Arcotheres coarctatus ( Bürger, 1895 )

Ahyong, Shane T. & Ng, Peter K. L., 2021, The pinnotherid crabs from the Gulf of Siam described by Rathbun (1909) (Decapoda: Brachyura): revisited and revised, Raffles Bulletin of Zoology 69, pp. 188-211 : 207-209

Arcotheres coarctatus ( Bürger, 1895)

( Fig. 17 )

Pinnotheres coarctatus Bürger, 1895: 369 , pl. 9 fig. 7, pl. 10 fig. 7 [type locality: Zamboanga, Philippines, from “ Cahebe ” = Polymesoda sp. ].

Arcotheres coarctatus — Ahyong & Ng, 2007b: 195, fig. 3A. — Ng et al., 2008: 248.

Pinnotheres parvulus — Rathbun, 1910: 331, 332, fig. 13, pl. 2 fig. 9. — Naiyanetr, 1980: 42. — Naiyanetr, 1998: 104. — Naiyanetr, 2007: 118. (not Pinnotheres parvulus Stimpson, 1858 ).

Material examined. ZMUC 284250 , ovigerous female (cl 7.6 mm, cw 8.8 mm), Lem Ngob, mangrove swamp, 23–27 December 1900 .

Description of female. Carapace slightly longer than wide, subcircular, sparsely and finely setose anteriorly; front weakly produced, anterior margin transverse in dorsal view; anterolateral margins poorly defined; dorsum smooth, with pair of fine longitudinally arcuate grooves extending from behind eyes to indistinct gastrocardiac groove; dorsal surface convex in profile. Epistome with narrow interantennular septum; median buccal margin with obtuse median point. Antennular sinus slightly larger than orbit; antennules folded slightly obliquely. Antennal articles 1 and 2 fused to epistome. Eyes partially visible in dorsal view, filling orbit, cornea pigmented.

Mxp3 ischiomerus length about twice width; outer margin strongly convex; inner proximal margin convex; inner distal margin bluntly rounded, approximately right-angled. Carpus shorter than propodus. Propodus tapering in distal half, apex rounded, dorsally and distally setose. Dactylus digitiform, distally setose, inserted slightly distal to propodal midlength, apex reaching to or slightly beyond end of propodus. Exopod inner margin almost straight, outer margin sinuous; flagellum with 2 articles, distally setose.

Chelipeds symmetrical from left to right, outer surface glabrous to sparsely setose. Dactylus and pollex relatively straight, crossing distally, without gape. Dactylus occlusal margin with blunt triangular tooth proximal to midlength, margin straight in distal half, finely denticulate, sparsely setose. Pollex occlusal margin with two, blunt triangular teeth proximal to midlength, straight margin in distal half, finely denticulate, sparsely setose; with fringe of short setae on inner ventral margin, extending onto inner surface of palm. Propodus palm dorsal margin 1.4× height, 1.2× length of dactylus; ventral margin almost straight. Carpus unarmed.

P2–5 unarmed, irregularly setose. P2, 3, and 5 similar, symmetrical from left to right; right P4 length 1.3× longer than left; relative lengths in decreasing order: P4> P5> P3> P2. Longer P4 merus about 0.5× pcl. P2–3 dactylus 0.4× propodus length, stout, evenly arcuate, distally spiniform, flexor margin unarmed; P3 dactylus slightly longer than P2. Longer P4 dactylus stout, strait, almost 0.5× propodus length, flexor margin unarmed, strongly setose; shorter P4 dactylus 0.4× propodus length, flexor margin unarmed, strongly setose. P5 dactylus 0.7× propodus length, slender, weakly curved, slightly longer than longer P4 dactylus; setose, longest on flexor margin; flexor distal margin of spines; extensor distal margin with 3 or 4 small denticles, otherwise unarmed. Relative dactylus lengths: P4> P5> P3> P2.

Thoracic sternum anterior margin shallowly concave medially; sternites 1–3 indistinguishably fused.

Pleon of 6 free somites and telson, extending to buccal region, covering bases of P2–P5.

Remarks. Ahyong & Ng (2007b) transferred P. coarctatus to Arcotheres . Re-examination of Rathbun’s (1910) specimen reported as Pinnotheres parvulus Stimpson, 1858 , showed it to be referrable to A. coarctatus , as currently understood, extending the known range of the species westwards to the Gulf of Thailand. Bürger (1895) reported “Cahebe” as the host of P. coarctatus . “Cahebe” appears to be a misspelling of the local name for a commonly harvested brackish water clam ( Geloina spp. ) ( Schmitt et al., 1973). In Tagalog and other Philippine languages, Geloina is known as “kabibe”, for which the old Hispanized spelling would be “cabebe” or “cabibe” depending on the regional patois (JCE Mendoza, pers. comm.).

Distribution. Philippines, Indonesia, and the Gulf of Thailand.

Figures

Fig. 17. Arcotheres coarctatus (Bürger, 1895), ovigerous female, cl 7.6 mm, cw 8.8 mm, Lem Ngob, Thailand, ZMUC 284250. A, dorsal habitus; B, anterior cephalothorax; C, anterior thoracic sternum; D, right chela; E–H, left P2–5, respectively; I–L, right P2–5, respectively; M, right maxilliped 3. Scale: A = 2.0 mm; B–L = 0.1 mm; M = 0.5 mm.