Most organisms are about as dense as water, roughly 1,000 kg m-3. Consequently, organisms living in water are supported by buoyant forces and have little need for supporting structures like bones or shells. Where supports do exist in aquatic animals, they usually are for some other purpose, like protection (in the case of a shell) or locomotion.
In air, obviously, the buoyant forces are much weaker and structures capable of supporting the organism's weight are essential. These structures do not come free. Cellulose, which is the principal supporting structure of plants, is essentially glucose, which must be manufactured by photosynthesis. The chitins that support the bodies of arthropods likewise are largely made of sugars. The production of mineralized supporting structures, like bones or shells, also incurs energy costs in the gathering and transport of the minerals. Finally, the bodies of some organisms, like earthworms, are supported by the internal pressure of fluids, which forms a so-called hydrostatic skeleton. Maintaining the high internal pressures needed to operate a hydrostatic skeleton incurs the costs of powering the heart muscles.
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