They absorb a controlled amount of water through the mouth and the gill membranes. Due to this intake of water, they produce large quantities of urine through which a lot of salt is lost. The salt is replaced with the help of mitochondria-rich cells in the gills.

Why is osmoregulation important in fishes?

Regardless of the salinity of their external environment, fish use osmoregulation to fight the process of diffusion and osmosis and maintain the internal balance of salt and water essential to their efficiency and survival.

How do fish maintain balance?

To maintain their water balance, marine fishes drink large quantities of seawater, retaining most of the water and excreting the salt. Most nitrogenous waste in marine fishes appears to be secreted by the gills as ammonia. Marine fishes can excrete salt by clusters of special cells (chloride cells) in the gills.