Gill morphology of two killifish species acclimated to the extreme euryhaline environment: A histological and ultrastructural analysis (Actinopterygii: Aphaniidae)

Abstract Killifishes (family Aphaniidae) in southern Iran demonstrate long‐time acclimation to extreme euryhaline environments. This study provides a detailed histological and ultrastructural characterization of gill morphology in two sympatric endemic aphaniids, Aphaniops furcatus and Aphaniops hormuzensis , inhabiting an ecologically extreme environment in southern Iran. Specimens were collected from the Khurgu hot‐sulphuric spring in the Hormozgan River System which drains into the Persian Gulf, and examined using haematoxylin and eosin (H&E) staining and scanning electron microscopy (SEM). Both species exhibited gill structures typical of teleosts, with primary and secondary lamellae essential for efficient gas exchange. In A. furcatus , gills displayed variations linked to size and gender, with numerous secondary lamellae and a variety of epithelial cells, including squamous, chloride, mucus, blood, and pillar cells. Air exposure led to interlamellar cell masses (ILCM) growth, reducing the lamellae surface area. In A. hormuzensis , gill arches featured a double row of primary filaments with secondary lamellae facilitating oxygen exchange, supported by pillar cells and dense distribution of mucous cells throughout the epithelium. The study highlights the adaptive mechanisms of A. furcatus in response to fluctuating environmental conditions and underscores the evolutionary conservation of gill morphology among teleosts. These findings provide crucial insights into respiratory efficiency and ecological adaptability, emphasizing eco‐evolutionary acclimation to extreme aquatic habitats.