Do fish drink? The answer depends on the fish type and its environment. Like all living organisms, fish must maintain a delicate balance of water and salts within their bodies to survive. Their methods for achieving this balance are diverse adaptations.
The Basic Challenge of Living in Water
All fish face osmoregulation, the process of maintaining an internal balance of salt and water. A fish’s body is largely fluid, encased in a semi-permeable membrane like skin and gills, constantly interacting with its fluid environment. This interaction is governed by osmosis, the natural movement of water across a semi-permeable membrane from higher to lower water concentration.
The internal fluids of a fish, specifically its blood, contain dissolved salts. Whether the fish lives in freshwater or saltwater, there is always a difference in salt concentration between its body and the surrounding water. This difference creates an osmotic pressure, causing water to constantly move across the fish’s permeable surfaces, particularly the gills, which are crucial for gas exchange but also susceptible to water movement. This continuous movement of water and salts presents a physiological hurdle that fish must overcome to maintain internal stability.
How Freshwater Fish Manage Water
Freshwater fish encounter an environment where the water outside their bodies is considerably less salty than their internal fluids. This means their bodies are “saltier” than the surrounding water, leading to a continuous osmotic influx of water into their cells and a tendency for salts to diffuse out. To counteract this constant influx, freshwater fish do not actively drink water. Instead, they absorb water passively through their skin and gills.
To manage the excess water, freshwater fish have highly efficient kidneys. These kidneys produce large volumes of very dilute urine, effectively expelling the surplus water. Simultaneously, to combat the loss of essential salts through diffusion and urination, freshwater fish possess specialized cells in their gills. These cells actively absorb ions like sodium and chloride from the surrounding water, transporting them into the bloodstream to maintain the necessary internal salt concentration. This active salt uptake and copious urination enable their survival in a dilute environment.
How Saltwater Fish Manage Water
Saltwater fish face the opposite challenge; they live in an environment far saltier than their internal body fluids. This osmotic gradient causes water to constantly leave their bodies and diffuse into the saltier external environment, leading to a risk of dehydration. To compensate for this continuous water loss, saltwater fish actively drink large amounts of seawater.
However, drinking salty water introduces an excess of salt into their bodies. Saltwater fish have specialized mechanisms to excrete this surplus salt. Their gills contain chloride cells, which efficiently pump out excess sodium and chloride ions into the surrounding seawater. Additionally, their kidneys play a role in expelling other excess ions like magnesium and sulfate. The kidneys of saltwater fish produce very small amounts of highly concentrated urine, helping them conserve water while eliminating waste products and excess salts. This combination of drinking seawater, active salt excretion via gills, and minimal, concentrated urine production allows saltwater fish to maintain their internal water and salt balance.