Discovery of a new cryptic Achalinus Peters, 1869 (Serpentes Xenodermidae) species from Hunan Province, China

A new species, Achalinus sheni sp. nov., from central Hunan Province is described, based on the results of molecular systematics and morphological characters according to five specimens. Our molecular phylogeny inferred from the mitochondrial CO1 gene fragment revealed that this new species is most closely related to A. yunkaiensis, but a considerable amount of genetic divergence exists between them (p-distance ranging from 5.8% to 6.4%) and much distinct genetic divergence exists compared with other known Achalinus species (p-distance ranging from 10.4% to 15.8%), supporting its validity. Morphologically, it can be distinguished from its congeners by: (1) dorsal scales strongly keeled, 23 rows throughout the body, the outmost row smooth and significantly enlarged; (2) tail relatively short, TaL/TL 0.183 similar to 0.224; (3) the suture between internasals subequal to the suture between prefrontals; (4) loreal one, subrectangular, LorH/LorL 0.53 similar to 0.57; (5) ventrals 161-170, anal entire, subcaudals 55-61, not paired; (6) the length of supraocular equal to or longer than the length of upper anterior temporal; and (7) vertebral line inconspicuous and subcaudal streak absent. Currently, 27 species of Achalinus are known in the world, amongst which 20 species are distributed in China. Moreover, a key to species of the genus Achalinus is provided in this study.