Plenotheres coarctatus ( Bürger, 1895 ), Ng & Ahyong, 2022

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 : 216-223

Plenotheres coarctatus ( Bürger, 1895) , new combination

( Figs. 63–68 )

Pinnotheres coarctatus Bürger, 1895: 369 , pl. 9 fig. 7, pl. 10 fig. 7 [type locality: Tipoeka, Timika, Indonesian Papua, Indonesia, by present neotype designation]; Tesch, 1918: 248 (list); Estampador, 1937: 546 (list); Silas & Alagarswami, 1967: 1197 (list); Serène, 1968: 93; Schmitt et al., 1973: 42.

Pinnoteres coarctatus — Balss, 1957: 1419 (list).

Arcotheres coarctatus — Ahyong & Ng, 2007b: 195, fig. 3A; Ng et al., 2008: 248; Ng et al., 2017: 1094; Trivedi et al., 2018a: 197; De Gier & Becker, 2020: tab. 1; Ahyong & Ng, 2021: 207–209, fig. 17.

Pinnotheres parvulus — Rathbun, 1910: 331, fig. 13, pl. 2 fig. 9; Naiyanetr, 1980: 42; 1998: 104; 2007: 118 (not Pinnotheres parvulus Stimpson, 1858 ).

Type material. Neotype (here designated): female (13.1 × 11.4 mm) ( ZRC 2020.13 ), Tipoeka, Timika, Indonesian Papua, Indonesia, in Polymesoda expansa , coll. D. L. Rahayu et al., 26 April 2000.

Other material examined. Indonesia: 2 males (2.7 × 2.8 mm, 3.3 × 3.4 mm) , 1 female (11.9 × 10.8 mm), 2 juvenile females (3.4 × 3.3 mm, 3.2 × 3.2 mm) ( ZRC 2018.765 ), same data as neotype ; 1 female (7.9 × 7.2 mm) ( ZRC 2019.1789 ), in Glauconome rugosa , from Kandang Kerbau market, Singapore, from Indonesia , coll. P. K. L. Ng, 29 December 2019 ; 2 females (11.3 × 10.7 mm, 10.2 × 9.5 mm) ( ZRC 2017.1284 ), Ajkwa, Timika Province, Indonesian Papua, no host data, coll. D. L. Rahayu et al., 20 January 2000 ; 1 female (11.3 × 10.2 mm) ( ZRC 2017.1285 ), Ajkwa, Timika Province, Indonesian Papua, in Geloina coaxans , coll. D. L. Rahayu et al., 11 January 2000 ; 1 female (13.2 × 11.6 mm) ( ZRC 2017.1286 ), Tipoeka, Timika Province, Indonesian Papua, in Polymesoda expansa , coll. D. L. Rahayu et al., 21 December 1999 ; 2 females (11.5 × 10.7 mm, 6.7 × 6.1 mm) ( ZRC 2018.766 ), in Batissa violacea , peat site, Timika, Indonesian Papua, Indonesia , coll. D. L. Rahayu et al., 27 November 1999 . Singapore: 1 female (14.1 × 13.1 mm) ( ZRC 2001.2237 ), Loyang mangroves, off Loyang Avenue , in Polymesoda expansa , coll. N. Sivasothi, 17–18 July 2001 ; 3 females (16.5 × 14.4 mm, 15.5 × 13.2 mm, 13.9 × 12.4 mm) ( ZRC 2003.681 ), in “ Geloina zeylanica (Lamarck, 1806) ” (= present Geloina coaxans ), coll. A. Ibrahim, June 2003 ; 2 females (14.4 × 13.2 mm, 13.3 × 11.9 mm) ( ZRC 2012.1223 ), in Polymesoda expansa , mudflats, Pulau Ubin, coll. G. Jahvel, 5 November 2012 . Peninsular Malaysia: 1 female (10.1 × 9.1 mm) ( ZRC 1995.438 ), in Glauconome rugosa , from market, coll. P. K. L. Ng, 21 December 1992 ; 5 females (11.7 × 11.2 mm, 9.5 × 8.8 mm, 9.4 × 9.0 mm, 8.7 × 8.2 mm, 6.6 × 6.3 mm) ( ZRC 2017.1283 ), in Glauconome rugosa , from Chinatown market, Singapore, from Malaysia, coll. P. K. L. Ng, 21 January 2018 ; 9 ovigerous females (6.8 × 6.5 mm, 8.0 × 7.1 mm, 8.1 × 6.7 mm, 8.1 × 7.6 mm, 8.4 × 8.6 mm, 9.0 × 8.0 mm, 9.1 × 8.0 mm, 9.9 × 8.8 mm, 10.5 × 9.2 mm) ( ZRC 2018.246 ), 2 ovigerous females (9.1 × 8.4 mm, 7.2 × 6.1 mm) ( AM P105903 ), in Glauconome rugosa, Chinatown market, Singapore, from Malaysia, coll. P. K. L. Ng, 11 February 2018 ; 4 females (8.0 × 7.6 mm, 9.1 × 8.0 mm, 10.2 × 9.5 mm, 11.9 × 10.6 mm) ( ZRC 2018.254 ), in Glauconome rugosa, Chinatown market, Singapore , from Malaysia, coll. P. K. L. Ng, 11 March 2018 ; 3 females (photographed 8.4 × 7.9 mm) ( ZRC 2020.10 ), in Glauconome rugosa , from Kandang Kerbau market, Singapore , coll. P. K. L. Ng, 29 March 2020 . Thailand: 1 ovigerous female (8.8 × 7.6 mm) ( NHMD Cr-284250), Lem Ngob, Gulf of Thailand , mangrove swamp, coll. Th. Mortensen, 23–27 December 1900 . Description. Carapace and pereopods poorly chitinised. Female: Carapace subcircular, slightly wider than long; appearing domed in frontal view; dorsal and lateral surfaces smooth, glabrous; with pair of fine longitudinally arcuate grooves almost always well defined, extending from behind orbits to indistinct gastrocardiac groove; front slightly projecting anteriorly beyond anterolateral margins, margin straight to gently convex; anterolateral margin smooth, convex, gently curving to join rounded angle with posterolateral margin ( Figs. 63D , 64A , 65 ). Eyes small, just visible in dorsal view in adults; mobile, filling orbit ( Figs. 63D–F , 64A, C ). Epistome with median part triangular, lateral margins concave ( Fig. 64C ) .

MXP3 outer surface with scattered short setae; ischiomerus completely fused, subrhomboidal, inner margin prominently angular (almost 90°) at widest point; carpus short; propodus conical, about 2.5 times as long as high, distinctly longer than carpus, tip rounded; dactylus slender, inserted at proximal one-third of propodus, tip slightly overreaching propodal apex; exopod relatively stout, with proximal part very wide, about two-thirds length of ischiomerus, flagellum 2-segmented ( Figs. 64C , 66I ).

Chela not prominently elongate, stout, dactylus exceeding two-thirds palm length; palm 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 almost straight; dactylus occlusal margin with blunt triangular tooth proximal to midlength, margin straight in distal half, finely denticulate, sparsely setose; pollex occlusal margin with 2 blunt triangular teeth proximal to midlength, straight margin in distal half, finely denticulate, sparsely setose, with fringe of short setae on inner ventral margin, extending to inner surface of palm ( Fig. 64D ).

P2–P5 dorsally, ventrally unarmed; margins of P2 and P3 covered with short setae; outer surface of P3 (especially merus) with dense long setae; ventral margins of dactylus with long dense setae; merus relatively longer, more slender, relative lengths of meri P4>P3>P2>P5; right (sometimes left) P4 merus and propodus distinctly the longer; P2–P4 dactyli relatively short, subequal, tip hooked, shorter than half propodus length; dactylus of longer P4 stout, weakly curved, longer than P2 and P3 dactylus, shorter than P5 dactylus; P5 merus 4.3–4.4 times longer than wide; P5 dactylus longest, gently falciform, ventral margin lined with short and long setae, distoflexor margin with relatively short row of 6–12 small spinules ( Fig. 66A–K ).

Pleon extending to buccal region, covering bases of P2–P5; telson slightly recessed into concave distal margin of somite 6 ( Fig. 64B ).

Male: Carapace almost circular, slightly longer than wide; dorsal surface smooth, inflated, lateral surfaces with setae; front projecting anteriorly, margin almost straight ( Fig. 67A, B ). Eyes distinctly visible in dorsal view ( Fig. 67A, B ). MXP3 as in female but dactylus shorter than propodus ( Fig. 68J–L ). 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. 67C , 68I ). 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 P3=P4>P2>P5; dactyli of P2–P4 progressively longer; left P4 merus slightly longer than right side; P4 and P5 dactylus subequal to that of P3, covered with short setae ( Fig. 68A–H ). Pleon triangular, widest at somite 3, lateral margins of somite 4 gently concave; somite 6 subrectangular; telson semicircular, slightly wider than long ( Fig. 68M ). G1 relatively stout, widely C-shaped, curved outwards, with elongate sharp subdistal dorsal flap at right angles to main structure ( Fig. 68N–P ). G2 short, with spatuliform tip; exopod long, curved ( Fig. 68Q ).

Variation. General setation is often less dense in larger specimens. The gastric grooves on the carapace are usually distinct, but less obvious in a few specimens, probably as a result of poor preservation. The single row of spinules on the ventral margin of the distal part of the P5 dactylus varies in number from 6 to 12, but the proximal ones are frequently very low and barely visible ( Fig. 66J, K ).

Colour. In life, the carapace varies from brown and grey to dirty white overall, with the ovaries appearing purplish and the eyes dull red ( Fig. 63 ).

Host. Confirmed from Geloina coaxans (Gmelin, 1791) , Polymesoda expansa (Mousson, 1849) , and Batissa violacea (Lamarck, 1818) ( Cyrenidae Gray, 1840 ); or Glauconome rugosa Hanley, 1843 ( Glauconomidae Gray, 1853 ). Both families are in the superfamily Cyrenoidea Gray, 1840 . Bürger (1895) recorded the host as “Cahebe” (= Polymesoda spp. ) ( Schmitt et al., 1973). Note that Bürger’s (1895) “Cahebe” is probably a misspelling of the local name for a commonly harvested brackish water clam ( Geloina sp. ), known as “kabibe” in Tagalog and other Philippine languages, or “cabebe” or “cabibe” under old Hispanised renderings (J. C. E. Mendoza, pers. comm.; Ahyong & Ng, 2021).

Remarks. Ahyong & Ng (2007b: 195) regarded Plenotheres coarctatus , new combination, originally described from Zamboanga, Philippines, and whose original types are lost, to be sufficiently recognisable as to not require a neotype designation at that time. Females of Pl. coarctatus as understood here are distinguishable from species of Arcotheres by the pair of anterior dorsal carapace grooves. Males of Pl. coarctatus , however, closely resemble male A. exiguus and A. rayi in most respects including the gonopod and P5 structure; they can be externally distinguished by the shape and elongation of the MXP3 propodus, which is shorter and conical in Pl. coarctatus , with the dactylus articulating slightly proximal to the midlength ( Fig. 68J–L ) (versus elongate and spatulate with the dactylus articulating slightly distal to the midlength in the two species of Arcotheres ; Figs. 44A , 45A , 52B ). In view of the strong similarities between males of A. exiguus , A. rayi , and what is here considered to be Pl. coarctatus ; and because this species is herein made the type species of a new genus, a neotype selection is required to fix its identity. This will stabilise the taxonomy of Pinnotheres coarctatus and the genus. The neotype selected is a female (13.1 × 11.4 mm; ZRC 2020.13) collected from Polymesoda expansa in Indonesian Papua. This location is southeast of the original type locality in southern Philippines.

Ahyong & Ng (2021) showed that Rathbun’s (1910) record of Pinnotheres parvulus from the Gulf of Thailand is referable to Pl. coarctatus .

Distribution. South China Sea from the Philippines, Indonesia, Peninsular Malaysia, Singapore, and Thailand.

Figures

Fig. 63.Plenotheres coarctatus (Bürger, 1895). A, B, specimens in situ (ZRC 2018.765) inPolymesoda expansa (Mousson, 1849) (Cyrenidae Gray, 1840), Indonesian Papua; C–G, female (11.7 × 11.2 mm) (ZRC 2017.1283), Peninsular Malaysia. C, in situ in Glauconome rugosa Hanley, 1843 (Glauconomidae); D, overall habitus; E, F, frontal view; G, ventral view. Photographs: A, B, Sigit Dwiono; C–G, Paul Y. C. Ng.

Fig. 64. Plenotheres coarctatus (Bürger, 1895), female (6.8 × 6.5 mm) (ZRC 2018.246), Peninsular Malaysia. A, overall dorsal view; B, ventral view of cephalothorax; C, frontal view of cephalothorax; D, outer view of right chela.

Fig. 65. Plenotheres coarctatus (Bürger, 1895), overall dorsal view. A, neotype female (13.1 × 11.4 mm) (ZRC 2020.13), Indonesian Papua; B, female (11.5 × 10.7 mm) (ZRC 2018.766), Indonesian Papua; C, female (13.9 × 12.4 mm) (ZRC 2003.681), Singapore; D, female (13.3 × 11.9 mm) (ZRC 2012.1223), Singapore; E, female (8.0 × 7.6 mm) (ZRC 2018.254), Peninsular Malaysia; F, female (11.9 × 10.6 mm) (ZRC 2018.254), Peninsular Malaysia.

Fig. 66. Plenotheres coarctatus (Bürger, 1895). A–I, neotype female (13.1 × 11.4 mm) (ZRC 2020.13), Indonesian Papua; J, K, female (14.1 × 13.1 mm) (ZRC 2001.2237), Singapore. A–D, left P2–P5, respectively; E–H, right P2–P5, respectively; I, left MXP3; J, distal part of left P5 dactylus (ventral view, soft setae omitted); K, distal part of right P5 dactylus (ventral view, soft setae omitted). Scales = A–H, 1.0 mm; I, 0.5 mm; J, K, 0.25 mm.

Fig. 67. Plenotheres coarctatus (Bürger, 1895), male (3.3 × 3.4 mm) (ZRC 2018.765), Indonesian Papua. A, overall dorsal view; B, frontal view of cephalothorax; C, outer view of right chela.

Fig. 68. Plenotheres coarctatus (Bürger, 1895), male (3.3 × 3.4 mm) (ZRC 2018.765), Indonesian Papua. A–D, left P2–P5, respectively; E–H, right P2–P5, respectively; I, outer view of right chela; J, left MXP3; K, palp of left MXP3 (outer view); L, palp of left MXP3 (inner view); M, pleon; N, left G1 (ventral view); O, distal part of left G1 (ventral view, setae omitted); P, left G1 (dorsal view, setae omitted); Q, left G2. Scales = A–I, M, 0.5 mm; J–L, N–Q, 0.2 mm.

Fig. 44. Arcotheres exiguus (Bürger, 1895), male (5.1 × 4.9 mm) (ZRC 2018.772), Peninsular Malaysia. A, overall dorsal view; B, right MXP3; C, frontal view of cephalothorax; D, anterior thoracic sternum; E, right chela; F–I, right P2–P5; J, pleon; K, L, left G1 (ventral and dorsal views, respectively); M, N, left G2 (ventral and dorsal views, respectively). Scale = A, 2.0 mm; B, K–N, 0.5 mm; C–J, 1.0 mm.

Fig. 45. Arcotheres exiguus (Bürger, 1895), male (4.5 × 4.2 mm) (ZRC 2018.768), India. A, left MXP3 (setae omitted); B, left chela and carpus; C–F, right P2–P5, respectively; G, male pleon; H, left G1 (dorsal view); I, left G1 (ventral view); J, left G2 with exopod. Scales = 0.5 mm.

Fig. 52. Arcotheres rayi Ahyong & Ng, 2007, male (4.2 × 3.8 mm) (ZRC 2013.1416), Peninsular Malaysia. A, overall dorsal view; B, right MXP3; C, frontal view of cephalothorax; D, right chela; E–H, left P2–P5; I–L, right P2–P5; M, pleon; N, O, right G1 (dorsal and ventral views, respectively); P, right G2. Scale = A, 2.0 mm; B, N–P, 0.5 mm; C–M, 1.0 mm.