Adaptive evolution characteristics of mitochondrial genomes in genus Aparapotamon (Brachyura, Potamidae) of freshwater crabs.

BACKGROUND: Aparapotamon, a freshwater crab genus endemic to China, includes 13 species. The distribution of Aparapotamon spans the first and second tiers of China's terrain ladder, showing great altitudinal differences. To study the molecular mechanisms of adaptive evolution in Aparapotamon, we performed evolutionary analyses, including morphological, geographical, and phylogenetic analyses and divergence time estimation. We sequenced the mitogenomes of Aparapotamon binchuanense and Aparapotamon huizeense for the first time and resequenced three other mitogenomes of Aparapotamon grahami and Aparapotamon gracilipedum. These sequences were combined with NCBI sequences to perform comparative mitogenome analysis of all 13 Aparapotamon species, revealing mitogenome arrangement and the characteristics of protein-coding and tRNA genes. RESULTS: A new species classification scheme of the genus Aparapotamon has been detected and verified by different aspects, including geographical, morphological, phylogenetics and comparative mitogenome analyses. Imprints from adaptive evolution were discovered in the mitochondrial genomes of group A, including the same codon loss at position 416 of the ND6 gene and the unique arrangement pattern of the tRNA-Ile gene. Multiple tRNA genes conserved or involved in adaptive evolution were detected. Two genes associated with altitudinal adaptation, ATP8 and ND6, which experienced positive selection, were identified for the first time in freshwater crabs. CONCLUSIONS: Geological movements of the Qinghai-Tibet Plateau and Hengduan Mountains likely strongly impacted the speciation and differentiation of the four Aparapotamon groups. After some group A species dispersed from the Hengduan Mountain Range, new evolutionary characteristics emerged in their mitochondrial genomes, facilitating adaptation to the low-altitude environment of China's second terrain tier. Ultimately, group A species spread to high latitudes along the upper reaches of the Yangtze River, showing faster evolutionary rates, higher species diversity and the widest distribution.

Freshwater crabs could act as vehicles of spreading avian influenza virus.

Avian influenza virus (AIV) possessed significant risk to various animals and human health. Wild birds, especially waterfowls are considered to be the natural reservoir of AIVs. The ecology of AIV is still far from being fully understood. Freshwater crabs are nonnegligible biotic factor in AIV ecosystem. We analyzed the ability of freshwater crabs accumulate and spread AIV. We found that AIV remain infectious in water only for 36 h but persist in crabs for 48 h. Crabs could accumulate AIV in their gills and gastrointestinal tracts. The AIV titers in crabs were higher than the surrounding contaminated water. Crabs could accumulate AIV from contaminated water, carry the virus and spread to naïve crabs via surrounding water. Our study identified freshwater crab as a novel transmission vehicle in AIV ecosystem.

The mitochondrial genome of Huananpotamon koatenense (Rathbun, 1904) (Brachyura, Potamidae) and phylogenetic analysis.

Freshwater crabs play essential roles in the well-functioning of the inland aquatic ecosystems. However, due to the lack of sufficient molecular resources, the study of freshwater crabs has been greatly hindered. In this study, the mitochondrial genome of Huananpotamon koatenense, a freshwater crab endemic to China, was sequenced for the first time. This mitogenome sequence is 15,528 bp long, and contains 13 protein-coding genes, 2 rRNA genes and 22 tRNA genes. Phylogenetic analyses based on 25 mitogenomes showed that H. koatenense was clustered with the known congeneric species of H. lichuanense.

[Investigation of the population diversity of intermediate host snails and crabs of Paragonimus along Jiulong River, Zhangjiang River and Dongxi River basins in southern Fujian Province].

OBJECTIVE: To investigate the population distribution of intermediate host snails and crabs of Paragonimus along the Jiulongjiang River, Zhangjiang River, and Dongxi River basins in Bopingling Mountain, southern Fujian Province, so as to provide baseline data for researches on parasitic disease prevention and control and enlargement of samples in the parasitic resource bank. METHODS: A total of 23 villages in 8 counties (districts) along the Jiulong River, Zhangjiang River, and Dongxi River basins in Zhangzhou City, Fujian Province were selected as survey sites during the period from November 2020 through March 2023, and snail and freshwater crabs were sampled from 1 to 3 streams and ditches neighboring residential areas in each village. Morphological identification of snails was performed according to the external morphological characteristics of collected snail shells, and the unidentified snail species sampled from the natural foci of paragonimiasis in Yunxiao County were subjected to se-quence analysis of the mitochondrial cytochrome oxidase 1 (CO1) gene. The crab species was identified by observing the morphological characteristics of the terminal segment of the first pleopod of male crabs, and Paragonimus cercariae and metacercariae were detected in collected snails. RESULTS: The shells of the unidentified snails sampled from the natural foci of paragonimiasis in Yunxiao County were approximately 50 mm in height and 18 mm in width, thick and solid, long tower cone-shaped, and had 8 to 10 whorls. CO1 gene sequence analysis identified the snail species as Sulcospira hainanensis. A total of 6 freshwater snail species belonging to 5 genera within 3 families, identified 23 survey sites, including Semisulcospira libertina, Paludomus zhangchouensis and S. hainanensis that belonged to the Family Pleurceridae, Tricula fujianensis and T. huaanensis that belonged to the subfamily Triculinae, Family Pomatiopsidae, and Melanoides tuberculata (Family Thiaridae), and 11 species of freshwater crabs belonging to 5 genera within 2 families were identified, including Sinopotamon genus of S. jianglense, S. pinheense, and S. zhangzhouense, Huananpotamon genus of H. planopodum and H. zhangzhouense, Nanhaipotamon genus of N. huaanense and N. longhaiense, and Minpotamon genus of M. nasicum and M. auritum that belonged to the Family Potamidae, and Somanniathelphusa genus of S. huaanensis and S. zhangpuensis (Family Parathelphusidae). In addition, the prevalence of P. westermani cercariae infections was 0.08% (2/2 317) in P. zhangchouensis from Danyan Village in Changtai District and 0.09% (1/1 039) in S. hainanensis from Jinkeng Village in Yunxiao County, and the prevalence of P. westermani metacercariae infections was 25.81% (8/31) in S. jianglense from Danyan Village in Changtai District, and 26.31% (5/19) in S. zhangzhouense from Jinkeng Village in Yunxiao County, respectively. CONCLUSIONS: There is a population diversity in the intermediate host snails and crabs along the Jiulongjiang River, Zhangjiang River, and Dongxi River basins in Bopingling Mountain, southern Fujian Province, and P. zhangzhouensis and S. hainanensis are, for the first time, confirmed as the first intermediate hosts of P. westermani.

Paragonimus paishuihoensis metacercariae in freshwater crabs, Potamon lipkei, in Vientiane Province, Lao PDR.

Among Paragonimus species, P. paishuihoensis is one of the most mysterious and poorly understood species. Metacercariae are characterized by having a unique dendritically branched excretory bladder. However, the morphology of the adult worm remains unknown. To date, metacercariae of this species have been reported only in China and Thailand. In this study, we first found P. paishuihoensis metacercariae in freshwater crabs, Potamon lipkei, in Hinheub District, Vientiane, Lao PDR, with a prevalence of 77.7% and the average intensity of 10.3 (range 1-28) metacercariae per crab. The molecular data based on ITS2 and CO1 markers indicated that P. paishuihoensis from Laos and Thailand were almost completely identical and were close to members of the Paragonimus bangkokensis/Paragonimus harinasutai complex. Attempts to infect experimental animals (cats, dogs, and rats) with P. paishuihoensis were unsuccessful, suggesting that these animals might be unsuitable definitive hosts for the species. Further studies are necessary to elucidate the taxonomic status and life cycle of P. paishuihoensis.

[Sequence analysis of Paragonimus internal transcribed spacer 2 and cyclooxygenase 1 genes in freshwater crabs in Henan Province].

OBJECTIVE: To investigate the sequences of internal transcribed spacer 2 (ITS2) and cyclooxygenase 1 (COX1) genes of Paragonimus metacercariae in freshwater crabs in Henan Province, identify the species of Paragonimus and evaluate its genetic relationships with Paragonimus isolates from other provinces in China. METHODS: Freshwater crabs were collected from 8 survey sites in Zhengzhou, Luoyang, Pingdingshan, Nanyang and Jiyuan cities of Henan Province from 2016 to 2021, and Paragonimus metacercariae were detected in freshwater crabs. Genomic DNA was extracted from Paragonimus metacercariae, and the ITS2 and COX1 genes were amplified using PCR assay, followed by sequencing of PCR amplification products. The gene sequences were spliced and aligned using the software DNASTAR, and aligned with the sequences of Paragonimus genes in the GenBank. Phylogenetic trees were created using the MEGA6 software with the Neighbor-Joining method based on ITS2 and COX1 gene sequences, with Fasciola hepatica as the outgroup. RESULTS: The detection rates of Paragonimus metacercariae were 6.83% (11/161), 50.82% (31/61), 18.52% (5/26), 8.76% (12/137), 14.29% (9/63), 17.76% (19/105), 18.50% (32/173) and 42.71% (41/96) in freshwater crabs from 8 survey sites in Zhengzhou, Luoyang, Pingdingshan, Nanyang and Jiyuan cities of Henan Province, with a mean detection rate of 19.46% (160/822), and a mean infection intensity of 0.57 metacercariae/g. The amplified ITS2 and COX1 gene fragments of Paragonimus were approximately 500 bp and 450 bp in lengths, respectively. The ITS2 gene sequences of Paragonimus metacercariae from 8 survey sites of Henan Province showed the highest homology (99.8% to 100.0%) with the gene sequence of P. skrjabini (GenBank accession number: MW960209.1), and phylogenetic analysis showed that the Paragonimus in this study was clustered into the same clade with P. skrjabini from Sichuan Province (GenBank accession number: AY618747.1), Guangxi Zhuang Autonomous Region (GenBank accession number: AY618729.1) and Hubei Province (GenBank accession number: AY618751.1), and P. miyazaki from Fujian Province (GenBank accession number: AY618741.1) and Japan (GenBank accession number: AB713405.1). The COX1 gene sequences of Paragonimus metacercariae from 8 survey sites of Henan Province showed the highest homology (90.0% to 100.0%) with the gene sequence of P. skrjabini (GenBank accession number: AY618798.1), and phylogenetic analysis showed that the Paragonimus in this study was clustered into the same clade with all P. skrjabini and clustered into the same sub-clade with P. skrjabini from Hubei Province (GenBank accession numbers: AY618782.1 and AY618764.1). CONCLUSIONS: Paragonimus species from freshwater crabs in Henan Province were all characterized as P. skrjabini, and the ITS2 and COX1 gene sequences had the highest homology to those of P. skrjabini from Hubei Province. The results provide insights into study of Paragonimus in Henan Province and China.

A novel gene order and remolded tRNAs revealed in the mitogenome of Asian gecarcinucid freshwater crabs (Brachyura, Gecarcinucidae).

Here we report the first mitochondrial genomes (mitogenomes) of four species of gecarcinucid freshwater crabs (FWCs) in two genera, two from China (Somanniathelphusa hainanensis and S. yangshanensis), one from Laos (Esanthelphusa dugasti), and one from Myanmar (Esanthelphusa keyini). A novel gecarcinucid mitochondrial gene order (GMGO2) that was only found in E. dugasti that contains a total of 42 genes, including one pseudogene, two remolded tRNAs and two duplicated tRNAs. The GMGO2 of E. dugasti was compared with the brachyuran ground-pattern mitochondrial gene order (BMGO), revealing the rearrangements of the positions of 10 tRNAs, two PCGs, and one mNCR. The three other gecarcinucids in this study were all found to possess a previously reported gecarcinucid mitochondrial gene order (GMGO1). The phylogenetic tree reconstructed using the secondary structures of 22 tRNAs of the mitogenomes of 41 species of FWCs provides insights into the evolution of the mitogenome of E. dugasti (GMGO2) which includes remolded and duplicated tRNAs.

New Genus and Two New Species of Semiterrestrial Crabs (Decapoda: Brachyura: Gecarcinucidae) from the Southern Western Ghats, India.

One new genus and two new species of semiterrestrial freshwater crabs are described from the Southern Western Ghats mountain range in India: Pavizham gavi gen., sp. nov. and Rajathelphusa brunnea sp. nov. The carapace of Pavizham n. gen. is superficially similar to Baratha and Snaha described from the same region, but it can be distinguished from Baratha in having no visible postorbital cristae, complete sutures between male thoracic sternites 2 and 3, the possession of a triangular male pleon and the elongate terminal segment of the male first gonopod; and from Snaha by the presence of a long flagellum on the exopod of the third maxilliped, a complete suture separating male thoracic sternites 2 and 3, and its triangular male pleon. Rajathelphusa brunnea sp. nov. can easily be distinguished from congeners in possessing a distinctly more slender and elongate male first gonopod.

Establishment of A New Bornean Genus of Gecarcinucidae (Crustacea: Brachyura), with Descriptions of Five New Species.

The freshwater gecarcinucid species from Borneo previously referred to Sundathelphusa Bott, 1969, are transferred to a new genus. Borneosa gen. nov. can be diagnosed by the presence of a distinct frontal median triangle that is shorter than the frontal margin, a relatively longer male thoracic sternum in which the sternopleonal cavity reaches more anteriorly, and a male first gonopod that has the terminal segment prominently covered with long setae. The eight species recognised-B. tenebrosa (Holthuis, 1979), B. aspera (Ng & Stuebing, 1989), B. brachyphallus (Ng, 2015), and five new species, B. niah, B. bario, B. kapit, B. sarawakensis and B. serrata-can be distinguished by characters of the frontal median triangle, epibranchial tooth, anterolateral margin, third maxillipeds, ambulatory legs, male sternum, male pleon and male first gonopod.

The complete mitochondrial genome sequence of the mountain crab Indochinamon bhumibol.

Indochinamon bhumibol has been found as the biggest freshwater crab in Thailand. In this study, we report the first complete sequence of mitochondrial genome from I. bhumibol encoding 13 protein-coding genes, 22 transfer RNAs, and 2 ribosomal RNAs. The nucleotide composition of I. bhumibol mitogenome showed a strong AT bias (70.4%) with a low GC content (29.6%). Comparative phylogenetic analysis with 28 crustaceans based on nine conserved genes demonstrated that I. bhumibol was closely related to members of the Potamidae family.

Paragonimus and its hosts in China: An update.

Paragonimiasis is an important foodborne parasitic disease. Over 50 species of Paragonimus have been reported worldwide, and China has the widest distribution and largest number of species. The detection of Paragonimus metacercariae from second intermediate hosts has been reported in 22 provinces and municipalities. The most frequently reported species are P. westermani, P. skrjabini, P. heterotremus and Euparagonimus cenocopiosus. In this review, we collected and reviewed relevant reports on the detection of Paragonimus metacercariae in second intermediate hosts from 1937 to 2020 from all areas of China. We provide an updated and current summary of Paragonimus species and their hosts in China. Data on the geographical range, species distribution, and second intermediate host species of Paragonimus were extracted. ArcGIS10.2 software was used to generate distribution maps of Paragonimus for four time periods: 1937-1990, 1991-2005, 2006-2020 and 1937-2020. We analyzed the geographic and spatiotemporal dynamics of Paragonimus prevalence in natural foci and provided a basis for further research and paragonimiasis prevention strategies in China.

[Investigation on freshwater crab populations and Paragonimus infections in the Minjiang River basin along the middle section of Wuyi Mountain].

OBJECTIVE: To investigate the populations of freshwater crabs, the intermediate host of Paragonimus and Paragonimus infections in freshwater crabs in the Minjiang River basin along the middle section of Wuyi Mountain, so as to provide baseline data for parasitic disease control and research and expansion of the parasite resource bank. METHODS: From November 2020 to April 2021, freshwater crabs were sampled from streams and ditches neighboring residential areas in Jianning County and its neighboring Ninghua, Shaowu, Jiangle and Shunchang counties. The crab species was identified based on the morphological features of the terminal segment of the first abdominal appendage of male crabs, and Paragonimus infections were detected in freshwater crabs. The Paragonimus metacercariae were isolated, and the types of metacercariae were identified based on the metacercaria size, cystic wall thickness, and the excretory bladder and intestinal tract morphology. In addition, the prevalence, intensity and index of metacercaria infections were calculated in freshwater crabs. RESULTS: There were seven crab species found in Jianning County and six neighboring water systems along the Minjiang River basin, including Sinopotamon jianglense, S. fukinense, Huananpotamon lichuanense, H. lini, H. shenni, H. planopodum, Bottapotamon engelhardti, and there were metacercariae of three Paragonimus species detected in these crabs, including P. westermani, P. skrjabini and P. sanpingensis, with a prevalence rate of 43.6% (125/287). The infection rates of P. westermani, P. sanpingensis and P. skrjabini were 57.1% (48/84), 26.2% (22/84) and 61.8% (21/34) in S. jianglense, and the infection rates of P. westermani and P. sanpingensis were 52.6% (51/97) and 30.9% (30/97) in S. fukinense, while the rate of P. westermani infection was 6.9% (5/72) in H. lichuanense, which is the first record of P. westermani infections in H. lichuanense. Mixed P. westermani and P. sanpingensis infections were predominantly found in freshwater crabs sampled from Jianning County, where the rate of Paragonimus infections was 70.4% (76/108), with 15.3 metacercariae identified in each crab with Paragonimus infections and 1.9 metacercariae found in each gram of crabs with Paragonimus infections, and the index of metacercariae infections was 20.5. In addition, P. westermani, P. skrjabini and P. sanpingensi metacercariae were found in freshwater crabs sampled from Jianning-neighboring counties, where the rate of Paragonimus infections was 52.3% (56/107), with 9.8 metacercariae identified in each crab with Paragonimus infections and 0.9 metacercariae found in each gram of crabs with Paragonimus infections, and the index of metacercariae infections was 4.6. CONCLUSIONS: There are multiple freshwater crab species and Paragonimus infection is high in freshwater crabs in Jianning County and its neighboring Minjiang River basin, which is a high-risk natural focus for Paragonimus infections.

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

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.

The Hillstream Decapod Crustaceans of Shenzhen, China, with Description of a New Species of Freshwater Crab (Crustacea: Brachyura: Potamidae) in the Genus Megapleonum Huang, Shih & Ahyong, 2018.

Freshwater decapod crustaceans are often ecologically important keystone species in their habitats. The freshwater hillstream decapods of Shenzhen City, Guangdong, China, were systematically surveyed for the first time from June to September 2019. We identified a total of 19 decapod species from 10 genera and six families. Of these, one Macrobrachium species and one potamid species could not be assigned to any known species. The latter clearly belongs to the monotypic genus Megapleonum Huang, Shih & Ahyong, 2018, and resembles the type species M. ehuangzhang in general external morphology. Striking differences in male gonopodal morphology along with genetic evidence based on mitochondrial 16S rDNA sequences strongly suggest that this species is distinct from the type species and it is herein described as Megapleonum shenzhen n. sp. Our survey data sheds light on the biodiversity of hillstream decapods in Shenzhen and highlights areas of conservation interest.

Description of a New Genus with Two New Species of Freshwater Crab (Decapoda: Brachyura: Gecarcinucidae) from the Southwestern Ghats, India.

A new genus of freshwater crab, Rajathelphusa gen. nov., and two species, R. ala sp. nov. (type species) and R. muni sp. nov., are described from the Kerala part of the Western Ghats biodiversity hotspot, India. The new genus is superficially similar to Arcithelphusa and Cylindrotelphusa from the same hotspot, but can easily be distinguished by the following suite of carapace and gonopod characters: an inflated carapace with distinct epigastric and postorbital cristae; the third maxilliped exopod has a long flagellum; male sternites 2 and 3 separated by a shallow median groove while sternites 3 and 4 are completely fused, demarcated by shallow oblique grooves; a male first gonopod that is almost straight with a short distal conical distal segment; and a male second gonopod that is as long as the first gonopod with a long and slender distal segment. The two new species can be differentiated from each other by the lateral margins of male pleonal somite 6 and the structure of the male first gonopod.

Two new species of freshwater crab of the genus Aparapotamon Dai & Chen, 1985 (Crustacea, Brachyura, Potamidae) from Yunnan, China.

Two new species of freshwater crab of the genus Potamid Aparapotamon Dai & Chen, 1985 are described from Yunnan Province, southwest China. Morphological comparisons were made between the two new species and type materials of other 11 species of Aparapotamon. Aparapotamonbinchuanense sp. nov. and A.huizeense sp. nov. can be separated from their congeners by the shape of the epibranchial tooth, the frontal view of the cephalothorax, the male first gonopod, and the female vulvae. The molecular analyses based on partial mitochondrial 16S rRNA gene are also included. This study brings the number of Aparapotamon species to 13.

Myxobdella socotrensis sp. nov., a new parasitic leech from Socotra Island, with comments on the phylogeny of Praobdellidae (Hirudinida: Arhynchobdellida).

The Socotra Archipelago (Yemen) is remarkable for unique ecosystems and high endemism, for which it is often referred to as the "Galápagos of the Indian Ocean". Here we describe a new parasitic leech Myxobdella socotrensis sp. nov. from Socotra, the largest island of the archipelago. The new species was found in a freshwater spring attached to the endemic crab Socotrapotamon socotrensis (Hilgendorf, 1883) (Crustacea: Potamidae). Based on its morphology, ecology and a phylogenetic analysis of DNA sequence data (12S, 18S, 28S and COI gene markers), the new leech species is classified into the highly diversified family Praobdellidae (Hirudinida: Arhynchobdellida), distributed in tropical regions of Africa, Asia and Central and South America. The praobdellid leeches are known to infest mammalian mucous membranes, however, some taxa are associated with other hosts, namely crabs, amphibians and birds. By its morphology, the new species fits quite well in the current concept of the genus Myxobdella Oka, 1917. However, the monophyly of Myxobdella was not supported here by molecular data, pointing at the need of a more comprehensive systematic revision of the genus and family. As far as known, Myxobdella socotrensis sp. nov. is endemic to the Socotra Island, but more data are needed to understand its evolutionary origin, biology and distribution.

Paragonimus westermani metacercariae in two freshwater crab species in Kagoshima Prefecture, Japan, as a possible source of infection in wild boars and sika deer.

Paragonimiasis is a particular foodborne parasitic disease that is endemic to southern Kyushu, including Kagoshima Prefecture, Japan. We previously detected Paragonimus westermani triploid larvae in meat samples obtained from wild boars and sika deer hunted in Akune City, Kagoshima Prefecture. These mammals act as paratenic hosts and their meat is a source of human paragonimiasis. Paratenic host mammals and humans become infected with the lung fluke, P. westermani, following consumption of second intermediate hosts, freshwater crab species, namely, Geothelphusa dehaani or Sawagani in Japanese, and Eriocheir japonica or Mokuzugani in Japanese. Therefore, this study aimed to investigate the current infection status of P. westermani in freshwater crabs in Akune City. We collected freshwater crabs from 15 locations and found that the prevalence of P. westermani metacercariae was 1.6% for Sawagani (15 of 941 examined) and 22.1% for Mokuzugani (21 of 95 examined). Based on the morphological characterization of metacercariae and molecular analyses of the internal transcribed spacer 2 region and mitochondrial 16S rRNA gene region using PCR-restriction fragment length polymorphism and sequencing, all metacercariae were identified as the triploid form of P. westermani. These results indicate that Sawagani and Mokuzugani serve as second intermediate hosts to maintain the life cycle of triploid P. westermani. Further, infection in crabs potentially leads to subsequent P. westermani infections in wild mammals, including wild boars and sika deer, both of which are considered important types of game meat in Japan.

Description of two new species of the genus Heterochelamon Türkay & Dai, 1997 (Crustacea: Decapoda: Brachyura: Potamidae), from southern China.

This study describes two new species of freshwater crab of the genus Heterochelamon Türkay & Dai, 1997 from southern China, H. huidongense from Guangdong Province and H. jinxiuense from Guangxi Zhuang Autonomous Region. The two new species can be differentiated from congeners by characters derived from the shape of the epibranchial tooth, external orbital angle, cheliped proportions and structure of the male first gonopod. The present study brings the number of Heterochelamon species to seven. We used the mitochondrial 16S rRNA gene for a molecular analysis and the results are consistent with the morphological features that support the recognition of two new taxa.

Rapid divergent coevolution of Sinopotamon freshwater crab genitalia facilitates a burst of species diversification.

One of the most striking radiations in brachyuran evolution is the considerable morphological diversification of the external reproductive structures of primary freshwater crabs: the male first gonopod (G1) and the female vulva (FV). However, the lack of quantitative studies, especially the lack of data on female genitalia, has seriously limited our understanding of genital evolution in these lineages. Here we examined 69 species of the large Chinese potamid freshwater crab genus Sinopotamon Bott, 1967 (more than 80% of the described species). We used a landmark-based geometric morphometric approach to analyze variation in the shape of the G1 and FV, and to compare the relative degree of variability of the genitalia with non-reproductive structures (the third maxillipeds). We found rapid divergent evolution of the genitalia among species of Sinopotamon when compared to non-reproductive traits. In addition, the reconstruction of ancestral groundplans, together with plotting analyses, indicated that the FV show the most rapid divergence, and that changes in FV traits correlate with changes in G1 traits. Here we provide new evidence for coevolution between the male and female external genitalia of Sinopotamon that has likely contributed to rapid divergent evolution and an associated burst of speciation in this lineage.