The principle problem with Pachytriton taxonomy historically, has been the assumption that color and pattern were reliable means of distinguishing taxa. As is often the case, that's false. It has been known for a couple of decades that the genus occurs in separate pockets, most notably P.labiatus in the southwest, “P.labiatus” in the northeast, and P.brevipes in the middle. The discovery of P.archospotus effectively split the range of P.brevipes in two.
A recent study of color and genetic variation from throughout the range of Pachytriton identified three major genetic groups, each being limited to one geographic region. Both mitochondrial and nuclear DNA was used, thus making it possible to identify cases of hybridization.
Clade A
In the southwest was the most genetical different population, the one which owns the name P.labiatus. This is a population of robust brown animals which sometimes have red lines. Nothing else can be called P.labiatus, since all other populations are more closely related to P.brevipes, and P.brevipes is the oldest name.
Clade B
In the inland center (Hunan) is the second most distinct group, corresponding in color, distribution, and anatomy to P.archospotus. This species has straight epibranchial bones and is brown with black spots. Other differences from the other species are more subtle, and the species description did not distinguish between the two distinct forms being called P.labiatus when making comparisons.
Clade C
The third genetic group consists of the eastern populations, including both typical P.brevipes and the slender animals commonly called P.labiatus and sold in the pet trade. There are several distinct subgroups, one of which corresponds to black-spotted tan P.brevipes, with or without red stripes (Mt. Junfeng, Mt. Wuyi). Specimens from Mt. Daiyun are geographically and morphologically grouped with P.brevipes, and are tan, but they are distinctive genetically. Mitochondrially, they are unique, but group with “P.labiatus”. Nuclear DNA is even more distinct, setting them apart from both populations. Remaining populations are slender animals predominantly from Zhejiang, and they group together in morphology, geography, mtDNA, and nuDNA. However, they include all color forms. These animals dominate the pet trade within China and internationally (collected from Hangzhou and Chengdu).
The status of P.granulosus
The problems with P.granulosus are these:
1) The type description was based on immature specimens, which thus do not show typical adult morphology.
2) The type specimens are lost.
3) A neotype was designated from the type vicinity, but this ALSO appears to be immature, and was furthermore not available for genetic testing.
Regardless of these issues, the name Pachytriton was validly proposed, and assigned to animals which were a) described, b) illustrated, c) from a specific locality, and d) from a locality where similar animals still occur. The type locality of P.granulosus is Jietouzhen, Tiantai City, Zhejiang. This is within the range of the slender pet trade animals of clade C and thus corresponds to the “unnamed” forms commonly confused with P.labiatus. Examination of Hypselotriton orientalis, Pachytriton brevipes, Pachytriton “labiatus” of all ages from Anhui and Zhejiang, and Paramesotriton chinensis, shows that the new specimens of “Pingia granulosa” correspond to juvenile Pachytriton, as does Chang's original description. The appropriate name for Zhejiang slender Pachytriton is P.granulosus. This is not affected by the need to examine the neotype of “Pingia granulosa”, although a NEW neotype would need to be designated if the former proves to be a valid species of Hypselotriton [which would then need to be re-described with a new name].
There is no evidence of microsympatry, and little evidence of interbreeding or introgression in any Pachytriton. At least not recently - The Mt. Daiyun specimens show different relationships acording to mtDNA than according to nuDNA. However, both DNA sets are distinctive. I would interpret this as a sign of ancient hybridization followed by independent evolution. The current isolation from P.granulosus populations would seem to prevent any continued introgression. Although otherwise included by morphology and geography with P.brevipes, I suspect that this population probably represents an unnamed species.
Mt. Dapan P.granulosus and Mt. Wuyi P.brevipes are relatively close geographically to one another. Mt. Dapan specimens are P.granulosus in morphology, distribution, and mtDNA, but are closer to Mt. Wuyi P.brevipes in nuDNA. Mt. Wuyi specimens are P.brevipes in all traits, but one specimen groups with P.granulosus by nuDNA. I would interpet this as indicating a narrow hybrid zone along the boundaries of the two species, though seemingly a rare occurence. This would require more genetic markers and a more extensive survey in southern Zhejiang and northern Fujian to determine better.
Variation
Overlap in coloration is believed due to a shared ancestral coloration (brown). The P.brevipes/P.archospotus line evolved a tan color, which has since been lost by P.granulosus. This is consistent with epibranchials being ancestrally straight in Asiatic newts, curved in Pachytriton, but reverting to being straight in P.archospotus. The hypothesis of intergrades has been safely rejected, as all variation is confined to populations which are genetically and geographically distinct. Where there is evidence of introgression, populations are consistent in color and morphology with the overall species [Mt. Wuyi are tan, Mt. Dapan are brown, Mt. Daiyun are tan; all variation can be found in unequivocal P.granulosus populations].
The phenotypes recognized in the pet trade over the years are mainly age-related. Small and young specimens tend to have bold ventral colors, which fades and blurs with age. All four of the main color themes can be found in the small areas of Zhejiang and Chengdu southwest of Shanghai, which supply the pet trade and are P.granulosus. Occasional specimens of P.brevipes may also reach the trade from nearby areas. Most pet trade newts seem to originate in this area, which drastically reduces the number of species available recently. The only other phenotypes have been determined to likely be Paramesotriton ermizhaoi and possibly another Paramesotriton. The latter are in a jar here somewhere, pending shipping off for testing.
In short, there are few reliable ways of identifying pet torrent newts. The main method is circumstancial - if most specimens are slender and brown, then likely all are P.granulosus, regardless of color. If H.orientalis and P.chinensis are also commonly available in the trade in your area, then it is likely that one supplier is involved, using a source in Shanghai that also supplies P.granulosus from the same region. P.archospotus could be identified by x-ray. P.brevipes could possibly be identified by careful measurements, with x-rays to rule out P.archospotus. P.labiatus could probably be identified by careful measurements.
Sources
Nishikawa, K., Jiang, J.-P., Matsui, M., & Chen, C.-S. (2009). Morphological variation in Pachytriton labiatus and a re-assessment of the taxonomic status of P. granulosus (Amphibia: Urodela: Salamandridae). Current Herpetology 28(2): 49–64
Wu, Y., Wang, Y., Jiang, K., Chen, X. & Hanken, J. (2010). Homoplastic evolution of
external colouration in Asian stout newts (Pachytriton) inferred from molecular phylogeny. Zoologica Scripta 39: 9–22.