Here's why:
* Osmosis: Water moves from areas of high water concentration to areas of low water concentration, across a semi-permeable membrane.
* Hyperosmotic: Means having a higher solute concentration (and therefore lower water concentration) than the surrounding environment.
* Hypoosmotic: Means having a lower solute concentration (and therefore higher water concentration) than the surrounding environment.
Saltwater fish live in an environment with a higher solute concentration than their internal fluids. This means water naturally wants to move out of their bodies (via osmosis) to try to reach equilibrium with the ocean. To counteract this, saltwater fish have evolved mechanisms to:
* Drink seawater: This allows them to ingest salts to maintain osmotic balance.
* Excrete excess salt: They have specialized cells in their gills that actively pump out excess salt.
* Produce concentrated urine: This helps to conserve water and excrete excess salts.
By doing these things, saltwater fish can maintain a stable internal environment even though they live in a hyperosmotic environment.