The genera Parahelice K. Sakai, Türkay & Yang, 2006, and Pseudohelice K. Sakai, Türkay & Yang, 2006, are widely distributed in estuarine, mudflats, or mangroves in the Indo-West Pacific, and species of them are often sympatric. Species identification of the two genera is generally difficult, mainly due to the similar morphology and large intraspecific variation. Our study is the first report of the two genera from Vanuatu, including one new species, Parahelice ngankeeae sp. nov., as well as Par. pilimana (A. Milne-Edwards, 1873) and Pseudohelice subquadrata (Dana, 1851). These species can be distinguished from similar species by the characters of the suborbital cristae, the male chelipedal manus, and the first gonopods. The new species is also supported by the molecular results from the cytochrome c oxidase subunit I (COI).

Two species of land hermit crabs, Coenobita longitarsis De Man, 1902 and C. pseudorugosus Nakasone, 1988 were described based on female specimens from Maluku, Indonesia and specimens from Cebu, the Philippines, respectively. However, no confirmed records of either species have been reported since their original descriptions. In this study, we examined specimens with typical morphological characters of C. longitarsis from Papua New Guinea and C. pseudorugosus from the Philippines and Indonesia, further supported by the analyses of the DNA barcoding marker, cytochrome oxidase subunit I (COI). The characters of male C. longitarsis are provided for the first time, with coxae of male fifth pereiopods subequal, without sexual tubes developed. Their slender morphology is suggested to be an adaptation to utilize native terrestrial snail shells in inland forests, which may also explain its rarity. Coenobita pseudorugosus is very similar to C. rugosus H. Milne Edwards, 1837, but can be distinguished by the adult sizes, as well as the morphology of sexual tubes of male fifth pereiopods and propodus of left third pereiopod. Morphological variation and the fresh coloration of C. longitarsis and C. pseudorugosus are also provided in this study.

S Parasesarma bidens (De Haan, 1835) is a common intertidal sesarmid crab that inhabits the estuarine and mangrove regions of East Asia, as well as Vietnam and the Philippines. Previous studies have suggested that P. bidens is a species complex composed of several species. Our findings indicated that the P. bidens species complex is monophyletic with six clades, as shown by reconstructed phylogenies based on mitochondrial 16S rDNA and cytochrome c oxidase subunit I, as well as nuclear 28S rDNA. Specimens of each clade exhibited different characters, including the morphology of the cheliped, anterior carapace, male gonopod 1, and female vulvae, as well as the coloration. Three species in this complex have been previously described: P. bidens (De Haan, 1835), P. cricotum (Rahayu & Davie, 2002), and P. sanguimanus Li, Shih & Ng, 2019. Here, we describe three additional species: P. continentale n. sp., P. insulare n. sp., and P. chiahsiang n. sp. Biogeographically, the range of P. bidens includes the southern margin of Korea, the main islands of Japan, and the Ryukyus, with a comparatively northern distribution. P. continentale is widely distributed from the continental coasts of southeastern and southern China (including northern Hainan Island) to southern Vietnam and is occasionally found in Penghu, Taiwan. Parasesarma insulare is an insular species distributed from Taiwan main island, the central Philippines, to northern Borneo. Finally, P. chiahsiang is mainly limited to the Penghu Islands (Pescadores) but is also sometimes found in Taiwan main island and the offshore Gueishan Island.

Based on recently collected material and records in the literature, 14 species of fiddler crabs (Crustacea: Ocypodidae: Gelasiminae) are reported from Vietnam. DNA barcoding analyses using the mitochondrial gene COI (cytochrome c oxidase subunit I) was performed to identify examined materials and their precise distributional range. Thirteen species-level taxa are identified and, with the exception of Galsimus borealis and G. vocans, have minimum interspcific divergences of at least 7.27%. The identified species include seven species of Tubuca Bott, 1973, three of Austruca Bott, 1973 and three of Gelasimus Latreille, 1817, and one Paraleptuca Bott, 1973. Two new records of Vietnam are herein reported: Tubuca rhizophorae and T. dussumieri. The southernmost distribution limits of East Asian G. borealis, T. acuta and T. arcuata are in northern Vietnam, A. lactea in central Vietnam, whereas northernmost limit of Southeast Asian T. rhizophorae and T. forcipata in southern Vietnam. A dichotomous key to identify the 14 Vietnamese species is provided.

Fiddler crabs (Brachyura: Ocypodidae: Gelasiminae) in Taiwan consist of 5 genera and 15 species, but knowledge of their larval development is limited to just 3 species, namely Austruca lactea, Tubuca arcuata, and Xeruca formosensis. In our study, the morphology of the first zoeal stage (zoea I) of the 15 species is described and compared to previous studies. The results show that the characters of zoea I can be used to distinguish the five studied genera and most species (except three groups, "Gelasimus borealis, G. jocelynae and G. vocans", "Paraleptuca crassipes and P. splendida" and "Tubuca arcuata, T. coarctata and T. paradussumieri"). The lateral spines on the carapace in zoea I are suggested to be a unique character in the Ocypodinae because they are absent in the Gelasiminae and Ucidinae, which supports the current systematics of the Ocypodidae.

Nine species of fiddler crabs (Crustacea: Ocypodidae: Gelasiminae) are known from the Arabian Sea and adjacent waters (Red Sea, Gulf of Aden, Gulf of Oman and Arabian/Persian Gulf): five species of Austruca, one Cranuca, two Gelasimus and one Tubuca. COI sequence data match morphological species boundaries and shows high connectivity within each. The fauna is highly endemic, with three species of Austruca (A. albimana, A. iranica and A. sindensis) confined to this region, and four others restricted to the Indian Ocean. restricted to the Indian Ocean. Austruca albimana and A. iranica speciated locally and now have narrowly overlapping ranges in Oman. Our results confirm the westernmost distributions of Austruca annulipes and Tubuca alcocki are Pakistan and the Red Sea, respectively. A key for the nine species is also provided to help identification.

Members of the genus Pseudohelice Sakai, Turkay & Yang, 2006 (family Varunidae) inhabit estuaries and high intertidal zone with sediments composed of mud and sand in tropical to temperate regions. Previous studies showed only one species, Pseudohelice subquadrata (Dana, 1851) in this genus, with a wide distribution in the Indo-West Pacific. In the present study, the species from the western Indian Ocean is shown to be a valid species, Pseudohelice latreillii (H. Milne Edwards, 1837), based on morphological and molecular evidence. The latter species had previously been incorrectly synonymised with Helice tridens (De Haan, 1835). The two species are very similar to each other in morphology, but can be distinguished by a suite of characters, including the structures of the infraorbital ridges in both sexes, male gonopods 1 and female vulvae. Based on the molecular evidence from cytochrome oxidase subunit 1 (COI), P. subquadrata and P. latreillii form two well-supported clades, and the intraspecific and interspecific divergences of COI are 0-1.9% and 3.3%-5.1%, respectively. Geographically, P. subquadrata is distributed in the western Pacific while P. latreillii occurs in the western Indian Ocean.

Xeruca formosensis is a species and genus of fiddler crab endemic to Taiwan, with limited distribution in western Taiwan and the offshore Penghu Islands. This study reports the complete mitochondrial genome (mitogenome) of this species using next-generation sequencing. The mitogenome contains 15,684 bp, comprising 13 protein-coding genes, 22 tRNAs, 2 ribosomal RNAs and a 750-bp intergenic space (control region). The nucleotide composition is biased toward A+T (69.4%). A phylogenetic analysis based on the concatenated protein-coding genes showed that the genera Xeruca Shih, 2015 and Tubuca Bott, 1973 are sister to each other. In addition, the phylogeny of the 16 available mitogenomes in the family Ocypodidae also supports the current systematics of this family based on one nuclear and two mitochondrial markers. As this species inhabits high intertidal mudflats with high temperature and high salinity, mitogenome analyses may help us understand the mechanisms of adaptation to extreme environments, as well as the connectivity of metapopulations based on mitogenomes from different populations.

A new genus and new species of potamid crab, Indawmon thanayei, is described from Indawgyi Lake in Kachin State, Myanmar. The taxon is characterised mainly by its male first gonopod structure which has the terminal and subterminal segments not clearly demarcated, with the terminal segment subcylindrical in shape, gradually tapering to a sharp tip and is bent at an angle of about 45-90 degrees. The new genus is compared with Indochinamon and Shanphusa, which it superficially resembles. The genetic data (based on the mitochondrial 16S rDNA) also supports the establishment of the new taxa.

Species of the varunid genus Parahelice Sakai, Türkay & Yang, 2006, inhabit high intertidal areas of oceanic islands of the tropical Indo-West Pacific region. As several species of Parahelice and Pseudohelice subquadrata (Dana, 1851) were found to be sympatric in some places, and their morphological differences are minor, especially in females, the misidentification of species is not uncommon in the literature. In this study, the DNA barcoding marker, mitochondrial cytochrome c oxidase subunit I (COI), was applied to confirm species identities and this was correlated with the specific characters of males and females. Distributions of three species of Parahelice were also updated, with Par. daviei (Sakai, Türkay & Yang, 2006), Par. pilimana (A. Milne-Edwards, 1873), and Par. pilosa (Sakai, Türkay & Yang, 2006) being new records to Taiwan, and Par. pilosa new to Bali, Indonesia.

Species in the brackish crab genus Ptychognathus are common in the seashore and estuary habitats with freshwater input. Due to their similar morphologies and dull colorations, it is always difficult to distinguish the species in this genus. In this study, the DNA barcode gene COI (cytochrome c oxidase subunit I) was used to help identify Ptychognathus from Taiwan. The results showed that the 10 species can be identified successfully based on COI, with intraspecific distances below 1.54% and interspecific distances of 12.2%-19.57%. In addition, two new species of Ptychognathus are described from Taiwan. Ptychognathus makii sp. nov. from southern Taiwan is similar to P. altimanus (Rathbun, 1914), and P. stimpsoni sp. nov. from southern Taiwan and the southern Philippines resembles P. aff. barbatus (A. Milne-Edwards, 1873) and P. pusillus Heller, 1865. Both species can be distinguished from other congeners by a suite of characters, including the carapace, orbital region, frontal region, telson of male pleon, male first gonopod, and setae on ambulatory legs.

Two new species of a new genus, Tortomon puer n. gen. n. sp. and T. gejiu n. gen. n. sp. are described from southern Yunnan, southwestern China, based on the morphology and mitochondrial 16S rDNA sequences. The new genus resembles Tenuipotamon Dai, 1990 and Parvuspotamon Dai & Bo, 1994, but can be separated by characters of the male thoracic sternum, male pleon, male first gonopod, and especially the coiled tip of the male second gonopod. The latter character easily separates Tortomon n. gen. from known potamid genera. Notes on the general biology of the two new species are also included.

The identity of the widely distributed mangrove and riverine sesarmid crab Sesarmops impressus (H. Milne Edwards, 1837) is clarified. The species has been reported from Madagascar to the South Pacific, but its taxonomy was previously confused. On the basis of morphological characters and available molecular evidence (using 16S rDNA and cytochrome oxidase subunit I), Sesarmops impressus is restricted to the western Indian Ocean and S. frontale A. Milne-Edwards, 1869 is confirmed to be its junior synonym. Sesarmops similis (Hess, 1865), long synonymised under S. impressus, is here recognised as a separate species and a neotype from Samoa is designated. The identity of the poorly known S. atrorubens (Hess, 1865), which has often been confused with S. impressus, is also clarified, and a neotype is selected from Fiji. Specimens from the eastern Indian Ocean and western Pacific are here identified as two new species, S. indicus sp. nov. and S. imperator sp. nov.

The identity of the widely distributed mangrove and riverine sesarmid crab

A new genus and new species of blind freshwater cave crab are described from Chongzuo City, Guangxi Zhuang Autonomous Region, China based on morphology and mitochondrial 16S rDNA sequences. The new genus, Phasmon gen. nov., is established for P. typhlops sp. nov., which is only the second blind cave crab known from China and East Asia. The combination of a very wide carapace, overall depigmentation, reduced orbits and vestigial unpigmented eyes of Phasmon immediately separates it from all known potamid genera. Molecular divergence estimates based on 16S rDNA suggest that the lineage to which the new genus belongs diverged from other potamids at the beginning of the Late Miocene (10.8 million years ago), much earlier than other Chinese cave crabs.

A new species of fiddler crab, Austruca citrus n. sp. from Fiji, Wallis & Futuna, and Samoa, in the South Pacific is described based on morphological and molecular evidence. This species is closely related to Austruca perplexa (H. Milne Edwards, 1852), but can be distinguished by a suite of characters, including adult size, carapace morphology, shape and coloration of the major chela, the ratio of major pollex length as a function of carapace width, and male first gonopod form. The molecular evidence of the mitochondrial 16S rDNA and cytochrome oxidase subunit I (COI) supports these morphological differences.

The poorly known sesarmid crab Sesarma (Holometopus) tangi Rathbun, 1931 was described from one male from Fujian, China, and was transferred to Neosarmatium Serene & Soh, 1970 by Schubart & Ng (2002). The collection of a good series of fresh specimens from near the type locality and Hong Kong allows us to reappraise its taxonomy. The new material shows that Sesarma tangi should be referred to its own genus, differing markedly from congeners in the detailed structure of its male and female chelae as well as in the form of the male first gonopods. This conclusion is strongly supported by genetic data.