Arcotheres similis ( 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 : 171-182

Arcotheres similis ( Bürger, 1895)

( Figs. 25–33 )

Pinnotheres similis Bürger, 1895: 373 , 374, pl. 9 fig. 14 [type locality: Ubay, Philippines]; Tesch, 1918: 250 (list); Silas & Alagarswami, 1967: 1210 (list); Serène, 1968: 94; Schmitt et al., 1973: 86.

Pinnotheres modiolicola Bürger, 1895: 370 , pl. 9 fig. 9, pl. 10 fig. 9 [type locality: Philippines, from Modiolus philippinarium ]; Lanchester, 1900: 761 ( Singapore); Estampador, 1937: 5 (list). [New synonymy]

Pinnotheres kamensis Rathbun, 1909: 110 ; 1910: 335, figs. 18, 19; Tesch, 1918: 249, 252; Suvatti, 1938: 69; 1950: 159; Silas & Alagarswami, 1967: 1200, 1223, 1225; Schmitt et al., 1973: 50; Naiyanetr, 1980: 42; 1998: 104; 2007: 11; De Gier & Becker, 2020: tab. 1.

Pinnotheres modiolicolus — Tesch, 1918: 249 (list); Silas & Alagarswami, 1967: 1203 (list); Serène, 1968: 94; Schmitt et al., 1973: 57; Dev Roy, 2013: 156, 157.

Pinnotheres spinidactylus Gordon, 1936: 169 , figs. 1, 2; Serène, 1968: 94;? Griffin & Campbell, 1969: 157, figs. 7, 8; Stephenson et al., 1970: 492; Schmitt et al., 1973: 88; Jones, 1990: 202; Davie, 2002: 428, fig. a, b, 434.

Arcotheres spinidactylus — Campos, 2001: 494; Ahyong & Brown, 2003: 10; Ng et al., 2008: 248; Ng et al., 2017: 1094; Trivedi et al., 2018a: 197, 248; De Gier & Becker, 2020: tab. 1.

Arcotheres similis — Ahyong & Ng, 2007b: 207, fig. 14; Ng et al., 2008: 248; Ng et al., 2017: 1094; Trivedi et al., 2018a: 197; De Gier & Becker, 2020: fig. 8B, tab. 1.

Arcotheres modiolicola — Ahyong & Ng, 2007b: 199, fig. 7; Ng et al., 2008: 248; Ng et al., 2017: 1094; Trivedi et al., 2018a: 197; De Gier & Becker, 2020: tab. 1.

Type material. Holotype: female (8.3 × 6.4 mm) ( SMF-ZMG 956), Ubay, Philippines, coll. C. Semper. Philippines: 1 damaged ovigerous female (8.0 × 6.5 mm) ( SMF-ZMG 168), from Modiolus philippinarum , coll. C. Semper (holotype of Pinnotheres modiolicola Bürger, 1895 ). Thailand: 1 juvenile male (1.8 × 1.6 mm) ( NHMD CR- 9396), west of Koh Kam, Gulf of Thailand, 9.1 m, coll. Th. Mortensen, 6 February 1900 (holotype of Pinnotheres kamensis Rathbun, 1909 ). Singapore: 1 ovigerous female (6.7 × 5.5 mm) ( NHM 1936.6.19.2), Siglap, from Modiolus philippinarum , coll. R. Winckworth (holotype of P. spinidactylus Gordon, 1936 ); 1 male (3.3 × 3.3 mm), 4 ovigerous females (5.8 × 4.7 mm, 6.7 × 5.8 mm, 5.7 × 4.9 mm, 6.3 × 5.3 mm) ( NHM 1936.6.19.3–7), Siglap, from Modiolus philippinarum , coll. R. Winckworth (paratypes of P. spinidactylus Gordon, 1936 ).

Other material examined. Singapore: 2 males (4.0 × 4.3 mm, 4.8 × 5.0 mm) ( NHM 1936.6.19.14–15), Siglap , coll. R. Winckworth ; 1 female (9.3 × 7.8 mm) ( ZRC 2017.1034 ), from Modiolus philippinarum , intertidal area, off Changi beach, next to Carpark 7, near ferry terminal, coll. S. K. Tan et al., 8 June 2016 ; 3 ovigerous females (7.1 × 5.9 mm, 7.6 × 6.4 mm, 8.3 × 6.7 mm) ( ZRC 2017.1035 ), from Modiolus philippinarum , intertidal area, off Changi beach, next to Carpark 7, near ferry terminal, coll. S. K. Tan et al., 3 August 2012 ; 1 male (5.5 × 5.8 mm), 5 females, 1 juvenile female ( ZRC 2017.1264 ) , 1 female ( AM P102287 ), from Modiolus philippinarum , intertidal area, off Changi beach, next to Carpark 7, near ferry terminal, coll. S. K. Tan et al., 26 June 2017 ; 1 female (6.5 × 5.2 mm) ( ZRC 2013.1429 ), in Modiolus philippinarum, Pulau Ubin , coll. S. K. Tan et al., 6 March 2012 ; 1 female (8.6 × 6.8 mm) ( ZRC 2017.1036 ), Tanah Merah Besar beach, coll. R. U. Gooding, 3 April 1966 ; 4 shrivelled females (largest ca. 7.6 × 6.3 mm) ( ZRC 1965.11.24.42–45), from “ Pinna sp. ”, coll. February 1938 ; 4 females ( ZRC 1965.11.24.42–45), “in Pinna ”, Pulau Senang, Southern Islands , coll. 1930s ; 2 females (8.8 × 8.1 mm, 5.8 × 5.1 mm) ( ZRC 2018.785 ) , 1 male (4.8 × 4.8 mm), 2 females (9.5 × 7.7 mm, 8.8 × 7.7 mm) ( AM P105906 ), from Modiolus philippinarum , intertidal area, off Changi beach, next to Carpark 7, near ferry terminal, coll. S. K. Tan et al., 25 July 2018 ; 2 males (4.7 × 4.9 mm, 4.5 × 4.3 mm), 3 females (larger 9.2 × 7.7 mm, 6.3 × 4.7 mm), 1 ovigerous female (6.3 × 5.2 mm) ( ZRC 2018.1369 ), from Modiolus philippinarum , intertidal area, off Changi beach, next to Carpark 7, near ferry terminal, coll. S. K. Tan et al., 25 December 2018 [largest pair male 4.7 × 4.9 mm, female 9.2 × 7.7 mm from 100.4 mm long Modiolus ]; 5 males, 13 females ( ZRC 2018.1389 ), from Modiolus philippinarum , intertidal area, off Changi beach, next to Carpark 7, near ferry terminal, coll. S. K. Tan et al., 25 December 2018 ; 9 males, 19 ovigerous females, 6 females (2 females with bopyrids under pleon) ( ZRC 2019.534 ), from Modiolus philippinarum , intertidal area, off Changi beach, next to Carpark 7, near ferry terminal, coll. S. K. Tan et al., 22 April 2019 ; 6 males, 1 juvenile female, 25 ovigerous females ( ZRC 2019.578 ), from Modiolus philippinarum , intertidal area, off Changi beach, next to Carpark 7, near ferry terminal, coll. S. K. Tan et al., 4 July 2019 ; 1 male, 22 females ( ZRC 2019.579 ), from Modiolus philippinarum , intertidal area, off Changi beach, next to Carpark 7, near ferry terminal, coll. S. K. Tan et al., 5 July 2019 ; 1 ovigerous female ( ZRC 2021.16 ), station DW57, probably from Modiolus sp. , east of Pulau Tekong , 01°25.342′N, 104°04.775′E – 01°24.949′N, 104°05.080′E, coll. Comprehensive Marine Biodiversity Survey, 22 October 2012 ; 1 male (5.5 × 5.5 mm), 10 females (1 photographed: 8.4 × 7.1 mm) ( ZRC 2021.79 ), in Modiolus philippinarum, Changi Bay , ex Red Cliff Shoal area , Singapore, coll. S. K. Tan et al., 13 January 2021 ; 3 females (1 dismembered, poor condition) ( ZRC 2013.574 ), no host record, East Coast Park, Bedok Jetty , Singapore, coll. S. Y. Chan, 31 July 2012 . Peninsular Malaysia: 3 females ( ZRC 2017.1282 ), no other data. Borneo : 1 female (7.3 × 6.0 mm; soft) ( ZRC 1965.11.24.47), in Vulsella vulsella , on deep sea cable, 45 fathoms, South China Sea , near Sarawak, 4°26′42″N, 112°16′55″E, coll. 16 October 1933 . Indonesia: 3 males (largest 5.7 × 5.6 mm) , 16 females (largest 8.5 × 7.0 mm) ( ZRC 2019.1026 ), in sand at Ekas, Lombok , probably discarded by artisanal shell collectors, Indonesia, coll. D. L. Rahayu, 14 May 2007 . India: 5 females (7.1 × 5.8 mm, 7.3 × 6.0 mm, 7.9 × 6.3 mm, 8.0 × 6.7 mm, 8.1 × 6.7 mm) ( ZRC 2017.1037 ), from Modiolus philippinarum, Tuticorin , Tamil Nadu, coll. R. Ravinesh, March 2017 . Uncertain locality: 5 females (6.3 × 4.8 mm, 7.4 × 5.6 mm, 8.6 × 6.9 mm, 9.6 × 7.5 mm, 9.9 × 7.9 mm) ( ZRC 1965.11.24.9–13), unidentified location, possibly from South Island, Cocos-Keeling Islands, Indian Ocean Territory , label badly damaged .

Description. Carapace and pereopods well chitinised. Female: Carapace subcircular, wider than long; dorsal and lateral surfaces smooth, glabrous; appearing domed in frontal view; front produced slightly anteriorly beyond orbits, margin gently concave to gently sinuous; anterolateral margin subparallel with frontal margin or gently sloping posteriorly to varying degrees, forming rounded angle with posterolateral margin ( Figs. 27A, C, D , 28A, C, D , 29 ). Eyes usually just visible in dorsal view in adults; filling orbit ( Figs. 27A , 29C, D ). Epistome with median part triangular, lateral margins gently concave ( Figs. 27D , 28D , 30K ).

MXP3 outer surface with scattered short setae; ischiomerus completely fused, subovate, inner margin usually rounded even 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 slightly proximal to midlength of propodus, tip not reaching propodal apex; exopod relatively slender, about two-thirds length of ischiomerus, flagellum 2-segmented ( Figs. 30J , 33C, D ).

Chela not prominently elongate, dactylus about two-thirds palm length; palm relatively slender, proximally narrower than distally or subequal; outer surfaces of palm, fingers (except for distal part) almost glabrous, with only scattered short setae; ventral margin of palm gently sinuous to almost straight; dactylus occlusal margin with distinct subproximal tooth; pollex occlusal margin with 1 low proximal tooth and denticles; tips of fingers sharp, hooked ( Figs. 27E , 30I ).

P2–P5 merus–propodus 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=P2>P5; right (sometimes left) P4 distinctly the longer; P2 and P3 dactyli subequal, weakly curved, exceeding two-thirds length of propodi, with or without flexor row of spinules; longer P4 dactylus weakly falciform, slightly shorter than propodus, with or without flexor row of spinules; P5 merus 4.6–5.2 times longer than wide; P5 dactylus longer than longer P4 dactylus, longer than propodus, margins lined with short and long setae, denser on ventral margin, distal part lined with 2 distinct rows of distoflexor spinules, upper row with 5–10 spinules, lower row with 8–31 spinules, increasing in size distally, lower row always with distinctly more spinules than upper row in adults ( Fig. 30A –H’).

Pleon extending to buccal region, covering bases of P2–P5; telson gently recessed into concave distal margin of somite 6 ( Figs. 27B , 28B ).

Male: Carapace longitudinally ovate, slightly longer than wide; dorsal surface appears punctate to smooth, gently inflated, dorsal surfaces convex, lateral surfaces with setae; front projecting anteriorly, margin sinuous to almost straight ( Figs. 31A, B , 32A ). Eyes distinctly visible in dorsal view ( Figs. 31A , 32A ). MXP3 similar to that of female, although ischiomerus with slightly more angular inner distal margin ( Fig. 32C ). 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 ( Fig. 31C ). P2–P5 legs 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, flexor margin with setae and row of small spinules; right P4 dactylus slightly longer, more slender than left side; P4 and P5 dactylus subequal or longer to that of P3, covered with short setae; P5 dactylus with 2 rows of distoflexor spinules, one row sometimes low, indistinct ( Fig. 32D –K’). Pleon slender, triangular, widest at somite 3 with distinctly convex lateral margins, lateral margins of somite 4 gently concave; somite 6 subtrapezoidal; telson semicircular with convex margins, wider than long ( Fig. 32B ). G1 relatively stout, arcuate, curved outwards, tip produced to differing lengths as short projection in line with main axis ( Figs. 32L–N , 33E–H, J, K ). G2 short, with spatuliform tip; exopod curved, two-thirds endopod length ( Figs. 32O , 33I ).

Variation. The eyes of A. similis are usually partially visible in dorsal view ( Fig. 27A ), although sometimes completely hidden ( Figs. 28A , 29 ).

Colour. In life, the colour is light brown to whitish brown overall, with the pereopods relatively lighter in colour ( Figs. 25 , 26 ).

Host. Almost always from Modiolus philippinarum (Hanley, 1843) ( Mytilidae Rafinesque, 1815 ). The one exception is a specimen (ZRC 1965.11.24.47) recorded from Vulsella vulsella (Linnaeus, 1758) ( Pteriidae Gray, 1847 ). This record from Vulsella , however, is highly unusual not only because of the disparate taxonomic and phylogenetic position of the hosts ( Combosch et al., 2017), but because it was taken at 45 fm (82 m) depth on an undersea cable, significantly deeper than any known record of A. similis , which is otherwise known only from intertidal and shallow subtidal depths. The accuracy of the Vulsella host record requires corroboration; it may represent an atypical vagrant association given the unusual habitat.

Remarks. We consider Pinnotheres similis Bürger, 1895 , Pinnotheres modiolicola Bürger, 1895 , Pinnotheres kamensis Rathbun, 1909 , and Pinnotheres spinidactylus Gordon, 1936 , to all represent the same species, Arcotheres similis . The confusion has stemmed from the combination of incomplete descriptions by Bürger (1895), the badly damaged type female of P. modiolicola , the minimal original description of P. kamensis , and the confused identity and mixed material of P. similis reported by Gordon (1936) (see Ahyong & Ng, 2007b; Ng, 2018a). Griffin & Campbell (1969: 161) had already noted the strong similarities between P. modiolicola and P. spinidactylus , and Ahyong & Ng (2007b: 207), in redescribing A. similis , observed that it may be synonymous with A. spinidactylus , although the number of distoflexor rows of spinules on the P5 dactylus seemingly differed (one row, based on Gordon’s 1936 account). Clarification of the identity of these four nominal species has only been possible following restudy of their respective type specimens. Reexamination of Gordon’s (1936) material of P. spinidactylus revealed that two rows of distoflexor spinules are present on the P5 dactylus in both sexes ( Fig. 30L ), as in A. similis ( Figs. 30 D’, H’, 32G’, K’). Ahyong & Ng (2021) showed that A. similis is a senior synonym of A. spinidactylus and A. kamensis . Likewise, our reassessment of A. modiolicola finds it to be indistinguishable from A. similis . Note that Ahyong & Ng (2007b) incorrectly indicated the presence of a single row of P5 dactylar spines, rather than two. The present reappraisal, which has examined types of all four nominal species, indicates they are conspecific.

Pinnotheres similis Bürger, 1895 , and Pinnotheres modiolicola Bürger, 1895 , were described in the same paper ( Bürger, 1895) so both names must be considered as simultaneously published under Article 24 of the ICZN (1999). As first revisers, we here select Pinnotheres similis Bürger, 1895 , to have priority over P. modiolicola Bürger, 1895 , when they are regarded as synonyms. We select P. similis to have priority because it is the more widely cited species and because its holotype is still in good condition, whereas that of P. modiolicola is badly damaged ( Ahyong & Ng, 2007b).

Gordon (1936: 169) described Pinnotheres spinidactylus on the basis of five females and one male from Siglap, Singapore, all from the mytilid Modiolus philippinarum (Hanley, 1843) . She compared her new species with Pinnotheres latus Bürger, 1895 (now in Magnotheres , new genus), Pinnotheres parvulus Stimpson, 1858 , the Chinese Pinnotheres sinensis Shen, 1932 (now in Arcotheres ), Pinnotheres tsingtaoensis Shen, 1932 (now in Nepinnotheres , see Remarks for Plenotheres , new genus), and Pinnotheres gordoni Shen, 1932 . The excellent series of specimens from Singapore agrees with the descriptions and figures of Gordon (1936) very well. The G1 figured by Gordon (1936: fig. 2c) agrees well in general shape with that of the present male except that the distal tip is straight (versus gently curved inwards; Fig. 33E, F ). In any case, the distal tip varies in length as well, from typically short ( Fig. 32L–N ) to sometimes long ( Fig. 33G, H, J, K ). Gordon’s (1936) P. spinidactylus is referable to A. similis , but her record of P. similis from Singapore is referable to A. placunicola (cf. Ng, 2018a).

Of the males available to Gordon (1936: 169), only the smallest male (3.3 × 3.3 mm; NHM 1936.6.19.3) was referred to the type series ( Fig. 31D ). She noted that the “two other [non-type] male specimens, measuring about 4.4 and 5.4 mm in length respectively [NHM 1936.6.19.14–15], have the second and third legs fringed with setae on the propodus and carpus much as in P. tsingtaoensis Shen (1932, p. 149) . The abdomen, however, and the dactylus of the external maxillipede are more like those of P. gordoni Shen (p. 152), but the first pleopod is heavily setose on both sides and is straight, not curved, at the apex” ( Gordon, 1936: 171). We examined all three males and they belong to the same species. The differences noted by Gordon (1936) can all be accounted for by size-associated change, the paratype male being smallest and yet to exhibit fully adult features. Fully adult male specimens have the G1 distinctly more setose ( Figs. 32L, M , 33G, H ), and ambulatory propodi and carpi that are also lined with long setae along the flexor margins ( Fig. 31A ). Gordon (1936: fig. 1c) did not remove the G1s from the males when she compared the G1, and their shape is actually very similar ( Fig. 33E–H ), except that the distal projection is relatively longer and the subdistal hump is relatively higher in the larger male ( Fig. 33G, H ; cf. Fig. 33E, F ).

Griffin & Campbell (1969: 157, figs. 7, 8) recorded “ Pinnotheres spinidactylus ” from two females and one male collected from the mytilid, Modiolus , in Moreton Bay in Queensland, Australia, commenting that although the leg proportions appeared to differ slightly, the carapace shape, front, and presence of distoflexor rows of spinules on the P5 dactylus are diagnostic for the species. Their figures agree closely with A. similis as characterised here except for the pleon of a 3.1-mm cw male, figured with somite 6 proportionately broader and the lateral margins prominently convex rather than gently concave (compared to the mature 5.5-mm cw male) ( Fig. 32B versus Griffin & Campbell, 1969: fig. 8B), and gonopods described as “immature, two pairs of biramous appendages” ( Griffin & Campbell, 1969: 159). The Griffin & Campbell (1969) male is a juvenile and the difference in pleon shape may be a function of its immaturity, but we are of the opinion that it is actually an immature female, especially since the authors describe it as having “two pairs of biramous pleopods”. The G1 of a 3.3 × 3.3-mm male (NHM 1936.6.19.3) from Singapore was already developed and clearly visible, so Griffin & Campbell’s “male” specimen and its pleon is similar to that of larger males, except that somite 6 is relatively shorter with the lateral margins of somite 6 gently concave ( Fig. 33L ). We also note that in the immature female of A. palaensis , the lateral margins of pleonal somite 6 are prominently convex ( Fig. 15C ), unlike that of similarly sized males which have the margins gently convex to sinuous ( Fig. 22J, O ). These observations suggest that Griffin & Campbell’s male of cw 3.1 mm is almost certainly a juvenile female instead. Without re-examination of the pleopods, however, we cannot be certain, but on the basis of the other female characters and its host, we provisionally include Griffin & Campbell’s (1969) record in the synonymy of P. similis ; this Australian record will need to be confirmed with fresh collections, especially since Queensland is some distance from the nearest confirmed records in Southeast Asia. Australian records of A. similis by Ward (1967) and Ahyong & Brown (2003) are actually referable to M. globosus , new combination (see account of M. globosus below; Ahyong, 2020b); that of Rathbun (1924) requires verification ( Ahyong, 2020b).

George & Noble (1970: 392) recorded Pinnotheres modiolicolus from Karwar and Kodibag in southwestern India, but it is probably misidentified (see also Trivedi et al., 2018b: 61). They recorded the species from Marcia opima (Gmelin, 1791) (as a Katelysia Römer, 1857 ) ( Veneridae ) and Mactra violacea Gmelin, 1791 (Mactridae) , both from the order Venerida . Although the MXP3 figured ( George & Noble, 1970: fig. 1.4) agrees with that of A. similis as defined here ( Figs. 30J , 32C , 33C, D ), the G1 is different, with the tip truncated ( George & Noble, 1970: fig. 1.5, 1.6) and without the recurved slender part as is typical for the species ( Figs. 32L–N , 33G, H, J, K ). The G1 differences, together with the fact that they were from venerid clams and not mytilids, indicates that their record is not correct. The Karwar and Kodibag specimens will need to be re-examined to determine their identity.

Dai et al. (1980: 131, fig. 3) reported “ Pinnotheres spinidactylus ” from the mytilid Mytilus crassitesta Lischke, 1868 , from Hainan Island. The figures in her paper (see also Dai et al., 1986: 393, fig. 208 (1–7); Dai & Yang, 1991: 424, fig. 208 (1–7)) superficially agree with A. similis . We have, however, examined photographs of the Hainan specimens in CAS, and while the carapace and pereopod features seem to agree with A. similis , there are two problems: the male pleon and G1 (neither figured by the above authors) are very different from those of A. similis s. str., and indicate that it is a different species. The male pleon of the Hainan specimens has somite 6 and the telson proportionately more elongate, and most significantly, the G1 is much longer and less curved, with the distal part very elongate and gradually tapering to a subtruncate tip. They are also not referable to Arcotheres boninensis ( Stimpson, 1858) or A. purpureus

( Alcock, 1900), which have a markedly wider, more rounded carapace and relatively shorter P2–P5 ( Ahyong & Ng, 2021). Nevertheless, Dai et al. (1980, 1986) and Dai & Yang (1991) recorded A. boninensis from China, and their figures conform to what we know about the species. The Chinese specimens of “ Pinnotheres spinidactylus ” will need to be restudied to ascertain their true identity.

Distribution. Western Pacific to the central Indian Ocean, including Australia, the Philippines, Indonesia, Singapore, Malaysia, Thailand, eastern India, and possibly the CocosKeeling Islands.

Figures

Fig. 25. Arcotheres similis (Bürger, 1895). A, ovigerous female (9.3 × 7.8 mm) (ZRC 2017.1034), Changi, Singapore; B, C, two females (ZRC 2017.1264) in situ on Modiolus philippinarum (Hanley, 1843) (Mytilidae); D, ovigerous female (7.9 × 6.3 mm) (ZRC 2017.1037), Tuticorin, India; E, female (6.5 × 5.2 mm) (ZRC 2013.1429), in situ on Modiolus philippinarum (Hanley, 1843) (Mytilidae), Pulau Ubin, Singapore. Photographs: A–C, E, Tan Heok Hui; D, R. Ravinesh.

Fig. 26. Arcotheres similis (Bürger, 1895), colour in life. A, B, ovigerous female (8.4 × 7.1 mm) (ZRC 2021.79), Pulau Ubin, Singapore; C, D, male (5.5 × 5.5 mm) (ZRC 2021.79), Tuticorin, India. Photographs: Paul Y. C. Ng.

Fig. 27. Arcotheres similis (Bürger, 1895), female (6.7 × 5.5 mm) (NHM 1936.6.19.2), Singapore (holotype female of Pinnotheres spinidactylus 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. 28. Arcotheres similis (Bürger, 1895), ovigerous female (9.3 × 7.8 mm) (ZRC 2017.1034), Changi, Singapore. A, overall dorsal view; B, ventral view of cephalothorax; C, dorsal view of carapace; D, frontal view of cephalothorax.

Fig. 29. Arcotheres similis (Bürger, 1895). A, female (8.1 × 6.7 mm) (ZRC 2017.1037), Tuticorin, India; B, female (8.0 × 6.7 mm) (ZRC 2017.1037), Tuticorin, India; C, female (7.4 × 5.6 mm) (ZRC 1965.11.24.10), Cocos-Keeling Islands (?); D, female (6.3 × 4.8 mm) (ZRC 1965.11.24.9), Cocos-Keeling Islands (?). A, B, overall view of carapace; C, D, dorsal view of carapace.

Fig. 30. Arcotheres similis (Bürger, 1895). A–K, ovigerous female (9.3 × 7.8 mm) (ZRC 2017.1034), Singapore; L, ovigerous female (6.7 × 5.5 mm) (NHM 1936.6.19.2), Singapore (holotype of Pinnotheres spinidactylus Gordon, 1936). A–D, left P2–P5, respectively; D’, distal part of dactylus of left P5 showing stiff subdistal spinules (subventral view, soft setae omitted); E–H, right P2–P5, respectively; H’, L, distal part of dactylus of right P5 showing stiff subdistal spinules (subventral view, soft setae omitted); I, outer view of left chela and carpus; J, outer view of left MXP3 (setae omitted); K, posterior margin of epistome. Scales = A–H, I, 1.0 mm; J, K, 0.5 mm; D’, H’ = 0.25 mm; L = 0.10 mm.

Fig. 31. Arcotheres similis (Bürger, 1895). A–C, male (5.5 × 5.8 mm) (ZRC 2017.1264), Singapore; D, male (3.3 × 3.3 mm) (NHM 1936.6.19.3), Singapore (paratype of Pinnotheres spinidactylus Gordon, 1936). A, D, overall dorsal view; B, frontal view of cephalothorax; C, outer view of right chela.

Fig. 32. Arcotheres similis (Bürger, 1895), male (5.5 × 5.8 mm) (ZRC 2017.1264), Singapore. A, dorsal view of carapace; B, pleon (setae omitted); C, left MXP3 (setae denuded); D–G, left P2–P5, respectively (setae omitted); H–K, right P2–P5, respectively; G’, distal part of dactylus of left P5 showing stiff subdistal spinules (dorsal view, soft setae omitted); K’, distal part of dactylus of right P5 showing stiff subdistal spinules (dorsal view, soft setae omitted); L, left G1 (ventral view); M, N, left G1 (dorsal view, setae omitted); O, left G2. Scales = A, D–K, 1.0 mm; B, C, L–O, 0.5 mm; G’, K’, 0.25 mm.

Fig. 33. Arcotheres similis (Bürger, 1895). A, B, ovigerous female (6.7 × 5.5 mm) (NHM 1936.6.19.2), Singapore (holotype of Pinnotheres spinidactylus Gordon, 1936); C, holotype female (8.3 × 6.4 mm) (SMF-ZMG 956 Go 950a), Philippines (holotype of Pinnotheres similis Bürger, 1895); D, female (7.3 × 6.0 mm; soft) (ZRC 1965.11.24.47), Sarawak; E, F, male (3.3 × 3.3 mm) (NHM 1936.6.19.3), Singapore; G–I, male (4.8 × 5.0 mm) (NHM 1936.6.19.14), Singapore; J, K, male (5.7 × 5.6 mm) (ZRC 2019.1026), Lombok, Indonesia; L, male (3.3 × 3.3 mm) (NHM 1936.6.19.3), Singapore (paratype of P. spinidactylus Gordon, 1936). A, left MXP3 (inner view); B, palp of left MXP3 (outer view); C, D, right MXP3 (outer view); E, G, J, left G1 (ventral view); F, distal part of left G1 (ventral view); H, K, distal part of left G1 (dorsal view); I, left G2; L, male pleonal somites 5, 6 and telson. A, B, E, F, after Gordon (1936: figs. 1c, 2a); C, after Ahyong & Ng (2007: fig. 14C). Scales = D, G–K, 0.5 mm; L, 0.2 mm.

Fig. 15. Arcotheres palaensis (Bürger, 1895). A–C, juvenile female (3.3 × 3.4 mm) (ZRC 2018.767), Thailand; D, female (8.2 × 7.0 mm) (ZRC 2020.9), Singapore; E, ovigerous female (6.6 × 5.2 mm) (ZRC 1965.11.24.32), Singapore. A, overall dorsal view; B, outer view of left chela; C, juvenile female pleon; D, E, female pleonal somites 5 and 6 and telson. Scales = 1.0 mm.

Fig. 22. Arcotheres palaensis (Bürger, 1895). A–N, male (3.6 × 3.6 mm) (ZRC 2019.1788), Indonesia; O, male (2.8 × 2.5 mm) (SMF- ZMG 172b) (paralectotype of Pinnotheres arcophilus Bürger, 1895), Philippines. A, left MXP3; B–E, left P2–P5, respectively; F–I, right P2–P5, respectively; J, pleon; K, left G1 (ventral view); L, M, left G1 (slightly different dorsal views); N, left G2; O, pleonal somites 3–5, telson, and left G1. Scales = A, K–N, 0.2 mm; B–J, 0.5 mm; O, 0.1 mm.