Arcotheres boninensis ( Stimpson, 1858 )

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 : 197-200

Arcotheres boninensis ( Stimpson, 1858)

( Figs. 8 , 9 )

Pinnotheres boninensis Stimpson, 1858: 108 . — Miers, 1886: 276. — Stimpson, 1907: 141, 142. — Tesch, 1918: 248, 251, 286. — Sakai, 1936: 197, 198, pl. 56 fig. 1. — Sakai, 1939: 588, 589, pl. 69 fig. 1. — Sakai, 1956: 50. — Sakai, 1965: 177, pl. 87 fig. 1. — Silas & Alagarswami, 1967: 1196, 1225. — Suzuki, 1972: 14, text-fig. 11. — Schmitt et al., 1973: 6, 10, 40. — Sakai, 1976: 568, 571, pl. 200 fig. 3. — Dai et al., 1980: 135, fig. 6. — Takeda, 1982: 202, fig. 598. — Miyake, 1983: 155, pl. 52 fig. 1. — Dai et al., 1986: 394, fig. 210. — Takeda & Konishi, 1988: 137. — Dai & Yang, 1991: 426, fig. 210. — Takeda, 1993: 60. — Konishi, 1996: 15. — Ho & Hung, 1997: 37. — Muraoka, 1998: 48. — Ng et al., 2001: 48. — Marumura & Kosaka, 2003: 67. — Hewitt, 2004: tables 1, 2. — Ng et al., 2008: 250. — Yang et al., 2008: 808. — Konishi, 2010: 32, 38. — Komatsu, 2011: 277. — Ng, Shih et al., 2017: 129.

Pinnotheres purpureus . — Sakai, 1933: 978, 979, fig. 2a.

Arcotheres boninensis . — Ahyong & Ng, 2020: 337.

Type material. Neotype (here designated): NSMT Cr 6567a, spent female (cl 6.0 mm, cw 8.8 mm), Ogasawara Islands, Japan.

Other material examined. JAPAN: NSMT Cr 6567b, 4 ovigerous females (cl 5.5 mm, cw 7.6 mm to cl 5.7 mm, cw 8.5 mm), 9 spent females (cl 3.8 mm, cw 5.0 mm to cl 6.2 mm, cw 9.2 mm), Ogasawara Islands . NEW CALEDONIA: AM P64781, 3 females (cl 5.8 mm, cw 8.2 mm to cl 6.2 mm, cw 9.2 mm), Ouvea, New Caledonia, 20°33′S, 166°27′E, in oysters on reef, coll. W. Burrell.

Description of female. Carapace wider than long, transversely ovate, inflated; front weakly produced, anterior margin transverse in dorsal view; anterolateral margins poorly defined; dorsum smooth, glabrous; dorsal surface convex in profile. Epistome with narrow interantennular septum; median buccal margin with low, obtuse median point. Antennular sinus of similar size or 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; inner distal margin with blunt, obtuse to almost right-angled. Carpus shorter than propodus. Propodus gently tapering in distal half, apex rounded, dorsally and distally setose. Dactylus digitiform, distally setose, inserted slightly distal to propodal midlength, apex reaching to or beyond end of propodus. Exopod inner margin almost straight, outer margin sinuous; 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 simple, not expanded. Dactylus occlusal margin with blunt triangular tooth proximal to midlength, straight margin in distal half, finely denticulate, sparsely setose. Pollex occlusal margin with two, low, irregular teeth proximal to midlength, margin straight in distal half, finely denticulate, sparsely setose; with fringe of short setae on inner ventral margin. Propodus palm dorsal margin 1.3–1.6× height, 1.1–1.4× length of dactylus; ventral margin almost straight. Carpus unarmed.

P2–5 unarmed, almost glabrous except for dactyli, densest on P5 dactylus. P2, 3, and 5 similar, symmetrical from left to right; right P4 length 1.2–1.3× longer than left; relative lengths in decreasing order: P4 (longer)> P5> P3> P2. Longer P4 merus about 0.5× pcl. P2–3 dactylus about 0.5× propodus length, evenly arcuate, distally spiniform, flexor margin minutely denticulate; P3 dactylus slightly longer than P2. Longer P4 dactylus slender, weakly curved, unarmed, 0.8–1.0× propodus length; shorter P4 dactylus, 0.6× propodus length, flexor margin minutely denticulate. P5 dactylus as long as propodus, slender, weakly curved, 0.8–0.9× P4 dactylus length; surfaces covered in upright setae, slightly longer on flexor margin; flexor distal margin with two rows of spines, otherwise unarmed. Relative dactylus lengths: P4 (longer)> 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. Arcotheres boninensis ( Stimpson, 1858) , together with A. pernicola ( Bürger, 1895) , A. pollus Ahyong & Ng, 2020 , and A. purpureus ( Alcock, 1900) appear to form a discrete group in the genus—here called the A. boninensis group—united by the strongly arched, transversely elliptical carapace, and long, slender, almost styliform dactyli of P4 and 5 that are about twice the length of those of P2 and 3 ( Ahyong & Ng, 2020). In addition, when the colouration has been recorded (not known for A. pollus ), the species of the group are generally purplish-black in life, and, except for A. pollus , are hosted by ostreid oysters ( Fig. 10 ; Alcock, 1900; Rathbun, 1909; Sakai, 1976; Trivedi et al., 2019). Distinctions between A. pernicola , A. boninensis , and A. purpureus , however, are not currently clear, partly due to the very poor condition of the type material of A. pernicola , and problems and ambiguities in diagnoses of the latter two species.

Arcotheres boninensis was described from the Bonin Islands (= Ogasawara Islands), Japan, and has since been reported from Taiwan, Hainan Island and northwestern Australia ( Hewitt, 2004; Yang et al., 2008). Material of A. boninensis from New Caledonia is included herein to document its presence there, making it the sixth species of pinnotherid to be recorded from New Caledonia ( Ng & Richer de Forges, 2007; Ahyong, 2020a). The taxonomy of A. boninensis , however, is problematical; the true identity of Stimpson’s (1858) species cannot be clearly determined because the original description is minimal and the type material lost ( Schmitt et al., 1973). Therefore, to progress taxonomic resolution in the A. boninensis group, we select a neotype for A. boninensis to fix the identity of the species. The neotype is an adult female (cl 6.0 mm, cw 8.8 mm, NSMT Cr6567a) from the Ogasawara Islands, Japan, corresponding to the form usually referred to Stimpson’s (1858) species. Variation within the present series of A. boninensis is slight, with the Mxp3 dactylus usually reaching beyond the end of the propodus (occasionally reaching only to the end). The length of the P5 dactylus varies allometrically, being proportionally longest in the largest specimens. In two specimens, P4 is damaged or lost, but in all other specimens, the right side P4 is longest.

Arcotheres boninensis appears to be closest to A. pernicola sharing the longer of the P4 dactyli being longer than the P5 dactylus, the presence of a double row of spinules on the distoflexor margin of the P5 dactylus, an unexpanded distal tip on the cheliped pollex, two well-developed, irregular teeth on the proximal flexor margin of the cheliped pollex, and a proportionally longer Mxp3 dactylus, which reaches to or beyond the tip of the propodus. Unfortunately, uncertainty also surrounds the identity of A. pernicola , whose holotype from the Philippines is in poor condition, being fragmentary and incomplete ( Ahyong & Ng, 2007b). What remains of the holotype of A. pernicola , however, accords well with material recently reported from western India and East Africa ( Trivedi et al., 2019), and we base our comparisons on this recent material.

Arcotheres pernicola (as presently understood) and A. boninensis appear to be almost indistinguishable, agreeing in almost all the diagnostic characters. The primary difference appears to be in the relative length of the Mxp3 dactylus. Bürger’s (1895: pl. 10 fig. 16) figure of the Mxp3 of A. pernicola depicts the dactylus as reaching the end of the propodus, although in western Indian Ocean material reported by Trivedi et al. (2019: fig. 2F), the dactylus varies from slightly underreaching to reaching the propodus tip as figured by Bürger (1895). In A. boninensis , the dactylus reaches to, or more usually, beyond the tip of the propodus ( Fig. 9B ), thus partially overlapping with that of A. pernicola . The distributions are also unusual, with no records of A. boninensis and A. pernicola in western Southeast Asia and the northeastern Indian Ocean, where the allied A. purpureus occurs instead. It cannot be excluded that more than one species is included in what is currently regarded as A. pernicola , so until complete material from the Philippines can be compared to A. boninensis and A. purpureus , all three nominal species are retained as separate.

Distribution. Japan, Taiwan, South China Sea (Hainan), New Caledonia, and northwestern Australia.

Figures

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.

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. 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).