In marine vertebrates, osmoregulation in gills is primarily mediated by which cells?

• A. Mucous cells
• B. Chloride cells (ionocytes)
• C. Goblet cells
• D. Osteocytes

Answer: (B)

Chloride cells, also known as ionocytes, are the gill cells specialized for osmoregulation in marine vertebrates. They carry the ion-transport machinery that moves Na+ and Cl− from the blood into the seawater, allowing the fish to excrete excess salts in a hypertonic environment. The basolateral Na+/K+-ATPase pump creates the necessary electrochemical gradients, while chloride exits into the water through apical chloride channels. Mucous and goblet cells mainly produce mucus to protect the gill surface, and osteocytes are bone cells, not involved in gill salt regulation.