Morphological features of the nasal cavities of hawksbill, olive ridley, and black sea turtles: Comparative studies with green, loggerhead and leatherback sea turtles.

We analyzed the internal structure of the nasal cavities of hawksbill, olive ridley and black sea turtles from computed tomography images. The nasal cavities of all three species consisted of a vestibule, nasopharyngeal duct and cavum nasi proprium that included anterodorsal, posterodorsal and anteroventral diverticula, and a small posteroventral salience formed by a fossa of the wall. These findings were similar to those of green and loggerhead sea turtles (Cheloniidae), but differed from those of leatherback sea turtles (Dermochelyidae). Compared to the Cheloniidae species, the nasal cavity in leatherback sea turtles was relatively shorter, wider and larger in volume. Those structural features of the nasal cavity of leatherback sea turtles might help to suppress heat dissipation and reduce water pressure within the nasal cavity in cold and deep waters.

Nasal Cavity of Green Sea Turtles Contains 3 Independent Sensory Epithelia.

The morphological and histological features of the nasal cavity are diverse among animal species, and the nasal cavities of terrestrial and semiaquatic turtles possess 2 regions lined with each different type of sensory epithelium. Sea turtles can inhale both of volatile and water-soluble odorants with high sensitivity, but details of the architectural features and the distribution of the sensory epithelia within the sea turtle nasal cavity remain uncertain. The present study analyzed the nasal cavity of green sea turtles using morphological, computed tomographic, and histological methods. We found that the middle region of the sea turtle nasal cavity is divided into anterodorsal, anteroventral, and posterodorsal diverticula and a posteroventral excavation by connective tissue containing cartilages. The posterodorsal diverticulum was lined with a thin sensory epithelium, and the anterodorsal and anteroventral diverticula were occupied by a single thick sensory epithelium. In addition, a relatively small area on the posteroventral excavation was covered by independent sensory epithelium that differed from other 2 types of epithelia, and a single thin bundle derived from the posteroventral excavation comprised the most medial nerve that joins the anterior end of the olfactory nerve tract. These findings suggested that the posteroventral excavation identified herein transfers stimuli through an independent circuit and plays different roles when odorants arise from other nasal regions.