The question of what fishes drink is complex, as different species solve this physiological problem in opposite ways. Fish are constantly surrounded by water, yet they must maintain a specific internal concentration of water and dissolved salts to function. This continuous interaction with their fluid environment requires a delicate balancing act for survival. Understanding how fish manage this internal chemistry involves the process of osmoregulation.
Osmoregulation: The Biological Challenge
All living organisms must maintain a stable internal environment, a process known as homeostasis. For fish, this involves keeping the concentration of salts and water within a narrow range, handled by osmoregulation. Osmosis is the passive movement of water across a semi-permeable membrane, such as the gills, moving from an area of lower solute concentration to an area of higher solute concentration. Water will naturally move to dilute the side with more dissolved salts.
Fish blood and body fluids contain electrolytes, commonly referred to as salts. If internal fluids are too diluted, cells will swell; if too concentrated, cells will shrink. The semi-permeable membranes of the gills are especially vulnerable because they must be thin enough for gas exchange (breathing). This makes the gills a primary site for uncontrolled water and salt movement, forcing fish to use energy to actively regulate their internal balance.
Marine Fish: Why They Must Drink
Marine fish live in saltwater, an environment much saltier than their internal body fluids. Because their blood has a lower solute content than the surrounding seawater, they constantly lose water passively through their gills and skin. This osmotic gradient creates a continuous risk of dehydration, meaning they are constantly thirsty.
To replace this lost water, most marine bony fish actively drink large amounts of seawater, sometimes consuming up to 35% of their body weight daily. Drinking saltwater introduces a massive load of excess salt into their system. Specialized cells in the gills, known as chloride cells, actively pump monovalent ions like sodium and chloride out of the blood and into the surrounding water.
Their kidneys are adapted to conserve water by producing very little urine. The kidneys primarily excrete divalent ions, such as calcium and magnesium. Meanwhile, the gills handle the majority of sodium chloride removal. This dual system allows them to replenish lost water while efficiently eliminating the excess salt ingested.
Freshwater Fish: How They Handle Water Intake
Freshwater fish face the opposite problem of marine species. They live in water far less salty than their internal fluids, making them hyperosmotic to their environment. This concentration difference causes water to constantly flow into the fish’s body through the semi-permeable membranes of the gills, creating a continuous risk of water logging.
Because they constantly absorb water passively, freshwater fish rarely drink the surrounding water. Their main biological challenge is to eliminate this constant influx of water without losing internal salts. They accomplish this using highly efficient, well-developed kidneys that are large relative to their body size.
These kidneys produce a large volume of very dilute urine, quickly flushing out the excess water. To counteract the inevitable salt loss that occurs through diffusion and urination, they also use specialized cells in their gills to actively absorb ions from the water. These gill cells use energy to scavenge salts like sodium and chloride from the surrounding low-salt water, ensuring their internal balance is maintained.