Arcotheres quadratus ( Rathbun, 1909 ) Ahyong & Ng, 2021

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 : 195-197

Arcotheres quadratus ( Rathbun, 1909) , new combination

( Figs. 6 , 7 )

Pinnotheres quadratus Rathbun, 1909: 110 . — Rathbun, 1910: 333, fig. 15. — Tesch, 1918: 250, 255. — Suvatti, 1938: 70. — Suvatti, 1950: 160. — Silas & Alagarswami, 1967: 1207, 1208, 1223, 1225. — Schmitt et al., 1973: 6, 83. — Naiyanetr, 1980: 42. — Naiyanetr, 1998: 105. — Naiyanetr, 2007: 118. — Ng et al., 2008: 251.

Questionable records: Pinnotheres quadratus . — Tesch, 1918: 261, 288, 289, pl. 17 fig. 2. — Chhapgar, 1957a: 506, pl. 12l, m. — Chhapgar, 1957b: 44, pl. 12l, m. — Chhapgar, 1958: 254, fig. 2a, b.

Type material. Holotype: ZMUC CRU-7921 , ovigerous female (cl 2.8 mm, cw 2.9 mm), Koh Chang, 1 fm (= 1.8 m), coral, January–March 1900.

Description of holotype. Carapace slightly wider than long, sub-oblong, front not produced, anterior margin weakly sinuous 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 to 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 obtusely, bluntly angular. Carpus shorter than propodus. Propodus spatulate, 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 sinuous; flagellum with 2 articles, distally setose.

Cheliped surfaces glabrous except inner, lower distal margin. Dactylus and pollex relatively straight, crossing distally, without gape; apices simple, not expanded mesially. Dactylus occlusal margin with blunt triangular tooth proximal to midlength, margin straight in distal half, finely denticulate, sparsely setose. Pollex occlusal margin with 2 rounded lobes proximally, weakly convex in distal half, finely denticulate, sparsely setose; with fringe of short setae on inner ventral margin. Propodus palm dorsal margin 1.7× height, 1.4× 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 1.4× longer than right; relative lengths in decreasing order: P4 (longer)> P5> P3> P2. Longer P4 merus about 0.4× pcl. P2–3 dactylus subequal, 0.5× propodus length, half length of P4 (longer) and 5 dactyli, evenly arcuate, distally acute, flexor margin unarmed, glabrous or with few short setae. P4 dactylus slender, weakly curved, about 0.9× propodus length. P5 dactylus slightly longer than propodus, slender, weakly curved, slightly longer than P4 dactylus; extensor margin with few short setae, flexor margin lined with long setae, distal flexor margin with row of 4 spinules. Relative dactylus lengths: P5> P4 (longer)> P3 = P2.

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

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

Remarks. Like Pinnotheres lanensis , P. quadratus is referrable to Arcotheres on the basis of the asymmetrical P4, the elongated and slender P4 and P5 dactyli that are longer than those of P2 and P3, and the perpendicular articulation between the P5 ischium and merus. Arcotheres quadratus , A. similis , and A. latus share the presence of elongated P2 and P3 dactyli that exceed half the length of the dactyli of P5 and the longer P4, and the presence of spinules on the distal flexor margin of the P5 dactylus. A single row of P5 spinules is present in A. quadratus and two rows in A. similis and A. latus . Arcotheres quadratus also differs from A. similis and A. latus in carapace proportions (about as long as wide versus distinctly wider than long), and the size at maturity (ovigerous at less than 3 mm cl versus 4.7 mm and 7.6 mm in A. similis and A. latus , respectively; Ng & Ahyong, in preparation). The quadrate carapace of A. quadratus might prove to be taxonomically useful, but until more specimens become available to assess variability in the species, carapace shape should be used with caution given its variability in some congeners such as A. palaensis and A. exiguus (see Ng & Ahyong, in preparation). The minute size at maturity of female A. quadratus (and A. lanensis ), with both being ovigerous at less than cl 3.0 mm, is conspicuous and, to our knowledge, unique in Arcotheres ; they may represent dwarf species in the genus.

Records of A. quadratus from outside the type locality require confirmation. Tesch (1918) recorded a male and ovigerous female from Indonesia (Lombok and Waigeu Island, respectively), hosted by Arca sp. , which can be referred to Arcotheres based on the elongated P4 and P5 dactyli that are “about twice as long” as the P2 and P3 dactyli. Unfortunately, Rathbun (1909) did not record the host of any of her pinnotherids, but Tesch’s (1918) brief account is not inconsistent with what we know of A. quadratus (although it could equally apply to A. lanensis ). Notably, Tesch’s (1918) ovigerous female from Waigeu Island, as in A. quadratus and A. lanensis , is minute (cl 2.9 mm, cw 3.2 mm) and might represent either of the two species. Tesch’s (1918: pl. 17 fig. 2) “male” from Lombok, however, is depicted with a comparatively wide, triangular pleon suggestive of a juvenile female rather than male. Given the immaturity at its size (cl 2.4 mm, cw 2.8 mm), the Lombok specimen is probably not A. quadratus , and might be referrable to other species of the genus known from there, such as A. palaensis ( Bürger, 1895) or A. ocularius Komai, Kawai & Ng, 2020 ( Komai et al., 2020). Chhapgar (1957a: pl. 12l, m) (repeated by Chhapgar, 1957b, 1958) figured a “male” of A. quadratus without locality in the context of a comparison with his new Indian species, Pinnotheres vicajii Chhapgar, 1957 (= Arcotheres exiguus ( Bürger, 1895) , see Trivedi et al., 2020); it appears to be re-drawn based on Tesch (1918: pl. 17 fig. 2) and does not constitute an independent record.

Distribution. Presently known with certainty only from the Gulf of Thailand; possibly from Indonesia.

Figures

Fig. 6. Arcotheres quadratus (Rathbun, 1909), ovigerous female holotype, cl 2.8 mm, cw 2.9 mm, Koh Chang, Thailand, ZMUC CRU- 7921. A, overall habitus; B, frontal view of cephalothorax; C, ventral view of cephalothorax.

Fig. 7. Arcotheres quadratus (Rathbun, 1909), ovigerous female holotype, cl 2.8 mm, cw 2.9 mm, Koh Chang, Thailand, ZMUC CRU- 7921. 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.