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Osmoregulation is necessary for euryhaline teleosts in maintaining internal ion and water balance in response to environmental salinity changes. In the epithelia of teleostean gills, the ionocytes (i.e. mitochondrion‐rich cells) are thought to be the primary site of osmo/ionoregulation. In the ionocytes, the Na+, K+‐ATPase (NKA) plays a crucial role in triggering ion‐transporting systems. Asian sea bass (Lates calcarifer) is a marine euryhaline teleost and an important economic species in Taiwan, Southeast Asia, and Australia that can be cultured in environments from seawater (SW) to fresh water (FW). However, the modulatory mechanisms of osmoregulation in gills of euryhaline Asian sea bass were not clear to date. Hence the purposes of the present study were to investigate changes in physiological parameters, morphology of apical openings of gill ionoctyes, as well as expression and localization of branchial NKA in the SW‐ and FW‐acclimated Asian sea bass. The results showed that the sea bass was able to maintain their internal physiological parameters including plasma osmolality and muscle water content in a tolerated range. Observed by the scanning electron microscope, different types of apical surfaces of ionocytes with distinct features were found in the afferent epithelia of gills in FW and SW milkfish. On the other hand, the specific activity of branchial NKA was lower in the FW milkfish compared to the SW individuals. The immunohistological observation further revealed that most NKA‐immunoreactive cells in gills of the FW‐ and SW‐acclimated Asian sea bass were distributed in the afferent and interlamellar regions of the filaments. Taken together, these results illustrated that the Asian sea bass was an efficient osmoregulator with these salinity‐dependent alterations to maintain homeostasis of ions and water in environments of different salinities. These findings contributed to the knowledge of phenotypic plasticity of ionocytes and changes in activity of NKA, the crucial enzyme for ionoregulation, in gills of the Asian sea bass upon salinity challenge.Support or Funding InformationThis study was supported by a grant to THL from the Ministry of Science the Technology (MOST), Taiwan (105‐2313‐B‐005‐027‐MY3).This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.