Do Sharks Sleep?

Scientists are looking deep into sharks’ behavior trying to explore as much as they can. But sadly, sharks are still one of the least studied marine animals though they have been living underwater for millions of years leaving behind even dinosaurs. It is in part because marine life research is quite a new discipline which is why even today we do not know much about sharks. These days marine experts are trying hard to solve the riddle about when do sharks go to rest and do they really relax or keep on swimming without interval. They are uncertain about their sleeping patterns and relaxing habits. However, what’s sure about them is that they do have both active and inactive periods.

Ram Ventilation | Do Sharks Sleep

Biologically, sharks are not similar to other fishes in that they do not have swim bladders which may allow them to float on water. Therefore, sharks must continue to swim (just like pelagic species of sharks do) or else they will sink and descend to the bottom of the ocean. Sharks have special gill slits located on both sides of their mouth. With the help of these gills, they extract oxygen from water. Water must pass through these gills all the time in order to inhale. Therefore, shark must constantly move so that oxygenated water may pass through its mouth and gills for respiration. This phenomenon in which water continually passes over their gills and mouths is known as ‘ram ventilation’ and the sharks that constantly swim are called ‘obligate ram ventillators’. Majority of sharks depend on ram ventilation. It is an effortless job for them to keep moving in seawater without taking rest. Marine scientists have observed that sharks inhale much more oxygen while swimming rather than at rest and in this way they conserve a lot of energy as well. But clearly sharks cannot remain in motion forever because after all they also need a short nap or some sort of rest like any other fish.

Whitetip Reef Sharks at the bottom of ocean - Do Sharks Sleep

Research and Sleeping Behavior of Sharks under Caves | Do Sharks Sleep

Some of the experts have witnessed an unusual behavior of sharks inside the caves of Mexico, Australia and Japan. According to the reports, these sharks are observed to having a rest for longer periods of time (possibly for hours) and because of this, scientists named them ‘sleeping sharks’. However, it is not completely true in that their eyes are seen to follow the divers during this posture which means they are certainly not snoozing.

Shark resting in cave - Do Sharks Sleep

There are some nocturnal shark species that are active during the late hours of darkness. During sonic telemetry study, it was observed that sharks like Tiger shark and Reef Whitetip shark stalk their prey at night meaning that these are sleepless sharks in any case. On the contrary, a number of sharks are noticed to sleep lightly inside the caves. When a careful study was carried out in these caves, it was observed that the content of oxygen in water under these caves was very high. This may be the reason why sharks are able to have a break in these areas. A similar sleeping pattern was observed with Great White shark who navigates narrow furrows in deep seawater and then takes a long breather. The water current in such channels was however passing over its gills probably allowing more oxygen for respiration during rest. It is therefore possible for Great White sharks to take a short nap while receiving oxygenated water at the same time. But again, there is not enough evidence or solid reasoning to support this assumption and the question of do sharks sleep is still up in the air.

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How can Sharks Inhale while Sleeping?

Typically, sharks keep on swimming and rumor has it that they do not take a break. Though it is true for most of the sharks but there are some species that are observed to snuggle down in water for hours without any movement and needless to say; they do take breaths as well. In reality, there is a small nostril-like opening behind the shark’s eye. This small hole is used to suck out more oxygen from water when the shark is stationary. It is known as spiracle. These spiracles help sharks (like nurse sharks) to stay put in water and continue to pump out oxygen from it. The spiracles play a crucial role for allowing sharks to take oxygen in static position. Ram ventilators lack these spiracles and so they must swim with no end in sight so as to survive.

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Is it Possible for Sharks to Swim while Sleeping?

To explore more about the swimming behavior of sharks, an experiment was conducted on a small shark known as spiny dogfish (Squalus acanthias). This research revealed that the shark’s movement in water was not controlled by its brain instead it was synchronized by its spinal cord. It was found that ‘central pattern generator’ (which was responsible for such movement) was located in the spinal cord. Perhaps this is why sharks may be able to swim while sleeping because their brain is not being used to coordinate movements in water. Seeing as majority of sharks keep on swimming, it is a demanding task for scientists to chase them and see when they take a break. But it is possible for sharks to swim while sleeping.

To wrap it up, sharks may or may not sleep like humans do but one thing is absolutely certain i.e. sharks both have active as well as relaxing periods and not all of them swim persistently without a break.

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References:

“Do Sharks Sleep?”. Dutch Shark Society, affiliated with Ghost Fishing Foundation. Published on 9 Jan. 2014. Web. Accessed on 14 Oct. 2014.

“Shark Basics”. Education-Most Commonly Asked Questions. ICHTHYOLOGY at the Florida Museum of Natural History. n.d. Web. 14 Oct. 2014.

“Biology of Sharks and Rays”. R. Aidan Martin. ReefQuest Centre for Shark Research. n.d. Web. Accessed on 14 Oct. 2014.

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