Characterization of the Adaptive Morphology of Japanese Stream Toad (Bufo torrenticola) Using Geometric Morphometrics

The order Anura (frogs and toads) is a group of amphibians and contains over 6500 extant species living in a variety of environments. Each frog species evolved body form adaptive for living and breeding in their own habitats. In Japan, four taxa of Bufo are living: Western-Japanese common toad (Bufo japonicus japonicus), Eastern-Japanese common toad (B. japonicus formosus), Miyako toad (B. gargarizans miyakonis) and Japanese stream toad (B. torrenticola). The former three taxa breed in still water as many other species of Bufo do. In contrast, B. torrenticola breeds in running water such as streams in mountainous area. Corresponding to their breeding in a stream environment, both adult and larva of B. torrenticola acquired unique morphological characters. However, few study have explored differences in the body form between B. torrenticola and closely-related Bufo species quantitatively, remaining the details about the morphological adaptation to a stream environment in this toad species poorly understood. In this study, we quantitatively compared the shape of the foot and skull between the adult male of B. torrenticola and its close relative B. j. formosus using landmark-based geometric morphometrics. Our analyses revealed that B. torrenticola has relatively longer toe phalanges with relatively larger foot webs and relatively shorter metatarsals and a narrower and more streamlined skull, compared to closely-related B. j. formosus. These morphological characteristics are considered adaptive for their breeding in mountain torrents.