Arcotheres palaensis ( 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 : 144-168

Arcotheres palaensis ( Bürger, 1895)

( Figs. 1A–C , 2–22 )

Pinnotheres obesus — Miers, 1880: 314, pl. 4 fig. 4 (not Pinnothera obesa Dana, 1852 ).

Pinnotheres palaensis Bürger, 1895: 372 , 373, pl. 9 fig. 12, pl. 10 fig. 12 [type locality: Palau]; Tesch, 1918: 249 (list); De Man, 1921: 260 (Amboina, Indonesia); 1929: 14, pl. 2 fig. 5, 5a, b (Indramaju, Java, Indonesia); Perez, 1921: 59 (Amboina, Indonesia) (parasitised by epicaridean); Gordon, 1936: 64 (discussion); Estampador, 1937: 547 (list); Miyake, 1939: 221 (list); Chuang, 1961: 189, pl. 94 fig. 1 ( Singapore, Malaysia); Silas & Alagarswami, 1967: 1205 (list); Serène, 1968: 94; Schmitt et al., 1973: 64 (list).

Pinnotheres rhombifer Bürger, 1895: 374 , pl. 9 fig. 15, pl. 10 fig. 14 [type locality: Ubay, Philippines]; Serène, 1968: 94. [New synonymy]

Pinnotheres latissimus Bürger, 1895: 373 , pl. 9 fig. 13a, b, pl. 10 fig. 13 [type locality: Manila, Philippines]; Gordon, 1936: 176, fig. 6; Serène, 1968: 94. [New synonymy]

Pinnotheres arcophilus Bürger, 1895: 371 , pl. 9 fig. 10, pl. 10 fig. 10 [type locality: Ubay, Philippines]; Lanchester, 1900: 762 ( Singapore); Tesch, 1918: 248 (list); De Man, 1921: 260, pl. 8 figs. 1–7 (Lombok, Indonesia); Perez, 1921: 61 (Lombok, Indonesia) (parasitism by epicaridean); Gordon, 1936: 164 (discussion); Silas & Alagarswami, 1967: 1196 (list); Schmitt et al., 1973: 39.

Pinnoteres palaensis — Balss, 1957: 1418 (list).

Pinnotheres paralatissimus Dai & Song, 1986: 55 , 61, 62, text-fig. 2 [type locality: Beihai, Guangxi, China]; De Gier & Becker, 2020: tab. 1. [New synonymy]

? Arcotheres alcocki — Takeda & Konishi, 1988: 138, figs. 1, 2f–h, 3; Watanabe, 2013: 47 (not Pinnotheres alcocki Rathbun, 1909 ).

Arcotheres guinotae Campos, 2001: 494 , figs. 1, 2 [type locality: Thailand]; Ng et al., 2008: 248; De Gier & Becker, 2020: tab. 1. [New synonymy]

Arcotheres palaensis — Pohle & Marques, 1998: 231 (part); Ahyong & Ng, 2007b: 200, fig. 9 (types); Ng et al., 2008: 248 (list); Ng et al., 2017: 1094; Trivedi et al., 2018a: 197; De Gier & Becker, 2020: tab. 1; Komai et al., 2020: 222.

Arcotheres rhombifer — Ahyong & Ng, 2007b: 205, fig. 12; Ng et al., 2017: 1094.

Arcotheres arcophilus — Ahyong & Ng, 2007b: 194, fig. 2; Ng et al., 2008: 248; Ng et al., 2017: 1093; De Gier & Becker, 2020: tab. 1.

? Arcotheres sp. — Watanabe & Henmi, 2009: 217, figs. 2, 3.

Arcotheres acrophilus — Trivedi et al., 2018a: 197 [misspelling].

Type material. Lectotype: female (10.2 × 8.4 mm) (SMFZMG 948a), Palau, coll. C. Semper. Paralectotypes: 3 poorly preserved females (10.3 × 8.3–10.8 × 8.9 mm; 1 with epicaridean) ( SMF-ZMG 948b), same data as lectotype; 1 female (10.0 × 7.0 mm, shrivelled) ( SMF-ZMG 947), same data as lectotype. Philippines: 4 females (9.5 × 7.4–11.8 × 8.3 mm, shrivelled) ( SMF-ZMG 177) (paralectotypes of Pinnotheres palaensis Bürger, 1895 ), Ubay, from “ Placuna sella ”, coll. C. Semper; 1 ovigerous female (8.5 × 6.8 mm) ( SMF-ZMG 179a) (simultaneous lectotype of Pinnotheres rhombifer Bürger, 1895 , and neotype of Pinnotheres latissimus Bürger, 1895 ), Ubay, from Pectunculus aurifluus ; 1 ovigerous female (7.4 × 5.7 mm) ( SMF-ZMG 179b) (paralectotype of Pinnotheres rhombifer Bürger, 1895 ), same data as lectotype; 1 female (5.3 × 4.7 mm) ( SMF-ZMG 172a) (lectotype of Pinnotheres arcophilus Bürger, 1895 ), Ubay, Philippines, from Arca, C. Semper, 1863 –1864; 1 male (2.8 × 2.5 mm), 1 shrivelled female (5.1 × 4.5 mm) ( SMF-ZMG 172b) (paralectotypes of Pinnotheres arcophilus Bürger, 1895 ), same data as lectotype. Thailand: 1 female (6.4 × 5.8 mm) ( MNHN-B 9498) (holotype of Arcotheres guinotae Campos, 2001 ) [photographs examined], from Barbatia sp. , coll. R. Serène.

Other material examined. Peninsular Malaysia: 1 female (8.1 × 6.6 mm) ( ZRC 2017.1252 ), Peninsular Malaysia or Singapore , from practical class, coll. 30 August 1967 ; 1 ovigerous female (7.3 × 5.8 mm), 1 non-ovigerous female (carapace length 6.0 mm; with bopyrid on right side of carapace), 1 non-ovigerous female (carapace length 5.7 mm; with bopyrid on left side of carapace) ( ZRC 1987.537 – 539 ), in Tegillarca granosa , from markets, coll. P. K. L. Ng, 29 November 1986 ; 1 female (6.1 × 5.2 mm) ( ZRC 1993.119 ), in Tegillarca granosa , from markets, coll. P. K. L. Ng, November 1987 ; 1 ovigerous female (8.5 × 7.2 mm) ( ZRC 1992.8378 ), in Tegillarca granosa , from markets in Singapore , coll. P. K. L. Ng, January 1992 ; 1 female (9.6 × 7.5 mm) ( ZRC 1999.975 ), probably Peninsular Malaysia, from food with cockles (Clementi Town Centre, Singapore ), coll. D. C. J. Yeo, 27 May 1999 ; 1 soft female (ca. 9.0 mm carapace width) ( ZRC 2000.2613 ), Batu Pahat , Johor, in Tegillarca granosa , mangroves, coll. H. K. Tan, 16 August 1983 ; 1 ovigerous female (13.3 × 11.2 mm) ( ZRC 2017.1281 ), probably Malaysia , in Tegillarca cf. rhombea, Chinatown market, Singapore, coll. 14 January 2018 ; 1 ovigerous female (7.9 × 6.6 mm) ( ZRC 2018.255 ), 1 ovigerous female (8.6 × 6.5 mm) ( AM P105905 ), probably Peninsular Malaysia, in Tegillarca granosa , from Chinatown market, Singapore, coll. P. K. L. Ng, 25 February 2018 ; 1 ovigerous female (7.5 × 5.8 mm) ( ZRC 2018.770 ), from food, probably from Tegillarca sp. , coll. B. Y. Lee, 6 May 2018 ; 1 ovigerous female (8.4 × 6.6 mm) ( ZRC 2018.759 ), probably Peninsular Malaysia, in Tegillarca granosa , from Chinatown market, Singapore , coll. P. Y. C. Ng, 4 November 2018 ; 1 ovigerous female ( ZRC 2019.1039 ), Johor, in Tegillarca granosa , from Chinatown market, Singapore , coll. P. K. L. Ng, 27 July 2019 ; 9 females (3 photographed live) ( ZRC 2020.1 ), Kandang Kerbau market, Singapore , in Tegillarca granosa , coll. 5 January 2020 ; 1 ovigerous female ( ZRC 2021.379 ), Kandang Kerbau market, Singapore , in Tegillarca granosa , coll. P. K. L. Ng, 12 January 2020 ; 1 ovigerous female ( ZRC 2020.2 ), Kandang Kerbau market, Singapore , in Tegillarca granosa , coll. 12 January 2020 ; 10 females (smallest 4.1 × 4.1 mm) ( ZRC 2020.9 ), Kandang Kerbau market, Singapore , in Tegillarca granosa , coll. P. K. L. Ng, 29 March 2020 ; 3 ovigerous females (10.2 × 7.8 mm, 10.3 × 7.9 mm, 11.6 × 10.3 mm) ( ZRC 2020.11 ), in Tegillarca granosa , from Chinatown market, Singapore , coll. P. K. L. Ng, 16 April 2020 ; 1 female (7.8 × 6.6 mm) ( ZRC 2021.0801 ), from Kandang Kerbau market, in Tegillarca granosa , coll. P. K. L. Ng, 5 December 2021 ; 1 ovigerous female (8.9 × 7.8 mm) ( ZRC 2021.814 ), in Tegillarca nodifera , from Yew Tee Market, Singapore , coll. H. H. Tan, 12 December 2021 ; 1 ovigerous female (7.9 × 6.4 mm), 2 females (6.3 × 5.0 mm, 6.4 × 4.8 mm) ( ZRC 2021.0821 ), in Tegillarca granosa , from Kandang Kerbau Market, Singapore, from Peninsular Malaysia , coll. P. K. L. Ng, 26 December 2021 ; 1 female (7.4 × 6.7 mm) ( ZRC 2022.42 ), in Tegillarca granosa , from Yew Tee Market, Singapore , coll. H. H. Tan, 23 January 2022 . Indonesia: 2 ovigerous females ( ZRC 1999.2404 ), Indonesian markets, in Tegillarca granosa , coll. C. M. Yang, 3 August 1983 ; 1 ovigerous female (7.1 × 5.7 mm), 1 non-ovigerous female (7.7 × 6.2 mm), 1 ovigerous female (carapace length 7.1 mm; with bopyrid) ( ZRC 1993.120 – 122 ), Tegillarca granosa , from markets, coll. P. K. L. Ng, 8 December 1987 ; 1 ovigerous female (5.1 × 4.3 mm) ( ZRC 1999.1202 ), between Pulau Berang and Pulau Ileu, Pulau Lingga , Riau Islands, coll. J. B. Sigurdsson, 28 March 1993 ; 1 female (8.8 × 7.8 mm) ( ZRC 2018.764 ), Teluk Kodek, Lombok, Indonesia , unknown host, coll. S. Dwiono, 2014 ; 1 female (carapace length 6.7 mm; with bopyrid) ( ZRC 2018.762 ), Teluk Kodek, Lombok, Indonesia , in Anadara antiquata , coll. S. Dwiono, 12 November 2013 ; 1 male (3.6 × 3.6 mm) ( ZRC 2019.1788 ), in Tegillarca granosa , from Kandang Kerbau market, Singapore , coll. P. K. L. Ng, 29 December 2019 ; 2 ovigerous females ( ZRC 2021.355 ), from supermarket, in Tegillarca granosa , coll. P. Y. C. Ng, 26 March 2021 . Singapore: 2 females ( ZRC 2014.217 ), coll. P. K. L. Ng, 2005 ; 10 females ( ZRC 1965.11.24.17–26), from market, probably from cockles, coll. August 1934 ; 1 female (6.7 × 5.0 mm) ( ZRC 2017.1038 ), Tanah Merah Ferry Terminal Beach, near Changi Point , intertidal, in Barbatia sp. , coll. S. K. Tan et al., 12–15 January 2017 ; 3 females (largest 9.5 × 7.7 mm) ( ZRC 1965.11.24.30–32), Pulau Senang , in “ Barbatia sp. J ”, coll. November 1934 ; 1 female (7.0 × 5.6 mm) ( ZRC 1965.11.24.33), Raffles Lighthouse , inside Barbatia decussata , coll. 1930s. Borneo : 1 female (8.7 × 8.1 mm) ( ZRC 2017.1260 ), off Kuching, Sarawak , from large Tegillarca sp. from mudflats, from seafood market, coll. P. K. L. Ng, June 2015 . Thailand: 3 females (3.3 × 3.4 mm, 5.0 × 4.2 mm, 11.1 × 9.8 mm) ( ZRC 2018.767 ), Bangkok market, from Tegillarca granosa , coll. P. Y. C. Ng, 8 April 2018 . Japan: 17 females (2.3 × 2.0–9.6 × 6.5 mm) , 1 male (4.2 × 3.7 mm) (NSMT-Cr 9363), Mukaishima, Hiroshima Prefecture, Seto Inland Sea , in Barbatia virescens , coll. K. Konishi, 18 June 1981 ; 6 females (4.5 × 3.8–8.6 × 6.1 mm) ( ZRC 2019.1879 ), Kawachi, Kumamoto, Kyushu, in Barbatia

RAFFLES BULLETIN OF ZOOLOGY 2022

virescens , coll. T. Watanabe, 15 October 2007. Others: 1 spent female (9.6 × 7.9 mm), 1 ovigerous female (9.3 × 8.2 mm) ( NHM 1860.6), “Indo-Malayan Seas”.

Description. Carapace and pereopods well chitinised, usually firm in post-hard stage. Female: Carapace subovate to subhexagonal, wider than long; dorsal and lateral surfaces smooth, glabrous; appearing domed in frontal view; front sometimes projecting anteriorly beyond orbits, margin gently sinuous to gently convex; anterolateral margin thickened, blunt but defined, sometimes subparallel with frontal margin or sloping posteriorly to various degrees, forming rounded angle with posterolateral margin ( Figs. 3 , 5A , 8A , 9A , 11–14 ). Eyes small, not or slightly visible in dorsal view in adults; mobile, completely filling orbit ( Figs. 3 , 5A , 8A, C , 9A, C , 13 , 14 ). Epistome with median part triangular, lateral margins gently concave ( Figs. 8C , 9C , 17O , 18M ).

MXP3 outer surface with scattered short setae; ischiomerus completely fused, subrhomboidal, maximum length 1.5–1.8 times maximum width, inner margin usually rounded, occasionally slightly 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, inserted at proximal one-third to near midlength of propodus, tip not reaching propodal apex; exopod relatively slender, about two-thirds length of ischiomerus, flagellum 2-segmented ( Fig. 16 ).

Chela not prominently elongate, dactylus about two-thirds palm length; palm relatively slender, narrower proximally than distally; outer surface of palm, fingers (except for distal part) almost glabrous, with only scattered short setae; ventral margin of palm gently convex to sinuous, sometimes with low bulge near midlength, extending onto mesial surface; 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. 8D , 9D , 15B ).

P2–P5 dorsally, ventrally unarmed; outer surface covered with scattered, very short setae or glabrous; ventral margins of propodus and dactylus slightly more setose; relative lengths of meri P4>P3>P2>P5; left (sometimes right) P4 distinctly the longer; P2 and P3 dactyli short, subequal, tip gently hooked, half propodus length or less; longer P4 dactylus elongate, slender, almost straight, sparsely setose distally; P5 merus 3.6–4.0 times longer than wide, P5 dactylus longest, margins lined with short and long setae, denser on ventral margin, distoflexor margin without rows of spinules, at most sometimes with 3–8 scattered low spinules ( Figs. 17A–N , 18A–L ).

Pleon extending to buccal region, covering bases of P2–P5; telson gently recessed into concave distal margin of somite 6 ( Figs. 5B , 6B , 9B ).

Male: Carapace almost circular, slightly wider than long; dorsal surface almost smooth, not prominently inflated, lateral surfaces with setae; front projecting anteriorly, margin sinuous to almost straight ( Figs. 20A, B , 21A, B ). Eyes distinctly visible in dorsal view ( Figs. 20A , 21A ). MXP3 as in female ( Fig. 22A ). 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. 20A, B , 21A, C ); ventral margin of palm gently to distinctly sinuous, sometimes with low bulge near midlength. 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; left P4 dactylus slightly longer, more slender than right side; P4 and P5 dactylus subequal to that of P3, covered with short setae ( Fig. 22B–I ). Pleon slender, triangular, widest at somite 3, lateral margins of somite 4 gently concave to almost straight; somite 6 trapezoidal or subquadrate; telson ovate to semicircular, slightly wider than long, widest near midlength ( Fig. 22J, O ). G1 relatively stout, arcuate, curved outwards, tip gently curved upwards or straight ( Fig. 22K–M, O ). G2 short, with spatuliform tip; exopod present as long curved structure ( Fig. 22N ).

Variation. Preservational effects notwithstanding, there is considerable variation in the shape of the carapace ( Figs. 13 , 14 ), from transversely ovate with the front barely protruding in dorsal view ( Figs. 5A , 6A , 11A, D , 12F , 13A–C ), to more rounded with a distinctly protruding frontal margin ( Figs. 11E, G, H , 12A–C, E, G, H , 13E–H , 14 ); and in some, the carapace is almost subtrapezoid ( Fig. 14E–H ). The good series of specimens demonstrates that there are many intermediate conditions that make precise carapace shape unreliable as a diagnostic character in A. palaensis . There is some variation in the length of the MXP3 dactylus, being relatively shorter in some specimens (e.g., Fig. 17D, G ); in most specimens, however, the dactylus almost reaches the tip of the propodus (e.g., Fig. 17H ), but never beyond. In most adult female A. palaensis examined (including small specimens of about 5 mm in carapace width), the distal margin of fused thoracic sternites 1–4 is proportionately narrower and the median part is deeply concave, appearing as a broad indentation. In the largest female of A. palaensis examined (13.3 × 11.2 mm; ZRC 2017.1281), however, the anterior thoracic sternum is also broad and medially shallow; and as such, this character may vary to some degree. In the smallest female examined (3.3 × 3.4 mm; ZRC 2018.767; Fig. 15A ), the pleon resembles that of males, being elongate with a semicircular telson ( Fig. 15C ). The male pleon varies slightly in shape, with the Philippine paralectotype male of P. arcophilus (2.8 × 2.5 mm; SMF-ZMG 172b) being slightly more hexagonal ( Fig. 22O ), whereas that of the Indonesian male is somewhat more rounded ( Fig. 20J ). Their G1 morphologies, however, agree in all key aspects ( Fig. 22K, O ).

Colour. The colour is variable, from pale to dirty white to pinkish or yellowish when alive, with the eyes pigmented red to black ( Figs. 2–4 ). In the freshly collected male (ZRC 2019.1788), the carapace is dirty white with numerous small brown flecks and the ventral surfaces are white ( Fig. 20 ).

Host. Known with certainty only from the Arcidae Lamarck, 1809 . Arcotheres palaensis has been found in Anadara antiquata (Linnaeus, 1758) , Tegillarca granosa (Linnaeus, 1758) , Tegillarca cf. rhombea (Born, 1778) , Tegillarca nodifera (Martens, 1860) , Barbatia decussata (Lamarck, 1819) , Barbatia virescens (Reeve, 1844) , and Barbatia sp. The one record from Tucitona aurifluus (Reeve, 1843) (previously in Pectunculus ) ( Glycymerididae Dall, 1908 ) (supposedly for two synonyms, Pinnotheres rhombifer Bürger, 1895 , and Pinnotheres latissimus Bürger, 1895 ) needs to be confirmed. Noteworthy, however, is that both Arcidae and Glycymerididae belong to the superfamily Arcoidea Lamarck, 1809 . As discussed earlier, records of “ A. palaensis ” from Placuna Lightfoot, 1786 (Placunidae) , and Mactra Linnaeus, 1767 (Mactridae) (cf. Schmitt et al., 1973: 65), almost certainly belong to other species.

Remarks. The large series of A. palaensis available for study, particularly from Singapore and Malaysia, has enabled documentation of the range of morphological variation in this species. The morphological range observed in the Singapore – Malaysia material alone fully encompasses that of the lectotype from Palau, giving confidence in species identification. Moreover, our observations based on the large sample size enabled reassessment of the taxonomy of nominal species of Arcotheres similar to A. palaensis , many of which are synonymised herein, as well as clarification of the identity of most published records in the region. Unfortunately, we have not been able to obtain additional specimens of A. palaensis from the type locality in Palau; despite a recent survey there to sample topotypic material of Brachyura (e.g., see Ng, 2019), the molluscs sampled did not yield any pinnotherids. Arcotheres palaensis is present in the adjacent Philippines (through the types and synonymisation of Pinnotheres latissimus and Pinnotheres rhombifer with it, their type localities being there; see later). We also have specimens from Borneo and Thailand, which connects the distribution of the species to Malaysia and Singapore. The records from Japan are also probably A. palaensis , though these may warrant further study (see later).

Ahyong & Ng (2007b: 205–206) synonymised Pinnotheres latissimus Bürger, 1895 , under Pinnotheres rhombifer Bürger, 1895 , commenting that there were no major characters separating them. The species had already been transferred to Arcotheres by Campos & Manning (2001). The good series of specimens of A. palaensis on hand now shows that both P. rhombifer and P. latissimus are its junior synonyms. No other characters apart from small differences in carapace proportions purportedly separate A. palaensis from A. rhombifer and A. latissimus , differences that are fully encompassed by the range in variation observed here. As all the synonymised species by Bürger were described in the same paper ( Bürger, 1895), the names are considered as simultaneously published under the zoological Code (ICZN, 1999). Under Article 24 of the Code (ICZN, 1999), we here regard Pinnotheres palaensis Bürger, 1895 , as having priority over P. latissimus Bürger, 1895 , and P. rhombifer Bürger, 1895 , when they are regarded as synonyms.

Arcotheres arcophilus is also certainly a junior synonym of A. palaensis . The types of A. arcophilus (a female and a male) are in poor condition ( Figs. 6A, B , 19A ), and appear to differ from the lectotype of A. palaensis only in that: the female carapace is relatively more hexagonal in shape with a relatively more distinct front ( Figs. 6A, B , 19A ), the anterior carapace margins of females are relatively less divergent ( Figs. 6A, B , 19A ), the eyes (for both sexes) are clearly visible in dorsal view ( Figs. 6A, B , 19A ), and the female ambulatory meri are slightly more slender ( Fig. 19A ) (see also Ahyong & Ng, 2007b: fig. 2A, C). One small ovigerous female specimen (5.1 × 4.3 mm) from the Riau Islands in Indonesia (ZRC 1999.1202) closely resembles the lectotype female in these characters ( Figs. 6C–E , 19D–M ); while a recently collected male (3.6 × 3.6 mm; ZRC 2019.1788) agrees very well with the paralectotype male of A. arcophilus (SMF-ZMG 172b), notably in the structures of the ambulatory legs, pleon, and G1 ( Figs. 20–22 ).

Carapace proportions and the anterior outline, however, have already been shown to be very variable in A. palaensis . Some specimens of A. palaensis superficially resemble A. arcophilus in possessing a more projecting frontal margin and in the less divergent anterior part of the lateral carapace margins. One female specimen (8.7 × 8.0 mm; ZRC 2020.1; Figs. 3A , 12C , 13E ) of A. palaensis has a carapace closely resembling that of the type female A. arcophilus ( Fig. 6A, B ), but its eyes are completely hidden by the carapace rim in dorsal view, the ambulatory meri are slightly more slender, the longer P4 propodus is somewhat shorter, and the chela is relatively more elongate. Eight other female specimens of A. palaensis from that lot (ZRC 2020.1) show a range of carapace shape variation ( Figs. 3B, C , 11D , 12D , 14A ). Most specimens of A. palaensis , however, have the same eye, leg, and chela features. Another female A. palaensis from Malaysia (ZRC 2018.770) has a carapace shape that is superficially like that of the type and other female specimen of A. arcophilus (ZRC 1999.1202), but the anterior parts of the lateral margins diverge more strongly and the eyes are underneath the anterior margins, completely invisible in dorsal view ( Figs. 11B , 14C ). The differences in the ambulatory legs are not significant and can easily be accounted for by variation. In the lectotype female of A. arcophilus , the MXP3 dactylus does not reach the apex of the propodus ( Fig. 19B ), but this character varies substantially in A. palaensis , with the dactylus sometimes almost reaching the tip of the propodus ( Fig. 16 ).

The one character of A. arcophilus that needs discussion is the visibility of the eyes of female specimens in dorsal view. The type specimens of A. arcophilus have part of the eyes visible in dorsal view ( Figs. 6A, B , 19A ), as does the more recent Indonesian female (ZRC 1999.1202; Figs. 6C, D , 19M ). The eyes in all other females we referred to A. palaensis are concealed under the frontal margins or, at most, scarcely visible in dorsal view. Komai et al. (2020) had used the dorsally exposed eyes as one of the diagnostic features of A. ocularius from Fiji and Lombok, but this feature was consistent in a large series of specimens of various sizes. In the case of A. arcophilus , however, the types and recent specimen from Indonesia are all relatively small (and the types are poorly preserved), and it would be more parsimonious to consider the dorsally exposed eyes of A. arcophilus as being at one end of a continuum of variation within the species, present at smaller sizes prior to maximum swelling of the adult carapace. It is not uncommon, in smaller A. palaensis , for the carapace margins lateral to the front to be sufficiently recessed that the eyes lie at the edge of the carapace, albeit remaining concealed in dorsal view; more rarely, however, are the eyes just visible from above, approaching the condition in the type material of A. arcophilus . In any case, A. ocularius can be distinguished by other characters of the carapace, pleon, and MXP3 ( Komai et al., 2020). Under Article 24 of the Code (ICZN, 1999), we here also regard Pinnotheres palaensis Bürger, 1895 , as having priority over P. arcophilus Bürger, 1895 , when they are regarded as synonyms.

Miers (1880) recorded Pinnotheres obesus with doubt from “Indo-Malayan Seas”, but Gordon (1936: 176) re-examined and identified them as P. latissimus instead (see also Schmitt et al., 1973: 51). Komai et al. (2020: 230), in revising P. obesa s. str., commented that Miers’ (1880) material was actually A. exiguus based on a personal communication by the present authors, but this is incorrect. Miers’ (1880) poorly preserved specimens (NHM 1860.6), re-examined here, are actually referable to A. palaensis , the chelae and ambulatory legs agreeing well with this species.

Tesch (1918) described Pinnotheres onychodactylus from three females collected from an unspecified host on a reef at 18-m depth, from between Gisser and Ceram islands in the Moluccas, Indonesia, the largest female being 8.8 × 6.8 mm. Tesch (1918: 259) comments: “This species much resembles P. rhombifer Bürger , which, perhaps, is nothing but P. palaensis Bürger , from which P. rhombifer is only distinguished by having the penultimate pair of legs shorter than the last pair. In the ‘Siboga’ specimens the carapace, the breadth of which is 1⅓ times its length, agrees with these of the two named species of Bürger; it is very much vaulted, with bulging hepatic and branchial regions, without sculpture, completely naked and very thin. The front is thickened, not prominent, and the eyes are not visible from above.” His figures and description, which note the diagnostic ambulatory dactylar structures, strongly suggest that his species is A. palaensis s. str. Unfortunately, he figures the legs (especially the P4) as symmetrical ( Tesch, 1918: pl. 17 fig. 5), with the left and right P4 dactyli the same size and form, and the MXP3 dactylus also exceeding the tip of the propodus ( Tesch, 1918: pl. 17 fig. 5A). Until the types can be re-examined to ascertain the accuracy of the P4 and MXP3 characters, it may be better to treat it as a distinct species for the moment. In any case, as discussed earlier, we regard A. rhombifer as a subjective junior synonym of A. palaensis . As we do not know whether the P4 is asymmetrical, we leave the species in Pinnotheres s. lato for the time being, although it is likely to be an Arcotheres .

De Man’s (1921) description and figure of a female specimen from Ambon, Indonesia, as well as his record of 11 females and two males from Java ( De Man, 1929), from a species of “ Arca ”, match those of A. palaensis very well and we have no doubt of his identification. De Man (1921) also figured a female Arcotheres specimen (as a Pinnotheres ) from Ambon and another from Lombok, referring them to A. palaensis and A. arcophilus , respectively (they are regarded as synonyms here). De Man based this mainly on the structure propodus, MXP3 dactylus, and the form of the chela, following Bürger’s (1895) original descriptions and figures. He noted that the carapace was in bad condition in both specimens and could not help the taxonomy. He discussed at length the asymmetry in the ambulatory legs and considered the possibility of it being anomalous or even the result of bopyrid infection. Ambulatory leg asymmetry was revisited in a later paper ( De Man, 1929). Perez (1921: 61), following on his correspondence with De Man (1921), briefly commented on this problem and followed De Man’s identifications of the Ambon and Lombok specimens. De Man’s (1921) figures for the two species are excellent and his figures of the third maxillipeds and chelipeds in particular agree very well with those of A. palaensis (and A. arcophilus ) here. The MXP3 dactylus of his “ A. arcophilus ” is short, terminating some distance from the tip of the propodus, and the chela is short and stout ( De Man, 1921: pl. 8 figs. 1, 2), while in A. palaensis , the MXP3 dactylus is slightly longer, almost reaching the tip of the propodus, and the chela is relatively more elongate and slender ( De Man, 1921: pl. 8 figs. 8, 9; cf. present Figs. 8D , 9D , 16 ). De Man’s (1921) record of “ A. arcophilus ” is also almost certainly from an Anadara or Tegillarca (then known as Arca ).

Pinnotheres paralatissimus Dai & Song, 1986 , was described from two females collected from an unspecified host in Beihai, Guangxi, southern China ( Dai & Song, 1986: 55, text-fig. 2; Fig. 10A–H ). Despite two separate searches in 2018 and 2019, the types could not be found in the Institute of Zoology, Chinese Academy of Sciences (Academia Sinica), where they were stated to have been kept (Meng Kai, Ng Ngan Kee, Lee Bee Yan, pers. comm.). The description and figures, however, are identical with what is here recognised as A. palaensis s. str.

Campos (2001) described A. guinotae from one female (6.4 × 5.8 mm; MNHN-B9498), collected from somewhere in Thailand from an unidentified species of Barbatia . He compared it with A. similis , A. spinidactylus , A. alcocki , and A. tivelae ( Gordon, 1936) , noting that “the acute and sinuous anterolateral margin of the carapace in A. guinotae n. sp., is not observed in all other known congeners” (Campos, 2001: 497). We have examined photographs of the type specimen, courtesy of A. Anker ( Fig. 7 ), who also kindly confirmed various key characters for us. The carapace shape of A. guinotae is somewhat unusual ( Figs. 1B , 7A ; Campos, 2001: fig. 1A), but we have on hand a female (6.7 × 5.0 mm; ZRC 2017.1038) collected from a Barbatia in Singapore, which is comparable in size to the holotype female of A. guinotae and agrees with it in all features, including the carapace shape ( Figs. 9 , 14H , 16G , 17K–O ). Our comparisons of the large series of A. palaensis , including specimens collected from Barbatia , found that the carapace shape variation fully encompasses that reported for A. guinotae . We have specimens that have carapace shape of intermediate conditions that transition into the more ovate form, also found in A. palaensis ( Figs. 11–14 ). The female telson of A. guinotae is proportionately narrower (Campos, 2001: fig. 1C) than those in most adult female specimens (e.g., Figs. 5B , 8B , 9B ), which we consider intraspecific variation. We have a few specimens of A. palaensis in which the female telson ( Fig. 15D, E ) is similar to that of the holotype of A. guinotae . Other specimens from the same lot or site have the more typically wide female telsons. There are no other characters that can separate the two species and the excellent description and figures by Campos (2001) leave no doubt. As such, we synonymise A. guinotae under A. palaensis .

The Indian record of “ Arcotheres alcocki ” by Lalitha Devi (1981: 216, figs. 1 (right), 2, 3) from Tegillarca granosa in Kakinada Bay in the Bay of Bengal may be A. palaensis as well. The figures of the female specimen are not all given in sharp detail ( Lalitha Devi, 1981: fig. 2), but nevertheless agree well with A. palaensis . The figured male agrees very well with male specimens examined here, including in the form of the pleon and G1 ( Lalitha Devi, 1981: fig. 3). The available evidence of “ Arcotheres alcocki ” presented by Lalitha Devi (1981) points to A. palaensis , but her material should be re-examined to determine if this is indeed the case. Trivedi et al. (2018a) recently described a species from southern India, A. shahi , ostensibly from oysters, that closely resembles A. palaensis , and necessary comparisons should be made to determine if they are related.

The Japanese records of “ A. alcocki ” are not that species. Ahyong & Ng (2007b: 193–194) observed that the record of “ A. alcocki ” by Takeda & Konishi (1988) is a different species close to A. sinensis ( Shen, 1932) , or perhaps even undescribed. We have since examined their specimens from the Hiroshima area as well as a series of females from nearby Kumamoto ( Figs. 12G, H , 14E, F ), and they agree best with A. palaensis as recognised here. As such, the records by Watanabe & Henmi (2009) and Watanabe (2013) should now also be referred to A. palaensis . The male reported by Takeda & Konishi (1988) is not in good condition but agrees with the present male from Indonesia ( Figs. 20–22 ) in most features. The carapace is slightly more rounded in the Japanese male (not evident from the figure) and its frontal margin is weakly emarginate (carapace wider with a straighter frontal margin in the Indonesian male; Figs. 20A , 21A ). The male pleon of the Japanese male, however, is unusual in being proportionately wider, with strongly concave margins, and with the telson relatively broader and subcircular in shape ( Takeda & Konishi, 1988: fig. 3c) (versus pleon less wide, margins weakly concave, and telson sub-hexagonal to semicircular in the Indonesian male; Fig. 22J, O ). The differences in carapace and pleon features may be due to size-associated variation—the Japanese male, at 4.2 × 3.7 mm, is larger than the Indonesian specimen (3.6 × 3.6 mm). In A. placunicola , however, smaller males have a relatively wider pleonal somite 6 and a somewhat more rectangular telson, with somite 6 becoming proportionately longer and the telson semicircular in larger specimens ( Ng, 2018a: 478, fig. 4B, C); their G1s, however, agree in relative stoutness. Takeda & Konishi (1988: fig. 3d, e) figured the tip of the G1 as having a small spine, but this proved to be an uneven evaginated fold caused by preservation; the gonopods are otherwise indistinguishable between Japanese and Malaysian specimens. The Japanese records, however, are the northernmost and only records of the species from temperate-subtropical waters, all others being from the tropics. It cannot be excluded that the morphological differences between the Japanese and Malaysian males possibly indicate more than one species; more work should be done to confirm this. The Japanese taxon, however, is certainly not A. alcocki s. str. (see later for this species).

We have on hand female specimens from Lombok, two of which were collected from specimens of Anadara antiquata (ZRC 2018.764, ZRC 2018.762) and belong to two separate species. Two specimens, one of which is infected by a bopyrid (carapace length 6.7 mm; ZRC 2018.762), match A. palaensis in all diagnostic features ( Fig. 12F ). The second specimen (9.4 × 8.4 mm; ZRC 2019.1025) is referable to A. ocularius Komai, Kawai & Ng, 2020 ( Fig. 23B ), as the MXP3 has a proportionately longer MXP3 dactylus, the front is more pronounced anteriorly, the eyes are clearly visible in dorsal view, and the female anterior thoracic sternum (fused sternites 1–4) is relatively broader with the median part shallow rather than deeper ( Komai et al., 2020). Our morphological identification of A. ocularius is corroborated by unpublished molecular comparisons currently underway in collaboration with L. M. Tsang.

In the ZRC there is one lot with a male and a female specimen initially identified as A. palaensis , collected in 1983 from Tegillarca (as Anadara ) in mangroves in Batu Pahat, Peninsular Malaysia (ZRC 2000.2613). These specimens had been loaned to Charles Pregenzer for his comparative studies (unpublished) and Gerhard Pohle for his phylogenetic work (see Pohle & Marques, 1998). The female specimen (ca. 9.0 mm carapace width) is in poor condition now, soft and with a deformed carapace, but is clearly A. palaensis as presently defined ( Fig. 18I, J ). The small male (3.7 × 3.3 mm), which is relatively hard and in good condition, however, cannot be this species even though it is labelled as being from the same host and location, because the P5 dactylus has a row of spinules ( Fig. 60H ) which A. palaensis lacks. In the shape and flatness of the carapace (when viewed frontally; Fig. 60D, E ), it closely resembles A. placunicola . In addition, the G1 is more similar in shape and features to that of A. placunicola , including the obliquely directed distal tip ( Figs. 60G , 62M, N ). We are confident this small male is actually A. placunicola and is here referred to that species with a new catalogue number (ZRC 2021.802). Although it cannot be excluded that the small male entered a Tegillarca instead of Placuna host, we consider it more likely that the specimen was mislabelled or misplaced. If this is the case, we do not know how the specimens of the two species got mixed in one lot, nor whether it was collection error or otherwise. It is important to note, however, that the male A. palaensis obtained by the authors from Tegillarca and Anadara has a clearly more elongate carapace, with the dorsal surface prominently convex and the front clearly more projected anteriorly ( Figs. 20A, B , 21A, B ).

Distribution. Western Pacific to the eastern Indian Ocean, from Palau and Japan to the South China Sea including the Philippines, Indonesia, Borneo, Malaysia, Singapore, Thailand, and eastern India.

Figures

Fig. 1. Original figures of selected species. A, A. palaensis (Bürger, 1895), after Bürger (1895: pl. 9 fig. 12); B, A. guinotae Campos, 2001, after Campos (2001: fig. 1A); C, A. arcophilus (Bürger, 1895), after Bürger (1895: pl. 9 fig. 10); D, A. modiolicola (Bürger, 1895), after Bürger (1895: pl. 9 fig. 9); E, Pl. coarctatus (Bürger, 1895), after Bürger (1895: pl. 9 fig. 7); F, M. latus (Bürger, 1895), after Bürger (1895: pl. 9 fig. 16).

Fig. 2. Arcotheres palaensis (Bürger, 1895), colour in life, in situ in Anadara and Tegillarca (Arcidae). A, ovigerous female (13.3 × 11.2 mm) (ZRC 2017.1281), Peninsular Malaysia; B, ovigerous female (8.7 × 8.0 mm) (ZRC 2020.1), Peninsular Malaysia; C, D, ovigerous female (11.6 × 10.3 mm) (ZRC 2020.11), Peninsular Malaysia; E, F, female (7.9 × 6.6 mm) (ZRC 2018.255), Peninsular Malaysia. A, D, E, in situ; B, dorsal view; C, ventral view. A, host A. cf. rhombea (Born, 1778); B–F, host T. granosa (Linnaeus, 1758). Photographs: Paul Y. C. Ng.

Fig. 3.Arcotheres palaensis (Bürger, 1895), colour in life, overall dorsal view.A, ovigerous female (8.7 × 8.0 mm) (ZRC 2020.1), Peninsular Malaysia; B, female (9.5 × 8.2 mm) (ZRC 2020.1), Peninsular Malaysia; C, female (9.3 × 8.3 mm) (ZRC 2020.1), Peninsular Malaysia; D, ovigerous female (11.6 × 10.3 mm) (ZRC 2020.11), Peninsular Malaysia; E, female (6.3 × 5.0 mm) (ZRC 2021.821), Peninsular Malaysia; F, ovigerous female (7.9 × 6.4 mm) (ZRC 2021.821), Peninsular Malaysia; G, female (6.4 × 4.8 mm) (ZRC 2021.821), Peninsular Malaysia; H, female (7.8 × 6.6 mm) (ZRC 2021.0801), Peninsular Malaysia. Photographs: Paul Y. C. Ng, P. K. L. Ng.

Fig. 4. Arcotheres palaensis (Bürger, 1895), ovigerous female (11.6 × 10.3 mm) (ZRC 2020.11), Peninsular Malaysia, colour in life. Two subventral views showing eyes, antennules, antennae, and MXP3.

Fig. 5. Arcotheres palaensis (Bürger, 1895). A, B, lectotype female (10.2 × 8.4 mm) (SMF-ZMG 948a), Palau. A, overall dorsal view; B, ventral view of cephalothorax.

Fig. 6. Arcotheres palaensis (Bürger, 1895). A, B, female (5.3 × 4.7 mm) (SMF-ZMG 172a) (lectotype of Pinnotheres arcophilus Bürger, 1895), Philippines; C–F, ovigerous female (5.1 × 4.3 mm) (ZRC 1999.1202), Indonesia. A, C, overall dorsal view; B, D, dorsal view of carapace; E, frontal view of cephalothorax; F, outer view of right chela.

Fig. 7. Arcotheres palaensis (Bürger, 1895), female (6.4 × 5.8 mm) (MNHN-B9498), Thailand (holotype of A. guinotae Campos, 2001). A, overall dorsal view; B, left cheliped; C, ventral view of distal part of P5 dactylus. Photographs: A, B, Arthur Anker; C, Sébastien Soubzmaigne.

Fig. 8. Arcotheres palaensis (Bürger, 1895), ovigerous female (8.5 × 7.2 mm) (ZRC 1992.8378), Peninsular Malaysia. A, overall dorsal view; B, ventral view of cephalothorax; C, frontal view of cephalothorax; D, outer view of right chela.

Fig. 9. Arcotheres palaensis (Bürger, 1895), ovigerous female (6.7 × 5.0 mm) (ZRC 2017.1038), Singapore. A, overall dorsal view; B, ventral view of cephalothorax; C, frontal view of cephalothorax; D, outer view of right chela.

Fig. 10. Arcotheres palaensis (Bürger, 1895), female (7.3 × 5.2 mm) (CAS), China (holotype of Pinnotheres paralatissimus Dai & Song, 1986); after Dai & Song (1986: fig. 2(1–7)). A, overall dorsal view; B, left MXP3; C, outer view of right chela; D–G, right P2–P5 dactyli; H, left P4 dactylus. Scales = A, 2.0 mm; B, D–H, 0.5 mm; C, 1.0 mm.

Fig. 11. Arcotheres palaensis (Bürger, 1895), overall dorsal view. A, ovigerous female (13.3 × 11.2 mm) (ZRC 2017.1281), Peninsular Malaysia; B, female (7.0 × 5.6 mm) (ZRC 1965.11.24.33), Singapore; C, female (9.6 × 7.5 mm) (ZRC 1999.975), Peninsular Malaysia; D, female (8.1 × 6.6 mm) (ZRC 2017.1252), Peninsular Malaysia; E, female (7.3 × 5.8 mm) (ZRC 1987.537), Peninsular Malaysia; F, ovigerous female (7.1 × 5.7 mm) (ZRC 1993.120), Peninsular Malaysia; G, female (8.4 × 7.3 mm) (ZRC 1965.11.24.17), Peninsular Malaysia; H, ovigerous female (7.9 × 6.6 mm) (ZRC 2018.0255), Peninsular Malaysia.

Fig. 12. Arcotheres palaensis (Bürger, 1895), overall dorsal view. A, female (8.7 × 8.1 mm) (ZRC 2017.1260), Sarawak; B, ovigerous female (7.5 × 5.8 mm) (ZRC 2018.770), Peninsular Malaysia; C, ovigerous female (8.7 × 8.0 mm) (ZRC 2020.1), Peninsular Malaysia; D, female (7.7 × 6.2 mm) (ZRC 1993.121), Indonesia; E, female (8.8 × 7.8 mm) (ZRC 2018.764), Lombok, Indonesia; F, female with bopyrid (carapace length 6.7 mm) (ZRC 2018.762), Lombok, Indonesia; G, female (8.0 × 5.8 mm) (ZRC 2019.1879), Japan; H, female (8.6 × 6.1 mm) (ZRC 2019.1879), Japan.

Fig. 13. Arcotheres palaensis (Bürger, 1895), dorsal view of carapace. A, ovigerous female (13.3 × 11.2 mm) (ZRC 2017.1281), Peninsular Malaysia; B, female (8.9 × 6.5 mm) (ZRC 1965.11.24.27), Singapore; C, ovigerous female (8.5 × 7.2 mm) (ZRC 1992.8378), Peninsular Malaysia; D, female (9.3 × 8.3 mm) (ZRC 2020.1), Peninsular Malaysia; E, ovigerous female (8.7 × 8.0 mm) (ZRC 2020.1), Peninsular Malaysia; F, ovigerous female (7.1× 5.7 mm) (ZRC 1993.120), Peninsular Malaysia; G, female (7.7 × 6.2 mm) (ZRC 1993.121), Indonesia; H, female (8.8 × 7.8 mm) (ZRC 2018.764), Lombok, Indonesia.

Fig. 14. Arcotheres palaensis (Bürger, 1895), dorsal view of carapace. A, female (9.5 × 8.2 mm) (ZRC 2020.1), Peninsular Malaysia; B, female (6.1 × 5.2 mm) (ZRC 1993.0119), Peninsular Malaysia; C, female (7.5 × 5.8 mm) (ZRC 2018.770), Peninsular Malaysia; D, ovigerous female (7.9 × 6.6 mm) (ZRC 2018.0255), Peninsular Malaysia; E, female (8.0 × 5.8 mm) (ZRC 2019.1879), Japan; F, female (8.6 × 6.1 mm) (ZRC 2019.1879), Japan; G, female (9.5 × 7.7 mm) (ZRC 1965.11.24.30), Singapore; H, ovigerous female (6.7 × 5.0 mm) (ZRC 2017.1038), Singapore.

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. 16. Arcotheres palaensis (Bürger, 1895), outer view of left MXP3. A, female (9.6 × 7.5 mm) (ZRC 1999.975), Peninsular Malaysia; B, ovigerous female (8.5 × 7.2 mm) (ZRC 1992.8378), Peninsular Malaysia; C, female (7.3 × 5.8 mm) (ZRC 1987.537), Peninsular Malaysia; D, female (6.1 × 5.2 mm) (ZRC 1993.119), Peninsular Malaysia; E, female (8.1 × 6.6 mm) (ZRC 2017.1252), Peninsular Malaysia; F, female (7.7 × 6.2 mm) (ZRC 1993.121), Indonesia; G, ovigerous female (6.7 × 5.0 mm) (ZRC 2017.1038), Singapore; H, I, ovigerous female (10.3 × 7.9 mm) (ZRC 2020.11), Peninsular Malaysia; J, female (8.8 × 7.8 mm) (ZRC 2018.764), Lombok, Indonesia. Scales = 0.5 mm.

Fig. 17. Arcotheres palaensis (Bürger, 1895). A–H, female (7.7 × 6.2 mm) (ZRC 1993.121), Indonesia; I–O, ovigerous female (6.7 × 5.0 mm) (ZRC 2017.1038), Singapore. A–D, left P2–P5, respectively; E–H, right P2–P5, respectively; I, J, left P4 and P5, respectively; K–N, right P2–P5, respectively; O, posterior margin of epistome. Scales = 0.5 mm.

Fig. 18. Arcotheres palaensis (Bürger, 1895). A–D, female (6.1 × 5.2 mm) (ZRC 1993.119), Peninsular Malaysia; E–H, M, ovigerous female (8.5 × 7.2 mm) (ZRC 1992.8378), Peninsular Malaysia; I, J, soft female (ca. 9.0 mm carapace width) (ZRC 2000.2613), Peninsular Malaysia; K, L, female (9.5 × 7.7 mm) (ZRC 1965.11.24.30), Singapore. A, B, left P4 and P5, respectively; C, D, right P4 and P5, respectively; E, F, left P4 and P5, respectively; F’, distal part of left P5 dactylus (subventral view, soft setae omitted); G, H, right P4 and P5, respectively; I, J, right P4 and P5, respectively; K, L, right P4 and P5, respectively; M, posterior margin of epistome. Scales = A–M, 0.5 mm; F’, 0.25 mm.

Fig. 19. Arcotheres palaensis (Bürger, 1895). A–C, female (5.3 × 4.7 mm) (SMF-ZMG 172a) (lectotype of Pinnotheres arcophilus Bürger, 1895), Ubay, Philippines; D–M ovigerous female (5.1 × 4.3 mm) (ZRC 1999.1202), Indonesia. A, overall dorsal view; B, right MXP3; C, right chela; D, left MXP3; E–H, left P2–P5, respectively; I–L, right P2–P5, respectively; M, frontal margin of carapace showing eyes in dorsal view. Scales = A–L, 0.5 mm; M, 1.0 mm.

Fig. 20. Arcotheres palaensis (Bürger, 1895), male (3.6 × 3.6 mm) (ZRC 2019.1788), Indonesia, colour in life. A, overall dorsal view; B, frontal view; C, ventral view. Photographs: Paul Y. C. Ng.

Fig. 21. Arcotheres palaensis (Bürger, 1895), male (3.6 × 3.6 mm) (ZRC 2019.1788), Indonesia. A, overall dorsal view; B, frontal view of cephalothorax; C, outer view of right chela.

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.

Fig. 23. Arcotheres ocularius Komai, Kawai & Ng, 2020, overall dorsal view. A, paratype ovigerous female (10.9 × 9.7 mm) (ZRC 2019.1877), Fiji; B, female (9.4 × 8.4 mm) (ZRC 2019.1025), Indonesia.

Fig. 60. Arcotheres placunicola Ng, 2018. A–C, paratype male (5.6 × 4.7 mm) (ZRC 2017.1010), Changi, Singapore; D–H, male (3.7 × 3.3 mm) (ZRC 2021.802), Batu Pahat, Peninsular Malaysia. A, D, overall dorsal view; B, E, frontal view of cephalothorax; C, F, outer view of left chela; G, left G1; H, distal part of P5 (ventral view). B after Ng (2018a: fig. 2B).

Fig. 62. Arcotheres placunicola Ng, 2018, male (3.7 × 3.3 mm) (ZRC 2021.802), Batu Pahat, Peninsular Malaysia. A, dorsal view of carapace; B, pleon (setae omitted); C, outer view of left MXP3 (setae denuded); D, outer view of left chela; E–H, left P2–P5, respectively (setae omitted); I–L, right P2–P5, respectively; M, left G1 (ventral view); N, left G1 (dorsal view, setae omitted); O, left G2. Scales = A, B, D–L, 0.5 mm; C, M–O, 0.2 mm.