Olfactory Organ of Anemonefishes of the Genus Amphiprion (Amphiprioninae, Pomacentridae)

Abstract—

The structure of the olfactory organ is studied in five species of anemonefishes of the genus Amphiprion: A. polymnus, A. clarkii, A. frenatus, A. perideraion, and A. ocellaris. All fish have one nostril, olfactory rosette of the arrow-shaped type with no secondary folding on olfactory lamellae, and two ventilation sacs (ethmoidal and larger lacrimal). The rosette is located on the medial (A. clarkii and A. perideraion) or ventromedial (A. polymnus, A. frenatus, and A. ocellaris) sides of the olfactory cavity; the number of lamellae in the rosette increases as a fish grows. The greatest total number of lamellae is 24 in A. polymnus. Intercalary (in four species) and dichotomous (in three species) lamellae are found in the rosette of most anemonefishes, which is an extremely rare case in other fishes. Atypical lamellae emerge later than regular ones in the ontogeny of the fish, but their number in different parts of the rosette differs between anemonefishes, and in some A. polymnus species, lamellae are predominantly atypical. The slope of the medial and lateral sides of the rosette toward the openings of the ventilation sacs (A. polymnus and A. frenatus) is considered a structural adaptation that improves water exchange near the surface of the olfactory lamellae and reception of odor information by the fish. The relationship between the structure of the olfactory organ and the generality of anemonefishes, their specialization, and strength of association with symbiotic sea anemones has not been revealed. The idea of an evolutionary trend from a simple structure of the olfactory organ (A. clarkii) to a morphologically more complex structure (A. polymnus and A. frenatus) in the genus Amphiprion after early separation from the general stem of the subgenus Actinicola (A. ocellaris and A. percula) is presented.