Swimming in water is much like flying in air, although the density and viscosity of water limit maximum speeds.
However, the density of water also permits animals to float, reducing metabolic requirements at rest. To optimize the ability to float, animal density should be equal to that of water. But since most animals contain some dense material (bone, shell, etc), the rest of the tissues must have a density lower than that of seawater.
How can this be done?
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