An ancient Polynesian legend tells that the gods condemned sharks to swim endlessly without ever stopping, depriving them of any rest, due to their bad behavior.
However, the true reason behind the constant movement of sharks is that, unlike other fish, they do not possess a swim bladder filled with gas that allows them to float motionless. If a shark stops its movement, it would simply sink to the bottom and suffocate because water would no longer flow through its gills, preventing it from obtaining the necessary oxygen from the water.
The evolution of sharks has provided a solution to their negative buoyancy by endowing them with a liver rich in oil (less dense than water), which can account for up to 20% of their body weight. The rest is achieved through a cartilaginous skeleton, which is much lighter than the typical bony skeleton of other fish, and low-density body tissues. These factors, combined with the streamlined shape of their bodies and the application of the Archimedes principle, enable sharks to swim with minimal effort. However, if they were to cease their movement, they would sink to the bottom.
Therefore, sharks cannot sleep, at least not in the way humans understand sleep. It is believed that sharks achieve some form of rest, at least muscle rest, by gliding in freefall towards the ocean floor. To accomplish this, they approach the surface and then let themselves fall towards the bottom without moving. Their pectoral fins act as wings, allowing them to glide slowly and spirally while remaining lethargic for a few minutes. During this time, their brains remain active, and water continues to circulate through their gills, providing them with oxygen.
In the 1970s, this theory seemed to be challenged when a Mexican diver discovered sharks near Isla Mujeres that appeared to be resting motionless in underwater caves. The «sleeping sharks» of Mexico are currently the subject of study and various theories. But that is another secret that the sea keeps reserved for us.
«You cannot defend what you do not love, and you cannot love what you do not know.»