Macromedaeus adelus, Mendoza, 2021

Mendoza, Jose Christopher E., 2021, Marine crabs new to Singapore, with a description of a new species of intertidal xanthid crab of the genus Macromedaeus Ward, 1942 (Crustacea: Decapoda: Brachyura), Raffles Bulletin of Zoology 69, pp. 463-480 : 469-477

Macromedaeus adelus , new species

(Figs. 5, 6, 8, 9A–D)

Xantho distinguendus Ow Yang, 1963: 232 [not Cancer (Xantho) distinguendus De Haan, 1835 ].

Macromedaeus distinguendus Yang, 1979: 21 (list). — Wee & Ng, 1994: 84 (list) [not Cancer (Xantho) distinguendus De Haan, 1835 ].

Material examined. All material collected from Singapore. Holotype: male, 17.6 × 10.9 mm ( ZRC 2017.0465), Punggol End, coll. K. L. Yeo, 11 March 1998. Paratypes: 2 males, 14.7 × 9.4 mm, 15.3 × 9.4 mm, 1 female, 14.5 × 9.1 mm ( ZRC 2009.0061 ), same data as holotype ; 6 males, 7.6 × 5.1–15.0 × 9.3 mm, 3 females, 9.0 × 6.0–10.6 × 6.7 mm, 2 juv. 4.5 × 3.1 mm, 4.7 × 3.3 mm ( ZRC 1985.1328 – 1338 ), Punggol , rocks etc. circa LWST, coll. D. S. Johnson, 17 March 1965 ; 4 males, 10.0 × 6.6–14.6 × 9.0 mm, 1 female, 11.8 × 7.3 mm ( ZRC 2017.0450 ), Punggol Beach , coll. anonymous, 7 February 1966 ; 1 male, 10.8 × 7.2 mm, 1 female, 13.1 × 8.1 mm ( ZRC 2017.0500 ), Punggol , just below LWST, coll. Menen, February 1969 ; 1 male, 15.9 × 9.9 mm ( ZRC 2021.0760 ), Punggol Jetty , on rocky shore, coll. R. Tan, 12 January 2013 ; 22 males, 7.9 × 4.9–12.1 × 7.4 mm, 4 females (1 with sacculinid), 7.7 × 4.9–12.4 × 7.9 mm, 4 ovig. females, 8.4 × 5.3–10.0 × 6.3 mm ( ZRC 1965.11.9.54–63), Changi, coll. M. W. F. Tweedie, June 1934 ; 1 male, 11.6 × 7.4 mm ( ZRC 1985.1492 ), Sentosa Island , reef, coll. P. K. L. Ng, 28 April 1992 ; 1 male, 11.0 × 7.3 mm ( ZRC 2017.0449 ), Cyrene Reef , coll. S. K. Tan & M. Low, 24 July 2009 ; 3 males, 11.8 × 7.7–16.0 × 10.3 mm, 1 female, 14.4 × 9.0 mm, 3 ovig. females, 10.6 × 7.0–16.8 × 10.3 mm ( ZRC 2017.0466 ), West Coast Park , intertidal, coll. H. H. Tan et al., 17 October 2016 . Others : 3 males, 8.7 × 5.8–12.1 × 7.7 mm, 1 female, 9.6 × 6.3 mm ( ZRC 1965.11.9.64–66), Telok Kurau village , coll. anonymous, February 1934 ; 1 male, 9.1 × 5.9 mm ( ZRC 1984.7971 ), Tanjong Teritip , coll. anonymous, 21 October 1963 .

Comparative material.

Macromedaeus distinguendus (De Haan, 1835) : 2 males, 17.1 × 11.2 mm, 17.7 × 11.9 mm ( ZRC 1999.0066 ), Amakusa , Matsushima, Kyushu, Japan, coll. Y. Takao, 13 March 1975 ; 1 male, 31.0 × 19.9 mm, 1 female, 23.1 × 15.6 mm (MNHN-IU-2017-2049 = MNHN-B22340 ), Amakusa , Matsushima, Kyushu, Japan, coll. T. Yamaguchi, 4 April 1988 ; 1 male, 8.4 × 5.9 mm, 3 females, 12.3 × 8.6–15.0 × 10.3 mm ( ZRC 2021.0774 ), eulittoral, Shilaoren , Qingdao, Shandong Province, China, coll. M. Türkay, 16 April 1992 ; 3 males, 16.4 × 11.1– 19.4 × 13.3 mm, 1 female, 18.3 × 12.5 mm ( ZRC 2017.0487 ), rocky intertidal, 36°14′46.96″N, 126°32′00.12″E, Boryeong , Korea, coll. J. Park, 27 October 2014 .

Macromedaeus voeltzkowi ( Lenz, 1905) : 1 male, 9.3 × 6.5 mm, 1 female, 6.7 × 4.6 mm, 2 ovig. females, 7.4 × 5.0 mm, 9.2 × 6.1 mm (MNHN-IU-2017-2021 = MNHN-B6628 ), under rocks, Nosy Bé, Madagascar, coll. A. Crosnier, 20 May 1958 ; 5 males, 6.4 × 4.5–11.1 × 7.8 mm, 4 females, 6.1 × 4.4–9.0 × 6.3 mm (MNHN-IU-2017-2022 = MNHN-B8416 ), Fort Dauphin , Madagascar, coll. Mission R. Decary, May 1932 .

Diagnosis. Carapace transversely subovate, fan-shaped, relatively broad, CW/CL ratio 1.6; regions well defined; dorsal surface granulose, with vague transverse striations formed by rows of granules; anterolateral margin with 4 broadly triangular teeth after external orbital angle. Anterior margin of ambulatory carpi rounded and granulose, not cristate. G1 slender, tapering, distal quarter curving laterally; distal tip with long narrow lobe armed with recurved spiniform setae, inserted into larger apical lobe; long, spiniform subdistal setae straight or slightly bent, arranged in double rows longitudinally.

Description. Carapace (Fig. 5A, C) transversely subovate, fan-shaped in outline with anterolateral margins convex and posterolateral margins slightly concave, greatest width approximately 1.6 times length at midline [mean 1.59 (n = 22), range of 1.50–1.64]; dorsal surface distinctly granulate, rugose, with larger granules laterally and on transverse ridges surmounting carapace regions; dorsal surface convex transversely and longitudinally; regions well defined; 1F, 2F distinct; 1M faintly separated from 2F by shallow furrow, fused to inner branch of 2M, 2M partially divided longitudinally, 3M entire, 4M indistinct, fused to posterior part of 3M; 1L indistinct, 2L, 3L partially confluent, 4L low but discernible, 5L, 6L distinct; 1P prominent, 2P with small cluster of round granules on either side. Front slightly advanced beyond level of orbits, slightly deflexed ventrally, Fig. 5. Macromedaeus adelus , new species, holotype, male, 17.6 × 10.9 mm (ZRC 2017.0465), Punggol End, Singapore. A, habitus, dorsal view; B, eyes, antennae, and third maxillipeds, anteroventral view; C, cephalothorax, anterior view; D, male thoracic sternum and pleon, ventral view; E, male thoracic sternum and pleon, posteroventral view; F, right P4 and P5, dorsal view; G, right (minor) chela, external view; H, left (major) chela, external view. Scale bars: A, C = 5.0 mm; B, D–H = 2.0 mm.

bilobate, median notch forming moderately deep, narrow V in dorsal view; anterior margin of lobes finely granulate, slightly convex, both mesial, lateral angles slightly produced; separated laterally from preorbital tooth by deep notch, appearing as small fossa from anterior view. Anterolateral margin broadly convex, with 4 coarsely granulate, broadly triangular teeth besides external orbital angle; first and second teeth somewhat bicuspidate, anterior cusp lower, sometimes broader than posterior cusp which is often more projecting and pointed; third, fourth teeth generally more acutely tipped and projecting furthest laterally. Posterolateral margins slightly concave, strongly converging posteriorly, subequal in length to anterolateral. Posterior margin somewhat convex, raised from adjacent section of carapace, granulate. Subhepatic and pterygostomian regions evenly covered with round granules.

Orbits (Figs. 5B, C, 6A) subcircular; supraorbital margin granulate, concave in dorsal view, with two small fissures laterally, inner orbital angle small, triangular, overhanging small fossa; external orbital angle not prominent, separated from infraorbital margin by distinct fissure; infraorbital margin granulate, concave in ventral view, ending mesially in a round-tipped triangular tooth. Eyes filling orbits completely, corneas well developed, pigmented, eyestalks short, stout. Antennules folding transversely, slightly obliquely; free articles short, stout; antennular fossae wider than long, subovate. Basal article of antenna subrectangular, slender; flagellum much longer than width of orbit, freely entering orbital hiatus. Posterior region of epistome broad, central part of posterior border with ventrally directed triangular projection, lateral parts convex, projecting ventrally. Endostomial ridges limited to posterior of endostome only, not reaching epistome.

Lacinia of first maxilliped moderately broad distally. Third maxilliped (mxp3) (Figs. 5B, 6B) granulate, not completely closing anterior portion of buccal cavity; ischium longer than wide, with submesial, longitudinal furrow, mesial margin irregularly dentate with submarginal row of stiff setae; merus subquadrate, antero-external angle not visibly projecting, anterior margin oblique, slightly notched mesially, with distinct depression near mesial margin; carpus, propodus, dactylus short, subcylindrical, successively tapering distally; mxp3 exopod relatively slender, slightly tapering distally, with subdistal tooth projecting mesially, well-developed flagellum.

Male thoracic sternum (Figs. 5D, E, 6C, D) relatively broad, mostly smooth, granulate laterally; st1, st2 fused completely; st2, st3 separated by complete suture; deep transverse groove between st3, st4, suture 3/4 reduced to short fissures on either side of thoracic sternum; sutures 4/5, 5/6 interrupted inside sternopleonal cavity; sutures 6/7 and 7/8 complete, clearly demarcated. Median line present on anterior exposed portion of st4, interrupted in exposed posterior part, but reappearing within sterno-pleonal cavity on posterior portion of st4, absent on st5, st6, complete on st7, st8. Tubercle (press-button) of sterno-pleonal locking mechanism on posterior half of st5, just anterior to suture 5/6; mesial margin of st7 with slight overhang which locks with depression on latero-internal surface of pleomere 3 when pleon is closed. St8 completely hidden from view when pleon closed.

Chelipeds (P1) (Fig. 5A, G, H) unequal, major and minor chela similar except for width of palm; supero-external surfaces rugose, coarsely granulate. Merus short, distal tip just exceeding the anterolateral carapace margin from dorsal view. Carpus trapezoidal, with blunt, granulate tooth on internal angle. Palm coarsely granulate on external surface (with transverse rugae on upper half in the major chela), finely granulate on internal surface, with broad, shallow longitudinal groove along supero-external surface, most prominent proximally near articulation with carpus. Fingers darkly pigmented, shorter than palm; external surfaces with one distinct longitudinal groove each; tips rounded and slightly hollowed out (spoon-tipped), occluding directly, not crossing; pigmentation of fixed finger extending to the palm in the male.

Ambulatory legs (P2–P5) (Figs. 5A, F, 6F, G) short, stout, sparsely setose; P2, P3 longest, coxa-to-dactylus length slightly greater than carapace width, P5 shortest, coxa-to-dactylus length 0.80 times carapace width. Meri subrectangular, dorsal surface mostly smooth, anterior margin lined with round granules, distal tip separated from rest by distinct, transverse groove. Carpi trapezoidal, anterior margin lined with small round or conical granules, with submarginal crest on dorsal surface in P2–P4 but not in P5. Propodi subrectangular, margins and dorsal surface granulate, dorsal surface with submedian longitudinal groove. Dactyli thin, subcylindrical, covered with round to conical granules, distal tip capped by chitinous claw; proximal end with small tubercle forming dactylo-propodal lock with large tuberosity on distal end of propodus; flexor margin with 1 or 2 small calcareous teeth subdistally.

Pleon of male (Figs. 5D, E, 6E) mostly smooth, short; tip of telson posterior to imaginary transverse line between posterior margins of sternal condyles of P1 coxae; lateral margins generally concave, somewhat undulating. Somite 1 wide, short, granulose, median region much shorter (thinner) than lateral regions, anterior margin concave, exposed lateral margins straight. Somite 2 similarly wide, short, granulose, anterior margin concave, posterior margin convex, lateral margins convex. Somites 3–5 immovably fused, vestiges of sutures seen as shallow notches on concave lateral margins; lateral margin of somite 3 with concave surface forming pleonal lock with episternite of st7; sternite 4 lateral margins slightly concave; sternite 5 lateral margins slightly convex. Somite 6 broadly subquadrate, anterolateral angles produced, lateral margins concave. Telson subtriangular, apex rounded, lateral margins straight to slightly convex; median height about 0.7 times basal width.

G1 ( Fig. 9A–C ) moderately long; stout proximally, tapering distally, curving outward; distal tip narrowly spathe-like, with shorter, smaller, narrow, tongue-like lobe inserted within and armed with incurving spiniform setae which continue proximally as much longer, straight to slightly curved spiniform setae; rest of distal tip armed with scattered, Fig. 6. Macromedaeus adelus , new species, Punggol End , Singapore. A–G, holotype, male, 17.6 × 10.9 mm ( ZRC 2017.0465 ); H, I, paratype, female, 14.5 × 9.1 mm ( ZRC 2009.0061 ). A, left eye and antennae, anteroventral view; B, left third maxilliped, external view; C, male thoracic sternum with exposed sternopleonal cavity, ventral view; D, left press-button tubercle, ventral view; E, male pleon, external view; F, right P4 propodus and dactylus, dorsal view; G, right P5 propodus and dactylus, dorsal view; H, female pleon, external view; I, left vulva, ventral view. Scale bars: C, H = 2.0 mm; A, B, D–G, I = 1.0 mm .

short spiniform setae. G2 ( Fig. 9D ) about one-third length of G1, sigmoidally curved, terminal segment about one-fourth length of subterminal segment. Penis emerging from sternal condyle of P5 coxa.

Female morphology. The morphology of the female is similar to that of the male, except in the sexual characters (Fig. 6H, I). The thoracic sternum has a wide sternopleonal cavity, with the median line of sternite 4 fully included within; sutures 2/3, 6/7, and 7/8 are complete and clearly demarcated; sutures 4/5 and 5/6 are both interrupted at the median region; suture 6/7 has a centrally located, decalcified, subtriangular area. The vulvae are crescent-shaped, with the posterior border notched; and they are positioned on the anterior portion of sternite 6, quite close to sternite 5. The adult pleon is much wider and longer, the tip of telson just reaches the imaginary transverse line connecting the coxosternal condyles of the chelipeds (P1), and all pleonal somites are freely articulated.

Etymology. The specific epithet of the new species is derived from the Greek “adelos”, meaning “unseen, unknown, and obscure”, because the true identity of this crab remained concealed for many decades. Used as a Latin adjective.

Live colouration. The carapace and pereopods are variably mottled orange, yellow, cream, and purplish brown on the dorsal and external surfaces. The chelae are dull olive-green or light brown, with dark brown fingers; in males this dark brown pigmentation invades the distal part of the palm adjacent to the fixed finger ( Fig. 8 ). Ventral and internal surfaces are generally whitish, but sometimes with purplish brown spots and tinges of yellow or pink.

Ecological notes. Specimens of Macromedaeus adelus , new species, were collected from the middle to lower intertidal zone, beneath rocks in sandy to silty-muddy substrate. The recent material (ZRC 2017.0466) were collected together with a larger and more common xanthid species, Leptodius affinis (De Haan, 1835) .

Remarks. The genus Macromedaeus was established by Ward (1942) to accommodate two species: M. punctatus , a new species from Diego Garcia Atoll (Chagos Archipelago, Indian Ocean) that he concurrently described and designated as type species, and Xantho nudipes A. Milne-Edwards, 1867 , originally described from New Caledonia and the Seychelles. Guinot (1968: 708–710) considered M. punctatus Ward to be a junior subjective synonym of X. nudipes A. Milne-Edwards after examining a series of specimens from the Indian and Pacific oceans. She further defined the diagnostic features for Macromedaeus , commenting on the apparent morphological similarities of this genus to Leptodius A. Milne-Edwards, 1863 , but separating these two genera primarily on the basis of the morphology of the basal article of the antenna, antennular fossae, third maxillipeds, and G1. She also included five more species previously placed in the genus Xantho Leach, 1814 : viz. X. distinguendus De Haan, 1835 (type locality: Japan); X. quinquedentatus Krauss, 1843 (type locality: South Africa); X. crassimanus A. Milne-Edwards, 1867 (type locality: New Caledonia); X. voeltzkowii ( Lenz, 1905) (type locality: Zanzibar); and X. demani Odhner, 1925 (type locality: Ternate, Maluku Islands, fide De Man, 1902). Serène (1984: 175–181, figs. 101–104, pl. 25 figs. A–F) treated the genus as well, concurring with Guinot (1968) regarding the synonymy of X. nudipes and M. punctatus . He provided a key to distinguish all six included species, primarily using characters such as the number of teeth or lobes in the carapace anterolateral margin, the degree of lobulation and/or subdivision of the dorsal regions of the carapace, and the morphology of the G1. This classification has been followed by subsequent authors to this day, with the genus included in the subfamily Xanthinae MacLeay, 1838 ( Ng et al., 2008). Furthermore, there is support from molecular phylogenetic analyses of the Xanthidae ( Lai et al., 2011) for the close relationship between Macromedaeus and Leptodius .

Macromedaeus adelus , new species, clearly belongs to the group within Macromedaeus which typically has four well-defined teeth on the carapace anterolateral margin after the external orbital angle, and which includes M. distinguendus and M. voeltzkowii ( Serène, 1984) . Other species of Macromedaeus have 10 ( M. nudipes ) or five welldefined teeth ( M. quinquedentatus and M. crassimanus ), or four poorly defined lobes, with the posterior two sometimes manifesting as small teeth (i.e., M. demani ) ( Serène, 1984).

De Haan (1835) described Cancer (Xantho) distinguendus from several specimens collected from Japan, providing a short Latin description as well as an accurate figure of the dorsal habitus of one of the syntypes and the external surface of its right chela (viz. De Haan, 1835: 48, pl. 13 figs. 7, 7a). As mentioned above, this species was subsequently transferred by Guinot (1968) to its current genus. Serène (1984) further clarified that the distribution of M. distinguendus was limited to the Sino-Japanese region, and that reports of this species from the Indian Ocean (viz. Alcock, 1898) and Red Sea (viz. Heller, 1861; Nobili, 1906; Klunzinger, 1913) should be properly identified as Medaeops neglectus ( Balss, 1922) instead. Subsequent to its description by De Haan (1835), Macromedaeus distinguendus was reported from several localities in the Japanese archipelago ( Sakai, 1935, 1939, 1965, 1976; Yamaguchi et al., 1976; Suzuki, 1979; Miyake, 1983), mainland China and its adjacent seas ( Stimpson, 1858, 1907; Alcock, 1898; Gordon, 1931; Shen, 1932; Forest & Guinot, 1961; Dai et al., 1986; Dai & Yang, 1991), and the Korean Peninsula ( Kamita, 1941; Kim, 1973; Kim & Chang, 1985). This species has also been reported from Taiwan ( Sato, 1936; Lin, 1949; Ng et al., 2001, 2017; Tseng et al., 2018), Palau (Sendler, 1923), and Western Australia ( Jones & Berry, 2000). There is also a record from Tahiti which is probably erroneous and probably due to a misreading of the record of Forest & Guinot (1961), which deals mainly with the brachyuran fauna of Tahiti and the Tuamotu Archipelago, for a specimen from Hong Kong (see Poupin, 1996).

As De Haan did not indicate a holotype, all of the material he examined are technically syntypes ( ICZN, 1999: Article 73). To stabilise the taxonomy of Macromedaeus distinguendus , Yamaguchi & Baba (1993) designated a lectotype among the five extant syntype specimens deposited at the Rijksmuseum van Natuurlijke Historie (RMNH; presently the Naturalis Biodiversity Center), in Leiden, the Netherlands. For the purposes of comparison in this paper, the concept of Macromedaeus distinguendus has been based on the following: the photographs of the lectotype (RMNH. CRUS.D.44657) and paralectotypes (RMNH.CRUS.D.42331; 42332) published in Yamaguchi & Baba (1993: fig. 163a, b; note that the lectotype is the specimen on the lower right of fig. 163b); the brief description and figure of one of the syntypes by De Haan (1835: 48, pl. 13 figs. 7, 7a); and the description of one of the syntypes (CW: 23 mm) by Buitendijk (1960: 330). From these, it can be gleaned that the key diagnostic features for this species are the subhexagonal carapace with a quadridentate anterolateral margin, the narrow frontal median fissure, the rugose and granular dorsal surface of the carapace and supero-external surfaces of the chelipeds, and the upper margin of the ambulatory carpus, described by Buitendijk (1960: 331) as “anteriorly with two blunt granular tubercles: the posterior with a granular ridge.” On this basis, it can be confidently said that the comparative material used in the present study (from Kyushu, Japan; Qingdao, China; and Boryeong, Korea) are all identical with M. distinguendus . Furthermore, the records from mainland Japan, China, and Korea, based on the available figures (cf. Sakai, 1939: pl. 58 fig. 4, pl. 91 fig. 4; 1976: fig. 221, pl. 153 fig. 2; Miyake, 1983: pl. 40 fig. 5; Shen, 1932: figs. 56, 58a, b, pl. 2 fig. 5; Dai et al., 1986: fig. 151-1, pl. 36 fig. 5; Dai & Yang, 1991: fig. 151-1, pl. 36 fig. 5; Kim, 1973: fig. 143A–C; respectively), are considered to be M. distinguendus primarily on the basis of the visible features of the carapace, chelipeds, and ambulatory carpi. It has to be stressed here, however, that while most of the illustrations of the G1 from these publications are not of sufficient magnification or detail to be useful for interspecific discrimination for the present work, the figure provided by Forest & Guinot (1961: fig. 46) is the most informative.

Macromedaeus adelus , new species, was previously reported from Singapore (Telok Kurau, Changi) by Ow Yang (1963) as “ Xantho distinguendus ”, and it has been listed by Yang (1979) and Wee & Ng (1994) in their respective catalogues. In fact, all the old ZRC material collected from Singapore in the 1930s to the 1970s were labelled “ Macromedaeus distinguendus ” or some earlier synonym. It is apparent, therefore, that M. adelus is most similar to M. distinguendus in the general form of the carapace, particularly in having four teeth on the anterolateral margin after the external orbital angle, and in the morphology of the G1. It can be distinguished from M. distinguendus , however, by the following characters: 1) the carapace is relatively wider, CW/CL mean of 1.59 (n = 22), range of 1.5–1.64 (Figs. 5A, 8) (vs. relatively narrower in M. distinguendus, CW /CL mean of 1.48 (n = 12), range of 1.42–1.56; Fig. 7A , cf. De Haan, 1835: pl. 8 fig. 7; Yamaguchi & Baba, 1993: fig. 163b lower right); 2) the median incision of the front is widely V-shaped (Figs. 5A, 8) (vs. median frontal incision is a thin slit in M. distinguendus ; Fig. 7A , cf. De Haan, 1835: pl. 8 fig. 7; Yamaguchi & Baba, 1993: fig. 163b lower right); 3) the chelae, especially the major chela, have longer fingers and a narrower palm (Fig. 5G, H) (vs. chelae with shorter fingers and wider (higher) palm in M. distinguendus ; Fig. 7G, H , cf. De Haan, 1835: pl. 8 fig. 7a); 4) the anterior margins of ambulatory carpi are rounded, granulate, and without large granular tubercles or cristiform lobes (Fig. 5A, F) (vs. anterior margins of ambulatory carpi of P2–P4 with granular tubercles and cristiform lobes in M. distinguendus ; Fig. 7A, F , cf. De Haan, 1835: pl. 8 fig. 7; Shen, 1932: fig. 56; Buitendijk, 1960: 331; Kim, 1973: fig. 143C; Miyake, 1983: pl. 40 fig. 5); and 5) the distal tip of the G1 is relatively more elongate and slender, and most of the subterminal spiniform setae on the mesial margin are either straight or slightly bent ( Fig. 9A–C ) (vs. distal tip relatively shorter and stouter; most of the subterminal spiniform setae are recurved in M. distinguendus ; Fig. 9E–G , cf. Shen, 1932: fig. 58b; Forest & Guinot, 1961: 58, fig. 46; Kim, 1973: fig. 143B; Dai & Yang, 1991: fig. 151-1).

There are indications of morphological variation in M. distinguendus as well. One of the paralectotypes (cf. Yamaguchi & Baba, 1993: fig. 163b lower left) appears to have a wide V-shaped median cleft on the frontal margin (instead of a narrow median fissure) and no obvious tubercles or crests on the anterior margin of the ambulatory carpi, although it is unclear from the photograph if these are artifacts of preservation or specimen damage. Buitendijk (1960) pointed out some differences in Gordon’s (1931) Chinese material (from Hong Kong and Pei-tai-ho), particularly Gordon’s observation of the lack of granulation in the external surfaces of the major chela and ambulatory legs. Gordon’s illustrations of the male pleon and G1 of her “ Xantho distinguendus ” (cf. Gordon, 1931: figs. 21, 22c) are also quite similar to the new species presently being described, especially in the subterminal spiniform setae of the G1 being erect instead of recurved. Specimens of “ Macromedaeus distinguendus ” reported by Tseng et al. (2018: figs. 3, 4B) from northeastern Taiwan do not have the characteristic granulose appearance of the typical form. Also, the frontal margin has a wide V-shaped median cleft instead of a narrow median fissure, and the anterior margin of the ambulatory carpi is relatively smooth and entire rather than armed with granular tubercles or lobes, which are features seen in the new species presently being described. The micrograph of the G1, while seemingly similar to the G1 illustrated by Forest & Guinot (1961), does not provide the discriminatory information required. Other available records from Taiwan (viz. Ho et al., 2000; Ng et al., 2001, 2017) do not provide figures for comparison. The aforementioned morphological differences suggest that one of the paralectotypes and the illustrated material of Gordon (1931) and Tseng et al. (2018) could be conspecific with M. adelus , new species, and that there may be an overlap in the range of M. distinguendus sensu stricto and M. adelus , new species, particularly in the southern coast of China and in Taiwan. As such, all these specimens will have to be re-checked carefully to ascertain this. Likewise, the records from Palau (Sendler, 1923) and Western Australia ( Jones & Berry, 2000) also lack figures and could not be checked independently at present. Their identities will have to be confirmed.

Macromedaeus adelus , new species, is also similar to M. voeltzkowii ( Lenz, 1905) primarily in having four teeth on the carapace anterolateral margin after the external orbital angle ( Lenz, 1905; Monod, 1938; Barnard, 1950; Serène, 1984; Ghotbedin & Naderloo, 2014; Naderloo, 2017). Lenz (1905: 353, pl. 47 figs. 6, 6a) described Leptodius voeltzkowii based on three male specimens collected from Zanzibar, off the eastern coast of Africa. He provided a brief description primarily focused on the chelar morphology, and figures showing the external and dorsal surfaces of the left chela. This species was subsequently reported from the Red Sea ( Monod, 1938), South Africa ( Barnard, 1950), Madagascar ( Serène, 1984), Somalia ( Galil & Vannini, 1990), the Gulf of Oman ( Naderloo, 2017), and the Persian Gulf (Ghotbedin & Naderloo, 2014; Naderloo, 2017). Interestingly, there is scant mention of Lenz’s type material, beyond mention of their probable location, “Lübeck Museum”, by Odhner (1925), in much of the literature on M. voeltzkowii . The types have not been further illustrated beyond Lenz’s figures of the left chela, nor is there evidence of them being examined further by other workers. Monod’s (1938) material from the Red Sea ( Egypt) for the most part agrees with the general description of the species, except for the form of the G1. The G1 of the Red Sea specimen was illustrated with an additional pronounced subdistal lobe ( Monod, 1938: fig. 16), which is otherwise absent in other specimens of M. voeltzkowii , particularly those from South Africa or Madagascar which are close to the type locality (cf. Barnard, 1950: fig. 42h; Serène, 1984: fig. 102). For the purposes of comparison, therefore, the form from Madagascar is used as the exemplar for this species for now. The comparative material from Madagascar used in the present study fully agrees with Lenz’s description, in particular, the presence of a double row of large, granulate tubercles on the upper surface of the chelar palm. The type locality, Zanzibar, is also relatively close enough to Madagascar, such that it is plausible that the two populations are conspecific.

Macromedaeus adelus can be distinguished from M. voeltzkowii by the following characters: 1) the carapace is proportionately wider, CW/CL mean of 1.59 (n = 22), range of 1.50–1.64 (Figs. 5A, 8) (vs. narrower, CW/CL mean of 1.45 (n = 13), range of 1.39–1.51 in M. voeltzkowii ; cf. Monod, 1938: fig. 16; Serène, 1984: pl. 25 fig. F); 2) the dorsal carapace region 6L is entire (Fig. 5A) (vs. 6L divided by transverse furrow in M. voeltzkowii ; cf. Serène, 1984: 177, pl. 25 fig. F); 3) the dorsal surface of the carapace is relatively less rugose (Fig. 5A) (vs. more rugose, due to distinct groups of granules on the areolae and subregions in M. voeltzkowii ; cf. Serène, 1984: pl. 25 fig. F); 4) the supero-external margin of the chelar palm is granular to rugose but does not have prominent granular tubercles (Fig. 5G, H) (vs. supero-external margin of chelar palm with four large, granular tubercles; cf. Lenz, 1905: pl. 47 figs. 6, 6a); 5) the ambulatory meri are relatively longer, L/W ratio: 2.1 (Fig. 5A, F) (vs. relatively shorter, L/W: 1.9, in M. voeltzkowii ); 6) the anterior margin of the ambulatory carpi is lined with small granules, with no distinct pattern (Fig. 5A, F) (vs. anterior margin of ambulatory carpi lined with row of large, conical granules in M. voeltzkowii ); and 7) the distal tip of the G1 is relatively more slender and less curved, with numerous, long spiniform setae, slightly curved and recurved, forming a double row ( Fig. 9A–C ) (vs. distal tip stouter and with a more pronounced curvature, but fewer spiniform setae, forming a single row in M. voeltzkowii ; cf. Serène, 1984: fig. 102; Naderloo, 2017: fig. 21.20f).

Figures

Fig. 9. Gonopods. A–D, Macromedaeus adelus, new species, holotype, male, 17.6 × 10.9 mm (ZRC 2017.0465), Punggol End, Singapore; E–G, Macromedaeus distinguendus (De Haan, 1835), male, 17.7 × 11.9 mm (ZRC 1999.0066), Amakusa Matsushima, Japan. A, E, left G1, sternal view; B, F, distal tip of left G1, sternal view; C, G, distal tip of left G1, pleonal view; D, left G2, pleonal view. Scale bars: A–G = 0.5 mm.

Fig. 8. Live colouration. Macromedaeus adelus, new species (ZRC 2017.0466), West Coast Park, Singapore. A, paratype, male, 16.0 × 10.3 mm; B, paratype, male, 11.8 × 7.7 mm. Scale bar: A, B = 5.0 mm.

Fig. 7. Macromedaeus distinguendus (De Haan, 1835), male, 17.7 × 11.9 mm (ZRC 1999.0066), Amakusa Matsushima, Japan. A, habitus, dorsal view; B, eyes, antennae, and third maxillipeds, anteroventral view; C, cephalothorax, anterior view; D, male thoracic sternum and pleon, ventral view; E, male thoracic sternum and pleon, posteroventral view; F, right P4 and P5, dorsal view; G, left (minor) chela, external view; H, right (major) chela, external view. Scale bars: A = 5.0 mm; B–H = 2.0 mm.