Why do marine bony fish drink seawater?

Marine bony fish drink seawater to counteract the continuous loss of water from their bodies caused by osmosis.12

Because the salt concentration in the surrounding seawater is higher than that within the fish's body, water naturally diffuses out through permeable surfaces like the skin and gills. To maintain homeostasis and prevent dehydration, these fish drink large quantities of seawater.231

To manage the resulting salt intake, marine bony fish utilize specialized physiological mechanisms:
* Kidneys: They excrete the excess salts while retaining as much water as possible, producing very little urine.12
* Gills: They possess clusters of special cells, known as chloride cells, that actively pump excess salts out of the body and into the surrounding sea.23

This process of osmoregulation is essential for their survival in a saline environment, as it allows them to balance their internal water and salt levels despite the constant osmotic pressure.12