Skin ionocyte remodeling in the amphibious mangrove rivulus fish (Kryptolebias marmoratus)

Amphibious fishes have evolved a variety of physiological modifications allowing them to survive in water and air. In air, the amphibious mangrove rivulus, Kryptolebias marmoratus, uses its skin as a site of ionoregulation. Skin ionocytes actively transport ions into/out of the body; however, it is unclear if there are specific morphological or functional changes occurring in skin ionocytes during air exposure. We used two microscopy techniques to describe skin ionocyte morphology and to investigate their plasticity after salinity challenges and air exposure. Immunohistochemical staining in air-exposed fish revealed ionocytes with Na ⁺ /K ⁺ ATPase (NKA), Na ⁺ /H ⁺ exchanger (NHE3b) and cystic fibrosis transmembrane conductance regulator (CFTR) immunoreactivity, whereas ionocytes from aquatic fish had only NKA (freshwater) or NKA and CFTR (brackish and hypersaline water). Following salinity challenges, we noted increases in the number and area of ionocyte apical surfaces, indicating that skin ionocyte activity increased in high salinity environments compared with control conditions. Furthermore, we show increased ionocyte area during air exposure suggesting increased ionocyte activity in all salinity conditions. Using energy dispersive X-ray spectroscopy to analyze the skin surface, we report decreases in magnesium, phosphorous, and sulfur after 7 days in air compared with fish in water, suggesting ionic movement in the skin surface during air exposure. Our study highlights morphological and functional features of skin ionocytes that are involved in ionoregulation in an air-exposed amphibious fish.