What is Sesarmops impressus (H. Milne Edwards, 1837) (Crustacea: Brachyura: Sesarmidae)?

© 2020 Academia Sinica, Taiwan Open Access What is Sesarmops impressus (H. Milne Edwards, 1837) (Crustacea: Brachyura: Sesarmidae)? Peter K. L. Ng1, Jheng-Jhang Li2,3, and Hsi-Te Shih4,* 1Lee Kong Chian Natural History Museum, National University of Singapore, 2 Conservatory Drive, Singapore 117377, Republic of Singapore. E-mail: [email protected] 2Department of Marine Biotechnology and Resources, National Sun Yat-sen University, 70, Lianhai Road, Kaohsiung 80424, Taiwan. E-mail: [email protected] 3East Peak Ecological Consultants, Inc., 22, Wanggong Road, Linyuan Dist., Kaohsiung 83249, Taiwan 4Department of Life Science and Research Center for Global Change Biology, National Chung Hsing University, 250, Kuo Kuang Road, Taichung 402, Taiwan. *Correspondence: E-mail: [email protected] (Shih) Received 28 March 2020 / Accepted 13 May 2020 / Published 10 July 2020 Communicated by Benny K.K. Chan 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. Key words: Taxonomy, New species, Sesarmops impressus, S. indicus, S. imperator, S. atrorubens, S. similis, Morphology, Mitochondrial 16S rDNA, Cytochrome oxidase subunit I. Citation: Ng PKL, Li JJ, Shih HT. 2020. What is Sesarmops impressus (H. Milne Edwards, 1837) (Crustacea: Brachyura: Sesarmidae)? Zool Stud 59:27. doi:10.6620/ZS.2020.59-27. BACKGROUND Serène and Soh (1970) established a new genus, Sesarmops, for six Indo-West Pacific sesarmid species: S. atrorubens (Hess, 1865), S. impressus (H. Milne Edwards, 1837) (designated as type species), S. intermedius (De Haan, 1835), S. mindanaoensis (Rathbun, 1914), S. sinensis (H. Milne Edwards, 1853) and S. weberi (De Man, 1892). In doing so, they expressed doubts about S. intermedius (De Haan, 1835) and S. sinensis (H. Milne Edwards, 1853), noting that they may need to be moved elsewhere in the future. Ng et al. (2008: 223) recognised six species in Sesarmops, commenting that the placement of S. intermedius, S. sinensis and S. weberi were provisional and they actually belonged to other genera. Paulay and Starmer (2011) subsequently transferred another species, Sesarma angustifrons A. Milne-Edwards, 1869, to the genus. Serène and Soh (1970: 400) defined Sesarmops as follows: carapace as long or longer than the width measured at the external orbital teeth, the frontal margin has a deep median concavity, the postfrontal lobes are prominent, the external orbital tooth is clearly separated from the rest of the lateral margin by a distinct cleft, with the gastric and cardiac regions well defined. In their key to the genera, in which they also cited the taxa as separate, Serène and Soh (1970: 391) had Sesarmops grouped with Pseudosesarma Serène & Soh, 1970, and Zoological Studies 59:27 (2020) doi:10.6620/ZS.2020.59-27 1 © 2020 Academia Sinica, Taiwan Bresedium Serène & Soh, 1970. Bresedium supposedly has the male telson deeply inserted into the distal margin of somite 6 and the chitinous distal part of the male first gonopod is distinctly elongate. Pseudosesarma and Sesarmops do not have these characters. The problem is that the type species of Sesarmops, S. impressus, actually has the male telson also inserted into the distal margin of somite 6 and the chitinous distal part of the male first gonopod is also long. The composition of Sesarmops and its close affinities with Bresedium Serène & Soh, 1970 have been discussed at length by several authors (Ng et al. 2008; Li et al. 2020; Schubart and Ng 2020), but until a complete revision is done, not much can be said. Schubart and Ng (2020) formally transferred S. intermedius and S. sinensis to a new genus with several other species previously placed in Chiromantes Gistel, 1848. Li et al. (2020) also noted that S. mindanaoensis (Rathbun, 1914) is not a member of Sesarmops as it has a very different male thoracic sternum, pleon and male first gonopod, and is also genetically distinct (see also Schubart and Ng 2020). The same situation exists for S. angustifrons which is genetically more allied with S. mindanaoensis than other Sesarmops species (Schubart and Ng 2020; unpublished data). In their revision of Labuanium Serène & Soh, 1970, Naruse and Ng (2020) treated S. weberi which Ng et al. (2008) believed was close to Labuanium trapezoideum (H. Milne Edwards, 1837). Naruse and Ng (2020) showed that S. weberi and L. trapezoideum are unlikely to be related but deferred to a decision about where to place S. weberi until a revision of the genus could be done. Christoph Schubart and his colleagues are now revising Sesarmops and Bresedium and will recognise more taxa in their ongoing attempts to derive a more monophyletic classification of sesarmid genera. Until this can be done, any revised diagnosis for Sesarmops would not be helpful. As such, the broad concept of Sesarmops used by Serène and Soh (1970) (see above) is followed in this study. One problem, however, that needs to be resolved here is with regards to the identity of the type species, Sesarma impressa H. Milne Edwards, 1837, whose type locality is unknown. Two species, Sesarma similis Hess, 1865 (from “Sydney”), and Sesarma frontale A. Milne-Edwards, 1869 (from Madagascar), are currently regarded as junior synonyms of this species (see De Man 1887; Crosnier 1965; Ng et al. 2008). Sesarmops impressus is one of the more common sesarmids inhabiting mangrove and fresh water habitats from Madagascar to the western Pacific and has often been reported in the literature. The first author has suspected there was a problem with the taxonomy of this species, as the life colours of the material from Madagascar, Comoros and Mayotte (e.g., Bouchard et al. 2013) is very different from those from the Philippines, Taiwan and Japan (e.g., Lee 2001; Liu 2009; Liu and Wang 2010; Li and Chiu 2013; W.-J. Chen and Lo 2014). Bouchard et al. (2013: 24) commented that “Sesarmops impressus is part of a species complex with at least two forms in the IWP [Indo-West Pacific]. The Indian Ocean population generally has white chela and a yellowish-white carapace. This is probably the real Sesarmops impressus. The ‘Sesarmops impressus’ reported from the East [sic] Pacific (e.g., Taiwan, China, Japan, Philippines, New Guinea, and eastern Indonesia) has dark brown carapaces with deep red chela. Their gonopods also differ.” The genetic study by Li et al. (2020) supports this observation. A revision of the species was also compounded by the uncertainty over the identity of Sesarmops atrorubens (Hess, 1865), which superficially resembles S. impressus. One of the problems with the genus is associated with the identities of the two species described by Hess (1865) ostensibly from “Sydney” – S. atrorubens and S. similis. Both species were described based on one specimen each and only briefly, with only S. atrorubens figured. The material was supposed to be deposited in Göttingen University. De Man (1887) re-examined Hess’ material, reidentified the species, and discussed at length the various taxonomic problems associated with his specimens. He managed to compare the types of S. atrorubens and S. similis with the type and other specimens of Sesarma impressa from Madagascar. He decided that S. atrorubens is a valid species while S. similis is synonymous with S. impressa. Although De Man’s (1887) discussion is detailed, he had few specimens available and did not provide any figures. Neither species has been reported in recent years, nor have they been described to modern standards, with the important male pleonal and gonopodal characters not known. The provenance of the two types is also not known. The extant decapod crustacean material of Hess (1865) is known to be deposited in the Senckenberg Museum, which incorporated the Göttingen University material as part of its collections (see Ahyong and Ng 2007; Ng 2012; K. Sakai and Türkay 2013; Naruse and Ng 2020). Some material is in The Naturalis Biodiversity Institute and was obtained by exchange (Fransen et al. 1997). The Swedish Museum of Natural History in Stockholm also has some hermit crab material (see Sandberg and McLaughlin 1993; Komai and Mishima 2003). A search of these museums with the help of the various curators failed to find the type of S. similis or S. atrorubens. The types must thus have been lost since 1887. As discussed by Ng (2012: 269), the reason why “Sydney” may have been indicated as the type locality is simply because it was the main port of call for many European vessels page 2 of 39Zoological Studies 59:27 (2020) © 2020 Academia Sinica, Taiwan trading in the European territories in that period, with collections actually originating from the South Pacific Islands. In the present work, neotypes were selected from the South Pacific islands to stabilise the taxonomy of these species. The authors examined the types of H. Milne Edwards and A. Milne-Edwards in Paris, as well as a good series of what has been traditionally called “Sesarmops impressus” from various parts of the Indo-West Pacific. Molecular evidence from the mitochondrial 16S rDNA and cytochrome c oxidase subunit I (COI) (see Li et al. 2019 2020; Shih et al. 2019a) was also used to support the morphological observations as far as possible. Material from the eastern Indian Ocean is also shown here to be different from S. impressus, and is referred to a new species. Most of the material from Southeast Asia and the western Pacific previously referred to as “Sesarmops impressus” is also considered a new species. The types of the two species described by Hess are lost. Fresh specimens matching Hess’ original description and figures of S. atrorubens were obtained from Fiji and the species is redescribed with the selection of a neotype. Sesarma similis is here recognised as a distinct species, a neotype is selected and the generic affinities are discussed. MATERIALS AND METHODS Specimens examined are deposited in the Muséum national d’Histoire naturelle, Paris (MNHN); Nationaal Natuurhistorisch Museum—Naturalis, Leiden (previously Rijksmuseum van Natuurlijke Historie, RMNH); Zoological Collections of the Department of Life Science, National Chung Hsing University, Taichung, Taiwan (NCHUZOOL); Senckenberg Museum und Forschungsinstitut, Frankfurt am Main (SMF); and the Zoological Reference Collection of the Lee Kong Chian Natural History Museum (previously Raffles Museum of Biodiversity Research), National University of Singapore (ZRC). Measurements provided, in millimetres, are of the carapace width and length, respectively. The abbreviations G1 and G2 are used for the male first and second gonopods, respectively. The terminology used follows Davie et al. (2015). Part of the sequences of the mitochondrial 16S rDNA and COI in Li et al. (2020) were used, including the sequences of two clades of S. impressus. In addition, we tried to sequence the specimens from India (ZRC 2015.0344a–c), Fiji (ZRC 2019.1070) and Samoa (ZRC 2017.0008) by following the method described by Shih et al. (2016). The sequencing for specimens from India and Samoa failed, probably because the specimens were too old or had previously been preserved in formalin. The sequences of the Fijian specimen and additional specimen from Mayotte (ZRC 2011.0005) have been deposited in the DNA Data Bank of Japan (DDBJ), under accession numbers LC547017-LC547020. For the combined 16S and COI dataset, the bestfitting models for sequence evolution of individual datasets were determined by PartitionFinder (vers. 2.1.1, Lanfear et al. 2017), selected by the Bayesian information criterion (BIC). The best models obtained were both GTR+I+G and were subsequently applied to the partitioned Bayesian inference (BI) analysis. The BI was performed with MrBayes (vers. 3.2.6, Ronquist et al. 2012). The search was run with four chains for 10 million generations and four independent runs, with trees sampled every 1000 generations. The convergence of chains was determined by the average standard deviation of split frequency values below the recommended 0.01 (Ronquist et al. 2005) and the first 600 trees were discarded as burnin. The maximum likelihood (ML) analysis was conducted in RAxML (vers. 7.2.6; Stamatakis 2006). Because RAxML does not accept the GTR + I model, the second-best model, GTR + G (i.e., GTRGAMMA), was used with 100 runs, and the best ML tree was found by comparing the likelihood scores. The robustness of the ML tree was evaluated by 1000 bootstrap pseudoreplicates under the model GTRGAMMA. Basepair (bp) differences and pairwise estimates of Kimura 2-parameter (K2P) distances (Kimura, 1980) for genetic diversities of COI between specimens were calculated with MEGA (vers. 10.0.5, Kumar et al. 2018). RESULTS TAXONOMY Family Sesarmidae Dana, 1851 Genus Sesarmops Serène & Soh, 1970 Type species: Sesarma impressa H. Milne Edwards, 1837, by original designation; gender masculine. Sesarmops impressus (H. Milne Edwards, 1837) (Figs. 1–6, 13A–D, 14A–C, 15A–G, 16A–G, N–P) Sesarma impressa H. Milne Edwards, 1837: 74 (no locality); H. Milne Edwards 1853: 186 (no locality); De Man 1887: 653, 671 (Madagascar); Lenz 1905: 370 (Zanzibar). Sesarma frontale A. Milne-Edwards, 1869: 27 (Madagascar). Sesarma frontalis – De Man 1887: 649 (Madagascar). Sesarma nodulifera – Lenz 1910: 562 (Comoros). ?Sesarma impressa – Lenz 1910: 561 (Madagascar). page 3 of 39Zoological Studies 59:27 (2020) © 2020 Academia Sinica, Taiwan Sesarma (Sesarma) impressum – Crosnier 1965: 63, figs. 82, 92, 104 (Madagascar, Comoros); Serène 1968: 106. Sesarma (Sesarma) impressa – Guinot 1967: 287 (list, Comoros). Sesarmops impressum – Serène and Soh 1970: 401, 406 (part), pl. 7 figs. A, B (no locality data). Sesarmops impressus – Keith et al. 2006: 36 (Comoros, Mayotte); Ng et al. 2008: 223 (list); Bouchard 2009: 6, 15 (Mayotte); Bouchard et al. 2013: 24, fig. 19 (Mayotte); Poupin et al. 2018: 72 (Mayotte). Sesarmops impressus 1 – Li et al. 2020: fig. 22 (Mayotte; Madagascar). Material examined: Lectotype: female (40.6 × 38.0 mm) (MNHN-IU-2000-3959 = MNHN-B3959) [dried], no locality. 1 male (28.1 × 26.0 mm) (MNHNIU-2000-3667 = MNHN-B3667) [dried, lectotype of Sesarma frontale A. Milne-Edwards, 1869], Nosy Bé, coll. Boivin (lectotype of Sesarma frontale A. Milne-Edwards, 1869); 1 male (29.7 × 25.6 mm), 1 female (26.0 × 22.1 mm) (MNHN-IU-2000-10929 = MNHN-B10929) [dried, paralectotypes of Sesarma frontale A. Milne-Edwards, 1869], same data as lectotype. Others: Madagascar: 1 young male (MNHNIU-2019-4518 = MNHN-B30336), Antanambé, coll. E. Chevreux, October 1896; 5 males, 1 young male, 1 young female (MNHN-IU-2019-4524 = MNHN-B16678), Antongil Bay, coll. E. Chevreux, 1899; 1 male (MNHN-IU-2019-4513 = MNHN-B16679), Antongil Bay, no other data; 1 ovigerous female (MNHN-IU-2019-4523), Mahomba, coll. E. Chevreux, October 1896; 1 female, 1 ovigerous female (MNHN-IU-2019-4515 = MNHN-B25740), Marosoroka River, 39 km south of Maroantsetra, coll. 12 September 1965; 2 males (30.2 × 27.8 mm, 24.8 × 22.3 mm) (MNHN-IU-2019-4517 = MNHN-B25776), Nosy Bé, coll. Millot, autumn 1922; 2 males (MNHNIU-2019-4520 = MNHN-B30338), Nosy Bé, coll. Millot, 1920s; 2 males, 1 female (MNHN-IU-2019-4516 = MNHN-B25743), Lokobe, Nosy Bé, coll. May 1966; 6 males (largest 32.2 × 29.2 mm), 2 females (27.6 × 25.2 mm, 23.8 × 21.5 mm), 3 ovigerous females (28.1 × 25.0 mm, 26.8 × 23.0 mm, 22.3 × 19.7 mm) (MNHNIU-2019-4521 = MNHN-B30343), Antongil Hiaraka, coll. R. Pereiras, November 1967; 3 small males (largest 21.7 × 19.9 mm) (MNHN-IU-2019-4519 = MNHN-B30337), Sainte Marie, coll. E. Chevreux, 1897; 1 male (MNHN-IU-2000-3958 = MNHN-B3958) [dried], west coast of Madagascar, Lanz coll. 1 female (22.1 × 20.8 mm) (ZRC 2010.302), Masoala Peninsula, eastern Madagascar, coll. W. Emmerson, June 2005. Mayotte: 3 males (largest 39.6 × 36.5 mm) (MNHNIU-2009-953 = MNHN-B32034), station 41a, near cultivated mangroves, 12°55'S 45°9'E, coll. J. M. Bouchard, KUW MAYOTTE 2009, 12 December 2008; 1 female (35.0 × 30.2 mm) (ZRC 2011.4), station 9, Bassin Mro Oua Ourovéni, coll. Tervuren Museum, 11 October 2000; 1 male (36.1 × 33.1 mm) (ZRC 2011.5), station 9, Bassin Mro Oua Ourovéni, coll. Tervuren Museum, 11 October 2000. Seychelles: 3 males (31.3 × 29.7 mm, 30.7 × 28.5 mm, 27.1 × 25.0 mm), 1 female (28.5 × 25.7 mm) (ZRC 2009.896), Mahé, coll. 2009; 1 male 1 female (MNHN-IU-2019-4514 = MNHN-B29630), Mahé Island, coll. P. Keith et al., 14 October 2003; 1 female (MNHN-IU-2019-4522 = MNHN-B28938), Mahé Island, coll. P. Keith et al., October 2003. Diagnosis: Male: Carapace almost quadrate; external orbital tooth with outer margin convex, especially along posterior part, separated from lateral margin by distinct cleft (Figs. 2A, 3A, 4, 6A, B, 13A–D); epibranchial tooth short but distinct, sometimes with low lobe posterior to it, never dentiform, sometimes undiscernible (Figs. 13A–D, 14A); outer surface of chela covered with numerous smooth rounded granules; inner surface granulated, with strong transverse median ridge on proximal part, lined with 7–11 tubercles and granules; dorsal margin of dactylar finger with randomly arranged granules, no trace of stridulatory row (Figs. 2C, 5F–I); ambulatory merus relatively short, wide, propodus and dactylus relatively short, wide (Figs. 3A, D, 4, 5A–C, 6A, B, 14B); male pleon transversely wide, somite 6 subtruncate, much wider than long with convex lateral margins; telson wider than long, distinctly inserted into concave distal margin of somite 6 (Figs. 3C, 5A–C, 14C); G1 relatively slender, elongate, distal half of stem expanded, inner margin with gentle but obvious subdistal hump, subdistal part of outer margin gently sloping with chitinous distal part long, gently curved to angle of ca. 60° from longitudinal axis (Figs. 15A–G, 16A–G, N–P). Female: telson inserted into distal margin of somite 6, lateral margins of somite 6 gently convex (Figs. 2B, 6C); vulvae anterior half of sternite 6, just touching margin with sternite 5, posterior edge forming bilobed sternal vulval cover, operculum low, non-protruding (Fig. 6D). Description of male: Carapace almost quadrate, widest point at edge of external orbital tooth posterolateral margin near base of third ambulatory legs (Figs. 2A, 3A, 4, 13A–D); dorsal surface rugose, small flattened granules and striae, regions clearly defined with distinct grooves, with only scattered short stiff setae on margins; not inflated in frontal view (Figs. 3A, B, D, 5D); surface adjacent lateral margins with distinct oblique striae; median gastro-cardiac groove distinct (Figs. 3A, B, D–E, 4). Front deflexed downwards, margin sinuous, bilobed, lobes separated by wide concavity, margin sinuous to gently convex (Figs. 3A, B, D, E, 4, 5D). Supraorbital margin separated from frontal margin by low angle, no lateral tooth, margin entire, page 4 of 39Zoological Studies 59:27 (2020) © 2020 Academia Sinica, Taiwan strongly convex, confluent with acutely triangular external orbital tooth, outer margin convex, especially along posterior part, separated from lateral margin by distinct cleft, sometimes deep; epibranchial tooth short but distinct, sometimes with low lobe posterior to it, never dentiform, sometimes undiscernible (Figs. 3A, D, 4). Anteroand posterolateral margins not demarcated, posterolateral area indicated only by presence of stronger striae, margin entire, gently sinuous to almost straight, gently divergent (sometimes subparallel) towards almost straight posterior carapace margin (Figs. 3A, D, 4). Epistome relatively narrow, transversely subtruncate, posterior margin with obtuse low median triangular tooth, lateral margins gently concave (Figs. 3B, E, 5D). Antennule large, transversely ovate, separated from base of antenna by low tooth, with distinct hiatus between them. Antenna relatively short; basal article subovate; flagellum entering orbit. Third maxilliped slender; ischium shorter than merus, with oblique submedian sulcus; merus subovate with distinct median ridge extending to anteroexternal angle of ischium; exopod slender, reaching to midlength of merus, with long flagellum, longer than width of merus (Fig. 5E). Chelipeds subequal, stout in adults (Figs. 3A, D, 4). Basis, ischium separated by distinct suture; inner margin with clusters of sharp granules (Fig. 5A– C). Inner margin of merus lined with sharp granules of varying sizes with distal one largest; margins not lamelliform; outer margin gently convex, granulated; inner surface with oblique row of stiff setae (Fig. 5A– C). Outer surface of carpus distinctly squamiform; inner distal angle weakly or not produced, lined with short tubercles or relatively long spines (especially in smaller specimens) (Figs. 3A, B, E, 4, 5A–C). Adult chela high, outer surface prominently covered with numerous smooth rounded granules; inner surface granulated, with strong transverse median ridge on proximal part, lined with 7–11 tubercles and granules (Figs. 3A–E, 4, 5A– C, F–I); dorsal margin of palm granulated (Figs. 3A– C, 4, 5F, H); ventral margin with median part concave, granulated (Figs. 5F–I, 3C, E); fingers as long as palm, slightly gaping when closed (Figs. 3E, 5F–I); propodal finger gently curved, cutting margin with 3–5 large teeth on proximal half, usually with 1 subdistal tooth (Figs. 3E, 5F–I); dactylar finger curved, more slender, dorsal Fig. 1. Sesarmops impressus (H. Milne Edwards, 1837). A, forest in Madagascar (photograph: Winks Emmerson); B, in riverbank in Comoros (photograph: Matthias Deuss); C, D, mangrove in Mohéli Island, Comoros (photographs: Gianni Di Marco). Specimens not collected. page 5 of 39 Zoological Studies 59:27 (2020) © 2020 Academia Sinica, Taiwan margin distinctly granulated along proximal third, remaining margin with low granules to almost smooth, granules not arranged in stridulatory row (Figs. 3E, 5F– I); tips of both fingers chitinised, gently excavated on inner surface (Figs. 3E, 5F–I). Ambulatory legs relatively short, stout, second pair longest (Figs. 3A, D, 4). Outer surfaces of merus, with distinct striae; outer surface of carpus and propodus smooth except for ridges (Figs. 3A, D, 4). Meri laterally flattened, dorsal margin gently cristate, gently serrate, with sharp, subdistal dorsal spine (Figs. 3A, D, 4). Outer surface of carpus of first to third legs with 2 low carinae; that of fourth leg with distinct carina (Figs. 3A, D, 4). Propodi of first to third legs with long, simple setae that partially obscures margins, that on third leg least setose; ventral margin of first propodus with very dense mat of short setae, absent on propodi of other legs; surface of fourth propodus almost glabrous except for scattered short setae (Figs. 3C, 4, 5A–C). Dactylopropodal lock not distinct. Inner surface of coxae of first to third ambulatory legs without tufts of dense short setae, with scattered short setae at most. Surface, margins of anterior thoracic sternites setose. Sternites 1, 2 completely fused, forming triangular structure (Figs. 3C, 5C), separated from sternite 3 by suture, median part shallow, concave toward buccal cavity; sternites 3, 4 fused but demarcated by low transverse setose groove (Fig. 5C). No visible pleonal locking tubercle on sternite 5, locking mechanism formed by enlarged rim-like anterior edge of sternopleonal cavity on sternite 4. Sternites 4/5, 5/6, 6/7, 7/8 medially interrupted, with median longitudinal Fig. 2. Sesarmops impressus (H. Milne Edwards, 1837), lectotype female (40.6 × 38.0 mm) (MNHN-IU-2000-3959), no locality. A, overall dorsal view; B, ventral view of cephalothorax; C, outer view of left chela. page 6 of 39 Zoological Studies 59:27 (2020) © 2020 Academia Sinica, Taiwan Fig. 3. Sesarmops impressus (H. Milne Edwards, 1837). A–C, male (28.1 × 26.0 mm) (MNHN-IU-2000-3667) (dried, lectotype of Sesarma frontale A. Milne-Edwards, 1869), Madagascar; D–F, female (26.0 × 22.1 mm) (MNHN-IU-2000-10929) (dried, paralecotype of Sesarma frontale A. Milne-Edwards, 1869), Nosy Be, Madagascar. A, D, overall dorsal view; B, E, F, frontal view of cephalothorax and chelae; C, ventral view of cephalothorax. page 7 of 39 Zoological Studies 59:27 (2020) © 2020 Academia Sinica, Taiwan Fig. 4. Sesarmops impressus (H. Milne Edwards, 1837), overall dorsal view. A, male (30.2 × 27.8 mm) (MNHN-IU-2019-4517), Madagascar; B, male (31.3 × 29.2 mm) (MNHN-IU-2019-45212), Madagascar; C, male (39.6 × 36.5 mm) (MNHN-IU-2009-953), Mayotte. page 8 of 39 Zoological Studies 59:27 (2020) © 2020 Academia Sinica, Taiwan groove on sternites 7 and 8. Sternopleonal cavity deep, reaching to just before margin of fused sternites 3, 4, to imaginary line joining anterior edges of coxae of chelipeds (Fig. 5A–C). Penis long, curved, partially chitinised. Pleon transversely wide, subtriangular; all somites, telson free (Figs. 3C, 5A–C). Somites 1–3 widest; somites 4, 5 transversely trapezoidal; somite Fig. 5. Sesarmops impressus (H. Milne Edwards, 1837). A, D, E, male (30.2 × 27.8 mm) (MNHN-IU-2019-4517), Madagascar; B, F, G, male (31.3 × 29.2 mm) (MNHN-IU-2019-45212), Madagascar; C, H, I, male (39.6 × 36.5 mm) (MNHN-IU-2009-953), Mayotte. A–C, ventral view of cephalothorax; D, frontal view of cephalothorax; E, right third maxilliped; F, H, outer view of left chela; G, I, inner view of left chela. page 9 of 39 Zoological Studies 59:27 (2020) © 2020 Academia Sinica, Taiwan Fig. 11. Sesarmops imperator sp. nov. A, C, male (24.7 × 23.4 mm) (ZRC 2019.1816), Sulawesi; B, D–F, male (41.8 × 41.1 mm) (ZRC 2001.307), Philippines. A, overall dorsal view; C, D, ventral view of cephalothorax; E, outer view of left chela; F, inner view of left chela. page 16 of 39Zoological Studies 59:27 (2020) © 2020 Academia Sinica, Taiwan 2001; 2 males (37.6 × 33.7 mm, 35.4 × 32.3 mm), 1 female (23.0 × 20.7 mm) (NCHUZOOL 15069), lower reaches of Gangkou R., Manjhou, Pingtung, coll. J.-J. Li, 6 July 2017; 1 male (46.9 × 43.1 mm) (NCHUZOOL 15070), lower reaches of Gangkou R., Manjhou, Pingtung, coll. J.-J. Li, 4 April 2018; 2 females (29.0 × 26.3 mm, 28.9 × 26.8 mm) (NCHUZOOL 15071), lower reaches of Gangkou R., Manjhou, Pingtung, coll. J.-J. Li, 18 May 2019; 1 male (40.1 × 38.0 mm) (ZRC 2001.32), Kenting National Park, Pingtung, coll. P.K.L. Ng, 7 November 2000; 1 male (ZRC 1998.400), Hengchun Peninsula, Pingtung, coll. H.-C. Liu, 19 May 1998; 1 male (ZRC 2000.1858), Lanyu Island, Taitung, coll. C.D. Schubart et al., 20 September 1999; 1 male (35.2 × 34.6 mm) (NCHUZOOL 15869), Lanyu, Taitung, 7 April 2002; 1 male (ZRC 1999.1276), near Nanliao Fishport, Lyudao (= Green Island), Taitung, coll. P.K.L. Ng, 6 June 1993; 1 female (ZRC 2002.427), at night, Changbin, Taitung, 23°18'22"N, 121°24'14"E, coll. P.K.L. Ng and H.-C. Liu, 22 June 2002; 1 male (ZRC 2017.0984), near shore, along banks, Nioushan, Hualien, coll. J.-J. Li and P.Y.C. Ng, 14 June 2017; 5 males (13.7 × 12.7 mm, 15.5 × 14.8 mm, 18.2 × 17.3 mm, 20.3 × 19.4 mm, 30.3 × 29.6 mm), 4 females (12.6 × 11.4 mm, 13.1 × 12.0 mm, 21.8 × 20.8 mm, 31.6 × 30.4 mm) (NCHUZOOL 15868), Meilun R. estuary, Hualien, 29 July 2014; 8 males, 2 females (ZRC 2017.0985), near shore, along banks, Meilun River estuary, Hualien City, Hualien, coll. J.-J. Li and P.Y.C. Ng, 14 June 2017. Others: Philippines: 2 females (ZRC 2013.1337), Luyang Cave Park, Catanduanes Island, Catanduanes Province, San Andres, coll. 9 July 2004; 1 young male (ZRC 2017.308), Dagubdub River, Sibuyan Island, San Fernando town, España Barangay, coll. E.Y. Sy, 11 February 2013; 1 male (35.9 × 34.7 mm) (ZRC 2017.477), under rocks, downstream from falls, Ambakan Falls, Jordan Town, Barangay Poblacion, Guimaras Island, coll. J.C. Mendoza and L. Ganancial, 1 June 2017; 8 males (largest 41.8 × 41.1 mm), 1 female, 3 ovigerous females (ZRC 2001.307), Kawasan Falls, Matutinao, Cebu, coll. P.K.L. Ng et al., 20 December 2000; 4 males, 1 ovigerous female (ZRC 2001.2318), Kawasan Falls, Matutinao, Cebu, coll. P.K.L. Ng, 25 November 2001; 2 males, 1 female (ZRC 2019.1822), base of Kawasan Falls, Matutinao, Cebu, coll. P.K.L. Ng, 30 July 2003; 1 male (29.2 × 29.2 mm) (NCHUZOOL 15871), Camiguin I., coll. H.-T. Shih, 30 August 2003. Indonesia: 2 males, 2 females, 1 juvenile female (ZRC 2019.1821), station THH 03-04, near Murex Resort, Manado, northern Sulawesi, coll. H. H. Tan, 24–27 May 2003; 1 male Fig. 12. Sesarmops imperator sp. nov. A, C, D, ovigerous female (30.1 × 27.8 mm) (ZRC 2001.307), Philippines; B, ovigerous female (26.0 × 24.0 mm) (ZRC 2001.307), Philippines. A, B, overall dorsal view; C, pleon; D, sternopleonal cavity and vulvae. page 17 of 39 Zoological Studies 59:27 (2020) © 2020 Academia Sinica, Taiwan Fig. 13. Carapaces. A, Sesarmops impressus (H. Milne Edwards, 1837), lectotype female (40.6 × 38.0 mm) (MNHN-IU-2000-3959), no locality; B, S. impressus (H. Milne Edwards, 1837), male (39.6 × 36.5 mm) (MNHN-IU-2009-953), Mayotte; C, S. impressus (H. Milne Edwards, 1837), male (36.1 × 33.1 mm) (ZRC 2011.5), Comoros; D, S. impressus (H. Milne Edwards, 1837), male (30.2 × 27.8 mm) (MNHN-IU-2019-4517), Madagascar; E, Sesarmops imperator sp. nov., male (34.8 × 32.7 mm) (ZRC 2016.252), Sulawesi; F, S. imperator sp. nov., male (35.9 × 34.7 mm) (ZRC 2017.477), Philippines. page 18 of 39 Zoological Studies 59:27 (2020) © 2020 Academia Sinica, Taiwan (24.7 × 23.4 mm) (ZRC 2019.1816), rocky area in front of Murex Resort, Manado, Sulawesi, coll. N.K. Ng and H.H. Tan, 16 July 2003; 2 males, 2 females (ZRC 2019.1820), 1 male (34.8 × 32.7 mm) (ZRC 2016.252) near Murex Resort, Manado, northern Sulawesi, coll. P.K.L. Ng, 17 July 2003; 1 female (ZRC 2019.1057), Bitung, Pulau Lembeh, Mawali Bay, area near to NAD Resort, Manado, Sulawesi, coll. H.H. Tan December 2018; 1 juvenile male (MNHNIU-2019-4512 = MNHN-B16680), Saluta, Halmahera, coll. W. Kükenthal, 1893–1894; 2 males (42.9 × 40.0 mm, 39.0 × 37.8 mm) (ZRC 2012.412), Sungei Okito, Wasile, northern Halmahera, Moluccas, coll. P. Robb, August 1994; 1 male (RMNH 26603), in office of Land Office, Hollandia harbour, Djayapura, ca. 500 m from sea, Indonesian Papua, coll. J. Ham and H.W. Moll, 19 October 1954. Papua New Guinea: 1 female (ZRC 2019.1819), Ioa Cave, Pomio District, New Britain, coll. F. Brehier, 25 February 2002. Vanuatu: 1 male (27.9 × 26.2 mm) (ZRC 2019.1818), Santo, coll. Santo Expedition 2006; 1 female (with moult, 38.1 × 34.1 mm) (ZRC 2019.1817), Riorua, Santo, coll. L. Fig. 14. A–C, Sesarmops impressus (H. Milne Edwards, 1837), male (24.8 × 22.3 mm) (MNHN-IU-2019-4517), Madagascar; C, S. impressus (H. Milne Edwards, 1837), male (21.7 × 19.9 mm) (MNHN-IU-2019.4519), Madagascar; D–F, S. indicus sp. nov., holotype male (22.7 × 22.0 mm) (ZRC 2015.344), Andamans; G–I, S. imperator sp. nov., paratype male (22.8 × 21.6 mm) (ZRC 2001.307), Philippines. A, D, G, right side of carapace; B, E, H, right fourth ambulatory leg; C, F, I, male pleonal somites 5, 6 and telson. page 19 of 39Zoological Studies 59:27 (2020) © 2020 Academia Sinica, Taiwan Fig. 15. Right G1. A–G, Sesarmops impressus (H. Milne Edwards, 1837), male (30.2 × 27.8 mm) (MNHN-IU-2019-4517), Madagascar; H–M, S. indicus sp. nov., holotype male (22.7 × 22.0 mm) (ZRC 2015.344), Andamans; N–S, S. imperator sp. nov., holotype male (37.7 × 36.3 mm) (NTOU), Taiwan. A, H, N, left G1 (dorsal view); B, I, O, left G1 (ventral view); C, J, P, distal part of left G1 (dorsal view); D, K, Q, distal part of left G1 (ventromesial view); E, L, R, distal part of left G1 (ventral view); F, G, M, S, distal part of left G1 (dorsomesial views). page 20 of 39Zoological Studies 59:27 (2020) © 2020 Academia Sinica, Taiwan Fig. 16. Right gonopods. A–G, Sesarmops impressus (H. Milne Edwards, 1837), male (30.2 × 27.8 mm) (MNHN-IU-2019-4517), Madagascar; H–M, S. indicus sp. nov., holotype male (22.7 × 22.0 mm) (ZRC 2015.344), Andamans; N–P, S. impressus, male (24.8 × 22.3 mm) (MNHN-IU-2019-4517), Madagascar; Q–S, S. imperator sp. nov., male (22.8 × 21.6 mm) (ZRC 2001.307), Philippines. A, H, N, Q, left G1 (dorsal view); B, left G1 (ventral view); C, I, O, R, distal part of left G1 (dorsal view); D, J, distal part of left G1 (ventromesial view); E, K, P, S, distal part of left G1 (ventral view); F, L, distal part of left G1 (dorsomesial view); G, M, left G2. Scale bars: A, B, H, M, N, Q = 1.0 mm; C–F, I–L, O, P, R, S = 0.5 mm. page 21 of 39Zoological Studies 59:27 (2020) © 2020 Academia Sinica, Taiwan Fig. 17. Right gonopods. Sesarmops imperator sp. nov. A–E, holotype male (37.7 × 36.3 mm) (NTOU), Taiwan; F–K, male (41.8 × 41.1 mm) (ZRC 2001.307), Philippines. A, F, left G1 (dorsal view); B, G, distal part of left G1 (dorsal view); C, H, distal part of left G1 (ventromesial view); D, I, distal part of left G1 (ventral view); J, distal part of left G1 (dorsomesial view); E, K, left G2. Scale bars: A, E, F, K = 1.0 mm; B–D, G–J = 0.5 mm. Deharveng and A. Bedos, 17 September 2006. Diagnosis: Male: carapace subtrapezoidal, longer than wide in adults; external orbital tooth with outer margin gently convex to almost straight, separated from lateral margin by cleft (Figs. 10A, 11A, B, 13E, F); epibranchial tooth short, may possess barely discernible very low lobe posterior to it but never distinct or dentiform (Figs. 13, E, F, 14G); outer surface of chela covered with numerous smooth rounded granules; inner surface granulated, with strong transverse median ridge on proximal part, lined with 7–11 tubercles and granules; dorsal margin of dactylar finger with randomly arranged granules, no trace of stridulatory row (Figs. 10D, E, 11E, F); ambulatory merus relatively short, wide, propodus and dactylus relatively short, wide (Fig. 14H); male pleon transversely wide, somite page 22 of 39Zoological Studies 59:27 (2020) © 2020 Academia Sinica, Taiwan 6 subtruncate, much wider than long with convex lateral margins; telson wider than long, distinctly inserted into concave distal margin of somite 6 (Figs. 10C, 11C, D, 14I); G1 relatively slender, elongate, stem straight with no obvious expansion along entire length, without subdistal hump along inner margin, subdistal part of outer margin with distinct angular hump, chitinous distal part long, straight, bent at angle of about 45° from longitudinal axis, relatively shorter, proximal and distal parts subequal in width when viewed laterally, with base longer (Figs. 15N–S, 16Q–S). Female: telson inserted into distal margin of somite 6, lateral margins of somite 6 more strongly convex, structure appears proportionately wider (Fig. 12C); vulvae anterior half of sternite 6, intrudes into margin with sternite 5, posterior edge forming weakly bilobed sternal vulval cover, operculum low, non-protruding (Fig. 12D). Colour: Carapace reddish-brown to purplishbrown, lateral and frontal margins yellow to orange; chelipeds usually purple, sometimes dark red, with tubercles white; ambulatory legs light brown; ventral surfaces dirty white (Fig. 9). This species (as Sesarmops impressus or S. impressum) has been well figured in many Taiwanese books (e.g., Lee 2001: 134; Liu and Wang 2010: 61; Li and Chiu 2019a: 89; Li and Chiu 2019b: 45). Etymology: The name is derived from the Latin for emperor, with which the colour purple has always been associated with in European culture. The name alludes to the colour of the species as well as the relatively large size when compared to the other sesarmids that occur in its habitat. The name is used as a noun. Remarks: The species is superficially similar to S. impressus s. str. with which it has long been confused. The differences, notably in the G1, however, are marked and leave no doubt it is a distinct species (see DISCUSSION). On the basis of the specimens examined, S. imperator sp. nov., has a very wide range from Sundaic Southeast Asia to the Ryukyus and Vanuatu in the western Pacific. We have also examined photographs of specimens from various parts of the Indonesian Moluccas and Papua New Guinea, and they are all S. imperator. On the basis of the distribution, we provisionally refer old records of “S. impressus”, “S. frontalis” and “S. atrorubens” from Flores and Timor by De Man (1887 1892), Nobili (1900) and Tesch (1917) to S. imperator as well. Ecology: Sesarmops imperator is a freshwater species, typically found among rocky areas in riverine habitats, usually with clean faster flowing water, often at the bases of waterfalls. They can be found near the shore where there is tidal influence all the way to several kilometres inland, even in highland sites (see also Liu 2009; Li et al. 2010; Liu and Wang 2010; Li and Chiu 2013; personal observations). In the central Philippines in particular, it can be found with two other species, S. mindanaoensis and S. mora Li, Shih & Ng, 2020. They have also been found near limestone caves in the Philippines (unpublished data), and one specimen (ZRC 2019.1817) from Riorua in Vanuatu was collected from next to a cave (see Lips et al. 2011). They are not known from mangroves. Distribution: Southeast Asia to Taiwan and Japan (Ryukyus), Palau, Indonesia (Moluccas, Lesser Sunda Islands, Sulawesi), Papua New Guinea and Vanuatu. Surprisingly, not yet known from Guam, other Mariana Islands and New Caledonia (cf. Paulay et al. 2003; Ng and Richer de Forges 2007). Sesarmops atrorubens (Hess, 1865) (Figs. 18–21, 23A–G, 24A–F) Sesarma atrorubens Hess, 1865: 149, pl. 6 fig. 12 (Sydney); Haswell 1882: 108 (list); De Man 1887: 653, 676, 678 (part) (Sydney); De Man 1890: 95 (Fiji); Ortmann 1894: 724 (Fiji). Sesarma (Sesarma) atrorubens – Tesch 1917: 131 (part) (Fiji). Sesarmops impressum – Serène and Soh 1970: 401, 406 (list). (not Sesarma impressa H. Milne Edwards, 1837). Sesarma (Sesarmops) impressum – McLay and Ryan 1990: 108, figs. 1A–G (Fiji). (not Sesarma impressa H. Milne Edwards, 1837). Sesarmops impressus – Ng et al. 2008: 223 (part; list). (not Sesarma impressa H. Milne Edwards, 1837). Material examined: Neotype (here designated): male (31.9 × 32.4 mm) (ZRC 2019.1069), fast flowing stream with large boulder rocks, stream of Twin waterfall, near Naba Village along Lavena Coastal Walk, Taveuni Island, 16°52'15.3"S, 179°54'06.6"W, Fiji, coll. B.Y. Lee, et al., 24 July 2019. Others: 1 male (26.1 × 26.6 mm), 1 female (26.8 × 26.5 mm) (ZRC 2019.1070) second waterfall, approx. 30–40 min hike along trail, Tavoro waterfalls, Bouma National Park, Taveuni Island, Fiji, coll. B.Y. Lee, S. Choy and B. Rashni, 25 July 2019. Diagnosis: Male: carapace subtrapezoidal, longer than wide in adults; external orbital tooth with outer margin convex, especially along posterior part, separated from lateral margin by distinct cleft (Figs. 19A, 20A, B); epibranchial tooth short, may possess barely discernible very low lobe posterior to it but never distinct or dentiform (Fig. 20B); outer surface of chela covered with small number of large round tubercles; inner surface with several large rounded granules, that on subdorsal surface largest, without transverse median ridge; dorsal margin of dactylar finger lined with stridulatory row of 26–30 granules, most transversely rectangular with median transverse depression, proximal ones more rounded, similar to surrounding rounded granules, distal ones low, poorly formed (Figs. page 23 of 39Zoological Studies 59:27 (2020) © 2020 Academia Sinica, Taiwan 19C, 20F–I); ambulatory merus relatively longer, more slender, propodus and dactylus long (Figs. 19, 20A); male pleon transversely narrower, somite 6 subtruncate, just wider than long with gently convex lateral margins; telson as long as wide, not inserted into straight distal margin of somite 6 (Figs. 19B, 20E); G1 relatively short, stout, stem with distal part distinctly wider than median parts, outer margin distinctly concave, with subdistal hump along inner margin, subdistal part of outer margin with distinct angular hump, chitinous distal part short, straight, subtruncate, spatuliform, bent at angle of about 45° from longitudinal axis (Figs. 23A– Fig. 18. Sesarmops atrorubens (Hess, 1865). A, B, in situ, Fiji (specimens not collected). A, ovigerous female (photograph: A. Ryan via Colin McLay); B, male (photograph: Richard Ploner). page 24 of 39 Zoological Studies 59:27 (2020) © 2020 Academia Sinica, Taiwan G, 24A–F). Female: telson inserted into distal margin of somite 6, lateral margins of somite 6 convex (Fig. 21B); vulvae anterior half of sternite 6, distinctly intrudes into margin with sternite 5, posterior edge forming low bilobed sternal vulval cover with prominent, protruding operculum (Fig. 21C). Colour: McLay and Ryan (1990: 110) noted that in life, the “Dorsal surface of carapace and limbs dark purple, ventral surface pale creamy yellow. Fingers and tubercles on outer face of cheliped propodus pale yellow against a dark purple background.” The present specimens agree in general with this description, though we would add the following: the lateral margins of the carapace are white, the ventral margins of the ambulatory meri are white, the palm of the chela is evenly purple except for the white tubercles, with the pollex completely white to pale yellow, the dactylar finger has the upper part of the proximal third purple with white tubercles, the carpus of the cheliped is dark purple with white granules, the ventral surfaces are dirty white to pale yellow, and the eyes bright yellow (Figs. 18, 19). The two sexes do not differ in colour. Fig. 19. Colour in life. Sesarmops atrorubens (Hess, 1865), neotype male (31.9 × 32.4 mm) (ZRC 2019.1069), Fiji. A, overall dorsal view; B, ventral view of cephalothorax; C, outer view of right chela. (photographs: B. Y. Lee). page 25 of 39Zoological Studies 59:27 (2020) © 2020 Academia Sinica, Taiwan “Vielleicht nur Weibchen des vorigen, was ich um so mehr vermuthe, da von den Exemplaren der Samm lung die ersteren nur Männchen, die letzteren nur Weibchen sind.” [The carapace is like Ses. atrorubens. The front is likewise divided into four ridges, which, however, does not protrude as far anteriorly. The legs as in the previous one species, only the meri are wider and shorter. The chelae of the female are very small. The palms are not covered with tubercles. Each finger carries three bigger teeth. Length 3.5 cm, Width 3.3 cm. Colour dark yellow-red… Maybe only females of the previous one {Ses. atrorubens}, which is likely, as from the collection, the former there are only males, the latter only females.]. No figures were provided. We do not agree with Hess that Sesarma similis is a female of S. atrorubens because we now have females of the latter, and its colour is like of males (purple and white), the postfrontal lobes still protrude prominently anteriorly like in males, the outer surface of the chelae is still covered with distinct large tubercles, and the ambulatory meri are not distinctly wider and shorter. As Hess specimen measures 33 mm in carapace width, it is quite large and not a juvenile. The type locality for S. similis is probably incorrect (see above discussion for S. atrorubens). Davie (2002: 226) commented that “(the Australian distribution for this species is based on the type locality of ‘Sydney’ for Sesarma similis Hess, and this is certainly erroneous; extensive field collecting across northern Australia has so far failed to recollect S. impressus; most probably it was collected from the islands of the south-west Pacific and sent to Hess via Sydney; however, as the northern coast of Australia is within the latitudinal range of the species. I will not yet exclude its possible occurrence.)” De Man’s (1887) comparisons of S. similis with S. impressus were made difficult by the fact that S. similis is known only from a female. On the basis of the present material we refer to these two species, we agree that they are superficially very similar. As noted by De Man, the carapaces are very close, with the shape, dorsal surface, lateral teeth, frontal margin and postfrontal lobes all similar. The ambulatory leg proportions, notably in the merus and dactylus, are also the same. Without a male of S. similis and relying only on carapace and pereopod features, it is not surprising De Man believed both species were synonymous. We believe; however, that they are separate species. In the ZRC is an old specimen from Samoa which the late Raoul Serène in 1970 had labelled as a new species, “Sesarmops samoaensis”. The name has never been published. The specimen is distinct species, with a very diagnostic G1 (Figs. 23H–Q, 24G–K). The specimen was the property or obtained by “Wroblewsky”, who is almost Johannes Julius Wroblewsky (or Wroblewski) (1820–1888). He was a Danish physician associated with several Danish zoologists such as Mørch and Steenstrup (MacGregor 2008: 236, 237) and probably purchased the specimen from the Museum Godeffroy (see also Schleisner 1889: 19). We here argue that Serène’s “Sesarmops samoaensis” is the same taxon as what has been described as Sesarma similis Hess, 1865. The carapace and ambulatory characters match in all respects. More significantly, most Sesarmops species, even S. atrorubens, have the outer surface of the female chelae granulated. It is less prominently granulated compared to males but still distinct. Hess (1865) described the chela as smooth, and this is for a large adult specimen. Li et al. (2020) showed that only in adult female S. mindanaoensis are the outer surfaces of the chelae almost smooth or with only few low granules. S. mindanaoensis (known from Philippines, Taiwan and Sulawesi), however, has distinctly more elongate ambulatory legs and a more slender G1 (see Li et al. 2020). In recognizing the synonymy of S. similis with S. impressus, Davie (2002: 226) noted that the zoological collections of the Göttingen University had two female specimens (catalogue number ZMG 602) which have been transferred to Senckenberg Museum. A search in the Senckenberg collections failed to uncover this material. This is clearly incorrect data as Hess (1865) himself had only one female specimen. As the type of Sesarma similis Hess, 1865 is lost and no longer extant, we here select the present male (32.5 × 30.0 mm) (ZRC 2017.8) from Samoa to be the neotype of the species. This will stabilise the taxonomy of these species names and allow the necessary generic revisions to be done in the future (see DISCUSSION). Ecology: Not known. Distribution: Known for certain only from Samoa. Molecular analyses A phylogenetic tree of the combined 16S and COI markers was reconstructed using BI and ML analyses (Fig. 25). Both Sesarmops imperator and S. impressus are well supported, and sister to each other, although the support values are low. The above two species, as well as Bresedium eurypleon Li, Shih & Ng, 2020 and B. brevipes (De Man, 1889), form a strongly supported clade, which is sister to S. mora Li, Shih & Ng, 2020. Both S. mindanaoensis and S. atrorubens are not closely related to the other species. The mean pairwise nucleotide divergence of K2P distances and bp differences of haplotypes of the four closely related species, Sesarmops impressus, page 32 of 39Zoological Studies 59:27 (2020) © 2020 Academia Sinica, Taiwan Fig. 25. Bayesian inference (BI) tree for species of Sesarmidae used in this study based on combined 16S rDNA and cytochrome c oxidase subunit I (COI) genes. Probability values at nodes represent support values for BI and maximum likelihood (ML). Only support values higher than 50% are shown in the trees. The numbers before localities of the species refer to the corresponding GenBank/DDBJ accession numbers. Table 1. Matrix of percentage pairwise nucleotide divergences with K2P distance (and value range) based on 658 bp of COI within and between four closely related species of Sesarmops and Bresedium. Lower-left values are K2P and upper-right ones are bp differences within species between species K2P bp difference S. imperator S. impressus B. eurypleon B. brevipes Sesarmops imperator 0.21 (0–0.49) 1.33 (0–3) 20.29 (19–21) 22.57 (19–25) 25.2 (21–28) Sesarmops impressus 0.46 (0–0.92) 3 (0–6) 3.2 (2.96–3.34) 21.13 (17–23) 25.9 (23–28) Bresedium eurypleon 0.54 (0.15–0.77) 3.5 (1–5) 3.58 (3.13–3.94) 3.31 (2.65–3.62) 23.45 (21–27) Bresedium brevipes 0.74 (0–1.23) 4.8 (0–8) 4 (3.51–4.42) 4.07 (3.6–4.42) 3.68 (3.28–4.26) page 33 of 39Zoological Studies 59:27 (2020) © 2020 Academia Sinica, Taiwan S. imperator, Bresedium eurypleon and B. brevipes, are shown in table 1. The intraspecific nucleotide divergences of the four species were all ≤ 1.23% (8 bp difference). The interspecific divergences among the four species were all ≥ 2.65% (17 bp difference). The interspecific divergence was at least 2.7-fold greater than intraspecific values (between B. brevipes and B. eurypleon), supporting our hypothesis that the four species are distinct. DISCUSSION In this study, species confused with or allied to Sesarmops impressus are revised, and two new species were established. Although we could not obtain the sequences from S. indicus sp. nov., or S. similis, the morphological differences diagnosed for the other three species, S. impressus, S. imperator sp. nov., and S. atrorubens are well supported by mitochondrial 16S and COI (Fig. 25). Sesarmops impressus s. str. is superficially similar to S. imperator and S. indicus in carapace features. The most reliable external way to distinguish S. imperator from S. impressus and S. indicus is the form of the external orbital tooth. In S. imperator, the outer margin of the external orbital tooth, is gently convex to almost straight (Figs. 13E, F, 14G). In S. impressus and S. indicus, the outer margin of the external orbital tooth is distinctly more convex, especially at the proximal part, with the cleft separating it from the lateral margin usually being more pronounced (Figs. 13A–D, 14A, D). In larger specimens, the carapace shapes are also slightly different, with that of S. impressus appearing more quadrate with the lateral margins slightly divergent to almost subparallel (Fig. 13A–D) while that of S. imperator tends to be more trapezoidal in shape (Fig. 13E, F). The lectotype of S. impressus is dried and slightly deformed, with the right side badly indented (Figs. 2A, 13A). The tips of the epibranchial teeth on both sides are also broken off (Fig. 13A) such that it appears lower. The shape of the female pleon is also useful for separating the species, but only when it is fully formed. In S. impressus, the telson is inserted into the distal margin of somite 6, the lateral margins of which are gently convex (Fig. 6C). In S. indicus and S. imperator, the lateral margins of somite 6 are more strongly convex, such that the structure appears proportionately wider (Figs. 8C, 12C). The shape of the female pleon of the dried lecotype female of S. impressus (Fig. 2B) agrees very well with that here figured for more recent specimens of S. impressus (Fig. 6C), confirming our identification of the present material. The present specimens of S. indicus are relatively smaller than those of S. impressus and S. imperator, but even when specimens of similar sizes are compared (Fig. 14), several major differences are observable. Externally, S. indicus can easily be separated from S. impressus and S. imperator by its proportionately longer ambulatory legs, notably in the merus and dactylus (Fig. 14E versus Fig. 14B, H). In addition, the male pleon of S. indicus is proportionately less broad (notably somite 6) when compared to those of S. impressus and S. imperator (somite 6 much broader with the telson distinctly inserted into the distal margin of somite 6) (Fig. 14F versus Fig. 14C, I) and the telson is not inserted into thedistal margin of somite 6 (Fig. 14F). These differences are also valid for large specimens of S. impressus and S. imperator. The G1s of the three species are distinct and the differences are valid even for small males. In S. impressus, the G1 has the distal half of the stem expanded, such that the inner margin has a gentle but distinct subdistal hump, and the subdistal part of the outer margin is gently sloping, with the chitinous distal part gently curved to an angle of about 60° from the longitudinal axis (Figs. 15A–C, E, F, 16A, B, C, E). In S. indicus and S. imperator, the entire G1 stem is straight with no apparent expansion along its entire length, there is no subdistal hump along the inner margin, the subdistal part of the outer margin has a distinct angular hump, and the chitinous distal part is straight but bent to about 45° from the longitudinal axis (Figs. 15H–J, L, N–P, R, 16H, I, K). The G1 of S. indicus, however, differs from S. imperator in that its chitinous distal part is relatively long and gently tapers when viewed laterally, with its base shorter (Figs. 15H–J, L, M, 16H, I, K versus Figs. 15N–P, R, S, 16Q–S, 17A, B, D, F, G, I). The above differences are valid even for small specimens of S. impressus (Fig. 16N–P) and S. imperator (Fig. 16Q–S) examined. Genetically, the identity of S. impressus and S. imperator are supported by the phylogenetic tree based on the combined 16S and COI markers, although their sister relationship is not strongly supported, and they form a larger clade including Bresedium brevipes and B. eurypleon (Fig. 25). With regards to the interspecific K2P distance of COI of the above four species, the values (≥ 2.65%) are larger than the minimum interspecific divergences among intertidal and terrestrial crabs (e.g., the families Dotillidae, Mictyridae, Ocypodidae, Sesarmidae and Varunidae) (see Chu et al. 2015; N. K. Ng et al. 2018; Shih et al. 2018 2019a b 2020; Shih and Poupin 2020). In this study, S. atrorubens and S. similis are retained in Sesarmops out of convenience, at least until the generic limits of the genus can be established. While the carapaces and general appearances of S. atrorubens page 34 of 39Zoological Studies 59:27 (2020) © 2020 Academia Sinica, Taiwan and S. similis superficially resemble S. impressus, there are major differences. As discussed under the remarks for S. atrorubens, the species is unusual in that it possesses a row of stridulatory tubercles on the dorsal margin of the male cheliped dactylus (Fig. 20H, I), a feature shared with Sesarmops weberi and Labuanium trapezoideum but absent in S. impressus, S. indicus and S. imperator. In addition, the male telson is not inserted into the distal margin of somite 6 (Fig. 20E), the inner surface of the male chela does not have a transverse ridge of granules (Fig. 20G), and significantly, the G1 is proportionately shorter and stouter, with the chitinous distal part short (Figs. 12A–G, 13A–E). The distal part of the G1 is closer to that of S. mindanaoensis (see Li et al. 2020), but the G1 stem of the latter is proportionately more elongate. While the male pleon of S. mindanaoensis is similar to that of S. atrorubens, the dactylus of the chela does not have a row of stridulatory granules (cf. Li et al. 2020). The phylogenetic relationship of S. atrorubens also supports the unusual morphology (Fig. 25). As discussed earlier, S. mindanaoensis will probably need to be separated into its own genus in the future, and on the basis of the present data, S. atrorubens will need to be moved as well. Sesarmops similis is another matter. The male telson is not inserted into somite 6 (Fig. 22E), the dactylus of the chela is not lined with stridulatory granules (Fig. 22G, H), and the G1 is unusually short and stout with a short and very wide chitinous distal part (Figs. 23H–Q, 24G–K). The relatively short and stout G1 structure resembles those of species of Pseudosesarma s. str. (cf. Ng and Schubart 2017; Schubart and Ng 2020), but the carapace of S. similis is different, with the external orbital tooth longer and more acutely triangular, the regions of the carapace clearly indicated with the dorsal surface rugose to granulose (external orbital tooth shorter with the carapace regions smooth and poorly indicated; cf. Ng and Schubart 2017; Schubart and Ng 2020). In addition, while Pseudosesarma species are known only from the Indian Ocean in Southeast Asia, S. similis is from some distance away from Samoa. There are no known Pseudosesarma species in the area between these sites. As such, for morphological and biogeographical reasons, it is perhaps not advisable to transfer S. similis to Pseudosesarma. Schubart and Ng (2020) established a new genus for three species (Sesarma (Sesarma) modesta De Man, 1902 [type species], and two new species from Indonesian Papua and Papua New Guinea), noting that it shares characters of Sesarmops, Bresedium and Pseudosesarma. The carapaces of these species are more similar to that of S. similis and the form of the male chela is also similar (Fig. 22G, H) (cf. Schubart and Ng 2020) but the G1 of the latter species is still much stouter and shorter, with the distal chitinous part too short and wide. Sesarmops similis may need to be transferred to this new genus at a later date. The generic status of S. atrorubens and S. similis would preferably be decided after the positions of S. weberi, S. mindanaoensis and S. angustifrons are ascertained, and the limits of Bresedium s. str. with Sesarmops s. str. established. For the moment, keeping both taxa in Sesarmops seems to be in the best interest of stability. The distribution of the species studied shows a common pattern observed for species occurring in the Indo-West Pacific, and this is probably because geographic barriers of land and/or currents affect larval dispersal. Sesarmops impressus is restricted to the western Indian Ocean, S. indicus to the eastern Indian Ocean, S. imperator in Southeast Asia and the western Pacific, S. atrorubens in Fiji, and S. similis in Samoa. Similar distributional patterns for Indo-West Pacific crabs, for example, can be found in fiddler crabs (Crane 1975; Shih et al. 2010 2016; Shih and Poupin in press) and gecarcinid land crabs (Lai et al. 2017; Ng and Shih 2014 2015). However, as the species of Sesarmops studied also mixed with species of Bresedium, further studies with more molecular markers and species will be necessary to clarify the issues of distribution and cladogenesis. Acknowledgments: This work and the reinstated species names have been registered with ZooBank under urn:lsid:zoobank.org:pub:B6FEC087-08F9406A-8DCD-2ADD6C18C38F. The authors are very grateful to many colleagues who helped make the present study possible. Winks Emmerson and Indraneil Das kindly helped with many queries and specimens. Christoph Schubart (University of Regensburg, Germany) and Peter Davie (Queensland Museum) participated in many important discussions on sesarmid taxonomy. Thanks are due to Satish Choy, Bindiya Rashni and Lee Bee Yan for specimens they obtained from Fiji. For access to collections, we are very grateful to Danièle Guinot, Paula Martin-Lefevre and Laure Corbari (MNHN) and Charles Fransen (RMNH). Ng Ngan Kee and Wan Jusof (ZRC) kindly helped with the examination and photography of the MNHN specimens. We are especially grateful to Andreas Allspach and the late Michael Türkay (SMF), who helped us search for Hess’ specimens; and to Rasmus Hovmöller (Swedish Museum of Natural History), who checked their holdings of Hess specimens and helped determine their origins. For the colour photographs, we are indebted to Joseph Poupin (Laboratoire d’Oceanographie, Ecole Navale, Brest), Matthias Deuss, Gianni Di Marco, page 35 of 39Zoological Studies 59:27 (2020) © 2020 Academia Sinica, Taiwan Richard Ploner, P.A. Ryan, Colin McLay, Hung-Chang Liu and Lee Bee Yan. This study was partially supported by a grant from the Ministry of Science and Technology (MOST 105-2621-B-005-002-MY3) Executive Yuan, Taiwan to HTS. Authors’ contributions: PKLN collated and processed the samples and performed the morphological comparisons. JJL collected samples and compared the material. HTS conceived the study and performed molecular analysis. All authors drafted, read and approved the final manuscript. Competing interests: The authors declare that they have no conflict of interests. Availability of data and materials: Sequences generated in the study have been deposited in the DNA Data Bank of Japan (DDBJ) database (accession numbers in Fig. 25 in the manuscript). 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