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 Author Ng, Peter K. L. Author Ahyong, Shane T. text Raffles Bulletin of Zoology 2022 2022-04-01 70 134 248 journal article 10.26107/RBZ-2022-0009 2345-7600 7174234 BE6164AE-1C24-4E01-8B7B-D80764F147B3 Arcotheres rayi Ahyong & Ng, 2007b ( Figs. 46–49 , 50H–L , 51 , 52 ) Arcotheres rayi Ahyong & Ng, 2007b: 203 , fig. 11 [ type locality: Mariveles or Bohol , Philippines ]; Ng et al., 2008: 248 ; Clark & Schram, 2009: 450 ; Ng et al., 2017: 1094 ; Trivedi et al., 2018a: 54 , 56; Trivedi et al., 2019: 590 , tab. 1; De Gier & Becker, 2020 : tab. 1. Fig. 46. Arcotheres rayi Ahyong & Ng, 2007 , holotype female (11.3 × 8.7 mm) (SMF-ZMG 952), Philippines. A, overall dorsal view; B, ventral view of cephalothorax; C, outer view of left chela. Type material. Holotype : female (11.3 × 8.7 mm ) ( SMFZMG952 ), Mariveles or Bohol , Philippines , coll. Semper ( paralectotype of Xenophthalmus latifrons Bürger, 1895 ). Other material examined. Peninsular Malaysia : 3 males (4.2 × 3.8–4.4 × 4.0 mm), 8 ovigerous females (5.4 × 5.1–8.4 × 7.0 mm), 5 spent females (5.6 × 4.0–6.9 × 5.2 mm ) , 10 females with bopyrids (5.4 × 3.8–10.4 × 6.2 mm ) ( ZRC 2013.1416 ), Bentong market, Pahang , in Paphia sp. , coll. M. Low , 10 May 2010 . Description. Female: Carapace and pereopods poorly chitinised, soft. Carapace subcircular to subhexagonal, wider than long; dorsal and lateral surfaces smooth, glabrous; appearing domed in frontal view; front in line with (larger specimens) or slightly produced anteriorly (smaller specimens) beyond transverse anterolateral margins, margin gently convex; anterior margin with (larger specimens) or without (smaller specimens) shallow notch above orbit flanking front ( Figs. 46A , 47A, C, D , 48A–C , 49A ). Eyes small, not or just visible in dorsal view in adults; mobile, completely filling orbit ( Figs. 46A , 47A, C, D , 48A–C , 49A, C ). Epistome with median part triangular, lateral margins gently concave ( Fig. 49C ). MXP3 outer surface with scattered short setae; ischiomerus completely fused, subrhomboidal, inner margin usually rounded, at widest point; carpus short; propodus about 3 times as long as high, subspatulate, distinctly longer than carpus, tip rounded to subtruncate; dactylus slender, inserted medially on propodus, tip underreaching or just reaching propodal apex; exopod relatively slender, about two-thirds length of ischiomerus, flagellum 2-segmented ( Fig. 49B ). Chela not prominently elongate, dactylus half to 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 convex to sinuous; dactylus occlusal margin with distinct subproximal tooth; pollex occlusal margin usually with 1 low proximal tooth (sometimes obsolete), 1 submedian tooth, and minute denticles; tips of fingers sharp, hooked ( Figs. 46C , 47E , 49E ). 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 longer, more slender, relative lengths of meri P4>P3>P5>P2; left (sometimes right) P4 distinctly the longer; P2 and P3 dactyli short, subequal, tip gently hooked; P4 longer dactylus elongate, weakly falciform, distinctly longer than half propodus length, longer than P5 dactylus; P5 merus 3.9–4.5 times longer than wide; P5 dactylus shorter than propodus extensor margin, longer than P2 and P3 dactyli, length/height <5.0, margins lined with short and long setae, denser on ventral margin, distoflexor margin without rows of spinules ( Figs. 49F–M , 50H–L ). Fig. 47. Arcotheres rayi Ahyong & Ng, 2007 , female (6.9 × 5.2 mm) (ZRC 2013.1416), Peninsular Malaysia. 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. 48. Arcotheres rayi Ahyong & Ng, 2007 . A, D, ovigerous female (7.6 × 5.8 mm) (ZRC 2013.1416), Peninsular Malaysia; B, C, E, female (6.2 × 4.4 mm) (ZRC 2013.1416), Peninsular Malaysia. A, B, overall dorsal view; C, dorsal view of carapace; D, E, right P5. Pleon extending to buccal region, covering bases of P2–P5; telson gently recessed into concave distal margin of somite 6 ( Figs. 46B , 47B , 49D ). Male : Carapace and pereopods well chitinised, firm. Carapace almost circular, slightly wider than long; dorsal surface almost smooth, not prominently inflated, lateral surfaces with setae; front projecting anteriorly, margin gently sinuous to almost straight ( Figs. 51A , 52A ). MXP3 as in female but dactylus shorter ( Fig. 52B ). Anterior thoracic sternum wide, sternites 1, 2 fused, partially sunken into buccal cavity; suture between sternites 2 and 3 shallow; sternites 3, 4 completely fused, separated only by shallow grooves. Chela relatively stout, shorter than in female ( Figs. 51C , 52D ). P2–P5 dorsally, ventrally unarmed; outer surface covered with short setae; P3 and P4 carpus and propodus with long natatory setae; left and right meri equal, relative lengths of meri P4>P3>P2>P5; dactyli of P2–P4 progressively longer; right P4 dactylus slightly longer, more slender than left side; P4 and P5 dactylus subequal to that of P3, covered with short setae ( Fig. 52E–L ). Pleon slender, triangular, widest at somite 3, lateral margins of somite 4 gently concave; somite 6 trapezoidal; telson semicircular, wider than long ( Fig. 52M ). G1 relatively stout, arcuate, curved outwards, with short triangular subdistal dorsal projection, tip rounded ( Fig. 52N, O ). G2 short, with spatuliform tip; exopod about two-thirds endopod length ( Fig. 52P ). Fig. 49. Arcotheres rayi Ahyong & Ng, 2007 , ovigerous female (7.6 × 5.8 mm) (ZRC 2013.1416), Peninsular Malaysia. A, overall dorsal view; B, right MXP3; C, frontal 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. Variation. The carapace shape varies somewhat, the most important being the prominence of the front, whose variation is broadly size-related. In the holotype of A. rayi , the largest known specimen of the species, the front does not protrude in advance of the general anterior margin, but is demarcated on either side by a shallow notch above each orbit ( Fig. 46 ). Other specimens, however, have a front that protrudes anteriorly beyond the general anterior margin, some retaining the notch on either side of the front ( Fig. 48B, C ), but most others without ( Figs. 47 , 48A , 49 ). The length of the longer P5 dactylus varies allometrically, being proportionately shorter in small specimens ( Table 2 ; Fig. 50H–L ). Fig. 50. Right female P5. A–G, Arcotheres exiguus ( Bürger, 1895 ) , Peninsular Malaysia: A, juvenile (3.8 × 3.6 mm) (ZRC 2018.772); B, ovigerous (4.5 × 4.1 mm) (ZRC 2018.775); C, non-ovigerous (4.6 × 4.2 mm) (ZRC 2018.772); D, spent (5.4 × 4.6 mm) (ZRC 2018.775); E, ovigerous (6.8 × 6.1 mm) (ZRC 2019.516); F, ovigerous (7.7 × 6.7 mm) (ZRC 2018.772); G, ovigerous (8.9 × 7.6 mm) (ZRC 2018.775). H–L, A. rayi Ahyong & Ng, 2007 , Peninsular Malaysia: H, juvenile (5.4 × 5.8 mm) (ZRC 2013.1416); I, non-ovigerous (5.6 × 4.3 mm) (ZRC 2013.1416); J, non-ovigerous (8.1 × 5.2 mm) (ZRC 2013.1416); K, spent (6.9 × 5.2 mm) (ZRC 2013.1416); L, ovigerous (8.4 × 7.0 mm) (ZRC 2013.1416). Scale = 1.0 mm. Host. Known only from Paphia sp. ( Veneridae Rafinesque, 1815 ). Remarks. Arcotheres rayi is morphologically very close to A. exiguus , but the good series of specimens has permitted evaluation of intraspecific variability, enabling more reliable recognition of interspecific differences. Females of A. rayi , in which the carapace front is demarcated by a notch in the anterior margin above each orbit ( Figs. 46 , 48B, C, E ), are readily distinguished from A. exiguus , which does not have the anterior carapace notches ( Figs. 33B, C , 35B, E ,