Yes, you can say that every fish swims, but unlike other fish, a significant portion of sharks are in danger of sinking. Sharks have to swim constantly to avoid sinking in the water.
Fishes are divided into two groups as cartilaginous and bony. Stingrays and sharks are cartilaginous fish and the remaining fish make up the group of bony fishes. While there is an air-filled pouch in bony fishes, sharks do not have it. These sacs, also called swim bladder, allow bony fish to stand still without swimming in the water.
Also, although the muscles around the sharks’ gills are active, they are not enough to pump water or stay still without swimming. Therefore, Sharks have to work their gills through swimming to get enough oxygen.
For these reasons, if sharks do not want to sink in the water, they have to swim constantly. Since stingrays often spend their lives on the ground, this is not a big problem for them.
How do they sleep if sharks have to swim always?
Until 2016, it was not possible to view a sleeping shark. This is because it has been really difficult to follow the relationships of aquatic creatures with sleep without disrupting their natural life. However, the Discovery Channel succeeded in recording this in mid-2016 with its advanced robot and underwater camera systems. During the shootings in the Guadalupe Islands of Mexico, the discovery team managed to catch a female great white shark, which they named “Emma”, while it is sleeping.
In the images, it is understood that the shark is sleeping very close to the bottom. But they continue to swim even while they are sleeping. It is important to note that sharks do not undergo deep sleep as humans do. They seem to be “sleep swimming,” with parts of their brain being less active, or “resting,” while the shark remains swimming.
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