How do marine bony fish remove excess salt from their bodies?

Marine bony fish live in an environment where the surrounding water has a higher salt concentration than their internal body fluids. Consequently, they face a constant risk of water loss through osmosis and an influx of excess salts.123

To maintain homeostasis, these fish employ the following mechanisms:

• Drinking Seawater: To compensate for the water lost to their environment, marine bony fish drink large quantities of seawater.123
• Active Salt Excretion: Because their kidneys are unable to produce urine more concentrated than their blood, they must rely on their gills to remove the excess salt ingested from seawater. Specialized cells in the gills, known as chloride cells, actively transport sodium and chloride ions out of the body and into the surrounding sea.231

This process of active transport is essential for survival in saline environments, though it requires a significant expenditure of energy to move ions against the diffusion gradient.12