Arcotheres exiguus ( Bürger, 1895 )

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 : 182-194

Arcotheres exiguus ( Bürger, 1895)

( Figs. 34–45 , 50A–G )

Pinnotheres exiguus Bürger, 1895: 377 , pl. 9 fig. 19, pl. 10 fig. 30 [type locality: Samar Island, Philippines]; Serène, 1968: 93.

Pinnotheres winckworthi Gordon, 1936: 177 , fig. 7; Serène, 1968: 95. [New synonymy]

Pinnotheres vicajii Chhapgar, 1957: 505 , 506, pl. 12 fig. n–q; Chhapgar, 1958: 253, fig. 2; Silas & Alagarswami, 1967: 1212; Schmitt et al., 1973: 81; Kumari & Rao, 1974: 165, figs. 1–19; Ng et al., 2008: 251; Pati et al., 2012: 384; Dev Roy, 2013: 156, 157. [New synonymy]

Pinnotheres sp. — Silas & Alagarswami, 1967: 1163, text-figs. 1, 2, pls. 1–3.

Pinnotheres gracilis — George & Noble, 1970: 392, fig. 1.1–1.3 [not Pinnotheres gracilis Bürger, 1895 ].

Pinnotheres casta Antony & Kutyamma, 1971: 60 , figs. 1, 2; Schmitt et al., 1973: 81; Dev Roy, 2013: 156, 157. [New synonymy]

Pinnotheres obscuridentata Dai & Song, 1986: 56 , 62, text-fig. 3 [type locality: Longmen, Guangxi, China]; De Gier & Becker, 2020: tab. 1. [New synonymy]

Arcotheres exiguus — Campos & Manning, 2001: 168; Ahyong & Ng, 2007b: 195, fig. 4; Ng et al., 2008: 248; Ng et al., 2017: 1093.

Arcotheres winckworthi — Ng et al., 2008: 248; Ng et al., 2017: 1093.

Arcotheres alcocki — Pati et al., 2015: 1067, fig. 1; Dev Roy & Rath, 2017: 103 (not Pinnotheres alcocki Rathbun, 1909 ).

Nepinnotheres vicajii — Ng & Kumar, 2015: 265; Trivedi et al., 2018b: 62.

Arcotheres casta — Ng & Kumar, 2015: 265; Ng et al., 2017: 1094; Kazmi et al., 2018: 136, fig. 5; Trivedi et al., 2018a: 197; Trivedi et al., 2018b: 61 (list); Trivedi et al., 2020: 505.

Viridotheres gracilis — Dev Roy, 2013: 156, 157; Trivedi et al., 2018b: 62 (not Pinnotheres gracilis Bürger, 1895 ).

Arcotheres vicajii — Trivedi et al., 2020: 497–507, figs. 1–8; De Gier & Becker, 2020: tab. 1.

Type material. Lectotype: female (5.6 × 4.6 mm), Samar Island, Philippines, coll. C. Semper, 1876. Peninsular Malaysia: 1 ovigerous female (9.3 × 7.6 mm) ( NHM 1936.6.19.12), Penang, Malay Peninsula, from Protapes gallus , coll. R. Winckworth, March 1933 (holotype of Pinnotheres winckworthi Gordon, 1936 ); 1 ovigerous female (8.8 × 7.3 mm) ( NHM 1936.6.19.13), Penang, Malay Peninsula, from Protapes gallus , coll. R. Winckworth, March 1933 (paratype of Pinnotheres winckworthi Gordon, 1936 ); 3 males (4.3 × 4.2–5.1 × 4.9 mm), 6 ovigerous females (6.3 × 5.4–7.7 × 6.7 mm). India: 1 ovigerous female (8.2 × 6.9 mm) ( ZRC 2021.784) (neotype of Pinnotheres casta Antony & Kutyamma, 1971 ), in Marcia cf. cordata , west coast of Tamil Nadu, India, coll. R. Ravinesh, 2016.

Other material examined. Peninsular Malaysia: 6 spent females (4.6 × 4.2–7.0 × 6.3 mm), 1 juvenile female (3.8 × 3.6 mm) ( ZRC 2018.772 ), Chinatown market, Singapore , from Marcia recens , coll. P. K. L. Ng, 25 February 2018 ; 1 male (4.9 × 4.8 mm) ( ZRC 2018.773 ), from venerid bought from Sheng Siong Supermarket , Singapore, coll. 27 September 2018 ; 2 males (3.4 × 3.4 mm, 3.7 × 3.7 mm), 1 ovigerous female (4.8 × 3.9 mm), 1 spent female (4.5 × 4.1 mm), 1 ovigerous female (4.3 × 4.0 mm) ( ZRC 2018.774 ), in Dosinia sp. , from local market, Singapore, coll. S. K. Tan, 10 December 2013 ; 1 male (4.7 × 4.6 mm), 1 ovigerous female (6.9 × 5.6 mm), 3 spent females (5.4 × 4.6–8.2 × 7.4 mm), 1 ovigerous female (4.5 × 4.0 mm) ( ZRC 2018.775 ), in Marcia sp. , from market, Singapore, coll. S. K. Tan, 10 December 2013 ; 1 ovigerous female ( ZRC 2018.1068 ), from clam, Yew Tee Market, Singapore , coll. H. H. Tan, 9 September 2018 ; 1 male (4.3 × 4.2 mm), 1 ovigerous female (6.9 × 6.1 mm) [photographed], 1 ovigerous female (7.9 × 7.3 mm) [photographed], 8 ovigerous females, 6 spent females, 2 juvenile females (2.1 × 1.6 mm, 1.6 × 1.5 mm) ( ZRC 2019.516 ), from Marcia recens , from market, Singapore , coll. P. Y. C. Ng, March 2019 ; 1 ovigerous female ( ZRC 2020.352 ), from Marcia recens, Yew Tee Market , Singapore , coll. H. H. Tan, 2 June 2019 ; 1 male (4.3 × 4.2 mm), 2 non-ovigerous females (5.7 × 4.9 mm, 5.7 × 5.1 mm), 2 ovigerous females (5.7 × 4.9 mm, 7.0 × 5.0 mm) ( ZRC 2020.14 ), in Marcia recens , from Sheng Shiong Supermarket, Singapore, coll. S. K. Tan, 23 March 2020 ; 2 ovigerous females ( ZRC 2020.66 ), in Marcia recens , from Chinatown market, Singapore , coll. P. K. L. Ng, 4 June 2020 . Singapore: 4 ovigerous females (6.2 × 5.2–7.0 × 6.3 mm) ( ZRC 2018.776 ), from kelong (artisanal fish traps), in “ Lala clams” (probably Marcia recens ), coll. B. Y. Lee, 25 March 2018 ; 2 ovigerous females ( ZRC 2020.390 ), in “Lala clams” ( Marcia sp. ), in seafood dish, Ah Hua Kelong Restaurant, Singapore , coll. M. S. Foo, 1 November 2020 ; 2 males (5.6 × 5.4 mm, 5.4 × 5.2 mm) ( ZRC 2020.419 ), from Meretrix meretrix , from Sheng Shiong Supermarket in Bukit Batok , Singapore, coll. W. Xu, 12 February 2017 ; 1 ovigerous female ( ZRC 2021.10 ), from clam in seafood restaurant, Singapore , coll. students, 2005 . Philippines: 1 female ( ZRC 2021.17 ), in Ruditapes philippinarum , from plate of noodles at seafood restaurant, purportedly from Korea but probably from Philippines or adjacent area, coll. S. K. Tan, 16 December 2020 . India: 3 males (4.3 × 4.1 mm, 4.5 × 4.2 mm, 4.7 × 4.4 mm), 1 non-ovigerous female (8.0 × 6.7 mm), in Marcia cf. cordata , Tamil Nadu, India, coll. R. Ravinesh, 2016 ( ZRC 2018.768 ) ; 1 juvenile female (4.0 × 3.6 mm) ( ZRC 2018.769 ), in Protapes cf. rhamphodes , Tamil Nadu, coll. R. Ravinesh, 2016 ; 1 female ( ZRC 2018.498 ), male (8.3 × 5.5 mm), Meretrix aurora , Mumbai, Maharashtra state, India , coll. S. Gosavi, 27 September 2018 .

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 slightly projecting anteriorly beyond orbits, margin gently sinuous to gently convex; anterolateral margin usually subparallel with frontal margin or sloping posteriorly to various degrees, forming rounded angle with posterolateral margin ( Figs. 34B , 35B, E , 36C , 37A, C, D , 38A–C , 39A, C–E , 41A, N ). Eyes small, not or just visible in dorsal view in adults; mobile, completely filling orbit ( Fig. 41A, N ). Epistome with median part triangular, lateral margins gently concave ( Figs. 38C , 39C , 41C ).

MXP3 outer surface with scattered short setae; ischiomerus completely fused, subrhomboidal, inner margin rounded to angular at widest point; carpus short; propodus about 3 times as long as high, subspatulate, distinctly longer than carpus, tip rounded to subtruncate; dactylus slender, varying lengths, inserted slightly proximal to midlength to near proximal one-third of propodus, tip not reaching or just reaching propodal apex; exopod relatively slender, about two-thirds length of ischiomerus, flagellum 2-segmented ( Figs. 36E , 41B, Q , 42A, L ).

Chela relatively short to 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 concave to 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. 36F , 37E , 38D, E , 41E , 42B ).

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, half propodus length; longer P4 dactylus elongate, broadly falciform, distinctly longer than half propodus length, slightly longer than P5 dactylus; P5 merus 4.1–4.5 times longer than wide; P5 dactylus shorter to longer (in specimens> 6.5 mm cl) than propodus extensor margin, length/height> 5.0 in specimens> 6.0 mm cl, margins lined with short and long setae, denser on ventral margin, distoflexor margin without distinct rows of spinules, sometimes with a row or patches of very low, spinule-like structures which are all similarly sized ( Figs. 36C, D , 41F–M, O, P , 42C–K, M , 50A–G ).

Pleon extending to buccal region, covering bases of P2–P5; telson gently recessed into concave distal margin of somite 6 ( Figs. 34C , 35F , 37B , 39B , 41D ).

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. 34D , 35C, D , 43A, B , 44A ). Eyes distinctly visible in dorsal view ( Figs. 34D , 35C, D , 43A, B , 44A ). MXP3 as in female ( Figs. 44B , 45A ). 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. 34F , 44D , 45B ). 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; P4 and P5 dactylus subequal to that of P3, covered with short setae ( Figs. 44F–I , 45C–F ). Pleon slender, triangular, widest at somite 3, lateral margins of somite 4 gently concave to almost straight; somite 6 trapezoidal; telson semicircular, wider than long ( Figs. 44J , 45G ). G1 relatively stout, arcuate, curved outwards, with distinct subdistal dorsal projection, tip rounded ( Figs. 44K, L , 45H, I ). G2 short, with spatuliform tip; exopod about half endopod length ( Figs. 44M, N , 45J ).

Variation. The length of the MXP3 dactylus varies somewhat, from falling short of ( Fig. 42A, C ) to reaching as far as, but not beyond, the propodus ( Figs. 36E , 41B ). In the lectotype female of A. exiguus , the MXP3 dactylus is relatively long ( Fig. 36E ; Ahyong & Ng, 2007b: fig. 4C). The degree of variation highlights the importance of a large series of specimens for accurate morphological characterisation. The MXP3 dactylus figured by Trivedi et al. (2020: figs. 6L, 7A) for two Indian specimens is inaccurate as the structure is actually much shorter (present Figs. 42C , 45A ), albeit still within the known variation in this species. The general features of adults vary little: the frontal margin is invariably slightly protruding in dorsal view, whereas the lateral margins are usually divergent but may be gently convex and subparallel ( Figs. 36C , 37A , 38A, B , 39A, D, E , 41A, N ). The P5 distoflexor margin is usually not armed with spinules, although it may be possible to sometimes discern a row of very low, spinule-like structures (e.g., Fig. 42M ). These structures, however, are unlike those in A. similis or A. placunicola because they are much weaker and all similarly sized, with the distal ones not elongate. Trivedi et al. (2020: fig. 4I) figured a P5 distoflexor margin (as A. vicajii , here treated as a junior synonym of A. exiguus ) that has several prominent spinules, but these are not arranged in a row and actually much smaller than drawn. As with A. rayi , the slenderness of the P5 dactylus on A. exiguus varies allometrically, becoming more slender with increasing body size. However, at any given size, the P5 dactylus is more slender in A. exiguus than in A. rayi ( Table 2; Fig. 50 ).

Colour. The colour is rather variable, with females usually pale yellowish- to cream-white, but the gonads, when mature, are bright orange ( Figs. 34A–C , 35A, B ). In some specimens, however, the overall carapace and appendages are pale yellow as well ( Fig. 35E, F ), the colour possibly being a consequence of host metabolism or food. Males are white to orange, with numerous small black spots ( Figs. 34D–F , 35C, D ).

Host. Almost always from Protapes gallus (Gmelin, 1791) , Protapes cf. rhamphodes (Oliver & Glover, 1996) , Marcia recens (Holten, 1802) , Marcia cf. cordata (Forskål, in Niebuhr, 1775), Meretrix aurora Hornell, 1917 , Meretrix meretrix (Linnaeus, 1758) , Dosinia sp. , and Ruditapes philippinarum Adams & Reeve, 1850 (all Veneridae Rafinesque, 1815 ). Kazmi et al. (2018: 136) reported the Pakistani host as Mercenaria Schumacher, 1817 , but this is a coldwater genus not known from Asia, so the authors were probably dealing with a different venerid.

Remarks. Pinnotheres exiguus Bürger, 1895 , transferred to Arcotheres by Campos & Manning (2001), was described from a small female (5.6 × 4.6 mm) from the Philippines, with the lectotype fixed by Ahyong & Ng (2007b: 195) ( Fig. 36A, B ). The lectotype ( Fig. 36C–F ) is now in poor condition, but agrees well, in all its MXP3 and ambulatory leg characters, with the type material of A. winckworthi ( Gordon, 1936) and the series of specimens on hand; the two species are herein synonymised. Arcotheres exiguus is distinctive in the genus in its combination (in females) of the elongated P5 dactylus (as long as or longer than the propodus and twice as long as the dactyli of the P2, P3, and shorter P4), which lacks a row of distinct distoflexor spinules on the distal part and is strongly setose only on the flexor margins, being glabrous or sparsely setose on the other surfaces. As discussed earlier (under Characters), the setation of the P5 dactylus of A. exiguus (as well as A. rayi ) contrasts with that of A. palaensis , A. ocularius , and A. similis , with the flexor margin distinctly more setose. Males of A. exiguus and A. rayi are unusual among congeners, of which males are known for having the articulation between the P5 ischium and merus diagonal rather than perpendicular to the segment axis, and a triangular subdistal lobe on the G1 (absent in others). In these respects, males of A. exiguus and A. rayi resemble male Plenotheres coarctatus . Arcotheres exiguus is most similar to A. rayi ; distinguishing features are discussed under the account of the latter.

Females of Arcotheres exiguus are ovigerous by 4.1 mm cl (ZRC 2018.775). The smallest male examined (cl 3.4 mm; ZRC 2018.774) is already mature.

Silas & Alagarswami (1967) described and figured in detail an unidentified species of Pinnotheres obtained from the venerid clam Meretrix casta (Gmelin, 1791) from Cochin, western India. Antony & Kutyamma (1971) subsequently collected the same species from the same host in Cochin, formally naming it Pinnotheres casta . On the basis of the characters, this is clearly a species of Arcotheres . The present specimens from southern India (ZRC 2018.768, ZRC 2018.769, ZRC 2021.784) agree very well with the accounts by Silas & Alagarswami (1967) and Antony & Kutyamma (1971) of P. casta . The figure of the overall female habitus by Antony & Kutyamma (1971: fig. 1A) is schematic, but those of the MXP3 and ambulatory legs clearly show the asymmetry and leg proportions ( Antony & Kutyamma, 1971: figs. 1B, 2) (see also Silas & Alagarswami, 1967: text-fig. 2). Silas & Alagarswami (1967) also had males of the species and figured their G1 and MXP3 ( Silas & Alagarswami, 1967: text-fig. 1(4–6)). Interestingly, Chhapgar (1957) had described Pinnotheres vicajii from western India, but the identity of his species was always in doubt because the original description was very brief and the figures too schematic. Ng & Kumar (2015) transferred the species to Nepinnotheres on the basis of Chhapgar’s (1957) figures of the ambulatory legs, which appeared to be symmetrical. Trivedi et al. (2020) re-examined the holotype male and fresh specimens of P. vicajii from the type locality and showed that P. casta was its junior subjective synonym.

Both sexes of P. vicajii (and P. casta ) agree very well with A. exiguus ( Fig. 42 ) and we regard them as conspecific. The structure of the G1, in particular, is diagnostic ( Figs. 44K, L , 45H, I ; cf. Trivedi et al., 2020: figs. 3D, 7H, I). As discussed by Trivedi et al. (2020), the types of Pinnotheres casta Antony & Kutyamma, 1971 , are no longer extant. In view of the complex taxonomy of the various species now synonymised under A. exiguus , to stabilise the taxonomy of these taxa, we here select an ovigerous female (8.2 × 6.9 mm; ZRC 2021.784) from the western shore of Tamil Nadu, India ( Fig. 39A–C ), as the neotype of P. casta ; it agrees with the type description and figures in almost all aspects. The neotype is from a location south of Kochi in the Indian state of Kerala, the original type locality, but is still in the southwestern part of India. Trivedi et al. (2020) redescribed the species at length, as A. vicajii .

Pinnotheres obscuridentata Dai & Song, 1986 , described from one female (no host recorded) collected in Longmen, Guangxi, southern China ( Fig. 40 ), is here provisionally considered to be a junior synonym of A. exiguus . Dai & Song (1986: 56, 62) commented that a distinguishing feature was the absence of a sub-basal tooth on the dactylar finger of the chela, but it seems more likely that it is simply broken or eroded; all other species of Arcotheres possess the dactylar tooth. In almost all other features as understood at present, it agrees well with A. exiguus . The MXP3 has a relatively short propodus with a short dactylus, and the chela is short and stocky ( Dai & Song, 1986: fig. 3-2, 3-3); all characters of A. exiguus and the proportions of the ambulatory legs also agree ( Dai & Song, 1986: fig. 3-1, 3-4–3-7). The carapace of P. obscuridentata is proportionally slightly wider than typical A. exiguus (at least as figured by Dai & Song, 1986: fig. 3-1), but the carapace in this species is weakly chitinised and if poorly preserved can appear to vary substantially ( Figs. 36A , 37A , 38A, B , 39A, D, E , 41A, N ). The concave posterior margin of the carapace of P. obscuridentata , as figured by Dai & Song (1986), is a result of contraction of the soft carapace cuticle and slight anterior tilt of the specimen when figured, which in A. exiguus , is usually almost straight but occasionally concave ( Fig. 39A ). The holotype of P. obscuridentata could not be found in the type repository, Institute of Zoology in the Chinese Academy of Sciences (Academia Sinica) ( Dai & Song, 1986), despite two separate searches in 2018 and 2019 (Meng Kai, Ng Ngan Kee, Lee Bee Yan, pers. comm.). The account by Dai & Song (1986: 56) is somewhat confusing, indicating that only the holotype female was available, yet providing two sets of carapace measurements, perhaps corresponding to more than one specimen or multiple points of measurement of the same specimen (carapace width 5.3–7.0 mm and length 3.9–4.8 mm). Their figured holotype specimen measures 6.8 × 4.8 mm according to the figure scale ( Dai & Song, 1986: fig. 3-1; Fig. 40A ).

The record of “ Pinnotheres gracilis ” by George & Noble (1970: 392) from Karwar in India is very likely A. exiguus . They did not illustrate the whole specimen, but the G1 figured ( George & Noble, 1970: fig. 1.2, 1.3) agrees very well with that of A. exiguus ( Figs. 44K, L , 45H, I ). The MXP3 (figured from the inner view), however, depicts a relatively long dactylus that extends beyond the tip of the propodus ( George & Noble, 1970: fig. 1.1); in A. exiguus , the dactylus is always shorter ( Figs. 36E , 41B , 42A, C , 44B , 45A ). The MXP3 figured shows its internal (rather than external) surface and does not appear to have been illustrated with the palp articles in the same plane, making the propodus appear shorter than it actually is. The G1 as figured is almost identical to that known for A. exiguus . Therefore, we provisionally refer their record to A. exiguus . It is noteworthy that George & Noble (1970) recorded the crab from the venerid clam Marcia opima (Gmelin, 1791) (as a Katelysia ). Pinnotheres gracilis s. str. is now in Viridotheres Manning, 1996 (see Ahyong & Ng, 2007b: 220; Trivedi et al., 2018b: 62), and occupies Solen sp. (Adapedonta) .

The record of “ Pinnotheres gracilis ” by Lalitha Devi (1981: fig. 1 (left)) from Kakinada Bay, Bay of Bengal, was from the pectenid scallop Amusium pleuronectes (Linnaeus, 1758) and is misidentified. The photograph of the specimen is not clear, but considering the host and pereopod proportions, it is almost certainly referable to Amusiotheres hanumantharaoi ( Devi & Shyamasundari, 1989) , known from the Bay of Bengal (see Devi & Shyamasundari, 1989; Ng & Ho, 2016a).

Arcotheres exiguus is superficially very close to A. cyclinus ( Shen, 1932) , especially in the form of the carapace, pereopods, and slender chelae ( Shen, 1932: figs. 80, 81c). The species was described from China from the venerid clam Cyclina sinensis (Gmelin, 1791) . The one major difference is the proportionately longer MXP3 dactylus, which extends beyond the tip of the propodus ( Shen, 1932: fig. 81a); the dactylus extends no further than the propodus tip in A. exiguus ( Figs. 36E , 41B , 42A, C , 44B , 45A ).

With clarification of the identity of A. exiguus and documentation of its geographic range, it is now evident that the species ranges widely from the South China Sea westwards to the northern Arabian Sea. Our scutiny of material from across the range has not revealed distinctions that would suggest that more than one species is present, as might be suspected on the basis of its wide distribution. Evidently, the distribution of A. exiguus appears to track that of its hosts. Nevertheless, should A. exiguus prove to be a composite taxon, four names are available that could be resurrected from synonymy.

Distribution. South China Sea, including the Philippines, to Singapore, Malaysia, India, and the northern Arabian Sea.

Figures

Fig. 34. Arcotheres exiguus (Bürger, 1895), colour in life and in situ in Marcia recens (Holten, 1802) (Veneridae), Peninsular Malaysia. A, male (4.3 × 4.2 mm) and ovigerous female (6.3 × 5.4 mm) (ZRC 2018.772) (placed together for photograph but originating from different shells); B, C, ovigerous female (6.3 × 5.4 mm) (ZRC 2018.772); D–F, male (4.3 × 4.2 mm) (ZRC 2018.772). Photographs: Paul Y. C. Ng.

Fig. 35. Arcotheres exiguus (Bürger, 1895), colour in life. A, B, ovigerous female (7.9 × 7.3 mm) (ZRC 2019.516), in situ in Marcia recens (Holten, 1802) (Veneridae), Peninsular Malaysia; C, male (4.3 × 4.2 mm) (ZRC 2019.516), Peninsular Malaysia; D, male (4.5 × 4.2 mm) (ZRC 2018.768), India; E, F, ovigerous female, from Meretrix aurora Hornell, 1917 (Veneridae), India (specimens not examined). Photographs: A–C, Paul Y. C. Ng; D–F, R. Ravinesh.

Fig. 36. Arcotheres exiguus (Bürger, 1895). A, B, after Bürger (1895: pl. 9 fig. 19, pl. 10 fig. 30); C–F, lectotype female (5.6 × 4.6 mm) (USNM 32432), Samar, Philippines, after Ahyong & Ng (2007: fig. 4). A, C, overall dorsal view; B, E, right MXP3; D, left P5 propodus and dactylus; F, right chela. Scale = B, 1.5 mm; C, 2.0 mm; E, 1.0 mm.

Fig. 37. Arcotheres exiguus (Bürger, 1895), ovigerous female (9.3 × 7.6 mm) (NHM 1936.6.19.12), Peninsular Malaysia (holotype of Pinnotheres winckworthi Gordon, 1936). A, overall dorsal view; B, ventral view of cephalothorax; C, dorsal view of carapace; D, frontal view of cephalothorax; E, outer view of right chela.

Fig. 38. Arcotheres exiguus (Bürger, 1895). A, C–E, ovigerous female (7.7 × 6.7 mm) (ZRC 2018.772), Peninsular Malaysia; B, female (8.2 × 7.4 mm) (ZRC 2018.775), Peninsular Malaysia. A, B, overall dorsal view; C, frontal view of cephalothorax; D, outer view of right chela; E, outer view of fingers of left chela.

Fig. 39. Arcotheres exiguus (Bürger, 1895). A–C, ovigerous female (8.2 × 6.9 mm) (ZRC 2021.784), India (neotype of Pinnotheres casta Antony & Kutyamma, 1971); D, non-ovigerous female (8.0 × 6.7 mm) (ZRC 2017.1037), India; E, juvenile female (4.0 × 3.6 mm) (ZRC 2018.769), India. A, D, E, overall dorsal view; B, ventral view of cephalothorax; C, frontal view of cephalothorax.

Fig. 40. Arcotheres exiguus (Bürger, 1895), female (6.8 × 4.8 mm) (CAS), China (holotype of Pinnotheres obscuridentata Dai & Song, 1986); after Dai & Song (1986: text-fig. 3). A, overall dorsal view; B, left MXP3; C, outer view of right chela; D–G, right P2–P5 dactyli. Scales = A, 2.0 mm; B–G, 0.5 mm.

Fig. 41. Arcotheres exiguus (Bürger, 1895). A–M, ovigerous female (9.3 × 7.6 mm) (NHM 1936.6.19.12), Penang (holotype of Pinnotheres winckworthi Gordon, 1936); N–Q, ovigerous female (8.8 × 7.3 mm) (NHM 1936.6.19.13), Penang (paratype of Pinnotheres winckworthi Gordon, 1936). A, N, carapace; B, Q, left MXP3; C, frontal view of cephalothorax; D, anterior thoracic sternum; E, right chela; F–I, left P2–P5; J–M, right P2–P5; O, left P4, dactylus, and propodus; P, right P4, dactylus, and propodus. Scale = A, N, 2.5 mm; B, Q, 0.625 mm; C–M, O, P, 1.25 mm.

Fig. 42. Arcotheres exiguus (Bürger, 1895). A–I, ovigerous female (8.2 × 6.9 mm) (ZRC 2021.784), India (neotype of Pinnotheres casta Antony & Kutyamma, 1971); J, K, non-ovigerous female (8.0 × 6.7 mm) (ZRC 2018.768), India; L, juvenile female (4.0 × 3.6 mm) (ZRC 2018.769), India; M, ovigerous female (4.5 × 4.0 mm) (ZRC 2018.775), Peninsular Malaysia. A, L, left MXP3 (setae omitted); B, left chela and carpus; C–F, left P2–P5, respectively (P3 carpus, propodus, and dactylus missing); G–I, right P2, P4, and P5, respectively; J, K, left and right P4, respectively; M, distal part of right P5 dactylus (ventral view, soft setae omitted). Scales = A–L, 0.5 mm; M, 0.25 mm.

Fig. 43. Arcotheres exiguus (Bürger, 1895), male (5.1 × 4.9 mm) (ZRC 2018.772), Peninsular Malaysia. A, overall dorsal view; B, frontal view of cephalothorax; C, outer view of right chela.

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. 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.