmoregulation strategy they use, most fish are limited to living within a certain range of water salinity. If a freshwater fish is moved into salt water or vice versa, they will likely die. However, there are number of exceptions, the most famous being salmon, which swim up freshwater rivers to spawn. But even more impressive is the Bull Shark, one of the few fish equally comfortable in both salt and fresh water. The Bull Shark owes its remarkable adaptability to a number of key adaptations. While most saltwater sharks transplanted into fresh water would immediately absorb large amounts of water while losing salt to their environment, Bull Shark kidneys have adapted to expel excess water as dilute urine while conserving precious salt. The sharks’ anal gland, which normally expels excess salt, also reduces its function, while the liver produces extra urea to further compensate for salt losses. Typically entering freshwater to breed, Bull sharks have been found in lakes and rivers around the world, including Lake Nicaragua, the Amazon River, the Assam River in India, and the Tigris River in Iraq. Individuals have also been spotted in the Potomac River in Maryland and up the Mississippi River as far north as Illinois.

#2: Two more key difference between sharks and bony fish are their skin and teeth. Instead of scales, shark skin is covered in tiny tooth-like structures called denticles or placoid scales, giving it the texture of sandpaper. Indeed, for centuries tanned sharkskin, known as shagreen, has been used as natural sandpaper or as a high-grip surface – for example, on the hilts of swords. While intuitively one might assume that such rough skin would slow a shark down, in fact the opposite is true. As a shark swims, the denticles break up the laminar flow of water over its body into thousands of tiny vortices, greatly reducing drag. Indeed, engineers have recently begun copying shark skin in order to improve the hydrodynamic efficiency of ships and other vehicles. For example, the Speedo Fastskin swimsuit worn by the US swimming team at the 2000 Sydney Olympics featured a pattern of silicone bumps on the chest meant to simulate sharkskin and promote turbulent flow. 83% of the swimming medals during those Olympics were won wearing FastSkin suits, and derivatives were used to break dozens of swimming records before FINA, the International Swimming Federation, banned their use in 2009, citing them as a form of “technological doping.”

Shark teeth are also unique, being continuously produced in rows within the shark’s mouth and pushed forward and replaced like a living conveyor belt. Sharks never stop growing teeth, and a single Great White Shark may go through as many as 20,000 in its lifetime. Interestingly, shark skin and teeth are directly related in evolutionary terms. Palaeontologists believe that the first sharks, which emerged during the Ordovician Period 450 million years ago, had skin covered in denticles but primitive mouths without teeth. According to this theory, over millions of years the denticles around the sharks’ mouths grew larger and larger, gradually evolving into full-size teeth. This would explain the unique replacement mechanism of shark teeth, which is identical to how denticles are replaced in the skin.

#3: Fish are typically classified as cold-blooded or poikilotherms, meaning that like reptiles and amphibians they cannot generate their own body heat and must instead absorb heat from the environment. However, several fast pelagic fish such as tuna, sailfish and great white, blue, and mako sharks are able to maintain their body temperature several degrees above the surrounding water. Unlike in proper warm-blooded animals like mammals and birds, however, this is accomplished via a complex network of blood vessels called the rete mirabile, or “miraculous net.” The rete mirabile is a form of what engineers call a counter-current heat exchanger, and is arranged in such a manner that heat always flows into the animal’s body, maintaining its internal temperature. While commonly associated with fish, rete mirabile are found in all kinds of animals, including humans. In cold temperatures, the body tries to reduce the temperature of the extremities like the hands and feet in order to limit heat loss. However, excess heat cannot simply be dumped into the environment as this would lead to a greater loss of body heat and potentially hypothermia. Instead, the veins of the hands and feet are wrapped around the arteries, allowing excess heat to be dumped into the veins and carried back to the body’s core. So the next time you’re cold and feel the painful onset of frostbite, take solace in the fact that your body totally has your back – if not your fingers and toes.

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