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Have you ever wondered how a fish drinks water? Your first reaction is probably something like, “It opens its mouth.” Like most things in life, it is not that simple.
All living things necessarily have some saltwater content in their bodies to keep chemical balance, allowing life to exist. The fluids inside an ocean-dwelling fish are only about a third as salty as the ocean itself. The water inside the fish's body tends to leave by osmotic pressure, which is the tendency of fluids to move through membranes toward higher concentrations. To avoid this loss of water, the fish does simply open its mouth and drink seawater. But that brings large amounts of salt into the fish's body. The salt concentration would be more than the fish's kidneys could handle. To aid the kidneys, the gills of ocean fish are designed to expel salt, so the fish is not pickled by it.
In freshwater fish, the osmotic pressure is reversed, so the fluids inside the fish are saltier than the water outside. The skin of a freshwater fish is designed so that water seeps in through its skin and gills. Therefore, the fish does not have to drink at all. When a salmon leaves the ocean and enters a freshwater stream, it merely stops drinking. Like freshwater fish, it depends on its skin to bring in its water needs.
God's design of life includes fitting living things with specialized equipment to survive in every environment. Fish are remarkable creatures specially equipped for the waterworld in which they live.
Source: National Wildlife, June/July 1995, page 30.
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