Orisarma neglectum
Creators: name: Schubart, Christoph D.
name: Ng, Peter K. L.
Publisher: name: Zenodo
Year: 2020
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Decapoda
Sesarmidae
Orisarma
Orisarma neglectum

description: Orisarma neglectum (De Man, 1887)
(Figs. 1E, 3C, 4C, E, G, 5G, 7H, 11H–L, 36D, 53 F)
Sesarma dehaani – Heller, 1865: 62. [not Sesarma dehaani H. Milne Edwards, 1853]
Sesarma neglecta De Man, 1887: 643, 661.
Sesarma (Sesarma) neglecta – Tesch, 1917: 178.
Sesarma (Holometopus) dehaani – Balss, 1922: 154 (part); Shen, 1932: 195, text-figs. 121–123. [not Sesarma dehaani H. Milne Edwards, 1853]
? Sesarma (Holometopus) neglecta – Shen, 1940a: 97.
Chiromantes dehaani – Liu & He, 2007: 161. [not Sesarma dehaani H. Milne Edwards, 1853]
Chiromantes neglectum – Ng et al., 2001: 41; Komai & Ng, 2013: 540, figs. 6B, 7D–F, H.
“ Chiromantes ” neglectum – Ng et al., 2008a: 220.
Material examined. Neotype (here designated): male (35.6 × 31.8 mm) (ZRC 1998.310 a), Shanghai, Qidong, Jiangsu Province, China, coll. Y.Y. Li, 2 May 1996. Others: CHINA – 15 males (smallest 22.7 × 20.0 mm, largest 35.5 × 32.5 mm), 6 females (smallest 16.7 × 14.5 mm, largest 27.2 × 23.7 mm) (ZRC 1998.310 b), 1 male (30.2 × 25.5 mm), 1 female (25.0 × 21.5 mm) (CBM-ZC 11459), Qidong, Lusi, Shanghai, coll. Y.Y. Li, 1 May 1996; 1 male (31.7 × 28.0 mm) (ZRC 1998.309), Qi Dong, Lusi, Shanghai, coll. Y.Y. Li, 1 May 1996; 3 males (ZRC), Chongming Island, Chongming Dao Bird Sanctuary, Shanghai, coll. N.K. Ng & L.K. Wang, 10 June 2005; 1 male (ZRC 2010.0420), Chongming, west of Shiqiao River, 31°38.37′N 121°21.496′E, coll. B.Y. Au, 15 July 2010.
Diagnosis. Carapace subtrapezoidal to quadrate; lateral margin entire, no trace of epibranchial tooth; posterolateral margins clearly converging towards posterior carapace margin; dorsal surface relatively higher, gastric region more swollen, grooves separating regions shallow, not prominent; frontal lobes relatively lower, median concavity between lobes shallow; ambulatory merus usually relatively more slender; chitinous part of G1 evenly cylindrical. In life, carapace grey, sometimes with dark median patches, chelae white and grey.
Colour. The dorsal carapace surface of fresh O. neglectum is generally a uniform grey to dark grey or brown, often with the gastric regions sometimes with symmetrically arranged dark grey blotches (Fig. 53F). The chelae are whitish with the granules grey, never purple (Fig. 53F).
Remarks. This species has long been regarded as a junior synonym of O. dehaani. In comparing O. neglectum with O. dehaani, De Man (1887) noted that one of the main differences was the higher carapace. Tesch (1917: 145) noted that the proportions of the carapace width, form of the lateral carapace margins and front were also different, and the inner surface of the palm of O. neglectum does not have a transverse row of granules, but it seems he did not have specimens and based this comparison primarily from De Man’s (1887) description.
We have examined a good series of O. neglectum specimens from the type locality, Shanghai in China, and we are confident it is distinguishable from O. dehaani. Most of the characters enumerated by De Man (1887) and Tesch (1917) are not valid, varying too much to be useful, although the difference in height of the carapace is valid. Smaller specimens of O. neglectum have lower transverse ridges on the inner surface of the palm, certainly much lower than in very large males. The major difference which distinguishes O. neglectum immediately from O. dehaani is its relatively higher carapace, with the gastric region more distinctly swollen, and the grooves separating the regions shallower and less prominent (Figs. 1E, 3C, 4C, E, G versus Figs. 1C, D, 3B, 4A, B, D, F). The shape of the carapace is different: in O. dehaani, the carapace is distinctly quadrate to transversely rectangular, with the posterolateral margins subparallel or gently converging (Fig. 4A, B) whereas it is subtrapezoidal to quadrate in O. neglectum, with the posterolateral margins clearly converging towards the posterior carapace margin (Fig. 4C). The structure of the frontal margin is quite different. In O. dehaani, the two prominent frontal lobes are separated by a deep and broad concavity (Fig. 4D). In O. neglectum on the other hand, the two frontal lobes are relatively lower and the median concavity is shallow (Fig. 4E). The G1s of the two species differ slightly with the chitinous part of O. dehaani having the basal part slightly constricted and the distal part broader (Fig. 11C–F). In O. neglectum, the distal chitinous part is evenly cylindrical (Fig. 11I–L). The relative length of the chitinous part varies slightly with size, being slightly longer in smaller specimens. The ambulatory legs of O. neglectum (especially the merus) are also typically more slender (Fig. 7H), with those of O. dehaani relatively broader (Fig. 7E–G); although it does vary and the difference is less easy to appreciate in smaller specimens. The live colours are also different. The dorsal carapace surface of freshly preserved or live O. dehaani often have large dark brown to black blotches medially (Fig. 53A, C), while in O. neglectum, it is usually a uniform grey, with the gastric regions sometimes with dark grey blotches (Fig. 53F).
Orisarma neglectum is only known with certainty from and around Shanghai, China, for the moment. As noted earlier, O. dehaani is now known from many parts of mainland China, Hong Kong, south of Taiwan, etc. (see Shen, 1940a, b; Dai et al., 1986; Dai & Yang, 1991), and detailed surveys of the southern coast of China are needed to establish the actual distributions of both taxa, especially since O. neglectum has long been regarded as a junior synonym. Shen (1940a: 97) actually recognised O. neglectum as a good species (as a Sesarma (Holometopus )) on the basis of specimens from Shanghai and southern China, but only commenting that “The specimens from Canton are not likely the young forms of Sesarma (Holometopus) dehaani H. MILNE-EDWARDS ”. The characters he used were not stated, and considering that he had specimens of what he regarded as O. dehaani from many parts of China (Shen, 1940a: 97), all the old material will need to rechecked to ascertain their identities. Certainly most of the older records will need to be re-examined (e.g., Stimpson [1907] recorded what was supposedly O. dehaani from Whampoa in southern China). It is possible that the uncertain identity of O. neglectum s. str. has contributed to the confusion. Fresh collections will also be needed to establish their actual distributions and where they may overlap. From what we have observed thus far at least, we have not seen any specimens or photographs/figures clearly referable to O. neglectum from south of Shanghai in China.
The whereabouts of the solitary type specimen of O. neglectum is not known. It is neither in Leiden, Amsterdam nor Paris, and we believe it is no longer extant (see also Ng et al., 2001). Since it is close to S. dehaani and can easily be confused (and has been), we feel that it is necessary to select a neotype for the species to stabilise its taxonomy. We hereby designate a male specimen 35.6 × 31.8 mm from Shanghai (ZRC 1998.310) as the neotype of Sesarma neglecta De Man, 1887.
Biology. Not much is known about the biology of O. neglectum s. str. except that it is common in mudflats and associated areas in mainland China, mainly in locations north of Fujian. Yang et al. (2014) reported on an interesting habit of some individuals from populations in Chongming Island outside Shanghai in China climbing shrubs to attack bird nests. We are confident these are O. neglectum as photographs of this species sent to us by W. Liu confirm this, and we have a specimen from near this location.
Distribution. From what is known, O. neglectum is found north of Fujian, China. Specimens we have examined from Hong Kong and southern China are all O. dehanni s. str. so far. Stefano Cannicci (pers. comm.), however, notes that it has been found in small numbers in Hong Kong. This is a pattern of distribution shared with other thoracotreme taxa like Eriocheir japonica, E. sinensis, and E. hepuesis (Varunidae, e.g., see Xu et al., 2009; Naser et al., 2012).
description: Published as part of Schubart, Christoph D. & Ng, Peter K. L., 2020, Revision of the intertidal and semiterrestrial crab genera Chiromantes Gistel, 1848, and Pseudosesarma Serène & Soh, 1970 (Crustacea: Brachyura: Sesarmidae), using morphology and molecular phylogenetics, with the establishment of nine new genera and two new species, pp. 891-994 in Raffles Bulletin of Zoology 68 on pages 906-910, DOI: 10.26107/RBZ-2020-0097, http://zenodo.org/record/5351295