A new species of freshwater crab of the genus Qianguimon Huang, 2018 (Decapoda: Brachyura: Potamidae) from Guangxi, Southern China

A new species of freshwater crab of the genus Qianguimon Huang, 2018, is described from Guangxi Zhuang Autonomous Region, southern China. It can be distinguished from congeners by the following characters: male first gonopods bent inward at about 45° at base of terminal segment, carapace regions distinct and rugged and the female vulva opening inwards and downwards. In addition, molecular evidence derived from the 16S rRNA gene supported the species described in this study as a new species of Qianguimon.

Qianguimon is a genus established by Huang (2018), with four species have been reported at present. The type species Q. aflagellum was originally described as Isolapotamon aflagellum by Dai et al. (1980) from Zhaoping, Guangxi Zhuang Autonomous Region. Afterwards, Huang (2018) recorded two additional localities for this species from Mengshan and Chengzhong, Guangxi, and placed it in the genus Qianguimon. Huang (2018) also reported another two new species of this genus: Q. splendidum from Yanghe, Guangxi and Q. elongatum from Leishan, Guizhou Province. Wang, Huang & Zou (2019) subsequently described the fourth species: Q. rongxianense from Rong, Guangxi. The prominent feature of this genus is the boot-shaped terminal (Hasegawa, Kishino & Yano, 1985), determined by MEGA ver.X.0 (Kumar et al., 2018) based on the Bayesian information criterion (BIC). The ML tree was built based on 1,000 bootstrap replicates in MEGA ver.X.0 (Kumar et al., 2018). The pairwise estimates of Kimura 2-parameter (K2P) distances (Kimura, 1980) among the five species of Qianguimon were calculated using MEGA ver.X.0 (Kumar et al., 2018). The electronic version of this article in portable document format will represent a published work according to the International Commission on Zoological Nomenclature (ICZN), and hence the new names contained in the electronic version are effectively published under that Code from the electronic edition alone. This published work and the nomenclatural acts it contains have been registered in ZooBank, the online registration system for the ICZN. The ZooBank LSIDs (Life Science Identifiers) can be resolved and the associated information viewed through any standard web browser by appending the LSID to the prefix http://zoobank.org/. The LSID for this publication is: urn:lsid: zoobank.org:pub:7BFE0C18-76EE-483C-9B5F-C0143C5B6A16. The online version of this work is archived and available from the following digital repositories: Peer J, PubMed Central, and CLOCKSS. Diagnosis. Carapace broader than long, regions distinct, anterolateral regions rugose; cervical groove and H-shaped groove deep, distinct; epigastric cristae conspicuous, postorbital cristae sharp. External orbital angle narrowly triangular, separated from anterolateral margin by gap; epibranchial teeth distinct; anterolateral margin lined with conspicuous granules. Third maxilliped merus median depression indistinct, exopod with vestigial flagellum. Chelipeds slightly unequal; outer surfaces of chelae smooth; fingers with very small gap when closed. Male pleon narrowly triangular, lateral margins gently concave; telson triangular, somite 6 transversely trapeziform. Male sterno-pleonal cavity very deep, median longitudinal suture of sternites 7/8 deep and relatively long.   (Fig. 1B). Sub-orbital regions covered with scattered rounded granules; sub-hepatic regions and pterygostomial regions covered numerous large granules (Fig. 1B). Epistome posterior margin narrow; median lobe triangular, lateral margins oblique (Fig. 1B).

Systematics
Third maxilliped merus about 1.3 times as broad as long, trapezoidal, median depression indistinct; ischium about 1.5 times as long as broad, rectangular, with distinct median sulcus; exopod reaching approximately 1/5 of merus length, with vestigial flagellum; dactylus not reaching the upper edge of ischium (Fig. 1C).
Chelipeds slightly unequal ( Fig. 2A). Merus cross-section trigonal, with inner-lower margin crenulated ( Fig. 2A). Carpus surface weakly wrinkled, with prominent sharp spine at inner-distal margin (Fig. 1A). Outer surfaces of chelae pitted, palm of larger chela about 1.2 times as long as high ( Fig. 2A). Movable finger approximately as long as the immovable finger; inner margin of fingers with numerous round and blunt teeth; fingers forming inconspicuous gap when closed ( Fig. 2A).
Ambulatory legs slender; the second pair longest and last pair shortest (Fig. 1A). Merus longest, without subdistal spine or tooth; carpus stout, dorsal margin with cristae (Fig. 1A). The fourth leg propodus about 1.9 times as long as broad, slightly shorter than dactylus; dactylus sharp, with several spines and numerous setae on the surface (Fig. 2E). G1 very slender, dorsal and ventral surface smooth, lateral margin without seta, terminal segment boot-shaped, distinctly sinuous, bend inward at a 45 angle medially, with blunt sub-distal projection (Figs. 4A-4D and 5A); tip of terminal segment exceeding sternites 4/5 suture (Fig. 2D); subterminal segment about 2.0 times as long as terminal segment. G2 elongate, almost equal to G1 in length; basal segment about 2.2 times length of distal segment, basal segment sub-ovate (Fig. 4E). Groove for G2 located medially on the ventral side of G1 subterminal segment, thin setae on distal regions of G1 subterminal segment (Fig. 4A).
Remarks. The new species is similar to other species Qianguimon, in its carapace broader than long, postorbital and epigastric cristae visible; exopod of the third maxilliped with short or no flagellum, male pleon triangular; G1 generally slender, terminal segment boot-shaped with sub-distal projection; vulvae medium-sized and reaching proximal three-quarters width of sternite 6. But Q. yuzhouense n. sp. can be differentiated from congeners by its regions distinct and dorsal surface rugged, narrowly triangular and sharp external orbital angle, blunt and broadly triangular epibranchial tooth, G1 very slender and bent inward at about 45 at base of terminal segment, tip exceeding sternites 4/5 suture in situ, female vulva opening inward at a 45 angle. Other differences are listed in Table 2 and Fig. 5.
Etymology. The new species is named after the type locality, Yuzhou District, Yulin City, Guangxi Zhuang Autonomous Region, China. Living color. Most of the carapace is dark brown. The chelipeds are brown to orange, while the ambulatory legs are brown. The overall color is consistent with the surrounding environment ( Figs. 6A and 6B).
Ecology. This species was found in a stream next to a mountain road. The stream has no obvious flowing water, and has lush weeds growing in it. We found the crab burrows by removing the weeds. The burrows are sandy and without much soil. We found the crabs after digging about 10 cm into the burrows (Figs. 6C and 6D).

Phylogenetic analyses
In this study, we obtained the 16S rRNA molecular data of three specimens collected from Yuzhou District, Yulin City, Guangxi Zhuang Autonomous Region, China. The alignment sequences were downloaded from GenBank and include 26 species from 18 genera of the subfamily Potamidae Ortmann, 1896 from Asia. The access numbers can be found in Table 1. We used the BI and ML methods to construct the phylogenetic tree. The topological structure of the trees showed a high degree of consistency (Fig. 7). The three mitochondrial 16S rRNA gene fragments of the new species are very close genetically, with the pairwise genetic distances zero (Table 3), which indicates that they are sequences from the same species and are consistent with the results of the morphological study. The new species are clustered together with Q. rongxianense, Q. aflagellum, Q. elongatum and Q. splendidum, and form an independent branch in the clade "Southern China" (Huang, Ebach & Ahyong, 2020), indicating that the five species are congeners of the genus Qianguimon. The minimum interspecific pairwise K2P genetic distances of the new species and other congeners is 0.037507 (Table 3).

DISCUSSION
Qianguimon yuzhouense n. sp. has the diagnostic features of Qianguimon, such as visible postorbital and epigastric cristae and male first gonopod generally slender with boot-shaped terminal segment (Huang, 2018). In this study, we collected mitochondrial 16S rRNA gene molecular data for all species of the genus, and based on this, established BI and ML phylogenetic trees. Phylogenetic analysis showed that the five species of the genus formed an independent branch. Both phylogenetic tree and genetic distances suggest that Q. yuzhouense is a new species. There are three clades within Qianguimon, however, support for these clades is not high. Considering the shared generic characters of these species, we believe that these species all belong to the same genus. The new species is found in Yuzhou District of Yulin City, Guangxi Zhuang Autonomous Region, which is within the distribution of Qianguimon. The other four reported species of this genus are all distributed in southern Guizhou Province or eastern Guangxi Zhuang Autonomous Region (Huang, 2018;Wang, Huang & Zou, 2019). In summary, the species reported in this paper is a new species of Qianguimon that is supported by molecular data, morphology and biogeography.

CONCLUSIONS
In this article, we reported a new species of freshwater crab from Yuzhou District, Yulin City, Guangxi Zhuang Autonomous Region, China. We found that it fits well within the definition of Qianguimon Huang, 2018, morphologically, and our molecular analysis also supports it as a new species of the genus Qianguimon.