Comparison of Head and Skull Shapes among Native Andrias japonicus, Introduced A. davidianus, and Their Hybrids in Japan (Urodela: Cryptobranchidae) based on Geometric Morphometrics

The Japanese giant salamander, Andrias japonicus, is one of the largest extant amphibians. It is endemic to Japan and has been designated as a special natural monument by the Japanese government. The genus Andrias is also protected by the Convention on International Trade in Endangered Species of Wild Fauna and Flora. The Japanese giant salamander and one of the congeners, the Chinese giant salamander, A. davidianus, were recently found to hybridize in several areas in Japan, due to human-mediated introduction of A. davidianus. In this study, we found major differences in head and skull shapes between the Japanese and Chinese giant salamanders, using geometric morphometrics. The hybrids showed intermediate morphology between the two species, but also possessed a wider mouth, at the skeletal level, than either of the parent species. This unique characteristic of hybrids is suggested as one of the reasons for the current dominance of the hybrids in the introduced areas and may be an example of heterosis in urodeles.