Sharks must constantly swim to breathe because they use a breathing mechanism called branchial respiration, which involves pumping water over their gills to absorb the oxygen present in the water.
Sharks breathe through their gills, which consist of several slits located on either side of their head. Unlike mammals that use lungs to breathe air, sharks extract oxygen directly from the water through these special gills. To absorb this oxygen, water must enter through the shark's mouth, pass through its gills, where the oxygen is absorbed and carbon dioxide is expelled, before being expelled through these gill slits. No lungs, no nostrils used for breathing: everything depends entirely on the efficiency of these gill structures. This anatomy therefore forces sharks to swim constantly (or almost) to maintain this vital water flow and thus ensure their oxygenation.
Sharks that use continuous swimming cannot simply rest on the bottom to catch their breath: they must constantly move forward to send oxygen-rich water over their gills. While swimming, they keep their mouths open, allowing water to flow in, pass through the gills, and exit through the gill openings. This continuous flow of fresh water ensures that their gills remain well-supplied with oxygen. If these sharks stop swimming, the water does not circulate effectively and they risk suffocation. Hence the necessity of always being in motion, even during their sleep. No swimming, no air: quite a demanding respiratory system!
The shark's gills are like well-made filters: they extract dissolved oxygen directly from the water. As the shark swims, water constantly flows through its gills, which capture the oxygen and transfer it into the blood. Each gill is made up of many tiny, very fine filaments, filled with tiny blood vessels: this allows for a huge exchange surface to absorb as much of the oxygen the shark needs. The faster it swims, the better this system works. But if it stops, the water flow ceases to circulate properly, the oxygen supply decreases drastically, and the shark begins to run out of air—well, oxygen. That's why it must keep moving to breathe.
Unlike sharks, some fish are calm; they can stay in place while breathing thanks to a system called active ventilation. For them, water enters through the mouth by actively pumping it with specific movements of the opercula (a kind of lid protecting the gills). This even allows them to breathe quietly while at rest or when they are napping. The shark, on the other hand, does not have this luck: its gills are directly exposed to the water without protection from opercula, forcing water to flow continuously with the movement of swimming. If it stops moving, it's like turning off the oxygen tap.
In some species of sharks that practice continuous swimming ventilation, stopping swimming can be fatal. Without movement, oxygen-rich water no longer circulates properly over their gills, preventing them from breathing effectively. As a result, the shark quickly begins to suffocate, its body enters a state of respiratory stress, and gas exchange becomes insufficient. Prolonged immobility thus leads to a cruel lack of oxygen (known as hypoxia). Only a few species of sharks, capable of ensuring active ventilation by pumping water while stationary, avoid these problems. But for many, stopping swimming is basically like holding one's breath indefinitely: it inevitably ends rather badly.
Some species of sharks, such as the nurse shark or the sleeper shark, do not need to swim constantly. They use active ventilation by sucking water through their gills while remaining stationary.
Most sharks do not have a swim bladder, the organ that allows bony fish to float. This is why they sink if they stop swimming, which drives them to remain in constant motion.
The largest known shark, the whale shark, can filter more than 6,000 liters of water per hour through its gills to obtain oxygen and capture its microscopic food.
The term 'obligate ram ventilators' refers to those sharks that must swim continuously to breathe effectively, such as the mako shark, the tiger shark, or the great white shark.
Some highly active bony fish such as tuna and swordfish also use ram ventilation or continuous swimming for respiration. This permanent respiratory method ensures a constant flow of oxygen essential for their high metabolism and ongoing activity.
For sharks that rely on constant swimming for ventilation, stopping means insufficient oxygenated water flowing over their gills. Prolonged interruption of swimming can lead to a rapid decrease in blood oxygen levels and, consequently, suffocation and death.
Sharks that ventilate through continuous swimming adopt an active resting state by swimming slowly and automatically thanks to their muscular and nervous system. Thus, even during their sleep, they continue to move slightly, allowing for a constant flow of oxygen-rich water over their gills.
No, sharks' gills are designed to extract oxygen solely from water. Out of their aquatic environment, the gill structures collapse, preventing any effective absorption of oxygen. Sharks can only survive for a few minutes out of water before being severely affected.
Active ventilation, or buccal pumping, allows certain species to remain motionless while actively pumping water through their gills. This ability is beneficial for species that dwell on the seafloor or in environments where continuous swimming would be energetically costly or challenging.
No, only certain species must constantly swim to ensure a sufficient flow of water through their gills. These species are referred to as continuously swimming ventilators (ram ventilation). However, other species of sharks, such as the nurse shark, use an active buccal pumping method and can remain still while breathing.
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