12. The Enigmatic Sleep Patterns of Sharks
For both marine biologists and ocean aficionados, shark sleep patterns have long been a topic of interest and enigma. Understanding the rest habits of these ancient predators—which have wandered the oceans for hundreds of millions of years—present a special difficulty. Sharks, unlike many land animals or even other marine life, have a basic biological restriction that profoundly affects their sleep pattern: they must breathe constantly from water passage across their gills.
Most shark species cannot just stop swimming and rest on the ocean floor as many fish do; this need for constant water flow dictates otherwise. Rather, they have developed an amazing capacity for motion-based sleep, which has perplexed researchers for decades. Their survival depends on this constant flow of oxygen-rich water over their gills, which lets them obtain the essential oxygen they need to operate.
Still, the realm of shark sleep is not a one-size-fits-all one. Smaller shark species, such the nurse shark, have evolved other strategies allowing them to slumber while staying still. Little holes called spiracles, which lie behind each eye, are unique organs found in some sharks. Even when the shark is not actively swimming, the spiracles force water over the gills like a pump. This adaptation helps nurse sharks and related species to lie motionless on the ocean floor for prolonged periods, therefore attaining a condition of rest more akin to what we usually regard as sleep.
2016 saw a major revolution in our knowledge of shark sleep when scientists obtained amazing video of a great white shark in what looked to be a sleeping state. Near Mexico’s Baja California Peninsula, this historic occurrence was recorded by a robotic submersible with advanced cameras. The video showed a female great white shark displaying hitherto unheard-of behaviour in this species.
The shark was seen swimming nearer the coast into shallower seas as evening fell. She positioned herself in this region facing directly into strong ocean currents, a behaviour thought to be absolutely vital by scientists. The shark lets the current naturally drive water over her gills by orienting herself in this way, with her mouth open, therefore preserving the oxygen supply required for her survival. By carefully controlling ambient conditions, the shark can lower her own energy use while nonetheless fulfilling her respiratory needs.
The obvious alteration in the shark’s swimming behaviour added great appeal to this observation. Her swimming pace clearly dropped while she stayed in the current, which led scientists to believe she had fallen asleep. For great white sharks, this decreased movement and the strategic orientation in the current were noted as a unique sleeping pattern.
One cannot exaggerate the importance of this finding. It offers the first hard proof of sleep-like activity among one of the most famous and terrifying predators in the ocean. Shared on Discovery Channel’s famed annual Shark Week programming, the video not only enthralled viewers but also created fresh opportunities for scientific study on the sleep patterns of major shark species.
This revolutionary discovery has caused marine biologists to reconsider shark behaviour and physiology. It begs interesting problems regarding the nature of sleep in these prehistoric organisms and how it might differ from sleep as we know it in other species. Sharks, for example, go through distinct phases of sleep, much as mammals’ REM and non-REM cycles show. Given the continual hazards in the ocean environment, how can they remain aware of their surroundings in this resting state?