Arcotheres alcocki ( Rathbun, 1909 )

Ng, Peter K. L. & Ahyong, Shane T., 2022, The pea crab genus Arcotheres Manning, 1993 (Crustacea: Brachyura: Pinnotheridae) from Singapore and Peninsular Malaysia, with a reappraisal of diagnostic characters and descriptions of two new genera, Raffles Bulletin of Zoology 70, pp. 134-248 : 202-204

Arcotheres alcocki ( Rathbun, 1909)

( Figs. 53 , 54 )

Pinnotheres parvulus — Bürger, 1895: 375, pl. 9 fig. 18, pl. 10 fig. 17 [not P. parvulus Stimpson, 1858 ].

Pinnotheres alcocki Rathbun, 1909: 114 (part; replacement name) [type locality: Burias, Philippines, by lectotype designation ( Ahyong & Ng, 2007b)]; 1910: 304; Serène, 1968: 93.

Arcotheres alcocki — Ahyong & Ng, 2007b: 193, 194, fig. 1; Ng et al., 2008: 248; Ng et al., 2017: 1094; Trivedi et al., 2018a: 197; De Gier & Becker, 2020: tab. 1.

Material examined. 1 female (9.6 × 7.0 mm) ( ZRC 2016.272 ), Pasir Panjang food terminal, Singapore, from Marcia recens , coll. J. C. Mendoza, 5 April 2016 .

Description. Carapace and pereopods well chitinised. Female: Carapace transversely ovate-subhexagonal, wider than long; dorsal and lateral surfaces smooth, glabrous; appearing gently domed in frontal view; front barely projecting anteriorly beyond orbits, margin gently convex; anterolateral margin gently sloping posteriorly to various degrees, forming rounded angle with posterolateral margin ( Figs. 53A, C, D , 54A ). Eyes small, partially visible in dorsal view in adults; filling orbit ( Figs. 53C , 54A ). Epistome with median part triangular, lateral margins gently concave ( Fig. 54C ).

MXP3 outer surface with scattered short setae; ischiomerus completely fused, subrhomboidal, inner margin rounded, even at widest point; carpus short; propodus about 3 times as long as high, subspatulate, distinctly longer than carpus, tip rounded; dactylus slender, inserted at slightly proximal to midlength of propodus, tip not reaching propodal apex; exopod relatively slender, about two-thirds length of ischiomerus, flagellum 2-segmented ( Fig. 54B ).

Chela not prominently elongate, dactylus about two-thirds palm length; palm relatively slender, proximally narrower than distally; outer surfaces of palm, fingers (except for distal part) almost glabrous, with only scattered short setae; ventral margin of palm gently sinuous; dactylus occlusal margin with distinct subproximal tooth; pollex occlusal margin with 1 low proximal tooth, 1 submedian tooth, and minute denticles; tips of fingers sharp, hooked ( Figs. 53E , 54E ).

P2–P5 dorsally, ventrally unarmed; outer surface covered with scattered, very short setae or glabrous; ventral margins of propodus and dactylus slightly more setose; merus relatively short, stout, relative lengths of meri P4>P3>P2>P5; right P4 longer than left; P2 and P3 dactyli short, subequal, tip gently hooked, half propodus length; right P4 dactylus longer, weakly falciform, shorter than propodus, shorter than P5 dactylus; P5 merus 4.3–4.4 times longer than wide; P5 dactylus longest, longer than propodus, margins lined with short and long setae, denser on ventral margin, distoflexor margin with 2 rows of 7–12 spinules ( Fig. 54F–M ).

Pleon extending to buccal region, covering bases of P2–P5; telson gently recessed into concave distal margin of somite 6 ( Figs. 53B , 54D ).

Male: Not known.

Variation. Not known.

Colour. Not known.

Host. The lectotype female did not have any host record ( Ahyong & Ng, 2007b: 193). The present specimen was from a clam, Marcia sp. (Veneridae) .

Remarks. The taxonomy of this poorly known species was discussed at length by Ahyong & Ng (2007b). Rathbun (1909, 1910) argued that specimens identified as “ Pinnotheres parvulus ”, recorded by Bürger (1895) from the Philippines, De Man (1887: 105; 1888: 383) from the Mergui Archipelago and Indonesia, and Alcock (1900), presumably from Indian waters, were not conspecific with Pinnotheres parvulus Stimpson, 1858 , and as such, she proposed a new name for them, Pinnotheres alcocki . No types were selected, rendering all of the material referred by these authors to P. parvulus as syntypes of P. alcocki (see Schmitt et al., 1973). Ahyong & Ng (2007b) studied one of Bürger’s (1895) “ P. parvulus ” specimens reported from Burias, Philippines, which they designated as the lectotype of Pinnotheres alcocki Rathbun, 1909 . They redescribed and figured the specimen and transferred the species to Arcotheres . Ahyong & Ng (2007b) argued that the syntype specimens from Mergui (De Man, 1887) were not conspecific with the Philippine lectotype, but were uncertain of their correct identity. As a result of the present restudy of Mergui material, these records and those of Alcock (1900) and Gordon (1936) are clarified here as Magnotheres globosus . De Man’s (1888) records of “ P. parvulus ” from Indonesia remain to be verified.

The present female specimen (ZRC 2016.272) of A. alcocki agrees very well with the lectotype in all key aspects. Ahyong & Ng (2007b: 194) indicated that A. alcocki s. str. was morphologically very close to A. sinensis ( Shen, 1932) from Chinese waters, and that the two may be synonyms. Detailed comparison of these two species is being undertaken as part of a separate study. We note, however, that several reports of Arcotheres sinensis (as Pinnotheres ) from Tamil Nadu, India ( Sethuramalingam & Khan, 1991: pl. 24 fig. i; Ravichandran & Kannupandi, 2007: 334; Kannappan et al., 2012: 45; see also Trivedi et al., 2018b: 61), cannot presently be determined as the original descriptions and/or figures are not sufficiently informative. Sethuramalingam & Khan (1991) and Ravichandran & Kannupandi (2007) did not record hosts, whereas Kannappan et al.’s (2012) specimens came from a gastropod ( Turritella attenuata Reeve, 1849 , family Turritellidae Lovén, 1847 ), and from his figure ( Kannappan et al., 2012: fig. 2) their species appears to be something quite different from A. alcocki or A. sinensis .

The precise provenance of the present specimen (ZRC 2016.272) is not known, as Marcia clams, although occurring in Singapore, are also imported as seafood; most imported stocks come from Peninsular Malaysia or Indonesia.

Distribution. South China Sea, from the Philippines and probably to the vicinity of Peninsular Malaysia or Indonesia.

Figures

Fig. 53. Arcotheres alcocki (Rathbun, 1909), spent female (9.6 × 7.0 mm) (ZRC 2016.272), Singapore. A, overall dorsal view; B, ventral view of cephalothorax; C, dorsal view of carapace; D, frontal view of cephalothorax; E, outer view of left chela.

Fig. 54. Arcotheres alcocki (Rathbun, 1909), spent female (9.6 × 7.0 mm) (ZRC 2016.272), Singapore. A, overall dorsal view; B, right MXP3; C, front view of cephalothorax; D, anterior thoracic sternum; E, right chela; F–I, left P2–P5; J–M, right P2–P5. Scale = A, 2.0 mm; B, 0.5 mm; C–M, 1.0 mm.