Sharks, like all animals, require periods of reduced activity to conserve energy and maintain physiological function. The question of how a creature that needs perpetual motion ever rests depends entirely on its unique biological adaptations for breathing. This ancient group of fish has evolved two distinct strategies for oxygen intake, leading to two completely different ways of achieving a restful state in the ocean.
The Short Answer: Defining Rest in Sharks
Sharks do enter a sleep-like state, though it does not involve the deep unconsciousness seen in mammals. This resting state is primarily characterized by physiological changes that reduce the shark’s energy expenditure. Recent studies on species like the draughtsboard shark have provided the first physiological evidence of this state, showing a significant drop in metabolic rate during periods of sustained inactivity.
Behaviorally, a resting shark demonstrates reduced responsiveness to external stimuli, which is a key indicator of sleep in the animal kingdom. For a shark, “sleep” is a state of lowered awareness and decreased oxygen consumption. This necessary biological function allows them to recover, even if their eyes remain open or they continue to move slowly.
The Role of Respiration in Resting Behavior
A shark’s ability to rest while stationary is determined by its method of moving water over its gills to extract oxygen. Sharks achieve this flow in two distinct ways.
The most energy-efficient method for fast-swimming species is ram ventilation, where the shark swims continuously with its mouth slightly open. This forward motion forces water over the gills, eliminating the need for active muscle control. Sharks that rely solely on ram ventilation are known as obligate ram ventilators, and they must keep swimming to breathe.
The alternative method is buccal pumping, which uses active muscle contractions in the mouth and pharynx to draw water in and push it over the gills. This muscular pumping allows a shark to remain completely stationary while maintaining a sufficient flow of oxygenated water. The presence or absence of this pumping ability dictates whether a shark rests while moving or remains motionless.
Sharks That Rest Motionless
Sharks capable of buccal pumping can achieve complete stillness when resting, a behavior commonly observed in bottom-dwelling species. These sharks, such as the Nurse Shark, Lemon Shark, and White-tip Reef Shark, often lie on the seafloor or gather in caves. While resting, they rhythmically open and close their mouths to actively pump water across their gills, ensuring uninterrupted respiration.
Some bottom-dwellers, like the Wobbegong, also use small openings behind their eyes called spiracles to draw in water. This prevents debris from entering the gills while they lie on the substrate. This stationary rest allows for a significant drop in metabolism, conserving substantial energy.
Even when motionless, the shark’s eyes may remain open, suggesting they maintain a degree of awareness to monitor their surroundings for potential threats.
Sharks That Rest While Swimming
Sharks that are obligate ram ventilators, such as the Great White, Mako, and Whale Shark, cannot stop moving. Therefore, their rest takes the form of continuous, low-speed swimming, sometimes called “sleep swimming.” These pelagic species achieve rest by reducing their physical output and slowing their pace to maintain the constant flow of water needed for breathing.
It is hypothesized that these sharks may utilize a mechanism similar to unihemispheric slow-wave sleep (USWS), a state seen in marine mammals like dolphins. In USWS, one half of the brain rests while the other half remains active enough to control the motor functions required for swimming and maintaining awareness. This allows the shark to continue the necessary rhythmic motion for ram ventilation while still experiencing a period of neurological rest. By moving into areas with strong currents, some species may also reduce their swimming effort by allowing the water flow to assist in respiration.