Atypical Structure of Olfactory Organ in Moon Wrasse Thalassoma lunare and Sixbar Wrasse T. hardwicke (Labridae).

The macromorphology of olfactory organ was studied in moon wrasse Thalassoma lunare and sixbar wrasse T. hardwicke. There are two nostrils in the olfactory organ. Thin walls of the anterior tubular nostril can easily close, the posterior nostril contains a valve. The olfactory rosette is absent and is replaced by the olfactory disk and vertical membrane, morphology structures of the olfactory organ previously unknown in fish. The disk is located on the rostral bottom part of olfactory cavity. Low crest-folds cover the disk surface that are probably the vestigial structures of primary olfactory folds. The expressiveness of crest-folds, disk thickness and shape are different in the moon wrasse and sixbar wrasse. In both species only one large ventilation lacrimal nasal sac is present adjacent below the olfactory cavity with the entrance at the cavity bottom caudal from the disk. A scheme of ventilation of the olfactory cavity is introduced. The possibility of receiving the olfactory information is discussed for wrasses staying in the ground at the time of avoidance of danger. [ABSTRACT FROM AUTHOR]