Life in Water
Can a fish crawl out of water? Are there animals that never drink? Can a frog survive being frozen solid? Since life on Earth began in water some 3.5 billion years ago, living organisms have evolved an amazing variety of techniques for surviving different water conditions. Water is one of life's essential ingredients. And life exists wherever there's water. Only in the driest parts of Chile's Atacama Desert, where rain hasn't fallen for decades, is life nearly absent. Everywhere else, animals and plants use water of all sorts, salty or fresh, hot or cold, plenty of water or almost no water at all.
Flooded With Water
You might think that freshwater fish have it easy, at least where water is concerned. But actually, fish have too much of a good thing: Water is constantly flowing into their bodies as they use their gills to breathe. To prevent their tissues from becoming too watery, their kidneys continually drain water into their urine. At the same time, these fish also must actively pump salt in from the water. Saltwater fish have the opposite problem--they have to bring in water while pumping out salt.