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. Ionocytes are interspersed between pavement cells which occupy the largest proportion of the gill epithelium. Ionocytes are highly metabolically active, as indicated by the large number of mitochondria (which produce energy in the form of ATP). They are also rich in Na+/K+ ATPases, in comparison to other cells. Ionocytes have an elaborate intracellular tubular system, continuous with the basolateral membrane (facing blood). The apical side (facing the environment) is typically invaginated below the surrounding pavement cells, forming apical crypts. Leaky paracellular pathways exist between the neighbouring ionocytes.