Morphology of urohyal bone and its importance in taxonomy of some fishes of the South basins of Caspian Sea

The urohyal bone is a single median solid bone with anterior tip generally connected to the ventral hypohyal and to the first basibranchial and the posterior end attached to the pectoral girdle by means of muscle. This bone has been generally used for age determination، but recently it has been distinguished that its morphology is diverse in different groups of fishes such that could be used in fish taxonomy. In the present study، the morphology of urohyal in 19 species fishes from 17 genera، 6 families and 5 orders of Caspian Sea basin has been studied. Results showed that the urohyals were varied in ventral، dorsal and lateral shape، presence and number of process in the anterior part، origin of dorsal keel (distance from anterior tip)، groove in ventral portion and some other attributes. In studied cyprinid fish، the ventral part of the urohyal forms a triangular facet، which may be with or without groove. All the members of Leuciscinae have the same urohyal with two-branched anterior part. But in Cyprininae، it is not two-branched. Base on the morphology of urohyal in different groups of studied fish، it seems that Acheilognathinae، Squalliobarbinae and Leuciscinae are close to each other that the other subfamilies. In Gobiidae and Esocidae، the urohyal lack ventral sides or it is reduced. Based on the urohyal bone morphology we prepared a pictorial key for identification of some fishes of Caspian Sea basin for the first time.