Abstract
AbstractHexatrygon fishes of Taiwan are represented by three species in the previous records, including H. taiwanensis、H. brevirostra and H. yangi respectively, which have been found on the north-eastern and south-western coast of Taiwan. Nowadays the classification of the three species depends mainly on the external morphology: the posterior tip of tail of H. yangi is naked, and snout angle is 45 degree. H. taiwanensis and H. brevirostra do not have a naked tail. H. brevirostra is similar to H. taiwanensis but distinguishable by having large percentage of snout length and disc width, longer snout and shorter tail. Because of the difficulty in catching Hexatrygon fishes, and the great differences between juveniles and adults, which give rise to the controversy of classification. Combining a total evidence of morphological features and 12S mtDNA data, the results follow (1) According to the newly recorded species of Hexatrygon longirostra, it increases to four species of Hexatrygon in Taiwan. However, the phylogenetic analyses show that four species have a high similarity in 12S rRNA sequence. The difference in genetic distance is very limited. It shows that the fishes of Hexatrygon in Taiwan are synonyms. (2) Snout is a limited organ for classification. The shape of snout, including width and thick, may be a perceptible feature for classification. But the snout angles of Hexatrygon overlap at interspecies level. (3) Besides the snout, other morphologic characters such as total length and disc width do not have obvious difference at interspecies level. Like snout, those characters fail to be the useful traits for classification.