Do Fish Ever Drink?

On a hot, sunny day, downing a bottle of water does a lot to rehydrate you. You might wonder if fish, surrounded by water, even need to drink it. Water in a fish̵7;s body is regulated through a process called osmoregulation. Whether they live in freshwater or salt water determines if fish actually need to drink to stay hydrated.

  1. Osmoregulation

    • Balancing the proper amounts of water and salt in a fish̵7;s body is a process called osmoregulation. Water can move in and out of a fish̵7;s body via osmosis, the movement of water through a cell wall. If the concentration of water is greater outside the fish than inside, water will move into the cells until the concentration of water is the same. All fish need a certain amount of water and salt in their bodies. Since the concentration inside the fish is different from the concentration in its environment, it̵7;s a constant battle to keep levels healthy.

    Freshwater Fish

    • Freshwater fish don̵7;t actively drink water through their mouths the way humans do. These fish have more salt inside their bodies than the surrounding water does. This means the water has a lower concentration of solvents and is hypotonic to the fish̵7;s blood. Water wants to move into the fish to dilute the salt to the same concentration as the outside water. Because water moves into the fish̵7;s body, they don̵7;t need to actively drink it. The excess water is dispelled via the fish̵7;s kidneys. They also have specialized chloride cells that allow them to absorb electrolytes, like salt, directly from surrounding water.

    Saltwater Fish

    • Fish that live in a saltwater environment have the opposite problem. In comparison to their blood, the water around them is hypertonic. They have a lower concentration of salt in their bodies than exists in the surrounding water, so they tend to lose water from their bodies. To compensate, marine fish will actively drink water from their environments, which leads them to ingest excess salt. The same chloride cells that absorb electrolytes in freshwater fish allow saltwater fish to release excess salt from their bodies.

    Fish Gills

    • Osmosis doesn̵7;t occur in a fish̵7;s skin as though it were a thin membrane. It occurs in a fish̵7;s gills. Whether it̵7;s a marine or freshwater fish, water is transferred in or out of the body as water flows over the gills. Fish gills have a network of capillaries, or tiny blood vessels, that absorb not only electrolytes and water, but oxygen as well. All fish do have water flowing into their mouths, but freshwater fish push it back out over the gills, while saltwater fish will take a sip.