 Monovalent ions (Na+, Cl−, K+) are excreted by the gills, so their U/P ratios in the urine are below 1.The excretion of inorganic ions by structures other than kidneys is called the extrarenal salt excretion.In southern bluefin tuna and other marine teleosts, specialized ion-transporting cells called ionocytes (previously known as mitochondrion-rich cells and chloride cells) is the primary sites of NaCl excretion
Ionocytes are usually found on the gill arch and filament, though in some cases can be also found on the gill lamellae when exposed to various environmental stressors.