Morphometric Variation Analyses and Revision of the Japanese Toads (Genus Bufo, Bufonidae)

Abstract

Two problems have been involved in the classification of the Japanese toads: (1) relationships with the continental forms, and (2) relationships among forms hitherto described within the islands. The present study was undertaken with the purpose of resolving the latter problem, and to achieve this, morphometric variation analyses were made of a large number of preserved specimens. The results were synthesized with the hitherto accumulated nonmetrical and non-morphological information so as to taxonomically revise the Japanese toads. A historical review of the classification of the Japanese toads revealed that the most complex problems were involved in the still-water breeding forms from the main islands (=the Japanese common toad), and that the taxonomic confusion was caused mainly by previous authors' classification through examination of a small number of specimens notwithstanding the great variability of external morphometric characters of the toads. The critical points for measurements for each character, and problems with measurement errors due to variant fixation and preservation are discussed. Allomorphic relations of 30 characters to SVL were analyzed for four age/sex groups of 147 Japanese common toads from Momoyama, Kyoto, with the result that many characters should be treated separately for three groups, i.c., young, adult males, and adult females. Besides, it was determined that simple ratios to SVL should not be calculated for many characters. For the Japanese common toad, SVL and relative value (ACV=theoretical value calculated by the allomorphic relationship of each character to SVL) of 10 selected characters for each of 2, 525 specimens belonging to 96 populations were analyzed, and the presence or absence of intrapopulational age and sexual variations and of interpopulational variations was noted. From these analyses, the Japanese common toad was divided into northeastern (A) and southwestern (B) types chiefly by the relative size of the tympanum diameter. For the two types of the Japanese common toad, presence of morphometric clines in relation to geographical and/or climatological parameters was investigated. Clear clinal tendencies were found mostly in type A, whereas few clear trends were detected in type B. Taxonomical meaning of the clines was discussed, and it was suggested that the Japanese common toad should be split into taxa below the species level. Similar morphometric analyses were made on the still-water breeding Miyako toad from the Ryukyu Archipelago. The results for 176 specimens from 4 populations indicated that this form was very uniform in external morphometry, that it greatly differed from the Japanese common toad in several characters, and that the differences were greater between this form and geographically adjacent type B Japanese common toads in some characters. Similar analyses were also performed on 265 specimens of 13 populations of the stream-breeding Japanese stream toad. Interpopulation variation in this form was very small. Comparisons with sympatric and allopatric populations of the Japanese common toad were made with the result that the two forms were found to be morphometrically almost completely separated, showing greatest differentiation in the zone of sympatry. Taxonomic conclusions were drawn with these results in mind. For the Japanese common toad, the type series or topotypes of each of the hitherto described forms were compared with the other populations and it was determined that neither type A nor type B can be split further. The syntypes of Bufo vulgaris japonicus, which is the oldest of all the described forms from Japan and is now kept in Leiden, were examined and a lectotype was designated and described. This form clearly belonged to type B. The nature of the intergradation of morphometric variations between type A and type B is discussed, and special attention is paid to the sudden change in the morphocline in the tympanum diameter. From this discussion, the relation of the two types is regarded as subspecific, and they are treated as B. japonicus formosus (type A) and B. j. japonicus (type B). For the Miyako toad, the morphometric difference from the Japanese common toad in the adjacent distribution range was studied, and the two forms are regarded as belonging to the different lineages. In addition, the known results of artificial hybridization support this idea, and therefore, the form is regarded as specifically different from japonicus and is treated as a subspecies (miyakonis) of the northeastern Chinese species B. gargarizans. The Japanese stream toad has been shown to have strong genetic compatibility with the Japanese common toad. Therefore, from the viewpoint of evolutionary taxonomy, the taxonomic value of the evidence from genetical sources is discussed. The two forms are distributed sympatrically, but are judged to seldom interbreed or show morphological intergradation in nature. Thus, notwithstanding the strong genetic compatibility of the artificial hybrids, the Japanese stream toad is regarded as a full species, B. torrenticola, as was originally described. In conclusion, the Japanese toads are taxonomically divided into three species, Bufo japonicus (j. japonicus Schlegel, 1838 and j. formosus Boulenger, 1883), B. gargarizans miyakonis Okada, 1931, and B. torrenticola M. Matsui, 1976.