Arcotheres purpureus ( Alcock, 1900 )

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 : 200-205

Arcotheres purpureus ( Alcock, 1900)

( Figs. 10–15 )

Pinnoteres purpureus Alcock, 1900: 339 . — Alcock & McArdle, 1902: pl. 62 fig. 6. — Hornell & Southwell, 1909: 102. — Guinot, 1967: 279.

Pinnotheres purpureus . — Borradaile, 1903: 431. — Laurie, 1915: 415. — Tesch, 1918: 250, 253, 287. — Silas & Alagarswami, 1967: 1207, 1214, 1216, 1222. — Schmitt et al., 1973: 6, 10, 83. — Ng et al., 2008: 251. — Ng & Kumar, 2015: 265. — Dev Roy, 2015: 88.

Pinnotheres nigrans Rathbun, 1909: 110 . — Rathbun, 1910: 334, figs. 16, 17. — Tesch, 1918: 249, 254. — Gordon, 1936: 173. — Suvatti, 1938: 69. — Suvatti, 1950: 159. — Silas & Alagarswami, 1967: 1203, 1223. — Schmitt et al., 1973: 6, 58. — Naiyanetr, 1980: 42. — Dai et al., 1980: 135, fig. 7. — Dai et al., 1986: 395, 396, fig. 212. — Dai & Yang, 1991: 427, 428, fig. 212. — Naiyanetr, 1998: 104. — Naiyanetr, 2007: 118. — Yang et al., 2008: 809. [New synonymy]

Arcotheres purpureus . — Ng, Clark et al., 2017: 1094. — Trivedi et al., 2018: 61. — Ahyong & Ng, 2020: 337.

Material examined. MALAYSIA: ZRC 1991.422 – 427 , 6 ovigerous females (cl 4.3 mm, cw 5.6 mm to cl 5.4 mm, cw 7.1 mm), Pulau Tioman , Malaysia, from oysters, coll. P. Ng, 15 April 1993 ; ZRC 1991.428 – 432 , 1 ovigerous female (cl 5.3 mm, cw 6.5 mm), 4 ovigerous females (cl 4.2 mm, cw 4.9 mm to cl 4.8 mm, cw 6.0 mm), Pulau Tulai, 1982 or 1983 ; ZRC 2018.1394 (with vial of zoea 1, 1985.1894), 6 ovigerous females (cl 4.5 mm, cw 5.7 mm to cl 5.2 mm, cw 6.9 mm), Pulau Tulai, 4 September 2000 . THAILAND: ZMUC CRU-9395 , ovigerous female (cl 6.6 mm, cw 8.2 mm), Koh Lan (west of Koh Chang ), 9 March 1900 (holotype of Pinnotheres nigrans Rathbun, 1909 ) . INDONESIA: ZRC 2019.0515 , 1 ovigerous female (cl 5.2 mm, cw 7.0 mm), Pulau Weh, Kampung Iboih, Aceh Province, Sumatra, Pulau Weh , 05°52.895′N, 95°14.708′E, intertidal, inside shingle oyster, THH19-01, coll. H.H. Tan & J.C.E. Mendoza, 10–14 February 2019 .

Description of female. Carapace slightly wider than long, transversely ovate to semi-oblong, inflated; front weakly produced, anterior margin slightly convex in dorsal view; anterolateral margins poorly defined; dorsum smooth, glabrous; dorsal surface convex in profile. Epistome with narrow interantennular septum; median buccal margin with acute, triangular, 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 weakly convex, almost straight; inner distal margin obtusely rounded. Carpus slightly shorter than propodus. Propodus tapering in distal half, apex rounded, dorsally and distally setose. Dactylus digitiform, distally setose, inserted slightly proximal to propodal midlength, apex not reaching end of propodus. Exopod inner margin almost straight, outer margin convex; flagellum with 2 articles, distally setose.

Chelipeds symmetrical from left to right, surfaces glabrous except inner, lower distal margin. Dactylus and pollex relatively straight, crossing distally, without gape; apices expanded mesially. Dactylus occlusal margin with small blunt triangular tooth proximal to midlength, margin straight in distal half, finely denticulate, sparsely setose. Pollex occlusal margin almost straight, finely denticulate, sparsely setose; with fringe of short setae on inner ventral margin. Propodus palm dorsal margin 2.0–2.2× height, 1.2–1.5× length of dactylus; palm ventral margin almost straight. Carpus unarmed.

P2–5 unarmed, largely glabrous, densest on P5 dactylus. P2, 3, and 5 similar, symmetrical from left to right; left P4 length 1.1× longer than right; relative lengths in decreasing order: P4> P5> P3> P2. Longer P4 merus about 0.5–0.6× pcl. P2–3 dactylus about 0.5× propodus length, evenly arcuate, distally spiniform; P2 flexor margin unarmed; P3 dactylus slightly longer than P2, flexor margin minutely denticulate. P4 dactylus of both sides slender, weakly curved, unarmed, at least 0.7× propodus length. P5 dactylus as long as propodus, slender, weakly curved, slightly shorter than longer P4 dactylus; surfaces covered in short setae, longer setae on flexor margin; flexor distal margin with single row of spines, otherwise unarmed. Relative dactylus lengths: P4 ~ P5> P3> P2.

Thoracic sternum anterior margin convex to sinuous medially; sternites 1–3 indistinguishably fused.

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

Remarks. Alcock (1900) and Alcock & McArdle (1902) provided a comparatively good account and good figures of A. purpureus ( Fig. 11 ). Re-examination of the holotype of Pinnotheres nigrans revealed that it is indistinguishable from A. purpureus and they are synonymised. Arcotheres purpureus was described from the Andaman Islands and, based on material reported herein, is now known to range to peninsular Malaysia, Indonesia (Aceh Province), and the Gulf of Thailand. Dai et al. (1980) also reported the species from Hainan Island (as P. nigrans ), where it overlaps with A. boninensis . Records from the Maldives ( Borradaile, 1903), Red Sea, and Djibouti require confirmation; these might be based on A. pernicola , which has also been recorded from the western Indian Ocean ( Trivedi et al., 2019). Arcotheres purpureus reported by Mohanty et al. (2018) from the Devi Estuary, Bay of Bengal, is based on what appears to be a species of Nepinnotheres similar to N. affinis ( Bürger, 1895) , judging from the figures provided; it is clearly not referrable to A. purpureus .

Although the type material of A. purpureus requires reexamination, the species is redescribed herein to facilitate more detailed comparison with other species of the genus. In the few specimens of A. purpureus that were sufficiently complete for determination, the left P4 was longer than the right, contrasting with A. boninensis , in which the right P4 appears to be longer. Whether the asymmetry of P4 is consistently handed in A. boninensis and A. purpureus remains to be determined by examination of a larger series of both species. The P4 dactylus of A. purpureus was at least 0.7× the respective propodus length, but given that many specimens had broken P4 dactyli, the degree of variation is currently difficult to assess.

As discussed under the account of Arcotheres boninensis , A. purpureus is a member of the A. boninensis group. Within the group, A. purpureus is closest to A. boninensis and A. pernicola , differing by a proportionally shorter Mxp3 dactylus that does not reach beyond the tip of the propodus ( Figs. 14B , 15B ) (versus dactylus reaching to or beyond the tip in A. boninensis and A. pernicola ; Fig. 9B ; Trivedi et al., 2019: fig. 2F), the presence of a single row of spinules on the distoflexor margin of the P5 dactylus ( Figs. 14J , 15I ) (versus two rows in A. boninensis and A. pernicola ; Fig. 9N ), the palm of the female chela is proportionately more elongate ( Figs. 12D , 14E , 15E ) (versus distinctly shorter in A. boninensis and A. pernicola ; Figs. 8D , 9F ; Trivedi et al., 2019: fig. 2D, E), and an expanded distal tip on the cheliped pollex ( Figs. 14F , 15F ) (versus unexpanded in A. boninensis and A. pernicola ; Fig. 9F ; Trivedi et al., 2019: fig. 2D, E). The carapace of A. purpureus also appears to have somewhat straighter lateral margins ( Figs. 11A , 12A , 13A , 14A , 15A ) than in A. boninensis ( Figs. 8A , 9A ).

Distribution. Western Southeast Asia, from Indonesia, Malaysia, the Gulf of Thailand, and the Andaman Sea; possibly from the western Indian Ocean.

Figures

Fig. 10. Arcotheres purpureus Alcock, 1900, photographed in life. A, ovigerous females, Pulau Tioman, Malaysia, ZRC 1991.423; B, ovigerous female cl 5.2 mm, cw 7.0 mm, Pulau Weh, Kampung Iboih, Aceh Province, Sumatra, Pulau Weh, ZRC 2019.0515 (photograph by H. H. Tan).

Fig. 11. Arcotheres purpureus Alcock, 1900, female (after Alcock & McArdle, 1902: pl. 62 fig. 6). A, dorsal habitus; B, anterior cephalothorax; C, left maxilliped 3.

Fig. 12. Arcotheres purpureus Alcock, 1900, female, cl 4.3 mm, cw 5.6 mm, Tioman, Malaysia, ZRC 1991.423. A, overall habitus; B, ventral view of cephalothorax; C, frontal view of cephalothorax; D, outer view of left chela.

Fig. 13. Arcotheres purpureus Alcock, 1900, ovigerous female holotype of Pinnotheres nigrans (Rathbun, 1909), cl 6.6 mm, cw 8.2 mm, Koh Lam, Thailand, ZMUC CRU9395. A, overall habitus; B, frontal view of cephalothorax; C, ventral view of cephalothorax.

Fig. 14. Arcotheres purpureus (Alcock, 1900), ovigerous female holotype of Pinnotheres nigrans (Rathbun, 1909), cl 6.6 mm, cw 8.2 mm, Koh Lam, Thailand, ZMUC CRU9395. A, dorsal habitus; B, right maxilliped 3; C, anterior cephalothorax; D, anterior thoracic sternum; E, right chela, lateral view; F, right chela pollex, dorsal view; G, left P4; H–J, right P3–5, respectively. Scale: A = 2.0 mm; B = 0.5 mm; C–J = 1.0 mm.

Fig. 15. Arcotheres purpureus (Alcock, 1900), ovigerous female, cl 5.7 mm, cw 7.0 mm, Tioman Island, Malaysia, ZRC 1991.422–427. A, dorsal habitus; B, right maxilliped 3; C, anterior cephalothorax; D, anterior thoracic sternum; E, right chela, lateral view; F, right chela pollex, dorsal view; G, left P2; H–I, left P4–5, respectively; J–M, right P2–5, respectively. Scale: A = 2.0 mm; B = 0.5 mm; C–M = 1.0 mm.

Fig. 9. Arcotheres boninensis (Stimpson, 1858), spent female neotype, cl 6.0 mm, cw 8.8 mm, Ogasawara Islands, Japan, NSMT Cr6567a. A, dorsal habitus; B, right maxilliped 3; C, anterior carapace margin; D, anterior cephalothorax; E, anterior thoracic sternum; F, right chela; G–J, left P2–5, respectively; K–N, right P2–5, respectively. Scale: A = 2.0 mm; B = 0.5 mm; C–N = 1.0 mm.

Fig. 8. Arcotheres boninensis (Stimpson, 1858), spent female neotype, cl 6.0 mm, cw 8.8 mm, Ogasawara Islands, Japan, NSMT 6567a. A, overall habitus; B, ventral view of cephalothorax; C, frontal view of cephalothorax; D, outer view of left chela.