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In many species of fish, neutral buoyancy is maintained by a gas-filled swim bladder which can be expanded or contracted as the circumstances require. A disadvantage of this method is that adjustments need to be made constantly as the water pressure varies when the fish swims higher and lower in the water column. An alternative buoyancy aid is the use of lipids, which are less dense than water--squalene, commonly found in shark livers, has a specific gravity of just 0.86.

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