Main airway throughout the nasal cavity of green sea turtles is lined by keratinized stratified squamous epithelium

•Sea turtle nasal cavity consisted of vestibule, main part and nasopharyngeal duct.•The vestibule and nasopharyngeal duct were all lined by keratinized epithelium.•Sensory epithelia in the main part transited into respiratory and then keratinized.•Keratinization of upper airway seemed a result of adaptation to marine life style. Structural and histological features of the nasal cavity of sea turtles are largely different from those of other Testudines species. The sea turtle nasal cavity is a pair of tubular structures with three diverticula and an excavation in the center, and three types of sensory epithelium are present in these four significant structures. To more clarify the adaptation of the nasal cavity to marine life style in sea turtles, non-sensory epithelium in the nasal cavity of green sea turtles (Chelonia mydas) were histologically determined from nostril to choanae in this study. Unlike many other animals including terrestrial turtles, the vestibular area and nasopharyngeal duct were all lined by keratinized stratified squamous epithelium. In the main nasal cavity, the margins of each sensory epithelium turned into respiratory epithelium with goblet cells, followed by keratinized stratified squamous epithelium. Keratinized epithelium appears more appropriate in sea turtle upper airway to protect against osmotic pressure when they release seawater through the nostrils, and thus this histological feature of upper airway might reflect adaptation to marine life style.