Magnotheres globosus (Hombron & Jacquinot, 1846) Ng & Ahyong, 2022

Ng, Peter K. L. & Ahyong, Shane T., 2022, The pea crab genus Arcotheres Manning, 1993 (Crustacea: Brachyura: Pinnotheridae) from Singapore and Peninsular Malaysia, with a reappraisal of diagnostic characters and descriptions of two new genera, Raffles Bulletin of Zoology 70, pp. 134-248 : 226-242

Magnotheres globosus (Hombron & Jacquinot, 1846) , new combination

( Figs. 72–86 )

Pinnotheres globosum Hombron & Jacquinot, 1846 : pl. 5 figs. 21–26 [type locality: Changi, Singapore, by present neotype designation]; Jacquinot & Lucas, 1853: 58; Ng & Corbari, 2019: 245, fig. 6.10–6.13 (discussion).

Pinnotheres rouxi —? Paulson, 1875: 70, pl. 9 fig. 2-2c (not Pinnotheres rouxi H. Milne Edwards, 1853 ).

Pinnotheres latus Bürger, 1895: 374 , 375, pl. 9 fig. 16, pl. 10 fig. 15 [type locality: Ubay, Philippines, by present lectotype designation, from Pinna sp. ]; Tesch, 1918: 249 (Sumba and Amboina, Indonesia); Gordon, 1936: 171 (discussion); Estampador, 1937: 547 (list); Miyake, 1939: 221 (list); Silas & Alagarswami, 1967: 1201 (list); Serène, 1968: 94; Schmitt et al., 1973: 52.

Pinnotheres parvulus — De Man, 1888: 105; Alcock, 1900: 339 [not Pinnotheres parvulus Stimpson, 1858 ].

Pinnoteres borradailei Nobili, 1906a: 410 (replacement name for Pinnotheres tenuipes Borradaile, 1903 ) [type locality: Minicoy, Lakshadweep Islands, India]; 1906b: 306 (part); Guinot, 1967: 279 (list).

Pinnotheres borradailei —? Lenz, 1910: 558, 576; Laurie, 1915: 415 (list); Tesch, 1918: 248, 253, 287 (list and key); Silas & Alagarswami, 1967: 1197 (list); Schmitt et al., 1973: 40 (list); Ng et al., 2008: 250 (list); Ng et al., 2017: 1080.

Pinnoteres ridgewayi Southwell, 1911: 226 , pl. 3 figs. 1, 2, 2a [type locality: Kondatchi Paar, Sri Lanka, by lectotype designation, Ng et al., 2017].

Pinnotheres ridgewayi — Tesch, 1918: 250, 251 (list); Gravely, 1927: 146, pl. 23 fig. 37; Prasad & Tampi, 1957: 22, fig. 1 (larvae); Sankarankutty, 1966: 349, 350, 358, 360, figs. 23, 28; Silas & Alagarswami, 1967: 1177, 1183, 1208, 1219, 1223, 1225, 1227 (discussion); Serène, 1968: 94; Schmitt et al., 1973: 84; Ng et al., 2008: 251 (list); Ng et al., 2017: 1080.

Pinnotheres similis — Rathbun, 1924: 15; Davie, 2002: 434 (not Pinnotheres similis Bürger, 1895 ).

Pinnotheres alcocki — Rathbun, 1909: 114 (part); Gordon, 1936: 176, fig. 5 [not Pinnotheres alcocki Rathbun, 1909 ].

Pinnotheres sp. — Ward, 1967: 62.

Pinnotheres globosa — Silas & Alagarswami, 1967: 1199, 1204; Schmitt et al., 1973: 46; Ng et al., 2008: 250.

Pinnotheres globosus — Serène, 1968: 93; Schmitt et al., 1973: 46 (part); De Gier & Becker, 2020: tab. 1.

Arcotheres similis — Ahyong & Brown, 2003: 9; De Gier & Becker, 2020: tab. 1 (part) [not Pinnotheres similis Bürger, 1895 ].

Arcotheres latus — Ahyong & Ng, 2007b: 198, fig. 6; Ng et al., 2008: 248; Ng et al., 2017: 1093; Trivedi et al., 2018a: 197; Ahyong, 2020b: 880; De Gier & Becker, 2020: tab. 1; Ahyong & Ng, 2021: 195, 207.

Nepinnotheres ridgewayi — Ng & Kumar, 2015: 265 (discussion).

Arcotheres borradailei — Ng & Kumar, 2015: 265; Ng et al., 2017: 1081, figs. 1–3, 8A; Trivedi et al., 2018a: 197; Trivedi et al., 2018b: 61; Trivedi et al., 2018c: 50; De Gier & Becker, 2020: tab. 1.

Arcotheres ridgewayi — Ng et al., 2017: 1085, figs. 4–7, 8B; Trivedi et al., 2018a: 197; Trivedi et al., 2018b: 61; Trivedi et al., 2018c: 50; De Gier & Becker, 2020: tab. 1.

Type material. Neotype (here designated): female (10.2 × 8.4 mm) ( ZRC 2016.164), off Changi Beach, Singapore, adjacent to National Service Recreational Country Club, from Pinna atropurpurea , intertidal area, coll. S. K. Tan et al., 6 July 2016. Myanmar: 1 ovigerous female (ca. 8.3 × 6.7 mm, carapace soft) ( NHM 1886.52), Mergui Archipelago, coll. J. Anderson (paralectotype of Pinnotheres alcocki Rathbun, 1909 ). Philippines: 1 female (10.6 × 8.7 mm) ( SMF-ZMG 956a) (lectotype of Pinnotheres latus Bürger, 1895 ), from Pinna sp. , Burias; 3 females (9.5 × 7.5–10.5 × 9.0 mm) ( SMF-ZMG 956b) (paralectotypes of Pinnotheres latus Bürger, 1895 ), same data as lectotype. India: 1 ovigerous female (8.0 × 6.8 mm) ( CUMZ I.63816) (holotype of Pinnotheres borradailei Nobili, 1906a , replacement name for Pinnotheres tenuipes Borradaile, 1903 ), Minikoi (present day = Minicoy), Lakshadweep Islands (previously known as the Laccadive, Minicoi, and Aminidivi Islands), India, previously part of Maldive and Laccadive Islands, supposedly from Mya , coll. J. S. Gardiner. Sri Lanka: 1 female (14.9 × 12.8 mm) ( ZSI / WGRC / IRINV 8102/10) (lectotype of Pinnotheres ridgewayi Southwell, 1911 ) [photographs examined], from Pinna bullata Gmelin, 1791 , pearl banks, Kondatchi Paar, Ceylon (= Sri Lanka), coll. T. Southwell, 1910; 2 females (14.4 × 11.9 mm, one with carapace in poor condition) [photographs examined], same data as lectotype (paralectotypes of Pinnotheres ridgewayi Southwell, 1911 ).

Other material examined. Singapore: 1 ovigerous female (13.2 × 9.9 mm), 1 non-ovigerous female (14.4 × 10.8 mm)

( ZRC 2017.1018), from Pinna atropurpurea , intertidal area, Tanah Merah Ferry Terminal Beach, near Changi Point, coll. S. K. Tan et al., 13 December 2016; 3 ovigerous females (11.9 × 9.3 mm, 13.1 × 11.0 mm, 13.6 × 10.5 mm) ( ZRC 2017.1019), from Pinna atropurpurea , intertidal area, Changi East, east of Tanah Merah Ferry Terminal Beach, near Changi Point, coll. S. K. Tan et al., 14–16 December 2016; 1 nonovigerous female (14.6 × 10.7 mm) ( ZRC 2017.1020), with Rhopalione sp. bopyrid parasite, from Pinna atropurpurea , intertidal area, Changi East, east of Tanah Merah Ferry Terminal Beach, near Changi Point, coll. S. K. Tan et al., 14–16 December 2016; 2 ovigerous females ( ZRC 2018.786), from Pinna atropurpurea , intertidal area, Tanah Merah Ferry Terminal Beach, near Changi Point, coll. P. K. L. Ng & S. K. Tan, 12–15 January 2017; 1 ovigerous female (13.6 × 10.4 mm) ( ZRC 2017.176), from Pinna atropurpurea , intertidal area, Tanah Merah Ferry Terminal Beach, near Changi Point, coll. P. K. L. Ng & S. K. Tan, 26 May 2017; 1 ovigerous female (13.2 × 10.1 mm) ( AM P105903), from Pinna atropurpurea , intertidal area, Tanah Merah Ferry Terminal Beach, near Changi Point, coll. S. K. Tan et al., 27 May 2017; 3 females ( ZRC 2018.786), from Pinna atropurpurea , intertidal area, Changi East, east of Tanah Merah Ferry Terminal Beach, near Changi Point, coll. S. K. Tan et al., 12 and 15 January 2017; 1 male (8.0 × 7.8 mm) ( ZRC 2020.12), station IT140 (collection number station SS-4132), intertidal, from Pinna atropurpurea, Pulau Tekukor , Southern Islands, 01°13.899′N, 103°50.265′E, 0.4–0.8 m, coll. Comprehensive Marine Biodiversity Survey, 31 May 2013. Philippines: 3 ovigerous females (9.8 × 7.6 mm, 11.0 × 8.9 mm, 11.8 × 9.5 mm), 1 ovigerous female (carapace broken, carapace length 11.9 mm), 1 non-ovigerous female (7.2 × 6.2 mm) ( ZRC 2017.1017), from Pinna sp. , Panglao Island, Bohol, Visayas, coll. Panglao 2004 Expedition, 5 July 2004; 1 ovigerous female (9.0 × 6.8 mm) ( ZRC 2018.781), station M11, host not recorded, rocky intertidal, fringe mangrove, and seagrass habitat, Sungcolan Bay, Panglao Island, Bohol, Visayas, 0–3 m, 9°38.3′N, 123°49.6′E, coll. Panglao 2004 Expedition, 6 June 2004; 1 ovigerous female (13.0 × 10.0 mm) ( ZRC 2018.782), station M18, in Pinna muricata , on sandy bottom and seagrass, Gak-Ang Islet, Panglao Island, Bohol, Visayas, 0–1 m, 9°33.0′N, 123°43.5′E, coll. Panglao 2004 Expedition, 10 June 2004; 1 male (5.4 × 5.5 mm) ( ZRC 2018.783), station M18, in Pinna muricata , on sandy bottom and seagrass, Gak-Ang Islet, Panglao Island, Bohol, Visayas, 0–1 m, 9°33.0′N, 123°43.5′E, coll. Panglao 2004 Expedition, 10 June 2004. Palau: 2 females (8.7 × 7.2 mm, 7.3 × 6.3 mm) ( SMF-ZMG 955), from Pinna nigrina , coll. Semper. Vanuatu: 1 female (13.5 × 10.7 mm) ( ZRC 2016.165), from unknown host, intertidal area, sand and muddy rock substrate, Belmoul Lagoon, Santo, 15°35.8′S, 167°06.1′E, coll. SANTO Expedition, 13 September 2006; 1 female (12.1 × 9.4 mm) ( ZRC 2016.166), specimen data BC 3888, from inside unidentified Pinnidae, Santo , coll. 9 October 2006. Australia: 2 males (5.1 × 5.1–6.8 × 6.8 mm), 7 females (4.0 × 4.3–9.0 × 11.7 mm) ( AM P7067), Fields Reef, Port Denison, Queensland, 20°01′S, 148°15′E, from young Pinna sp. in shallow water, coll. E. Rainford, 1923; 1 female (8.9 × 7.1 mm) ( AM P12173), Michelmas Cay, Great Barrier Reef, Queensland, 16°35′S, 146°02′E, from Pinna sp. on coral reef, coll. G. P. Whitley & T. Iredale, June 1926; 1 female (12.2 × 9.0 mm) ( AM P64685), Lindeman Island, Queensland, dredged, coll. M. Ward, December 1928; 1 female (11.5 × 9.2 mm) ( AM P20161), East Arm, Darwin Harbour, Northern Territory, in mytilid Stavilla [sic] horrida (= Stavelia subdistorta (Reclúz, 1852)) , coll. O. Cameron, 19 December 1971. India: 1 female (17.7 × 13.6 mm) ( ZSI / WGRC / IR-INV 8235), from Pinna atropurpurea, Kovalam , 8.3°N, 77.2°E, Thiruvananthapuram, Kerala, southwestern India, 5–10 m depth, coll. A. B. Kumar & R. Ravinesh, 2014; 1 ovigerous female (14.7 × 11.2 mm) ( ZRC 2016.184), same locality and host as preceding, coll. R. Ravinesh, early 2016; 1 ovigerous female (16.6 × 12.3 mm) ( ZRC 2016.185), same locality and host as preceding, coll. R. Ravinesh, June 2016.

Description. Carapace and pereopods well chitinised. Female: Carapace subcircular to transversely subhexagonal, wider than long; dorsal and lateral surfaces smooth, glabrous; appearing domed in frontal view; front occasionally projecting anteriorly beyond orbits, margin straight to gently convex; anterolateral margin gently convex ( Figs. 72A , 73A , 74A–C , 75A, B , 76 , 79A, C , 80A, C , 81A–E , 82 ). Eyes small, usually not clearly visible in dorsal view in adults, filling orbit ( Figs. 72A , 73A , 74A, B , 75A , 76 , 79A , 80A , 81A–D , 82 , 84A ). Epistome with median part triangular, lateral margins gently concave ( Figs. 79C , 80C , 81E ).

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, spatuliform, distinctly longer than carpus, tip rounded to subtruncate; dactylus slender, inserted near or slightly proximal to ventral midlength of propodus, not or just reaching propodal apex; exopod relatively slender, about two-thirds length of ischiomerus, flagellum 2-segmented ( Figs. 74D , 83I–K , 84B–G ).

Chela not prominently elongate, dactylus about half to threequarters palm length; palm relatively slender, proximally narrower than distally; outer surface 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 denticles; tips of fingers sharp, hooked ( Figs. 79D , 80D ).

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 slender, relative lengths of meri P4>P3>P2>P5; left or right P4 merus slightly longer than other side, propodus and dactylus not clearly bilaterally asymmetrical; P2–P4 dactyli relatively long, subequal, weakly curved, longer than half propodus length, P4 dactylus similar in shape but slightly longer than P2 and P3 dactyli; P5 merus 4.5–4.6 times longer than wide; P5 dactylus longest, margins lined with short and long setae, denser on ventral margin, with 2 rows of distoflexor spinules, upper row usually with 5–8 spinules, lower row with 12–18 small spinules, increasing in size distad ( Figs. 81F , 83A–H ).

Pleon extending to buccal region, covering bases of P2–P5; telson gently recessed into concave distal margin of somite 6 ( Figs. 72B , 79B , 80B ).

Male: Carapace almost circular, slightly wider than long; dorsal surface almost smooth, not prominently inflated, lateral surfaces with setae; front projecting anteriorly, margin almost straight ( Figs. 72C , 77A, B , 85A, B , 86A ). Eyes distinctly visible in dorsal view ( Figs. 72C , 77A , 85A, B , 86A ). MXP3 as in female ( Fig. 86D ). 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 ( Figs. 77C , 85C , 86C ). Chela relatively stout, shorter than in female ( Figs. 85D , 86E ). 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; P5 dactylus falciform, long, covered with short setae ( Figs. 85A , 86F–I ). Male pleon relatively narrowly triangular, widest at somite 3, lateral margins of somite 4 gently concave; somite 6 trapezoidal; telson broadly trapezoidal, distal margin sinuous, much wider than proximal one ( Figs. 77C , 85C , 86J ). G1 very slender, proximal two-thirds straight, broadly curved at distal onethird, with sharply tapering tip ( Fig. 86K ). G2 short, with spatuliform tip; exopod as long as endopod ( Fig. 86L ).

Variation. The variation in carapace shape in this species is quite substantial, from transversely ovate with a low front (e.g., Fig. 82F ) to more subhexagonal with a rounded, more produced front (e.g., Fig. 82D ). The holotype of Pinnotheres latus has a relatively low truncate front (cf. Ahyong & Ng, 2007b: fig. 6A) as does another specimen from the Philippines (ZRC 2017.1017) ( Fig. 84A ), but in most specimens, the front is broadly convex (e.g., Fig. 82A, B ). The shape of the MXP3 ischiomerus varies to some degree, with the inner angle at the widest point usually angular and relatively well defined (e.g., Figs. 83J, K , 84A, B ), but in several specimens, it is more rounded (e.g., Fig. 84D, E ) with one appearing almost broadly curved ( Fig. 84F, G ). The slender MXP3 dactylus varies slightly in length, usually not underreaching ( Figs. 83I , 84B, E ), but occasionally reaching the apex ( Figs. 83J, K , 84C, D ). The number of spinules on the two rows on the ventral distal part of P5 dactylus is quite variable. In the neotype female, there are only six spinules on the outer row ( Fig. 83 D’) and 14 spinules on the inner row on the left side ( Fig. 83D ”), but on the right side, there are six spinules on the outer row ( Fig. 83 H’) and only seven very short, barely visible spinules on the inner row ( Fig. 83H ”). The spinules on the inner row of the right side seem abraded, very short, and barely visible ( Fig. 83H ”).

Colour. The colour of live females varies from pale brown, with numerous small, almost black flecks, to pale or bright purple ( Figs. 72A, B , 73 , 74 , 75A, B , 76 ), with the eyes dark brown to red and white ( Figs. 74C, D , 75B ). In life, males are light brown with pale ventral surfaces ( Fig. 77 ).

Host. Known with certainty from Pinna (Subitopinna) atropurpurea Sowerby, 1825 , or Pinna (Quantulopinna) muricata Linnaeus, 1758 ( Pinnidae Leach, 1819 ; Pinnoidea Leach, 1819). We have not found more than one specimen in any single Pinna collected. In Europe, another species, Nepinnotheres pinnotheres (Linnaeus, 1758) , lives in Pinna (see Becker & Türkay, 2010; Trigos & Vicente, 2018; Acarli et al., 2019).

Remarks. The authorship and year of publication of Pinnotheres globosum was discussed by Clark & Crosnier (2000) and Holthuis (2002); we follow their recommendations to recognise the taxon as being validly published from the early published plate, i.e., the taxon should be cited as Pinnotheres globosum Hombron & Jacquinot, 1846 . The text by Jacquinot & Lucas (1853) that actually described the species appeared later. Jacquinot & Lucas (1853: 59) apparently only had one 20 × 18-mm female, commenting that “Nous ne connaissons pas le mâle de ce Pinnotheres , qui a été pris dans une grosse Modiole à Singapour [We do not know the male of this Pinnotheres , who was caught in a big mussel in Singapore]” ( Jacquinot & Lucas, 1853: 60). The French name for the host, “Modiole”, is not a specific reference to Modiolus , but a vernacular reference to a mussel of some kind, which, at that time, would have included pinnids as well as mytilids. On the basis of the size and description of a “big mussel”, it is almost certain that they obtained their specimen from a large species of Pinna (even today often called horse mussels), and that the crab species is what is currently known as Arcotheres latus ( Bürger, 1895) (see Ng & Corbari, 2019). This is one of the two largest pinnotherids known from Singapore, with the largest known recent local specimen measuring 14.6 × 10.7 mm (ZRC 2017.1020), and the largest known specimen reaching 17.7 × 13.6 mm (from India). The carapace of this species is rather variable, with some almost circular in outline ( Fig. 82 ).

Hombron & Jacquinot’s (1846: pl. 5 fig. 21) figure of the species (present Fig. 78A–C ) is very schematic, showing a round carapace with all the ambulatory legs symmetrical and the dactyli short and equal in length. Similarly, his figure of the MXP3 is inaccurate, with the presence of a faint suture between the ischium and merus, and the exopod long, exceeding the length of the ischiomerus (Hombron & Jacquinot, 1846: pl. 5 fig. 22). No species of pinnotherid from Singapore and Peninsular Malaysia has this kind of carapace, ambulatory legs, and MXP3. In fact, the MXP3 structure (shape as well as separation of the ischium and merus, and length of the exopod) is not found in any pinnotherid. Henri Milne Edwards (1853: pl. 11 fig. 6) depicted the MXP3 of a specimen from Vavao in the Solomon Islands more accurately, showing a long ischiomerus and relatively short dactylus which does not reach the tip of the propodus ( Fig. 78D ), features that agree very well with what is currently identified as A. latus (cf. Figs. 83I–K , 84B–G ). Hombron & Jacquinot’s (1846) figures are inaccurate and cannot be relied on.

The types of Pinnotheres globosus Hombron & Jacquinot, 1846 , cannot be located in the MNHN and are almost certainly lost. In view of the complex and often confusing taxonomy of this species, uncertain identity, and the revised synonymy recognised here, a neotype should be selected. We here designate a recently collected female specimen (10.2 × 8.4 mm; ZRC 2016.164) from Singapore as the neotype of Pinnotheres globosus Hombron & Jacquinot, 1846 . This neotype is a recently collected, fully intact specimen from a known host and ethanol-preserved.

Comparisons of a large series of specimens of M. globosus , new combination, from many parts of the Indo-West Pacific, show that it is in fact a senior synonym of Pinnotheres latus Bürger, 1895 (type locality: Philippines), Pinnoteres ridgewayi Southwell, 1911 (type locality: Kondatchi Paar, Sri Lanka), and P. borradailei Nobili, 1906a (type locality: Lakshadweep, India). The lectotype of P. latus is relatively well preserved, but now relatively soft. Bürger (1895: pl. 9 fig. 16) figured the front as somewhat triangular, while Ahyong & Ng (2007b: fig. 6A) showed it to be truncate (see Variation). Ng et al. (2017: 1093) distinguished A. ridgewayi from A. latus by the proportionately longer MXP3 ischiomerus and differing proportions of the ambulatory dactyli, but the large series of specimens on hand now show that the differences are well within the range of variation for one species. Sankarankutty’s (1966: text-figs. 23, 28) figures of the distal half of the G1 and male pleon of a specimen from southern India (as P. ridgewayi ), although somewhat simple, show the characteristic slender G1 and trapezoidal male telson of M. globosus , new combination.

The taxonomy of Arcotheres borradailei ( Nobili, 1906a) (a replacement name for Pinnotheres tenuipes Borradaile, 1903 ) from the Maldives was revised by Ng et al. (2017: 1093), who separated it from A. ridgewayi in possessing denser setation on the occlusal surfaces of the fingers of the chela, more rounded inner angle of the MXP3 ischiomerus, and proportionately shorter and stouter P3 and P4. Following our assessment of M. globosus based on the extensive series examined here, it is evident that the supposed distinguishing features of A. borradailei and A. ridgewayi are well within the range of variation of M. globosus , so the two species are herein synonymised. The original host record by Borradaile (1903: 432) is uncertain, said to be a “? Mya ” ( Myidae Lamarck, 1809 ), but this is probably a misidentification as the family is primarily a temperate one.

Rathbun (1909) proposed the name, Pinnotheres alcocki , for specimens reported as P. parvulus Stimpson, 1858 , by De Man (1887, 1888) from Indonesia and the Mergui Archipelago, Bürger (1895) from the Philippines, and Alcock (1900) from an unspecified locality (presumably in Indian waters), rendering the “ P. parvulus ” material of all of these authors as syntypes. To fix the identity of the species, Ahyong & Ng (2007b) designated one of Bürger’s (1895) Philippine specimens as the neotype of Pinnotheres alcocki and transferred it to Arcotheres . Gordon (1936: fig. 5) reported and partially figured one of the Mergui specimens of De Man (1887) as Pinnotheres alcocki , but after reexamination of the specimen (NHM 1886.52, a paralectotype of P. alcocki ), we refer the specimen instead to M. globosus . Indeed, De Man (1887) had already noted the similarity of his material to M. globosus (as Pinnotheres ) based on the limited type description and figures despite ultimately referring it to P. parvulus . The Mergui specimen, an ovigerous female, is in poor condition, appearing to have been fixed in formalin given the “stiffness” of the appendages and cuticle texture. The carapace is collapsed and the frontal margins sunken in, but the walking legs are intact with the right P4 only slightly longer than the left. De Man (1887) reported the host as Pinna atropurpurea Sowerby, 1825 , and Pinna vexilllum Born, 1778 (now Atrina ), consistent with what we know of M. globosus . The present record constitutes the first confirmation of M. globosus from the Andaman Sea.

Females of several species of Arcotheres superficially resemble M. globosus in sharing similar MXP3 form and a similarly elongated P5 dactylus with two distoflexor rows of spinules on the dactylus: A. alcocki , A. similis , and A. placunicola . Although the differences in P4 symmetry and carapace shape are distinctive in intact, well-preserved specimens, incomplete, poorly preserved specimens, or specimens with regenerating pereopods can be more difficult to separate generically. In such a situation, the distally hooked P2 and P3 dactyli will separate A. alcocki and A. placunicola from M. globosus , which has broadly curved dactyli. Both A. similis and M. globosus share similarly broadly curved P2 and P3 dactyli, and are difficult to separate without reference to the P4 and carapace. In female M. globosus , however, the buccal margin of the epistome is transverse with a narrowly triangular median point ( Figs. 79C , 81E ), rather than very broadly angular in A. similis ( Figs. 28D , 30K ).

The Australian specimens of “ A. similis ” reported by Ahyong & Brown (2003) and Ward (1967), mostly from Pinna , were restudied and referred to A. globosus by Ahyong (2020b). Among these is a large female (AM P20161) with the host recorded as the mytilid Stavelia subdistorta (Récluz, 1852) ; the host data for this specimen may be incorrect given that M. globosus is otherwise not known from the Mytilidae .

Distribution. Western Pacific to central Indian Ocean, including Palau, Vanuatu, Australia, Philippines, Singapore, Myanmar, Sri Lanka, and India.

Figures

Fig. 72. Magnotheres globosus (Hombron & Jacquinot, 1846), colour in life. A, ovigerous female (13.0 × 10.0 mm) (ZRC 2018.782), Philippines; B, ovigerous female (9.0 × 6.8 mm) (ZRC 2018.781), Philippines; C, male (5.4 × 5.5 mm) (ZRC 2018.783), Philippines. Photographs: A, Chan Tin-Yam.

Fig. 73. Magnotheres globosus (Hombron & Jacquinot, 1846). A, B, colour in life and in situ in Pinna sp., ovigerous female (11.8 × 9.5 mm) (ZRC 2017.1017), Panglao, Philippines; C, female in situ in Pinna atropurpurea, Singapore, coll. 12 January 2017 (not collected). Photographs: A, Chan Tin-Yam; B, C, Tan Heok Hui.

Fig. 74. Magnotheres globosus (Hombron & Jacquinot, 1846), colour in life. A, non-ovigerous female (14.4 × 10.8 mm) (ZRC 2017.1018), Changi, Singapore; B–D, ovigerous female (13.2 × 9.9 mm) (ZRC 2017.1018), Changi, Singapore. A, B, overall dorsal view; C, D, frontal view of cephalothorax. Photographs: Tan Heok Hui.

Fig. 75. Magnotheres globosus (Hombron & Jacquinot, 1846), colour in life, non-ovigerous female (14.6 × 10.7 mm) (ZRC 2017.1020), with bopyrid parasite (Rhopalione sp.), Changi, Singapore. A, overall habitus; B, frontal view of cephalothorax showing asymmetrical pleon; C, female pleon with left side folded inwards; D, ventral surface of pleon showing folding of left side; E, ventral view of Rhopalione sp.; F, dorsal view of Rhopalione sp. Photographs: Lee Bee Yan.

Fig. 76. Magnotheres globosus (Hombron & Jacquinot, 1846), colour in life. A, ovigerous female (12.1 × 9.4 mm) (ZRC 2016.166), Vanuatu; B, ovigerous female (13.5 × 10.7 mm) (ZRC 2016.165), Vanuatu. Photographs: Chan Tin-Yam.

Fig. 77. Magnotheres globosus (Hombron & Jacquinot, 1846), male (8.0 × 7.8 mm) (ZRC 2020.12), Singapore. A, overall dorsal view; B, frontal view of cephalothorax; C, ventral view of cephalothorax. Photographs: Arthur Anker.

Fig. 78. Magnotheres globosus (Hombron & Jacquinot, 1846), type female (20.0 × 18.0 mm). A, overall dorsal view; B, D, left MXP3; C, outer view of right chela. A–C, after Hombron & Jacquinot (1846: pl. 5 figs. 21–23); D, after Milne Edwards (1853: pl. 11 fig. 6).

Fig. 79. Magnotheres globosus (Hombron & Jacquinot, 1846), neotype female (10.2 × 8.4 mm) (ZRC 2016.164), Changi, Singapore. A, overall dorsal view; B, ventral view of cephalothorax; C, frontal view of cephalothorax; D, outer view of left chela.

Fig. 80. Magnotheres globosus (Hombron & Jacquinot, 1846), ovigerous female (11.8 × 9.5 mm) (ZRC 2017.1017), Panglao, Philippines. A, overall dorsal view; B, ventral view of cephalothorax; C, frontal view of cephalothorax; D, outer view of left chela.

Fig. 81. Magnotheres globosus (Hombron & Jacquinot, 1846). A, ovigerous female (12.1 × 9.4 mm) (ZRC 2016.166), Vanuatu; B, E, F, ovigerous female (13.5 × 10.7 mm) (ZRC 2016.165), Vanuatu; C, ovigerous female (11.9 × 9.3 mm) (ZRC 2017.1019), Changi, Singapore; D, ovigerous female (13.6 × 10.5 mm) (ZRC 2017.1019), Changi, Singapore. A–D, overall dorsal view; E, frontal view of cephalothorax; F, left P2–P5.

Fig. 82. Magnotheres globosus (Hombron & Jacquinot, 1846), dorsal view of female carapace. A, neotype ovigerous female (10.2 × 8.4 mm) (ZRC 2016.164), Singapore; B, ovigerous female (13.2 × 9.9 mm) (ZRC 2017.1018), Changi, Singapore; C, ovigerous female (13.6 × 10.5 mm) (ZRC 2017.1019), Changi, Singapore; D, ovigerous female (11.9 × 9.3 mm) (ZRC 2017.1019), Changi, Singapore; E, ovigerous female (13.5 × 10.7 mm) (ZRC 2016.165), Vanuatu; F, ovigerous female (12.1 × 9.4 mm) (ZRC 2016.166), Vanuatu.

Fig. 83. Magnotheres globosus (Hombron & Jacquinot, 1846), neotype ovigerous female (10.2 × 8.4 mm) (ZRC 2016.164), Singapore. A–D, left P2–P5, respectively; D’, distal part of dactylus of left P5 showing stiff subdistal spinules (dorsal view, soft setae omitted); D”, distal part of dactylus of left P5 showing stiff subdistal spinules (ventral view, soft setae omitted); E–H, right P2–P5, respectively; H’, distal part of dactylus of right P5 showing stiff subdistal spinules (dorsal view, soft setae omitted); H”, distal part of dactylus of right P5 showing stiff subdistal spinules (ventral view, soft setae omitted); I, outer view of right MXP3 (basal part with breakage line, exopod, and setae omitted); J, left MXP3 (setae omitted); K, left MXP3 (slightly tilted, exopod and setae omitted). Scales = A–H, 2.0 mm; D’, D”, H’, H”, 0.2 mm; I–K, 1.0 mm.

Fig. 84. Magnotheres globosus (Hombron & Jacquinot, 1846). A, ovigerous female (carapace width 11.9 mm, carapace broken) (ZRC 2017.1017), Panglao, Philippines; B, ovigerous female (13.6 × 10.5 mm) (ZRC 2017.1019), Changi, Singapore; C, ovigerous female (11.8 × 9.5 mm) (ZRC 2017.1017), Panglao, Philippines; D, ovigerous female (11.0 × 8.9 mm) (ZRC 2017.1017), Panglao, Philippines; E, ovigerous female (16.6 × 12.3 mm) (ZRC 2016.185), Kerala, India; F, G, ovigerous female (14.7 × 11.2 mm) (ZRC 2016.184), Kerala, India. A, frontal margin of carapace; B, C, E–G, left MXP3 (setae denuded); D, right MXP3 (setae denuded). F, G, drawn at different angles. Scales = A, 1.0 mm; B–G, 0.5 mm.

Fig. 85. Magnotheres globosus (Hombron & Jacquinot, 1846), male (5.4 × 5.5 mm) (ZRC 2018.783), Philippines. A, overall dorsal view; B, frontal view of cephalothorax; C, ventral view of sternum and pleon; D, outer view of right chela.

Fig. 86. Magnotheres globosus (Hombron & Jacquinot, 1846). A–L, male (8.0 × 7.8 mm) (ZRC 2020.12), Singapore; M–R, female (13.0 × 10.0 mm) (ZRC 2018.782), Philippines. A, M, overall dorsal view; B, O, frontal view of cephalothorax; C, P, anterior thoracic sternum; D, R, right MXP3; E, Q, right chela; F–I, right P2–P5; J, abdomen; K, left G1, abdominal view; L, left G2, abdominal view; N, carapace front, oblique dorsal view. Scales = A, 4.0 mm; B–J, 2.0 mm; K, L, 1.0 mm; M, N, 5.0 mm; O–Q, 2.5 mm; R, 1.25 mm.

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. 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.