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 isn’t 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 isn’t 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 doesn’t 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.
Now that you know how a fish drinks water, the next question would be about other creatures that spend their time in the sea. Birds like albatrosses and petrels can spend more than a year at sea, and whales and seals live in the ocean 24/7/365. How can they avoid being poisoned by the salt? We’ll discuss that tomorrow.
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.
— John N. Clayton © 2020
Data from National Wildlife magazine June/July 1995, pages 30-34.