Which physiological mechanism do fish use for osmoregulation in saltwater environments?

In saltwater environments, fish face the challenge of osmoregulation due to the higher salinity of the surrounding water. To maintain their internal fluid balance, marine fish primarily use the mechanism of excreting excess salts through their gills. Their gills have specialized cells known as chloride cells, which actively transport excess ions, particularly sodium and chloride, out of their bodies. This process allows the fish to rid themselves of the surplus salts while conserving water, which is a critical adaptation for survival in the hyperosmotic conditions of seawater.

The method of salt excretion through gills is crucial because marine fish tend to lose water to their environment through osmosis, requiring them to drink seawater to prevent dehydration. By effectively excreting salts, they maintain an appropriate internal osmotic balance and ensure their physiological processes can function optimally. Other options such as filtering water through their gills, absorbing water through the skin, or producing dilute urine are not applicable in the context of saltwater fish, as these mechanisms do not align with the requirements for osmoregulation in high-salinity environments.