Editing Shark (section)

===Buoyancy===
Unlike bony fish, sharks do not have gas-filled swim bladders for buoyancy. Instead, sharks rely on a large liver filled with oil that contains [[squalene]], and their cartilage, which is about half the normal density of bone.<ref name=Cartilagious/> Their liver constitutes up to 30% of their total body mass.<ref name="Collins">{{cite book
|last1=Compagno |first1=Leonard
|last2=Dando |first2=Marc
|last3=Fowler |first3=Sarah
|title=Sharks of the World
|publisher=Collins Field Guides
|year=2005
|isbn=978-0-00-713610-0
|oclc=183136093
}}</ref> The liver's effectiveness is limited, so sharks employ [[dynamic lift (fish)|dynamic lift]] to maintain depth while swimming. [[Sand tiger shark]]s store air in their stomachs, using it as a form of swim bladder. Bottom-dwelling sharks, like the [[nurse shark]], have negative buoyancy, allowing them to rest on the ocean floor.

Some sharks, if inverted or stroked on the nose, enter a natural state of [[tonic immobility]]. Researchers use this condition to handle sharks safely.<ref name=ParttGruberTaniuchi>{{cite book
| last1=Pratt|first1=H. L. Jr|last2=Gruber |first2=S. H. |last3=Taniuchi |first3=T
| title=Elasmobranchs as living resources: Advances in the biology, ecology, systematics, and the status of the fisheries
| publisher=NOAA Tech Rept.
| year=1990}}</ref>