Hyper-regulators: life in fresh water
Freshwater habitats are filled with abundant animal life from numerous phyla. Because the osmotic concentration of fresh water is extremely low compared to both extracellular and intracellular fluids, all freshwater animals are hyper-regulators. The influx of water across the integument presents a difficult and costly physiological challenge. Freshwater animals have evolved numerous mechanisms for producing very dilute urine as a means of ridding themselves of this excess water. A second challenge is the requirement to obtain ions from an environment in which ionic concentrations are exceptionally low. Ion uptake is achieved by specialized transport processes in both the integument and the gut. Examples from both invertebrate and vertebrate groups are provided to illustrate the physiological and morphological diversity that exists in freshwater organisms.
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