Geosesarma amphinome De Man 1899
Creators: name: Ng, Peter K. L.
Publisher: name: Zenodo
Year: 2015
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Decapoda
Sesarmidae
Geosesarma
Geosesarma amphinome

description: Geosesarma amphinome (De Man, 1899)
(Figs. 1, 2)
Sesarma (Sesarma) amphinome De Man, 1899: 133, pl. 12 fig. 16; Tesch 1918: 129 (list); Serène 1968a: 1087 (list); Serène 1968b: 106 (list).
Geosesarma amphinome — Serène & Soh 1970: 402 (list); Ng 1988: 119 (list); Fransen et al. 1997: 123; Ng et al. 2008: 220 (list).
Material examined. Lectotype: female (11.1 × 8.5 mm) (NNM 1198), Sintang, Borneo, coll. M. Moret, 1893– 1894. Paralectotypes: 2 males (larger 14.4 × 8.7 mm), 1 female (NNM 23283), same data as lectotype.
Diagnosis. Carapace subtrapezoidal, wider than long, width to length ratio 1.3–1.6, lateral margins gently diverging (Figs. 1 A, B, 2A); dorsal surface with visible regions, anterior regions densely covered with low, small rounded, flattened granules (Fig. 1 A–D); front deflexed, frontal lobes broad with subtruncate margins; postfrontal, postorbital cristae low but distinct (Fig. 1 C, D); external orbital tooth acutely triangular, directed obliquely, outer margin gently convex to almost straight, tip extending beyond lateral margin (Figs. 1 A, B, 2A). Merus of third maxilliped subovate, subequal in length to ischium; exopod slender with long flagellum (Fig. 2 B). Outer surface of palm of adult male covered with small rounded granules; inner surface with transverse comb-like ridge; dorsal margin of dactylus with cluster of small rounded granules on proximal part, 3 or 4 slightly larger granules after it, unarmed on distal half (Figs. 1 F, 2F). Ambulatory legs with relatively slender merus, with sharp subdistal spine on dorsal margin, surfaces gently rugose (Figs. 1 A, B, 2D). Male abdomen triangular; somite 6 wide, with convex lateral margins; telson semicircular (Figs. 1 E, 2C). G1 relatively slender, distal pectinated part elongated, gently curved, expanded, spatuliform, tip medially clefted (Fig. 2 G–K).
Colour. Not known. Ecology. Not known.
Remarks. The species was described on the basis of two male and three female specimens collected probably from Sintang, along the Kapuas River in western Kalimantan, Borneo (De Man 1899: 133). Serène (1968a: 1089) identified a female specimen as the lectotype without discussion (see Fransen et al. 1997: 123).
The lectotype female is an adult; while the larger paralectotype male is mature, its chelae are still relatively small. Its male abdomen and G1, however, are fully formed. One characteristic feature of this species is that although the dorsal margin of the dactylus is mostly unarmed except for a few small granules on the proximal part (Figs. 1 F, 2F), there is a prominent transverse comb-like structure on the inner surface of the chela (Fig. 2 F).
Geosesarma amphinome is superficially similar to G. sarawakense (Serène, 1968) (from central Sarawak), G. peraccae (Nobili, 1903) (from Singapore and southern Peninsular Malaysia), and G. pylaemenes sp. nov. (from Anjungan, Kalimantan Barat), with similar trapezoidal carapace shapes and the exopod of their third maxillipeds possess a long flagellum (Figs. 1 A, B, 2A, B, 3A, B, 4A, 5A, B, 6A, B, 7A; Serène 1968a: pl. 1 fig. 3; Ng 1988: fig. 56A, C). Geosesarma amphinome, G. peraccae and G. pylaemenes sp. nov. possess a prominent transverse comblike structure on the inner surface of the chela (Figs. 2 F, 3F, 5F; Ng 1988: fig. 56A). The chela of the holotype of G. sarawakense is small (cf. Serène 1968a: pl. 1 fig. 3) but also has a low transverse ridge (Fig. 6 F).
Geosesarma amphinome can be distinguished from G. sarawakense by the dorsal surface of the anterior part of the carapace being more prominently granular (Figs. 1 A–C, 2A versus Fig. 6 A–C; Serène 1968a: pl. 1 fig. 3), and the distal pectinated part of the G1 is more curved, spatuliform and relatively longer (Fig. 2 G–K) (the distal pectinated part of the G1 is straight and relatively shorter in G. sarawakense, Fig. 7 D–F; Serène 1968a: figs. 7, 8). The dorsal surface of the carapace, male abdomen and G1 of G. peraccae is distinctly different from G. amphinome, with the dorsal surface prominently more granular, somite 6 proportionately broader and the G1 being very slender, with the distal pectinated part almost straight (Fig. 5 A, B, D; Ng 1988: fig. 56D–F) (the carapace less granular, the abdominal somite 6 less broad, and the distal pectinated part wider and gently curved in G. amphinome, Figs. 1 A– D, 2C, G–K).
Although both G. amphinome and G. pylaemenes sp. nov. occur in Kalimantan Barat, their type localities (Sintang and Anjungan, respectively) are almost 200 km apart. Geosesarma amphinome can be separated from G. pylaemenes sp. nov. in having the anterior half of the dorsal surface of the carapace relatively more granular (Fig. 1 A–C versus Fig. 3 A, B), the external orbital tooth is more acutely triangular (Figs. 1 A–C, 2A) (more broadly triangular in G. pylaemenes sp. nov., Fig. 3 A, B), abdominal somite 6 is proportionately less broad and the telson is not recessed into its distal margin (Figs. 1 E, 2C) (somite 6 broader and the telson is partially recessed into its distal margin in G. pylaemenes sp. nov., Fig. 4 B), and the G1 is relatively shorter with the distal pectinated part proportionately broader and more spatuliform, and with the median part deeply clefted (Fig. 2 G–K) (G1 relatively more slender with the distal pectinated part narrower and not strongly clefted medially in G. pylaemenes sp. nov., Fig. 4 D–G).
Geosesarma sarawakense and G. peraccae are both lowland species, typically associated with freshwater swamps or muddy forested habitats. Considering their similarity with G. amphinome, the latter species probably lives in similar habitats. Geosesarma peraccae has also been observed to forage in cups of pitcher plants (Nepentheceae) (Tan & Ng 2008), and G. amphinome may have similar phytotelmic habits. In any case, other species of Geosesarma have been reported from this habitat: G. malayanum Ng & Lim, in Ng, 1986, from Peninsular Malaysia (Ng & Lim 1987); G. nemesis Ng, 1986, from southern Peninsular Malaysia; G. b a u from Sarawak (see comparative material examined) and G. gracillimum from Brunei (S. Choy, pers. comm.).
It is noteworthy that the holotype of G. sarawakense was actually collected with another species of Geosesarma. This specimen, an ovigerous female (11.2 × 10.9 mm) (ZRC 1965.7.29.5), has a quadrate carapace, longer ambulatory legs and is clearly a separate species. In the absence of a male, it is provisionally referred to G. bau for the time being.
description: Published as part of Ng, Peter K. L., 2015, Semiterrestrial crabs of the genus Geosesarma De Man, 1892 (Crustacea, Brachyura, Sesarmidae) from western Borneo, Indonesia, with descriptions of three new species, pp. 37-56 in Zootaxa 4048 (1) on pages 38-41, DOI: 10.11646/zootaxa.4048.1.2, http://zenodo.org/record/290075