TerrestrialVersusAquatic Biology: The Medium and Its Message

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Abstract
Many basic biological functions are constrained by the physical properties of the fluids in which organisms live. Here I explore four selected examples in which physical differences between air and water have contributed to the functional divergence of terrestrial and aquatic organisms. 1. Water is about 800 times as dense as air. As a result, while the cost of locomotion is generally less for aquatic organisms, the hydrodynamic forces they encounter are larger. 2. The combined effects of density and viscosity insure that the capture of suspended particles is mechanically more effective on land than in water. 3. The speed of sound is four times greater in water than in air, requiring aquatic organisms to use higher frequency sounds in echolocation systems. 4. The resistivity of air is 16 orders of magnitude larger than that of seawater, which might explain why aquatic animals use electrical sensing organs to detect prey but terrestrial animals do not.

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