Arcotheres similis ( 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 : 205-207

Arcotheres similis ( Bürger, 1895)

( Fig. 16 )

Pinnotheres similis Bürger, 1895: 373 , 374, fig. 14. — Chhapgar, 1957b: 42. — Silas & Alagarswami, 1967: 1210, 1216, 1219. — Schmitt et al., 1973: 86.

Pinnotheres kamensis Rathbun, 1909: 110 . — Rathbun, 1910: 335, figs. 18, 19. — Tesch, 1918: 249, 252. — Suvatti, 1938: 69. — Suvatti, 1950: 159. — Silas & Alagarswami, 1967: 1200, 1223, 1225. — Schmitt et al., 1973: 50. — Naiyanetr, 1980: 42. — Naiyanetr, 1998: 104. — Naiyanetr, 2007: 118. [New synonymy]

Pinnotheres spinidactylus Gordon, 1936: 169 , figs. 1, 2. — Griffin & Campbell, 1969: 157, figs. 7, 8. — Stephenson et al., 1970: 492. — Jones, 1990: 202. — Davie, 2002: 434.

Arcotheres spinidactylus . — Ahyong & Brown, 2003: 10.

Arcotheres similis . — Ahyong & Ng, 2007b: 207, 208, fig. 14. — Ng et al., 2008: 248.

Not Arcotheres similis . — Ahyong & Brown, 2003: 9, 10 [= Arcotheres latus ( Bürger, 1895) ]

Material examined. ZMUC CR- 9396, juvenile male (cl 1.6 mm, cw 1.8 mm), west of Koh Kam, 5 fm (= 9 m), coll. Th. Mortensen, 6 February 1900 (holotype of Pinnotheres kamensis Rathbun, 1909 ); NHM 1936.6.19.2, 1 ovigerous female (cl 5.5 mm, cw 6.7 mm), Siglap, Singapore, from Modiolus philippinarum , coll. R. Winckworth (holotype of Pinnotheres spinidactylus Gordon, 1936 ); NHM 1936.6.19.3–7, 1 male (cl 3.3 mm, cw 3.3 mm), 4 ovigerous females (cl 4.7 mm, cw 5.8 mm to cl 5.8 mm, cw 6.7 mm), Siglap, Singapore, from Modiolus philippinarum , coll. R. Winckworth (paratypes of P. spinidactylus Gordon, 1936 ).

Description of holotype of Pinnotheres kamensis . Carapace subcircular, slightly longer than wide, widest at midlength, weakly arched; pterygostomial surface glabrous. Front produced, anterior margin weakly concave in dorsal view; width about 0.3× cw. Dorsum, glabrous, sparsely punctate. Epistome interantennular septum triangular; median buccal margin with triangular median point. Antennular sinus of similar size to orbit; antennules folded obliquely. Antennal articles 1 and 2 fused to epistome. Eyes visible in dorsal view, filling orbit, cornea pigmented.

Mxp3 ischiomerus length about twice width; outer margin convex; inner concave, distal margin bluntly obtuse. Carpus slightly shorter than propodus. Propodus spatulate, apex rounded, dorsally and distally setose, length almost 3× dactylus length. Dactylus digitiform, distally setose, inserted near propodal midlength, apex not reaching end of propodus. Exopod margins convex; flagellum with 2 articles, distally setose.

Chelipeds symmetrical from left to right, glabrous except for inner ventral margin and occlusal margins of dactylus and pollex. Dactylus and pollex relatively curved, crossing distally, with slight gape, apices simple, neither expanded distomesially. Dactylus and pollex occlusal margins straight, sparsely setose, each with blunt triangular tooth proximal to midlength and row of minute finely graded denticles. Pollex distomesial surface with sparse fringe of setae on inner ventral margin. Propodus palm dorsal margin 1.1× height, as long as dactylus; ventral margin gently convex. Carpus unarmed, glabrous.

P2–5 subequal from left to right, unarmed, with natatory setae, otherwise largely glabrous. Relative lengths: P4> P3> P2> P5. Pereopod 2 basis anterior surface smooth. P2–4 dactyli strongly curved, falcate, apex sharp, glabrous or sparsely setose, flexor margin with row of 2–4 minute spines; pereopod 2–4 dactyli 0.6× propodus length. P5 dactylus weakly curved, 0.9× propodus length, sparsely setose, flexor margin with 2 rows of spines. Relative dactylus lengths: P3 = P4> P2> P5.

Thoracic sternum anterior margin medially emarginate; sternites 1–3 indistinguishably fused, setose.

Pleon narrow, triangular, of 6 free somites and telson; widest at somite 2; telson semi-circular, wider than long, wider than anterior margin of somite 6. Pleopods 1 and 2 undeveloped.

Remarks. The identity of Arcotheres similis was significantly clarified by redescription of the type material ( Ahyong & Ng, 2007b), prior to which, only the rather minimal type description and inaccurate figure were available ( Bürger, 1895). As a result, Gordon’s (1936) record of the species from Singapore proved to be referrable to A. placunicola Ng, 2018 , and records from Australia of Ahyong & Brown (2003) were determined as A. latus ( Bürger, 1895) (see Ahyong, 2020b). Alongside her misidentified record of A. similis, Gordon (1936) described A. spinidactylus , also from Singapore, notably with a diagnostic row of spinules on the P2–5 dactyli. Ahyong & Ng (2007b), however, observed the singular apparent difference between A. spinidactylus and A. similis to be the presence of one instead of two distoflexor rows of spinules, respectively, on the P5 dactylus, and that Gordon (1936) possibly overlooked the second row in her species. Our re-study of the type material of A. spinidactylus and both sexes of A. similis as part of a wider study of regional Arcotheres (Ng & Ahyong, in preparation) revealed two rows of spinules on the P5 dactylus. Arcotheres spinidactylus and A. similis are indistinguishable and the two species are herein synonymised. As such, A. similis remains in the Singaporean and Australian faunas on the basis of previous records of A. spinidactylus from the respective localities ( Gordon, 1936; Griffin & Campbell, 1969; Jones, 1990). Accordingly, the spinulose P2–4 dactyli of males (and often females), on which A. spinidactylus was originally recognised, are now diagnostic of A. similis . Rathbun’s (1924) record of P. similis from Western Australia is probably based on another species given that it was identified based on Bürger’s (1895: pl. 9 fig. 14) inaccurate figure and said to be hosted by an Ostrea sp. (in Western Australia, currently Crassostrea sp. or Saccostrea sp. ). The only Australian pinnotherid currently known to be hosted by ostreids is A. boninensis , which has a proportionally much wider carapace than reported by Rathbun (1924). Rathbun’s (1924) specimen should be re-examined; it could be referrable to (or near) A. shahi Trivedi, Campos & Vachrajani, 2018 , from southern India, which resembles Bürger’s (1895) figure of A. similis and is hosted by a species of Crassostrea ( Trivedi et al., 2018) .

The holotype of Pinnotheres kamensis is a juvenile male and the gonopods are undeveloped. Nevertheless, the spatulate Mxp3 propodus, single row of spinules on the flexor margin of the P2–4 dactylus, and double row of flexor spinules on the P5 dactylus indicate that P. kamensis is referrable to Arcotheres similis ( Bürger, 1895) , confirming the presence of the species in the Gulf of Thailand. Even as a juvenile, the holotype of P. kamensis agrees closely with mature males of A. similis in all diagnostic features, apart from the gonopods, which are as yet undeveloped. Records of A. similis (as P. spinidactylus ; Dai et al., 1980, 1986; Dai & Yang, 1991) from Hainan Island, China, require verification; they appear to represent a different species (Ng & Ahyong, in preparation).

Distribution. Philippines, Indonesia, Malaysia, the Gulf of Thailand, Singapore, Australia, and India.

Figures

Fig. 16. Arcotheres similis (Bürger, 1895), juvenile male holotype of Pinnotheres kamensis Rathbun, 1909, cl 1.6 mm, cw 1.8 mm, west of Koh Kam, Thailand, ZMUC CR-9396. A, dorsal habitus; B, left maxilliped 3; C, lower buccal margin; D, anterior thoracic sternum; E, pleon; F, right chela; G–J, right P2–5, respectively. Scale: A = 1.0 mm; B = 0.25 mm; C–J = 0.5 mm.