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I've seen this claim several places:

If a shark stops swimming it will die. (from wikianswers, alongside contradictory text)

[I]f they do not swim, then they will have no oxygen and they will drown. (from ask.com)

Do sharks drown and die if they stop swimming?

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In general no, but there are some species of sharks that require movement to circulate water over their gills. NOVA had this question addressed to Dr. Samuel "Sonny" Gruber who responded as follows:

NOVA: Do sharks have to keep swimming to breathe?

Gruber: Several species do, including hammerheads and mackerel sharks. Typically, pelagic sharks that never encounter the bottom are adapted to swim all their lives. But the vast majority of sharks have a buccal (mouth) pump and are not so-called "obligate ram ventilators." (Ram ventilators like hammerheads and great whites must swim to pass water over their gills.) So the answer is no, emphatically. There are even some sharks with spiracles, holes on the top of their head that allow water to enter their gill chambers when the mouth is on the bottom.

This is corroborated by science and educational websites online as well as in the biology textbooks such as "Biology of Sharks and Their Relatives, Second Edition."

Wobbegong shark lying on the bottom of the ocean

The Wobbegong shark, a member of the carpet sharks family. As an ambush predator, the Wobbegong shark demonstrates this behavior by lying in wait on the ocean floor among rocks and other materials waiting for smaller fish to swim closer.

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  • mind that needing waterflow doesn't mean they have to swim. They could be stationary in flowing water instead...
    – jwenting
    Aug 27, 2013 at 15:26
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    As a wonderful example, see the Wobbegong shark - which sits on the bottom, camouflaged, waiting motionless awaiting its prey.
    – Rory Alsop
    Aug 28, 2013 at 8:43
  • @RoryAlsop Ah! Good one!
    – rjzii
    Aug 28, 2013 at 13:21
  • But it seems there are some sharks that would (Great whites and hammerheads)
    – Chad
    Aug 28, 2013 at 18:40
  • I think, although science has long believed certain sharks needed constant movement, I remember a few instances (very rare) where this has been found not entirely true! ack.net/sharks080510.html and sharkwatchsa.com/en/blog/category/482/post/1099/… What I really want to find is the article where the great whites were found sleeping in some underwater cave, but I can't!!!
    – AthomSfere
    Aug 30, 2013 at 0:49

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