Specialized epithelial cells involved in the maintenance of osmotic homeostasis. They are characterized by abundant mitochondria and ion transporters. In amniotes, they are present in the renal system. In freshwater fish, ionocytes in the skin and gills help maintain osmotic homeostasis by absorbing salt from the external environment. [ http://www.ncbi.nlm.nih.gov/pubmed/19268451 http://www.ncbi.nlm.nih.gov/pubmed/17555741 ]