Geographic Variation of Acoustic and Morphometric Traits in Ryukyu Kajika frog (Buergeria japonica)

Delimitating species is a central concern in evolutionary biology, taxonomy, and biological conservation. Acoustic and morphological traits play key roles in frogs, especially in species recognition and reproduction. Buergeria japonica is widely distributed from Ryukyu Archipelago to Taiwan. Recent studies indicated the extremely high genetic differentiation with non-monophyletic relationship among populations in Taiwan, which revealed the necessity to reevaluate the taxonomic status of this species across their distributional range. In this study, we sampled advertisement calls from 17 localities and morphology from 13 localities across Ryukyu and Taiwan to examine the variation among populations. By analyzing their advertisement calls, we found an exclusively unique type of long call occurring in the southwestern and eastern populations in Taiwan. Moreover, these populations also represent faster call duration and shorter call rise time compared to those from the other regions. On the other hand, the dominant frequency in the northwestern populations in Taiwan was higher than the others. Morphological measurements with discriminant analysis indicated that the species should be separated into two forms, which correspond to Taiwanese clade (the southwestern and eastern populations in Taiwan) and Japanese clade (Ryukyu and the northwestern populations in Taiwan) defined by mitochondrial sequences. Principal component analysis showed that traits on the head from Taiwanese clade are significantly higher than the other. These results indicated that there is a cryptic species in Taiwan, which represents different acoustic signal but with similar morphology from B. japonica sensu stricto.