Orisarma magnum
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 magnum

description: Orisarma magnum (Komai & Ng, 2013)
(Figs. 1F, 11M–P)
Sesarma dehaani – Stimpson, 1858: 106 (part); Stimpson, 1907: 134 (part); Parisi, 1918: 111 (part); Asakura et al., 1993: 10.
Holometopus dehaani – Takeda & Miyake, 1976: 113 (list).
Sesarma (Holometopus) dehaani – Ooishi, 1970: 95.
Chiromantes dehaani – Marumura & Kosaka, 2003: 66 (part); Takeda & Ueshima, 2006: 93 (part); Komatsu, 2011: 277 (list).
Chiromantes magnus Komai & Ng, 2013: 539, figs. 1A, 2–5; Toyota & Seki, 2014: 191.
Material examined. Holotype: male (52.2 × 47.2 mm) (CBM-ZC 11452), Renju Valley, Chichi-jima Island, Ogasawara Islands, coll. H. Tachikawa, 23 September 2008. Paratypes: 1 male (50.1 × 44.4 mm) (CBM-ZC 11456), 1 male (50.6 × 45.6 mm) (ZRC 2013.0173), same data as holotype; 1 male (41.8 × 37.9 mm) (CBM-ZC 11453), Kominato, Chichi-jima Island, coll. T. Komai, 10 December 2005; 1 male (28.2 × 25.5 mm) (CBM-ZC 11454), Kopepe Beach, Chichi-jima Island, coll. T. Komai, 11 December 2005; 1 male (45.2 × 40.8 mm) (RUMF-ZC 2160), same locality, coll. Y. Fujita, 22 June 2010; 1 ovigerous female (43.5 × 40.2 mm) (RUMF-ZC 2362), same data as above; 1 male (49.8 × 44.0 mm) (RUMF-ZC 2360), same locality as above, coll. Y. Fujita, 27 June 2010; 4 males (smallest 18.2 × 15.8 mm, largest 28.5 × 26.1 mm), 1 female (27.9 × 25.2 mm), CBM-ZC 11455, Chichi-jima Island, coll. K. Iwasaki et al., August 2008; 1 male (50.3 × 45.5 mm) (CBM-ZC 11457), Okumura River, river mouth, coll. H. Tachikawa, 28 September 2008; 1 female (CBM-ZC 11600), Kiyose, Chichi-jima Island, coll. T. Komai, 14 July 2009; 1 male (28.3 × 25.3 mm) (RUMF-ZC 2361), same locality as above, coll. Y. Fujita, 25 June 2010.
Diagnosis. Carapace subquadrate; lateral margin entire, no trace of epibranchial tooth; posterolateral margins gently diverging towards posterior carapace margin; dorsal surface relatively lower, gastric region less swollen, grooves separating regions deep, prominent; frontal lobes separated by deep, broad concavity; ambulatory merus relatively long; chitinous part of G1 evenly cylindrical. In life, carapace with mottled pattern of black-brown and gray-brown or yellowish gray; chelae light olive or yellowish gray.
Colour. Komai & Ng (2013: 547) noted that in life “the carapace has a mottled pattern of black-brown and gray-brown or yellowish gray with scattered whitish spots; the carpus and the dorsal part of chela are light olive or yellowish gray; striae on the ambulatory meri are dark brown”.
Remarks. The taxonomy of this species was treated at length by Komai & Ng (2013). The main differences between O. magnum and the related O. dehaani and O. neglectum (other than its much larger adult size) are gently diverging posterolateral margins which give it a more subtrapezoidal appearance (Fig. 1F) and the proportionately longer and more slender ambulatory meri (Fig. 1F). Their colours in life also differ, with that of O. magnum being unusually mottled.
In the current genetic dataset, there are no obvious genetic differences between O. magnum, O. neglectum, and O. dehaani, which should normally be expected for intraspecific relationships. We see a similar pattern for the closely related O. intermedium and O. sinense. The morphological differences, however, are such that all three of them must be recognised as distinct species. The relative gigantism of O. magnum may be due to the effects of island life as the Ogasawara Islands are very isolated and over 1000 km away from mainland Japan, but this is probably not enough to explain its much larger size.
Biology. Komai & Ng (2013: 547) comment that “Individuals occur in estuarine areas to middle parts of rivers or streams where tidal influence is completely absent, sometimes extending to adjacent marsh areas, and sometimes found in burrows under rocks”. This is not dissimilar to O. dehaani which has also been found some distance from the sea. Nevertheless, the water table (which they can reach through their burrows) is probably still at least brackish in content.
Distribution. Known only from the Ogasawara Islands thus far (Komai & Ng, 2013).
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 910-912, DOI: 10.26107/RBZ-2020-0097, http://zenodo.org/record/5351295