Effects of Salinity on Gill Ultrastructure of Alligator Gar Atractosteus spatula

The alligator gar, Atractosteus spatula (Lacépède, 1803), is a euryhaline, bimodal breather, occupying a wide range of habitats. The gills and swim bladder are both respiratory organs, creating a potential trade-off for fish inhabiting saline environments where gills must also be used to facilitate osmo- and ionoregulation. Therefore, to understand whether gill adaptations in alligator gar differ from other euryhaline basal fishes and teleosts, alligator gar were acclimated to fresh water (salinity: 0; n = 5) or saline water (salinity: 20; n = 5) for over 4 weeks and gill ultrastructure and localization of the key sodium and proton motive ATPases [Na⁺, K⁺-ATPase (NKA) and vacuolar-type H⁺-ATPase (VHA)] were evaluated. Gills were analyzed by scanning and transmission electron microscopy to evaluate the external surfaces and internal cellular structures of epithelial cells, and by immunofluorescence microscopy to identify ionocyte types. In alligator gar acclimated to fresh water, ionocytes extended beyond pavement cells, contained numerous microvilli and mitochondria, and were generally located on the gill filaments at the base of lamellae. Following acclimation to saline water, ionocytes were recessed compared to adjacent pavement cells, contained numerous mitochondria, and were generally located on the gill filament epithelium at the base of lamellae and in interlamellar spaces. In the gills, there were numerous NKA immunoreactive cells in saline-acclimated fish, with fewer in freshwater-acclimated fish. Notably, VHA-immunoreactive cells were present in both salinity groups and occurred in a separate, less abundant subpopulation than those containing NKA. Therefore, the alligator gar has similar gill adaptations as many teleosts for salinity acclimation, with the presence of VHA resembling other basal euryhaline actinopterygians.