A new species of freshwater crab of the genus Nanhaipotamon Bott, 1968 (Crustacea, Decapoda, Brachyura, Potamidae) from Longhai, Fujian Province, China

Abstract A new species of freshwater crab of the genus Nanhaipotamon Bott, 1968 is described from Xiaye Village, Chengxiang Town, Longhai County, Zhangzhou City, Fujian Province, China. The new species is distinguished from congeners by the combination of characters of its carapace, third maxilliped, unequal chelipeds, triangular male abdomen and unique male first gonopod. Molecular evidence derived from partial mitochondrial 16S rRNA and COI genes also support the species as new.

In 2019, during a survey of freshwater crab resources in Longhai, Fujian Province, the first author collected several specimens of the genus Nanhaipotamon. In August 2020, we made another collection trip to obtain additional samples. After morphological comparison, we found the Longhai specimens to be distinct from known species of Nanhaipotamon. Molecular evidence based on the 16S rRNA and COI genes also support it as new. Therefore, we herein describe a new species, Nanhaipotamon longhaiense sp. nov.

Materials and methods
Specimens were collected from Longhai, Fujian Province by Mao-Rong Cai and preserved in 95% ethanol. The holotype and allotype were deposited at the Department of Parasitology of the Medical College of Nanchang University, Jiangxi, China (NCU MCP). Other examined materials were deposited at the Center for Disease Control and Prevention of Zhangzhou City, Zhangzhou, China (ZZCDC) and the National Tropical Disease Research Center, Shanghai, China (TDRC). Carapace width and length were measured in millimeters. The abbreviations G1 and G2 refer to the first and second gonopod. The terminology used herein primarily follows that of Dai (1999) and Davie et al. (2015).
Chelipeds strongly unequal. Merus cross-section trigonal, inner-lower margin crenulated. Carpus surface weakly wrinkled, with longitudinal depression and sharp Ambulatory legs slender, second leg longest, merus 0.5-0.6 times as long as carapace length; last leg with propodus 2.1 times as long as broad, slightly shorter than dactylus. Dactylus gently curved, with sharp spines on the surface (Figs 2C, 3A).
Etymology. The new species is named after the county where is located, Longhai County, Zhangzhou City, Fujian Province, China.
Distribution. Longhai County, Zhangzhou City, Fujian Province, China. Ecology. The new species occurs in the wetlands of low-elevation hills and mountains, amongst dense vegetation where there is little to no water flow year-round (Fig.  7B). During the day, the crabs usually hide in mud burrows close to the water source (Fig. 7A) or hide under rocks under water. We observed a berried female in August, suggesting the time around this month to be a part of the breeding season (Fig. 3C).
Remarks. With a convex carapace dorsal surface, unequal chelipeds and triangular male abdomen, Nanhaipotamon longhaiense sp. nov. fits the diagnosis of Nanhaipotamon. Like some species within this genus, N. longhaiense sp. nov. shows intraspecific variation in G1 morphology, the distal margin of the G1 terminal segment is flat to oblique (Fig. 6A-C). In the holotype, the distal margin is flat (Fig. 6A), whereas in some adult specimens, the distal margin is oblique (Fig. 6B, C), and the inner margin of the G1 terminal segment is slightly convex to distinctly convex ( Fig. 6A-C).
We make comparisons between the new species and seven species of Nanhaipotamon, among which N. wuping and N. macau are morphologically similar to this new species, N. yongchuense, N. huaanense and N. nanriense are geographically close (Dai 1999), and N. guangdongense and N. hepingense are from Guangdong near Fujian (Dai 1999). Nanhaipotamon longhaiense sp. nov. can be differentiated from its congeners by its unique G1 (Fig. 5A). Compared to N. longhaiense sp. nov., which has a semicircular G1 terminal segment inner distal angle, N. guangdongense, N. hepingense, N. yongchuense, N. nanriense, and N. huaanense differ in having instead a bluntly triangular G1 terminal segment inner distal angle (Fig. 5B-F). The G1 terminal segment inner distal angle is also semicircular in both N. wupingense and N. macau (Huang et al. 2018a); however, the terminal segments in these two species are proportionately larger. In N. macau, the G1 terminal segment distal margin is sinuous to V-shaped (cf. Huang et al. 2018a: fig. 5D, E). In N. wupingense, the G1 terminal segment distal margin is sinuous to an inverte V-shaped (cf. Cheng et al. 2003: fig. 7;Huang et al. 2018a: fig. 6D). In N. longhaiense sp. nov., however, the G1 terminal segment distal margin is flat to oblique (Fig. 6A-C). The detailed differences between the new species and congeners are presented in Table 2.

Phylogenetic analyses
In this study, we obtained the partial mitochondrial 16S rRNA and COI genes from specimens of Nanhaipotamon collected from Xiaye Village, Chengxiang Town, Longhai County, Fujian Province, China. A total of 37 546 bp 16S rRNA gene sequences and 22 658 bp COI gene sequences were used to construct the BI and ML trees. The topological structures of the 16S rRNA and COI trees are similar. Both trees show that N. longhaiense sp. nov. and 11 other species of Nanhaipotamon are clustered into one clade (Figs 8,9). In the 16S rRNA tree, four sequences of N. longhaiense sp. nov. form a small branch within Nanhaipotamon, while in the COI tree, the N. longhaiense sp. nov. clade and N. wupingense are sister to each other, indicating a close phylogenetic relationship between N. longhaiense sp. nov. and N. wupingense. The pairwise distances between the 12 species of Nanhaipotamon were calculated based on the COI gene. The result shows that the pairwise genetic distances between Nanhaipotamon range from 0.0239 to 0.1552 (Table 3), while distances between N. longhaiense sp. nov. and its congeners are from 0.0880 to 0.1423. Therefore, the genetic distance is large enough to support N. longhaiense sp. nov. as new. Both the phylogenetic position and genetic divergences provide evidence supporting the recognition of N. longhaiense sp. nov. as a new species.

Discussion
Nanhaipotamon is endemic to China and mainly distributed in the low-elevation coastal areas or islands in southeastern China. Due to the isolating effect of mountain   Dai, 1997, CB 05141 C N. hepingense, Dai, 1997, CB 05106 D N. yongchuense, Dai, 1997, CB 05104 E N. nanriense, Dai, 1997, CB 05103 F N. huaanense, Dai, 1997, CB 05105. ranges, Nanhaipotamon is restricted to an area east of the Wuyishan Range and south of the Nanling Range (Shih et al. 2011). With 18 species, including N. longhaiense sp. nov., species diversity in Nanhaipotamon is the highest among sympatric genera (Longpotamon, Somanniathelphusa, Huananpotamon, Bottapotamon, Minpotamon, Heterochelamon, Cantopotamon, Cryptopotamon, Eurusamon, Yarepotamon, Yuebeipotamon) except Geothelphusa. Huananpotamon is followed by Nanhaipotamon, with 15 species distributed on both sides of the Wuyishan Range (Fujian and Jiangxi Provinces) (Shih et al. 2011;Chu et al. 2018). While all the other sympatric genera consist of fewer than 10 species. Therefore, Nanhaipotamon has important value as part of the regional biodiversity.
In the morphological classification of freshwater crabs, the G1 character provide important morphological identification features (Dai 1999). Intraspecific variation in G1 morphology has been reported in some species of Nanhaipotamon, such as N. guangdongense from different localities (Huang et al. 2012;Huang et al. 2018a). In N. longhaiense sp. nov., intraspecific variation of G1 morphology was also found. Several questions have arisen due to G1 intraspecific variation: Dai (1997) described N. hepingense and N. pinghense, both from Heping County, Guangdong Province. Shih et al. (2011) provided molecular evidence that they are synonymous and many scholars agree with this (Huang et al. 2012;Chu et al. 2018). Huang et al. (2012) described N. zhuhaiense in Zhuhai, Guangdong Province, where N. guangdongense is also found. Later, Huang et al. (2018a) indicated that N. zhuhaiense and N. guangdongense are probably conspecific, but they did not have sufficient material on which to take taxonomic action. These problems were caused by intraspecific variation, which makes it difficult to classify species based    on morphology alone. Therefore, when describing a new species of this genus, it is recommended that morphological classification be combined with molecular analysis. There are likely other problems with some species in this genus, and therefore a revision is necessary.

Conclusion
In this article, we report a new species of Nanhaipotamon collected from Xiaye Village, Chengxiang Town, Longhai County, Fujian Province, China. Nanhaipotamon longhaiense sp. nov. can be distinguished from congeners by the combination of carapace, third maxilliped, and male first gonopod characters. Molecular evidence based on the mitochondrial 16S rRNA and COI genes also support it as a new species of the genus Nanhaipotamon.