Physiological Attributes of Freshwater Marine and Terrestrial Animals

When we apply a physiological criterion to the problem of water balance, we find, surprisingly, that the physical world is divided not into just two environments, aquatic and terrestrial, but three: freshwater aquatic, marine or saltwater aquatic, and terrestrial. Each has associated with it a particular suite of physical challenges and physiological adaptations, which can mark an animal as "belonging" in one of those habitat types (Box 7A).

To illustrate, let us take a by now familiar example: kidneys. Generically, kidneys are organs that maintain the proper content of water and salts within the body. As we saw in Chapter 2, the water inside animals' bodies frequently differs in composition from the waters they live in. In accordance with the Second Law, a difference in solute concentration will drive fluxes of salts and water between an animal and its environment. To do their jobs, kidneys and the other water balance organs must do physiological work against these thermodynamically favored fluxes of salts and water. A freshwater animal must produce large quantities of very dilute urine, so that it may offset a large osmotic influx of water and diffusional loss of salts. An

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