Cutaneous respiration and osmoregulation in amphibious fishes

In his early career, August Krogh made fundamental discoveries of the properties of cutaneous respiration in fish, frogs and other vertebrates. Following Krogh's example, the study of amphibious fishes provides an excellent model to understand how the skin morphology and physiological mechanisms evolved to meet the dual challenges of aquatic and terrestrial environments. The skin of air-exposed fishes takes on many of the functions that are typically associated with the gills of fish in water: gas exchange, gas sensing, iono- and osmoregulation, and nitrogen excretion. The skin of amphibious fishes has capillaries close to the surface in the epidermis. Skin ionocytes or mitochondrial-rich cells (MRCs) in the epidermis are thought to be responsible for ion exchange, as well as ammonia excretion in the amphibious mangrove rivulus Kryptolebias marmoratus. Ammonia gas (NH₃) moves down the partial pressure gradient from skin capillaries to the surface through ammonia transporters (e.g., Rhcg) and NH₃ is volatilized from the mucus film on the skin. Future studies are needed on the skin of amphibious fishes from diverse habitats to understand more broadly the role of the skin as a multifunctional organ.