It is a common misconception that sharks must constantly swim to survive. While some sharks do need continuous movement, their rest is complex and depends on unique physiological adaptations. Sharks engage in periods of reduced activity, differing significantly from human sleep. Their diverse behaviors and breathing mechanisms reveal how these ancient creatures find repose in the vast ocean.
The Nature of Shark Rest
Sharks do not experience sleep like humans or many other mammals, characterized by unconsciousness and prolonged immobility. Instead, their “sleep” involves periods of reduced activity and responsiveness. During these times, sharks remain aware of their surroundings, and their eyes often stay open.
Recent studies on draughtsboard sharks provide physiological evidence of sleep in some species. Researchers observed that when these sharks rested for over five minutes, they exhibited a flatter body posture and a reduced metabolic rate, indicating energy conservation. Draughtsboard sharks were observed to sleep with their eyes open, especially at night, suggesting eye closure might be a response to light rather than a direct indicator of sleep.
Breathing Mechanisms During Rest
A shark’s ability to rest is closely tied to its method of respiration. Sharks primarily use two breathing methods: ram ventilation and buccal pumping. Ram ventilation requires the shark to swim continuously with its mouth open, forcing water over its gills to extract oxygen. Sharks relying solely on ram ventilation, such as great white, mako, and whale sharks, must keep moving to breathe and will suffocate if they stop. Their speed helps push more water through their gills, increasing oxygen intake.
In contrast, many other shark species use buccal pumping, allowing them to remain stationary. These sharks use muscles in their cheeks to actively draw water into their mouths and pump it over their gills. Some buccal pumping sharks also possess spiracles, small respiratory openings behind their eyes, which assist in pulling water over the gills, especially when their mouths are closed or buried in sand. Nurse and tiger sharks are examples that employ buccal pumping, enabling them to rest motionless on the seafloor. Some sharks, like reef sharks, can switch between both ram ventilation and buccal pumping, adapting their breathing to activity level.
Diverse Resting Behaviors
Varied breathing mechanisms lead to diverse resting behaviors across shark species. Sharks relying on buccal pumping are often found resting motionlessly on the seafloor or within caves and crevices. Nurse sharks, for instance, spend much of their day resting in groups on the ocean bottom, becoming more active at night to hunt. Whitetip and Caribbean reef sharks are also observed resting stationary in caves or under ledges, utilizing buccal pumping to maintain oxygen flow. These species exhibit reduced responsiveness to external stimuli during these periods, aligning with resting or sleep-like states.
For obligate ram ventilators, such as great white and mako sharks, resting involves a different approach since they cannot stop swimming. While they do not exhibit stationary rest, observations suggest they may enter a state of reduced activity while still in motion. Some researchers propose that large, pelagic sharks might engage in unihemispheric sleep, where one half of their brain rests while the other remains active, allowing them to continue swimming and stay aware. Anecdotal evidence suggests some ram-ventilating sharks, like basking and blue sharks, can enter unresponsive, low-activity states while in motion, but concrete evidence linking these to true sleep is still emerging. Great white sharks have been observed engaging in “synchronized swimming” at consistent speeds, suggesting they might be resting while moving.