The idea of a shark resting often seems contradictory to their image as constant, fearsome ocean predators. This raises a compelling biological question: if sharks are always moving, how do they get the rest necessary for life? Exploring this reveals the diverse ways these marine animals manage periods of inactivity, challenging common misconceptions.
Understanding Shark Rest
Sharks do not “sleep” like humans or other mammals, which involves extended unconsciousness and immobility. Instead, they enter a state of reduced activity. This state is characterized by decreased responsiveness to their environment and a lower metabolic rate, allowing them to conserve energy. The belief that all sharks must continuously swim to breathe has historically fueled misunderstandings about their resting habits.
Breathing During Rest
A shark’s ability to rest is closely tied to its method of respiration. Sharks primarily use two methods to move water over their gills for oxygen exchange. Ram ventilation requires the shark to swim continuously with its mouth open, forcing water over its gills. Many active, pelagic sharks, like great whites, mako sharks, and whale sharks, are obligate ram ventilators, meaning they must continuously move to breathe and will suffocate if they stop.
Conversely, some shark species employ buccal pumping, using cheek and neck muscles to actively pump water into their mouths and over their gills. This allows them to breathe while remaining stationary. Many species can switch between ram ventilation and buccal pumping depending on their activity level. Sharks that utilize buccal pumping, such as nurse sharks and angel sharks, can rest motionless on the seafloor. Some also possess spiracles, small openings behind their eyes, which assist in drawing water over their gills.
Diverse Resting Behaviors Across Species
The variety in respiratory mechanisms leads to diverse resting behaviors across different shark species. Sharks relying on buccal pumping, such as nurse sharks, white-tip reef sharks, and lemon sharks, are frequently observed resting motionlessly on the seafloor or within caves. These sharks often exhibit relaxed postures while maintaining oxygen flow over their gills. This ability allows them to conserve energy, a key aspect of their resting state.
For obligate ram ventilators like great white sharks and mako sharks, resting involves a different strategy since they cannot stop swimming. While direct observation of “sleep” in these species is challenging, evidence suggests they may enter a sleep-like state while in motion. This could involve “sleep swimming,” where parts of their brain are less active while their spinal cord coordinates movements. Observations suggest these sharks exhibit periods of reduced responsiveness, such as basking sharks floating almost motionless at the surface.
What Science Says About Shark Brains
The scientific understanding of shark “sleep” from a neurological perspective is still developing. Unlike mammals, which exhibit distinct brainwave patterns during different sleep stages, research on shark brain activity during rest is limited. However, studies have begun to explore physiological changes that occur when sharks are in a restful state. For instance, research on draughtsboard sharks and Port Jackson sharks indicates that during periods of inactivity, they show reduced responsiveness and a decrease in oxygen intake, suggesting a lower metabolic rate. This metabolic reduction is consistent with some definitions of sleep across the animal kingdom.
While some sharks have been observed closing their eyes during these restful periods, others keep them open, indicating that eye closure is not a universal indicator of rest. The ability of some sharks to swim continuously while potentially resting their brains suggests that movement coordination might be handled by the spinal cord rather than requiring full brain engagement. This highlights that shark rest is a complex biological process, differing significantly from mammalian sleep, yet serving the purpose of energy conservation and recovery.