Do Fish Drink Water?

What is the airspeed velocity of an unladen swallow?

–The Bridgekeeper

Life, as we currently understand it, depends on water. Perhaps, somewhere in the universe, a form exists that does not require the liquid; but, on our pale, blue orb, anything alive needs dihydrogen monoxide to survive.

You and I largely require ingesting liquid to hit our water necessities. Some creatures don’t need to drink water to persist, such as the sand cat. These types of critters still necessitate water; they simply get their fill from the food they consume.

But what about those beasts whose home is the water itself? Do fish drink water? Are they constantly gulping their surroundings?

The answer is yes.

The answer is also no.

The answer is it depends on where the fish lives.

This ambiguity arises thanks to a topic you might recall from a biology class: osmosis. This phenomenon occurs when a selectively permeable membrane, such as skin or cell walls, separates two areas. Osmosis is the spontaneous movement of solvent or solute molecules through the membrane, a process that equalizes the concentrations of solvents. Imagine a membrane separating two regions that both contain water and different concentrations of salt. If water can pass through the membrane, it will do so until the concentration of the salt is the same on both halves.

Humans do not have to worry much about osmosis in our outer layer. Fish, however, are living osmotic systems.

Consider the freshwater fish. Blood and fluids inside a freshwater fish are much saltier than the water that surrounds them. Therefore, water flows into their bodies via osmosis across their skin and their gills. So, despite the fact that these fish are surrounded by perfectly potable fluid, they do not need to drink water! The water that enters their mouths when they eat does not go to the digestive system. Instead, they let it flow back to the ambient system. When it comes to water, freshwater fish live the easy life. Just chill and life-sustaining water flows into you.

The saltwater fish doesn’t have the same luxury. Their blood and fluids are less salty than the oceans, which means they have a big problem. Water actively leaves their bodies, hoping to equalize the solvents across the membrane. To compensate for this situation, saltwater fish drink water in copious amounts. As you probably know, drinking saltwater from the ocean will eventually kill a human. Yet fish manage to survive on a steady diet of salt.

How?

Fish are fantastic osmoregulators. Osmoregulation is the process by which an organism maintains hydro-homeostasis. Since osmosis happens naturally, most organisms can’t just roll the dice. Without osmoregulation, the saltwater fish would die via shriveling, as the water seeped from their bodies. Conversely, osmosis running rampantly in a freshwater fish would cause it to bloat into a water-filled balloon, eventually bursting.

Fish have special cells that continuously pump salt ions into or out of their blood, depending on the situation. Freshwater fish constantly add salt, whereas saltwater fish spew it out. In the case of saltwater fish, they also have hale kidneys, which filter a lot of the salt they drink.

One fish, however, towers above all the others when it comes to osmoregulation. Salmon famously inhabit both saltwater and freshwater zones. They spawn in freshwater streams and rivers but spend most of their lives in the ocean. Before they die, they return to the spot they were born to produce offspring. How do they manage to adapt to the polarizing forces of osmosis?

Incredibly, salmon have evolved cells that can reverse how they pump salt ions. Starting life in freshwater, the salmon pumps ions into the blood and produces dilute urine. When they reach the fresh-salt border, they cannot immediately switch the function of the important cells. Instead, they spend a certain amount of time in brackish water, slowly acclimatizing to the salt. They start to drink water that contains salt, pushing slowly into areas with higher saline concentrations. The cells adapt. Suddenly, ions are pumped outward and the fish begins to excrete small amounts of highly concentrated urine. When it’s time to return to the freshwater, the process happens again. The fish remain in border waters for days or weeks until their cells can revert to pumping ions into their blood.

In Monty Python and the Holy Grail, the Bridgekeeper attempts to trick King Arthur with a seemingly unanswerable question. “What is the airspeed velocity of an unladen sparrow?” In the film, an incorrect answer would doom Arthur to death. But Arthur realized the question wasn’t so straightforward; he flipped the question on the old man: African or European swallow?

If you ever encounter a Bridgekeeper, who asks, “do fish drink water?” you’ll know to respond like Arthur. Saltwater or freshwater fish? The answer depends, unless, of course, you’re dealing with salmon!