The idea that all sharks must swim constantly or suffocate is a widespread misconception that simplifies the remarkable diversity found across hundreds of species. While it is true for some recognizable species, it is not the rule for all sharks. Whether a shark must keep swimming depends entirely on the specific respiratory physiology that evolved for its environment. The distinction lies in the two primary methods sharks use to draw oxygen-rich water over their gills.
Respiration by Ram Ventilation
For highly active, pelagic sharks, continuous forward motion is the only way they can breathe, a process known as obligate ram ventilation. This method requires the shark to swim with its mouth slightly open, forcing or “ramming” water into its oral cavity and over the gills. The shark’s movement creates the necessary flow of water across the gill filaments where oxygen exchange occurs.
Sharks that rely solely on ram ventilation have lost or reduced the muscular mechanisms needed to actively pump water. They must maintain a minimum swimming speed to ensure an adequate supply of oxygen. Great white sharks, mako sharks, and whale sharks are examples of obligate ram ventilators that must keep moving to survive.
If these species stop swimming for too long, the lack of water flow means they cannot extract sufficient oxygen, leading to suffocation. This reliance on motion means their entire physiology is adapted for a life of continuous movement. Even when resting, these animals must find ways to reduce energy expenditure while maintaining forward momentum.
Respiration by Buccal Pumping
In contrast to obligate ram ventilators, many shark species possess a muscular mechanism that allows them to actively breathe while remaining stationary, a process called buccal pumping. These sharks use the muscles in their mouth and pharynx to expand and contract their buccal cavity, creating a pressure gradient. This action draws water in through the mouth and forces it out over the gills, independent of forward movement.
This active ventilation system provides a distinct advantage, allowing species to inhabit environments where stopping is common, such as the seafloor or coral reefs. Nurse sharks, angel sharks, and Port Jackson sharks are well-known examples of species that employ buccal pumping and can be observed resting motionless on the bottom. Some bottom-dwelling sharks also utilize small openings behind their eyes called spiracles, which draw in water when their mouth is closed or buried in the sand.
Many shark species are facultative ram ventilators, meaning they can switch between both breathing methods depending on their activity level. When swimming quickly, they use the more energy-efficient ram ventilation, but they can switch to buccal pumping when they slow down or come to a stop. Research has indicated that ram ventilation can be more energy-efficient than the muscular effort required for buccal pumping.
How Resting Sharks Conserve Energy
Sharks that can breathe without swimming use their ability to stop moving as a primary method of conserving energy, entering a state similar to sleep. Studies on species like the draughtsboard shark show that when they become inactive for sustained periods, their metabolic rate drops significantly. This lowered oxygen consumption is a physiological indicator that rest functions to reduce energy expenditure.
For species that must swim continuously, like the grey reef shark, energy conservation is achieved by leveraging environmental factors. These sharks have been observed “surfing” natural currents or updrafts, positioning themselves to swim against the flow with minimal effort. Utilizing these natural water movements allows them to maintain the required speed for ram ventilation while reducing muscular energy expenditure by at least 15%.
Whether resting on the bottom or using currents, the state is characterized by a sustained period of immobility and reduced responsiveness. Sharks capable of buccal pumping often seek out sheltered locations, such as caves, rocky crevices, or sandy bottoms, for their resting periods. Combining a flat, recumbent posture with a lowered metabolic rate allows these behaviors to conserve energy between active periods of hunting and traveling.