With more than 500 species described today, sharks are one of the oldest animals on Earth, with fossil records dating back 400 million years. In this way, the ancestors of sharks swam the seas before dinosaurs inhabited the land. Being one of the main predators of marine ecosystems, these cartilaginous fish are essential to maintain the natural order of these ecosystems.
Sharks sleep, yes or no?
Despite all the research that has been carried out on sharks over many years and even today, the shark dream has become one of its most mysterious characteristics. Regarding the answer to this question about whether sharks sleep or not, we can indicate that sharks are animals that do not sleep as we humans do, entering a state of unconsciousness, nor in the strict sense of the word, but rather rest differently. : Sharks alternate active periods and rest periods and, in addition, it is something that changes according to the species.
Due to their physiology, there are species of sharks that need to be in motion almost all the time to make water enter through their gills and thus be able to breathe, since they lack an organ responsible for making water enter through their gills. These types of sharks are known as obligate ram breathers (or obligate ram fans), as they attract water through their mouths and expel it through their gills, and include species such as the great white shark. This type of breathing could be a problem for them when due to circumstances they stop swimming, for example, if they are caught in a net, which would cause them to suffocate.
However, the need to keep moving applies only to a small minority, as forced ram breathers would represent only about two dozen species. So, although there are species of sharks that need to keep moving constantly to receive the vital supply of oxygen, there are also species of sharks that have spiracles , small openings located behind each eye which allow sharks to breathe while they remain stationary. In the deep sea. These structures force water through the gills of sharks, allowing the shark to be still when resting.. The spiracles are especially useful structures for shark species that typically inhabit bottoms, such as the stingray shark or the wobbegong shark .
Now that you know that sharks sleep but in their own way, that is, they do it in a very different way from ours or even from other marine beings.
For shark species that do not have spiracles, the sleeping process is based on alternating active periods with rest periods and, therefore, they present an appearance as if they were swimming asleep, since sharks sleep while swimming .
Sharks often look for a current to rest , so they stay in the current, drifting and letting the water pass over their gills without having to move and allowing them to rest. During this process, the shark has parts of its brain less active or resting while the shark continues to swim or drift with the current. In fact, it is the spinal cord that coordinates swimming while the shark sleeps, rather than the brain. This control would allow the sharks to swim while they are practically unconscious and, also, the rest of their brain.
Do sharks sleep with their eyes open or closed?
Although regardless of the breathing method, sharks can have periods of rest while lying still, they do not sleep in the strict sense of the word. Sharks sleep with their eyes permanently open , as they do not have eyelids . Also, his pupils continue to monitor the movement of the creatures swimming around him.
However, most species of sharks have developed a membrane similar to the eyelids that performs the same function: the nictitating membrane , it is a thin layer of translucent tissue that covers the eyeball.
While there are sharks that sleep a few hours a day , others can sleep for long periods of time . Most sharks are nocturnal animals , but their precious time of activity depends on the species of shark.
On the one hand, species such as the nurse shark, the horned suno or the balloon shark show nocturnal activity while sleeping during most of the day. On the other hand, there are also many other species that sleep during the day but exhibit twilight activity, for example the dogfish. This generally allows sharks to hunt at night and early in the morning, when the fish are most active.