The southern bluefin tuna acquires its water by drinking seawater: its only available water source. Since the osmotic pressure of the fluids in the tuna must be hyposmotic to the seawater that has been taken up, there is a net loss in ions from the tuna. Ions diffuse across their concentration gradient from the fluids of the tuna to the external seawater. The result is a net movement of water into the fluid of the bluefin tuna, with the net movement of ions being into the seawater. Southern bluefin tuna, along with other marine teleost fish, have acquired a variety of proteins and mechanisms which allow the secretion of ions through the gill epithelium.