Do Sharks Stop Swimming?
- bioworld090
- Jul 23, 2022
- 3 min read
Updated: Aug 4, 2022
Image Credit: PIRO4D
Sharks are a group of cartilaginous fish consisting of more than 500 species. They are well-known predators of the marine world. In fact, the great white shark is considered the apex predator of the oceans.
There's a common belief about sharks that they would drown and die if they stop swimming. But is it a fact or just a belief? Do sharks really die if they stop swimming?
The answer to the question is that it depends upon the species of shark and how they breathe. There are shark species that need to swim constantly to breathe and some that don't. So, does it mean that different species of sharks breathe differently?
The answer is yes. Sharks have two different ways of breathing: ram ventilation and buccal pumping.
RAM VENTILATION
Ram ventilation requires a shark to constantly swim. The shark breathing through the ram ventilation method swims forward with its mouth opened. The water enters through the mouth, passes over the gills and flows out through the gills. The exchange of gases takes place when the water passes over the gills. Ram ventilation is more energy efficient than buccal pumping.
There are around 24 species of sharks that are referred to as "obligate ram ventilators". They cannot switch between the two breathing methods and rely only on ram ventilation for breathing. These are the sharks that need to constantly swim to breathe and live. Obligate ram ventilators include the great white shark, mako shark, salmon shark, sandbar shark, thresher shark, porbeagle shark, hammerhead shark, bull shark and whale shark.
BUCCAL PUMPING
Buccal pumping involves drawing water into the mouth with the help of buccal muscles. In simple terms, the shark opens and closes its mouth rhythmically to draw water inside its mouth. The opening of the mouth means lowering the floor of the mouth which increases the volume of the mouth. Water is drawn in and then the shark closes its mouth which means raising the floor of the mouth. This decreases the volume of the mouth and water is pushed towards the gills.
Benthic sharks(sharks that live near the bottom of the sea) such as nurse sharks, bullhead sharks and carpet sharks use buccal pumping to breathe.
Most sharks can switch between the two breathing methods.
Image Credit: saving the sharks
Some species of sharks also have spiracles. Spiracles are respiratory openings located a little behind the eyes and are used to pump water over the gills.
Some common questions come to mind after reading about the ways of breathing in sharks. Do sharks sleep? If yes, then how do they, especially “obligate ram ventilators”, sleep? Do they sleep in the same way as humans or the meaning of sleep is different for them?
Sharks do sleep but not like humans. Sleep has a different meaning for sharks. For sharks, sleep means conserving energy, low metabolic rate and less activity. It would be more appropriate to say that sharks go in a resting phase rather than sleep.
There is evidence of "sleep" in “buccal pumping” sharks. According to an article, nurse sharks spend much of their time resting on the seafloor during the day. Sharks like Port Jackson, Caribbean reef, whitetip reef shark, etc., have a higher metabolic rate and show higher activity at night as compared to during the day. Lemon sharks also have a higher metabolic rate at night.
Ram ventilating sharks show very little changes in their activity. These sharks show something called cyclic diel vertical migrations(DVMs) which is inhabiting different depths of the sea at different times of the day. Sharks like shortfin mako, blue, whale, etc., live in greater depths during the day as compared to the night. DVMs may be indicative of rest in ram ventilating sharks but it might also be not.
Sharks exhibit a behaviour, called “yo-yo” movement, which is characterized by repeated active ascent towards the surface and descent towards the bottom. In simple words, the shark swims towards the surface and then glides downwards without using much energy. This behaviour is thought to aid in rest and energy conservation. However, a study on tiger sharks fitted with cameras and accelerometers showed images of encounters with prey fish. This indicates that the “yo-yo” movement could be a hunting tactic instead of an energy conservation behaviour or it could be a combination of both.
It is also said that it is the spinal cord, instead of the brain, that controls the swimming in sharks. This led to the hypothesis that sharks can rest their brain while swimming.
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