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A new supergiant species of Bathynomus A. Milne-Edwards, 1879 from Vietnam is described. Bathynomus vaderi sp. nov. is characterised by its wide, rectangular clypeal region with parallel lateral margins, concave distal margin, and narrowly acute apex; the distally narrowing and posteriorly curved coxa of pereopod 7; and the presence of 11 upwardly curved pleotelson spines. The new Bathynomus is the fourth species with upwardly curved pleotelson spines and the second supergiant in the South China Sea. The taxonomy of B. jamesi Kou, Chen & Li, 2017 from the South China Sea is also discussed based on a large series of specimens. Previously reported differences in body form and pleotelson spines, which suggest that there may be two forms or species, are regarded as intraspecific variation for the time being. The contemporary culinary trend and fishing of Bathynomus in Vietnam, which have contributed to this discovery, are also discussed.

The goneplacid crab, Carcinoplax mistio Ng & Mitra, 2019, was originally described from West Bengal, India, in the northern Indian Ocean. Additional material of C. mistio from off Tamil Nadu in the southeast of India revealed a high degree of size-associated variation in the structures of the anterolateral tooth of the carapace, chelipeds, and male and female pleons. In addition to an in-depth morphological examination of C. mistio, this study also records the natural coloration of the species and conducts a genetic comparison (with mitochondrial COI and 16S rRNA genes) with its close relatives, C. haswelli (Miers, 1884) and C. purpurea Rathbun, 1914. Molecular comparison of C. mistio with its morphologically closest congener, C. haswelli from northern Australia and the western Pacific, corroborates their status as separate species. The genetic sequence of C. mistio, however, is similar to that of C. purpurea from the West Pacific, although these two species can easily be distinguished by distinct carapace and ambulatory leg characters. The present study provides some possible explanations for the genetic and morphological incongruence observed between C. mistio and C. purpurea and highlights the need for a detailed molecular study for Carcinoplax H. Milne Edwards, 1852, to appreciate the evolution of various morphological characters in the genus.

Abstract The poorly known xanthiform crab Pilumnus palmeri Garth, 1986, is known only from two poorly preserved specimens collected from Ecuador. Garth had only provisionally placed the species in Pilumnus Leach, 1816, and it was subsequently referred to a new genus, Garthopilumnus Števčić, 2011 and a new family Garthopilumnidae Števčić, 2011. Garthopilumnus currently sits as a valid genus within Pilumnidae Samouelle, 1819. A re-examination of the holotype male, now in poor condition, shows that it is a juvenile, and key aspects of the original figure of the species given in the account of the type are inaccurate. Given the incompleteness and immaturity of available material, we could not confidently assign the species to an existing family, but we are also not convinced a new family for the species is warranted. The morphological features of Garthopilumnus palmeri suggest it is a member of the Portunoidea, where we provisionally place it.

In the American Museum of Natural History, New York, there is an old specimen labelled as “Holodromia harrisonwilliamsi Boone, MS name” that was collected from Cocos Island in 1925. This name has never been published. An examination of the specimen shows that it is a juvenile specimen of Homolodromia robertsi Garth, 1973, a species described from Peru, which has since been reported from Chile and off the coast of Ecuador. This paper reports on the specimen Homolodromia robertsi Garth found in Costa Rica for the first time, a discovery that extends the known range of this species northwards.

The spiny spider crab, Samadinia pulchra (Miers in Tizard, Moseley, Buchanan & Murray, 1885) (type locality: Bohol Sea, Philippines) has long been considered to be widespread in the western Pacific and Indian Ocean, with Anamathia livermorii Wood-Mason in Wood-Mason & Alcock, 1891, described from the Andaman Sea, in its synonymy. Reassessment of Samadinia pulchra from throughout its purported range indicates that it comprises a complex of three species: S. pulchra sensu stricto, occurring in the northwestern Pacific, from the South China Sea to Japan; S. livermorii, occurring in the Indian Ocean, ranging from southwestern Indonesia to East Africa; and a new species, S. jimlowryi sp. nov., ranging from northwestern Australia to southeastern Indonesia.

A new species of spider crab of the genus Leptomithrax Miers, 1876, is described from New Caledonia. Leptomithrax lowryi sp. nov. belongs to a group species that includes L. bifidus (Ortmann, 1893) and L. sinensis Rathbun, 1916, but differs in the structure of the carapace spines and teeth, ischium of the third maxilliped, male thoracic sternal structure, shape of the male telson and the first gonopod.

Abstract The poorly known pilumnid, Vellumnus penicillatus (Gordon, 1930), is redescribed by examining the Hong Kong types. A new species, Vellumnus tki n. sp., is described from Macclesfield Bank, South China Sea, and it is distinguished from its closest congener, Vellumnus minabensis (Sakai, 1969) from Japan, by the setal pattern on the carapace and pereiopods, carapace proportions, structure of the front and anterolateral teeth, armature on the chelipeds and relatively shorter ambulatory legs.

The systematics of four species of the homolodromiid genus Dicranodromia A. Milne-Edwards, 1880, from East Asia and the Philippines is reappraised: D. danielae Ng & McLay, 2005, D. doederleini Ortmann, 1892, D. karubar Guinot, 1993, and D. martini Guinot, 1995; and key characters such as the epistome, gonopods, and spermatheca are figured in detail. Two new species, D. erinaceus sp. nov. and D. robusta sp. nov., are described from Taiwan and the Philippines, respectively. Dicranodromia erinaceus sp. nov. resembles D. spinulata Guinot, 1995, and D. delli Ahyong, 2008 (from New Caledonia and New Zealand) but can be separated by its distinctly spinulated carapace surfaces and proportionately shorter fifth ambulatory legs. Dicranodromia robusta sp. nov. is superficially similar to D. baffini (Alcock & Anderson, 1899) and D. karubar Guinot, 1993, but can easily be separated by possessing a broad dorsoventrally flattened infraorbital tooth. A genetic study of the species using the mitochondrial cytochrome c oxidase I gene confirms that the taxa are distinct, with D. erinaceus sp. nov. coming out in a well-supported clade from congeners. The megalopa of D. doederleini is also reported for the first time.

The leucosiid crab Orientotlos iishibai Sakai, 1980 was described from one female collected off western Japan and had never been reported since. The species is now recorded from southwestern Taiwan for the first time, and is redescribed and figured at length. Although Sakai argued that Orientotlos Sakai, 1980, is closely related to Oreophorus Rüppell, 1830 and Atlantotlos Doflein, 1904, the genus is actually morphologically most similar to Merocryptus A. Milne-Edwards, 1873. The two genera, however, still differ markedly in a number of key carapace and cheliped characters.

A new species of semi-terrestrial crab of the genus Geosesarma (Sesarmidae) is described from a limestone cave in central Sarawak, Malaysian Borneo. Geosesarma sodalis sp. nov. is characterised by its quadrate carapace, absence of a flagellum on the exopod of the third maxilliped, presence of 10 or 11 sharp tubercles on the dactylus of the chela and a diagnostic male first gonopod structure. This is the sixth species of Geosesarma reported from Sarawak, and the first member of the genus collected from inside caves.

The taxonomy of the potamid freshwater crabs of the Johora tahanensis (Bott, 1966) species group (Potamoidea) is revised. Seven species are recognised, all from Peninsular Malaysia and southern Thailand, three of which are described as new. The three new species were previously identified as J. tahanensis but can be distinguished by characters of the carapace, male first gonopod, and vulva. A revised key to the 18 recognised species of Johora Bott, 1966, is provided.

The taxonomy of the deep-water spider crabs of the genus Scyramathia A. Milne-Edwards, 1880, is revised and four extant species are recognised from the Atlantic and western Indian Ocean: S. carpenteri (Norman, in Thomson 1873) (type species), S. umbonata (Stimpson, 1871), S. hertwigi Doflein, in Chun 1900, and S. tenuipes sp. nov. Scyramathia tenuipes sp. nov. from the Mediterreanean is easily distinguished from its congeners by its slender and elongate ambulatory legs. All species are diagnosed and figured. The taxonomy of two allied genera from the Atlantic and Mediterranean, Rochinia A. Milne-Edwards, 1875, and Anamathia Smith, 1885, are also treated and their type species redescribed and figured.

The giant isopod genus Bathynomus A. Milne-Edwards, 1879, is recorded for the first time in Indonesian waters, from deep waters off southern Java in the Indian Ocean. Bathynomus raksasa sp. nov. is described and notes on juvenile specimens of an unidentified species found in the same locality are also provided. Bathynomus raksasa sp. nov. is characterized by the large size (averaging at 330 mm), narrowly rounded clypeus apex, prominent longitudinal carina on the clypeus, convex lateral margins of the uropodal exopod and endopod, produced distolateral corners of the uropodal exopod and endopod which have acute ends, an uropodal exopod with a setal fringe of medium length (69%), a pleotelson 1.6 times wider than long with the posterior margin medially concave, and the large number (11–13) of spines on the pleotelson.

Abstract Five species of pilumnid crabs assigned to Eurycarcinus A. Milne-Edwards, 1867, Heteropanope Stimpson, 1858, and Pilumnopeus A. Milne-Edwards, 1867, have been reported from India: E. orientalis A. Milne-Edwards, 1867, E. bengalensis Deb, 1999, H. glabra Stimpson, 1858, H. neolaevis Deb, 1995, and P. convexus (Maccagno, 1936). The identity of E. bengalensis is confused and the species had been provisionally transferred to Heteropanope. Examination of the types however, confirms the affinities of the species with Eurycarcinus and consequently extends the range of the genus to the eastern Indian Ocean. A re-examination of the types of H. neolaevis show that it is a junior subjective synonym of Aniptumnus quadridentus (De Man, 1895), and is the first record of the genus from India. Eurycarcinus integrifrons De Man, 1879, is also formally recorded for the first time from India.

Abstract Mursia spiridonovi Karasawa, 2018 known only from the western Indian Ocean, is the smallest species of the genus, measuring only 18 mm in carapace length. The species is redescribed from fresh material collected in Tamil Nadu in southern India. It is compared with similarly sized specimens of M. bicristimana Alcock and Anderson, 1894, all of which are immature. Mursia spiridonovi is characterised not only by its occurrence in shallow waters (less than 300 m depth) and small adult size but also by the prominently granular carapace and structure of the cheliped merus, male pleonal somites and gonopods.

A new species of freshwater crab of the genus Indochinamon Yeo & Ng, 2007 (family Potamidae), is described from highlands north of Myitkyina in Kachin State, Myanmar. Indochinamon khinpyae sp. n. is distinguished from congeners by its very rugose carapace, broad male pleon and distinctively structured male first gonopod; and is the first potamid species recorded from northern Myanmar.

The identity of the majid species of Paramaya De Haan, 1837, in the Indian Ocean is clarified with the collection of fresh specimens from the Bay of Bengal. Previously identified as P. spinigera (De Haan, 1837) which is known only from Japan, Taiwan, and Korea, the material from eastern India is here referred to a new species, P. mulli sp. n. The new species can easily be distinguished from all congeners by its relatively shorter pseudorostral and carapace spines, more swollen branchial regions, distinctly granulated male thoracic sternum, and the G1 is not prominently curved with the dorsal projection on the sub distal part short and the tip rounded.

Seven species of freshwater crabs from three families are recorded from and around the Danum Valley Conservation Area in Sabah, Malaysian Borneo: Thelphusula capillodigitus sp. n., Thelphusula dicerophilus Ng & Stuebing, 1990, Arachnothelphusa terrapes Ng, 1991, Terrathelphusa secula Ng & Tan, 2015, Parathelphusa valida Ng & Goh, 1987 (new record) (Gecarcinucidae); Isolapotamon ingeri Ng & Tan, 1998 (Potamidae); and Geosesarma danumense Ng, 2002 (Sesarmidae). The new species of Thelphusula Bott, 1979, can be distinguished from all congeners by a unique combination of morphological features, most notably the presence of dense patches of short setae on the fingers of the adult male chelipeds, as well as the structure of the male first gonopod. Arachnothelphusa terrapes is confirmed to be a phytotelm species. A key to all species in the conservation area is provided.

A new species of freshwater chasmocarcinid crab, Australocarcinus insperatus sp. n., is described from the Seychelles Islands in the Indian Ocean. This is the first record of the genus and the subfamily Trogloplacinae Guinot, 1986, from the Indian Ocean, with all other members previously recorded from Australia, New Britain, New Caledonia, and Palau in the Pacific Ocean. The disjunct distribution of Australocarcinus is unexpected considering all trogoplacines are believed to practice direct development, lacking free-swimming larval stages. The new species is morphologically most similar to A. riparius Davie, 1988, from Queensland, Australia, but can be distinguished from its three congeners on the basis of the structures of its carapace, ambulatory legs and male first gonopod.

A new species of deep-water oregoniid spider crab of the genus Pleistacantha Miers, 1879, is described from the Indian Ocean. The species is distinct in possessing a prominently inflated carapace in which the median parts of the branchial regions almost meet. It can also be distinguished from its closest congeners, P. moseleyi (Miers, 1885), P. pungens (Wood-Mason, in Wood-Mason and Alcock 1891), and P. ori Ahyong & Ng, 2007, in its more elongate and less spinose chelipeds and ambulatory legs, shorter third maxilliped, trapezoidal male pleon and a male first gonopod which is relatively stout with a short subdistal dorsal papilla.

A new species of lowland freshwater crab of the family Gecarcinucidae, Salangathelphusa peractio, is described from Langkawi, an island off the northwestern coast of peninsular Malaysia. Salangathelphusa peractio sp. n. can be separated from S. brevicarinata (Hilgendorf, 1882) in having a proportionately broader external orbital tooth, a distinctly concave posterolateral margin, and the terminal segment of the male first gonopod is not distinctly bent laterally outwards; and from S. anophrys (Kemp, 1923) by its more quadrate carapace and the terminal segment of the male first gonopod possessing a relatively longer and less curved distal part. This is sixth wholly freshwater brachyuran species known from the island.

Two brachyuran species of the families Dynonemidae and Iphiculidae are reported from red coral beds in northern Taiwan. The dynonemid Acanthodromia margarita (Alcock, 1899) has hitherto been reported from the Andaman Sea, Japan, and Philippines and the species is here recorded for the first time from Taiwan. A new species of iphiculid, Pariphiculus stellatus sp. n., is also described. The new Pariphiculus, which also occurs in the Philippines, is superficially similar to P. agariciferus Ihle, 1918, a species known from Indonesia, Japan, Philippines, South China Sea, Taiwan, and Vanuatu, but can be distinguished by distinct carapace, pleonal and male first gonopod features.

Abstract The rare raninid crab Umalia trirufomaculata (Davie and Short, 1989) is recorded from India for the first time. It is redescribed and compared at length with the related but poorly known U. misakiensis (Sakai, 1937) on the basis of a good series of specimens from the Philippines. Some of the supposed differences which have been documented between the two taxa (e.g., carapace proportions and features of the cheliped) are not valid; while new distinguishing characters associated with the male pleon and gonopods are observed.

Abstract Trawls from the Bay of Bengal obtained material of a new species of the goneplacid crab, Carcinoplax mistio, the sixth species of the genus known from India. The new species superficially resembles C. purpurea Rathbun, 1914, and C. sinica Chen, 1984, from the western Pacific. Records of C. sinica from the Persian Gulf are also here referred to the new species. Carcinoplax mistio n. sp. differs from its closest congeners mainly in the form of the carapace, anterolateral armature, proportions of the ambulatory legs and structure of the male pleon.

The pea crab genus Serenotheres Ahyong & Ng, 2005 (Pinnotheridae) is currently only represented by one species, S. besutensis (Serène, 1967). A new species is now assigned to this genus, described from a date mussel Leiosolenus obesus Carpenter, 1857 (Mollusca: Bivalvia: Mytilidae: Lithophaginae) collected in the Solomon Islands. Serenotheres janus sp. n. differs from S. besutensis in possessing a conspicuously broader carapace, with the lateral margins of the dorsal lamellum distinctly produced and the posterolateral part deeply concave, the dorsal lamellum being highest at the median cleft, the rostrum is relatively more prominent, the surfaces of the anterolateral margin and hepatic region are less prominently pitted and eroded, the ischiomerus of the third maxilliped is relatively more rectangular, and the P2 merus is proportionately longer.

Abstract Two boxes in the U.S. National Museum of Natural History, Smithsonian Institution, were found to contain the dried type material of the land crabs Cardisoma hirtipes Dana, 1851, and Cardisoma obesum Dana, 1851 (Gecarcinidae). The types of the common Pacific land crab, C. hirtipes, were believed to have been lost and a neotype had been designated in earlier revisions. Cardisoma obesum Dana, 1851 is currently regarded as a junior subjective synonym of Cardisoma carnifex (Herbst, 1796).

Abstract A new species of the freshwater crab genus Sundathelphusa Bott, 1969 is described from the Indonesian island of Lombok. Sundathelphusa tuerkayi, new species, appears to be most closely related to Sundathelphusa aruana (Roux, 1911), which is known for certain only from the Aru Islands, but differs from it and all other congeners by a unique combination of morphological characters. The new crab hosts a small ectosymbiotic temnocephalid flatworm, possibly belonging to the genus Temnosewellia Damborenea and Cannon, 2001 (Platyhelminthes: Temnocephalida).