why do sharks keep swimming at night?

The Deep Dive

Shark respiration is a tale of two strategies. Most bony fish use buccal pumping, actively drawing water into the mouth and over the gills. Sharks, however, have evolved a spectrum of methods. At one end are the obligate ram ventilators—species like the great white, mako, and whale shark. Their mouths are often positioned ventrally, making active pumping inefficient. Instead, they must constantly swim forward with their mouths slightly open, allowing the flow of water to pass over their gills and extract oxygen. If they stop, they suffocate. This biological imperative operates around the clock, day and night. At the other end are sharks like the nurse shark or horn shark, which possess large, muscular pharynxes that can actively pump water, allowing them to rest motionless on the ocean floor. For the obligate swimmers, nighttime is not a period of rest but of continued necessity. However, their activity often shifts; many are crepuscular or nocturnal hunters, using the cover of darkness to ambush prey. To reconcile the need to move with the need for rest, sharks exhibit unihemispheric slow-wave sleep, where one hemisphere of the brain rests while the other remains alert to maintain swimming and vigilance. Their highly efficient, oil-rich livers also provide buoyancy, reducing the energy cost of this perpetual motion.

Why It Matters

Understanding why sharks swim constantly is crucial for their conservation and our safety. It informs fishing practices; for example, sharks caught on longlines that are held immobile for hours can suffer severe physiological stress. This knowledge guides the development of safer release techniques and bycatch reduction devices. For ecotourism, it explains why certain species are always on the move, affecting how dive operators plan encounters. Biologically, it showcases a remarkable evolutionary adaptation to aquatic life, inspiring engineers in designing more efficient underwater propulsion systems that minimize energy use. Ultimately, it underscores that sharks are not mindless machines but complex animals with specific physiological needs that must be respected in a changing ocean.

Common Misconceptions

A prevalent myth is that all sharks will die if they stop swimming. In reality, only a subset of species, the obligate ram ventilators, require constant forward motion to breathe. Many others, like the bottom-dwelling catsharks, can pump water over their gills while stationary. Another misconception is that sharks are perpetually active, hyper-aggressive hunters with no downtime. Research shows they have clear rest periods and can engage in unihemispheric sleep, allowing them to conserve energy while still moving. They are not indiscriminate eaters but strategic predators that often feed at specific times, not continuously.

Fun Facts

• The great white shark can enter a state of 'tonic immobility' when flipped upside down, becoming completely still and catatonic for several minutes.
• Some shark species, like the swell shark, can gulp air at the surface and store it in their stomachs, allowing them to float motionless while digesting a meal.