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How Do Saltwater Fish Deal With Osmosis?

| Updated September 26, 2017

Saltwater fish live in a natural environment where the levels of salt are much higher than they have in their bodies. Because salt is an agent that attracts and draws out water, the surrounding salt water draws water from inside the fish so they must constantly drink water to replace the lost water in their system. The process of water being drawn out of the body by the surrounding water of higher salt levels is known as osmosis. Both saltwater fish and freshwater fish rely on osmosis and diffusion to keep their tissues in healthy condition.

Water for Replacement

School of fish, underwater view
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Salt water fish are perfectly adapted to their salty environment and need osmosis to live. The replacement fluid taken on to replace the lost water is desalinated by a process known as diffusion. Diffusion allows fish to live in a state of constant osmosis.

If a saltwater fish is placed in an environment that does not contain the excess levels of salt outside, such as in freshwater, their natural inclination to pull in water and dispel all salt from that water will result in the fish exploding with water that is not being removed from them by their environment.

Water Overload

Salmon Leaping, River Tay, Scotland, UK
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Freshwater fish, on the other hand, are subjected to a reverse effect. The salt levels inside their bodies are higher than the surrounding water outside because that water is constantly being drawn inward by the salt content in their system. Freshwater fish do not drink water as a result. If placed in a saltwater environment, they would literally drown in a sea of water because they are not geared to pull in extra water.