Opecarcinus ngankeeae Wong, Meij & Chan, 2024, sp. nov.

Opecarcinus ngankeeae sp. nov.

(Figs. 3, 6–8)

? Troglocarcinus (Troglocarcinus) crescentus.— Fize & Serène, 1957: 62 [part], fig. 10. [Not Cryptochirus crescentus Edmondson, 1925]

Opecarcinus SET 6.— Xu et al. 2022: fig. 2.

Type material. Holotype: female (2.2 × 2.8 mm; ASIZCR-000463), on Pavona varians, Gongguanbi, Green island, Taiwan, 28 Mar 2021, coll. K. Wong. Paratypes: 1 female (2.3 × 3.2 mm; ASIZCR-000464), data same as holotype; 2 males (1.7 × 2.1 mm, 2.1 × 2.7 mm), 9 females (2.4 × 2.9 mm to 3.4 × 3.9 mm) (ASIZCR-000465), on P. decussata, off Zhaori Hot Spring, Green Island, Taiwan, 27 Aug 2022, coll. K. Wong, M.-C. Chang.

Description of holotype female (ASIZCR-000463). Carapace approximately 1.3 times longer than broad, longitudinally ovate in outline, posterior half rounded, laterally inflated, broadest at posterior 2/3 of CL, surface moderately sculptured, anterior half covered with numerous rounded and conical tubercles; anterior 1/3 of carapace moderately deflected at about 40°, strongly depressed; front broadly concave between both IOA, each armed with 1 distinct acute spine, lined with about 10 stout conical tubercles, median one likewise acute; protogastric region strongly sunken as a broad invert-V shaped depression, medially lined with 2 prominent acute spines, laterally extending to hepatic regions, mesogastric region slightly inflated, armed with 2 medial and 1 lateral spines above the protogastric depression, gastric and cardiac regions separated by a shallow but distinct H-shaped groove, posteriorly furnished with small, well-separated small granules; ALA triangular, distinctly lower than, and anteriorly as protruding as front, externally lined with small conical tubercles, devoid of prominent spines, posterior of ALA lined with a small patch of conical spines on hepatic region along the anterolateral margin; ALA laterally confluent with posterolateral margin, entire, unarmed, convex (Fig. 6A). Pterygostomial region mildly granular, completely fused dorsally with carapace, devoid of suture (Fig. 6C, D).

Surface of basal plate of antennular peduncle granular, dorsally depressed, elongated triangular, mesial margin bearing distinct basal lobe, mesial-distal angle acute (Fig. 6B). Eyestalk short and stout, dorsally lined with numerous small conical spines, cornea terminal (Fig. 6B). Buccal frame broad, quadrate, not completely covered by both MXP3, leaving a medial void as broad as the external maxilliped. Epistome medially of a rounded elevation, surface longitudinally crested, lateral of which each lined with well-defined longitudinal crest (Fig. 6D). MXP3 depressed, mildly granular, ischium mesial-distal lobe semi-circular, mesial margin convex; merus mesially lined with one plumo-denticulate setae on internal surface (Fig. 6G). MXL1 mesially of 6 elongated and 4 shorter simple setae along mesial margin, 2 short simple setae on basal margin, and 1 elongated setae on external surface (Fig. 6H).

Chelipeds symmetrical; merus longitudinally ovate, granular along dorsal and ventral margins; carpus triangular in lateral view, externally armed with numerous conical spines, distally armed with one prominent spine; palm dorsally with approximately 9 conical tubercles, externally depressed, proximally of flattened granules, internally inflated and proximally elevated, granular patch extending to about 2/3 of palm height; fingers shorter than palm, moderately deflexed, much compressed, distally criss-crossing, dactylus proximally armed with 2 small conical tubercles (Fig. 7A). P2 to P5 armed from coxae to propodi, on coxae to ischium ventrally of stout tubercles, on meri to propodi also of conical tubercles along dorsal (external) surfaces; each dactylus articulated dorso-ventrally, tapering into a fine tip (Fig. 7B–J). P2 merus subquadrate, 1.5 times as long as broad, dorsally armed with numerous conical tubercles, increasing in size laterally, distal-ventral angle distinctly produced anteriorly, strongly tuberculate, posterior surface lined with short prominent denticulate crest subparallel to distal-ventral margin; carpus likewise stout, distally produced as a tuberculate rim overhanging articulation with propodus; propodus externally armed with small conical tubercles; dactylus unarmed (Fig. 7B–D). P3 merus trapezoidal, 1.2 times as long as broad, strongly spinose, distally lined with longitudinal depression on posterior surface; carpus distally rimmed but less prominent than P2; dactylus armed proximally by 2 indistinct rounded tubercles (Fig. 7E, F). P4 merus subquadrate, 1.2 times as long as broad; merus to propodus along extensor margin lined with numerous small conical tubercles; dactylus unarmed (Fig. 7G, H). P5 merus subtriangular, 1.4 times as long as broad; merus and carpus covered with small conical tubercles; propodus 1.4 times as long as broad, surfaces smooth; dactylus unarmed (Fig. 7I, J).

Thoracic sternum broad, anterior plate subquadrate, somites 3 and 4 fused, approximately 1.5 times as broad as long, anteriorly rimmed, transversely lined with a band of five stout tubercles, tips rounded, laterally each armed with two close-set tubercles, medial of the fused somite sparsely furnished with approximately 12 small rounded granules, of which four slightly larger; lateral margins of somites 4 and 5 (opposite to respectively P1 and P2 coxae) lined with rounded tubercles; somites 4 to 7 medially depressed; somite 6 bearing obliquely ovate gonopore, laterally sheltered by an anteriorly convex hood, somite 7 lined with distinct median line; sutures 4/5, 5/6 and 7/8 medially interrupted, suture 6/7 nearly confluent, progressing into a mesial rhomboid depression (Fig. 6E). PLP1 to PLP3 uniramous (Fig. 7K–M).

Note on variation in ornamentation of female thoracic sternites. Among the nine intact female individuals examined (CW 2.2 to 3.4 mm), ornamentation on the anterior plate of thoracic sternum shows some variation (Fig. 6E, I–L). The transverse row is comprised of around 10, but less than 12, stout tubercles with rounded tips; transverse row might be medially confluent or briefly interrupted. Immediately below this transverse row numerous small rounded granules are often present, up to 10, and at least two to four might be more easily observed, all sparsely-spaced from each other. Thoracic sternite 5 is laterally lined with rounded granules along the margins, and also on the lateral part of the sternite (Fig. 6I–L).

Description of paratype male, 1.7 × 2.1 mm (ASIZCR-000465). Carapace ornamentation similar but less elaborate than in females; front concave, lined with numerous denticles, median spine wanting; IOA each armed with prominent spine; protogastric depression laterally extending to hepatic region; posterior half sparsely covered with low granules (Fig. 8A).

P1 more robust than in females, merus short, prismatic, dorsally spinose; carpus short, rhomboid, distally likewise armed with one prominent spine; chela externally depressed, dorsal half of low conical tubercles, internally inflated dorsal to medial surface granular (Fig. 8B); fingers shorter than palm, internally excavated, mildly deflexed, dactylus granular on proximal 1/4 (Fig. 8C). P2 merus 1.8 times as long as broad, subquadrate, dorsally strongly spinose, distal-anterior margin armed with conical tubercles; carpus and propodus likewise externally granular, granules on the anterior surface more prominent (Fig. 8D, E). P5 merus longitudinally ovate, 1.5 times as long as broad, dorsally spinose, ventrally armed with spinules on distal angle; carpus to dactylus surfaces smooth, unarmed (Fig. 8F, G).

Thoracic sternum similar to that of female (Fig. 8H). Pleon overall outline longitudinally ovate, lateral margins of telson to somite 3 confluent, arched; telson twice as broad as long (Fig. 8I). G1 slender, dorso-ventrally compressed, tapering into a fine, rounded tip, curved gently laterally, mesial margin rimmed on distal 1/3, lateral margin lined with several fine setae along lateral margin (Fig. 8J, K). G2 likewise strongly compressed, about 1/3 length of G1, inserted at base, tip slightly notched medially (Fig. 8L).

Live coloration (females). Anterior depression dark brown, posterior of which lined on each side two oblique chestnut brown streaks of varying length and intensity on an off-whitish background, giving a gradient of variation between distinct brown streaks on a whitish carapace (Fig. 3E–J, L), to overall light brown, oblique stripe on branchial, and patch on cardiac and intestine regions off-white (Fig. 3K) and anteriorly tinted with rusty brown with white markings not apparent (Fig. 3M); ambulatory leg segments each with one pale brown band (Fig. 3).

Recorded hosts and form of domiciles. Xu et al. (2022) reported that O. sp. SET6 inhabits plate-forming Leptoseris and Pavona species. Our material from Green Island, Taiwan was collected from Pavona varians (Verrill, 1864) and P. decussata (Dana, 1846). Both host species have massive, submassive or encrusting forms (Fig. 2A, B). Domiciles openings crescent-shaped as typical of the genus, depths less than 1 cm, perpendicular or oblique to the surface, and markedly dorso-ventrally flattened as of the general physique of the occupant (Fig. 2C, D).

Etymology. We take this opportunity to honor the late Dr. Ngan-Kee Ng, for her remarkable contributions to brachyuran taxonomy, particularly grapsoid crabs.

Distribution. So far known from the Coral Triangle to Japan: Lembeh and Ternate in Indonesia, Semporna and Kudat in Malaysia and Okinawa, Japan (Xu et al. 2022); Green Island, Taiwan (this study), and possibly Nha Trang, Vietnam (Fize & Serène 1957; see below).

Placement in the genus Opecarcinus. Opecarcinus ngankeeae sp. nov. was collected from Pavona corals, and recorded from Leptoseris and Pavona by Xu et al. (2022), which is in agreement with the coral family Agariciidae as an exclusive host for cryptochirids of the genus Opecarcinus (Kropp 1989). Morphological features as depicted above, especially the elaborate distal-mesial expansion on P2 merus, pterygostomial region completely fused with the vase-shaped carapace, and tuberculate anterior plate of thoracic sternum, warrant its placement in the genus Opecarcinus as previously defined (Kropp & Manning 1987; Kropp 1989, 1990).

Comparison with congenerics. Xu et al. (2022) provided genetic data for all described Opecarcinus species, except for O. granulatus (likely O. SET16), and O. aurantius. After further examination of the material putative species O. SET10 likely represents O. aurantius (Xu et al. in prep). Both species do not cluster close to O. ngankeeae sp. nov. (Fig. 5, Xu et al. 2022: fig. 2). Based on genetic data, Opecarcinus ngankeeae sp. nov. is closest to O. SET5 from Xu et al. (2022). Opecarcinus SET 5 predominantly inhabits Leptoseris, but has also been recorded from Pavona explanulata and Pavona spp. corals. There are marked differences in colour pattern of the two species (Xu et al. 2022: fig. 2); further morphological comparisons will be included in a revision of the genus (Xu et al. unpublished).

Based on morphology, Opecarcinus ngankeeae sp. nov. shares some resemblance with described congenerics, particularly with O. crescentus, O. granulatus and O. peliops. The geographic distribution of these species includes the West Pacific (see Kropp 1989), and thus overlaps with the distribution range of O. ngankeeae sp. nov. The anterior plate of the female thoracic sternum of these species are all lined with fewer than ten tubercles, that are all well-spaced from each other. Opecarcinus peliops can be distinguished from O. ngankeeae sp. nov. by the anteriorly notched P2 merus, and its strict association with Pavona duerdeni (Kropp, 1989; Xu et al. unpublished). Below we discuss the affinities of O. ngankeeae sp. nov. with O. crescentus and O. granulatus.

The species Cryptochirus crescentus was described by Edmondson (1925) based on material from Johnston Island. This species was subsequently reported largely from Pacific waters by Edmondson (1933: Christmas Island (now Kiribati), Pacific Ocean), Hiro (1938: Hawaii), Utinomi (1944: Central Pacific), Fize & Serène (1957: Nha Trang, Vietnam), Serène (1962: Johnston Island), Amerson & Shelton (1976: Johnston Atoll), Takeda & Tamura (1981 1983: Yaeyama, Wakayama and Sagami Bay, Japan), Kropp (1989, 1990, 1994: across Indo-Pacific from Phuket (Thailand), Vietnam, Moluccas (Indonesia), Central Pacific to Pacific coast of Mexico), Ng & Davie (2002: Phuket, Thailand), Paulay et al. (2003: Mariana Ils), and Wei et al. (2006: Orchid Island (Lanyu), Taiwan). As shown by Xu et al. (2022), Opecarcinus exhibits substantial unrevealed diversity. For most of these records, whether each record represented the "real" O. crescentus remains unverified. In Taiwan, O. crescentus has so far only been reported from Orchid Island by Wei et al. (2006). As for OTUs supported by genetic data, two specimens from Green Island matched O. SET16 by Xu et al. (2022) from Ternate, Indonesia. This specimen likely represents O. granulatus (Xu et al. unpublished). We have not yet assessed or examined the material of O. SET16 from Taiwan in detail.

Compared with illustrations of the female holotype provided by Kropp (1989), O. crescentus resembles O. ngankeeae sp. nov. by the overall shape of carapace, and granulation on anterior plate of female thoracic sternum (Kropp 1989: fig. 1b, d), but can distinguished by the following discernible characters in females: (1) front of O. ngankeeae is medially armed with one slender spine (vs. devoid of prominent median tubercles; Kropp (1989: fig. 1a); (2) protogastric depression deeply excavated, medially aligned with two slender spines (vs. depression moderate, sparsely lined with small conical tubercles; Kropp (1989: fig. 1a); (3) female thoracic sternite anterior plate laterally armed with two closely set tubercles, and posterior of the granular row scattered with about 12 small low rounded tubercles (vs. anterior plate laterally of single rounded tubercle, and unornamented posterior to the granular row; Kropp (1989: fig. 1d); (4) MXP3 merus internally lined with single plumo-denticulate seta (vs. 2; Kropp (1989: fig. 1j); (5) MXL1 mesially lined with 5 medium-length simple setae (vs. 5 short setae), one elongated setae on external surface (vs. 1 short seta), and one short seta along basal (posterior) margin (vs. absent; Kropp (1989: fig. 1k); (6) longer chela palm of 1.7 times as long as high (vs. 1.2 times; Kropp (1989: fig. 1f), and (7) P5 merus distally arched along dorsal margin (vs. nearly straight, produced into an acute angle; Kropp (1989: fig. 1h).

As for male crabs, both species differ by (1) G1 morphology, that of O. ngankeeae sp. nov. is comparatively slender, tapering into a fine tip, oriented anterolaterally at about 40°, and slightly constricted along the mesial margin at the medial 1.3 (vs. relative stout, distally rounded, oriented anterolaterally at about 30°; Kropp (1989: fig. 2e); and (2) P2 merus transversely quadrate (vs. subovate; Kropp (1989: fig. 2g).

Among past records enumerated above, apart from topographic (and type) material, specimens from Vietnam and Japan were illustrated by, respectively, Fize & Serène (1957) and Takeda & Tamura (1981) in considerable detail. In line drawings of the overall female habitus, provided by Fize & Serène (1957: fig. 10) the individual was clearly shown to be ornamented with multiple distinctly acute spines among conical tubercles, concave front, protogastric depression medially armed with acute spines, as well as an distally arched P5 merus. This matches O. ngankeeae sp. nov. instead of O. crescentus, indicating their material from Nha Trang might at least partially include the herein described new species. In comparison, the illustration by Takeda & Tamura (1981: fig. 1A) showing the overall habitus of O. crescentus depicted a carapace ornamented by conical tubercles and lower granules, devoid of any more slender or acute, H-shaped groove not indicated, and P5 merus dorsally nearly straight and distal angle slightly produced, do not correspond to the herein defined species O. ngankeeae sp. nov. The SEM image of the carapace of a female identified as O. crescentus from Taiwan, presented by Wei et al. (2006: fig. 2G), shows a concave front, lined with numerous small conical tubercles but none being prominent, protogastric depression rather shallow, also probably do not represent O. ngankeeae sp. nov. Genetic evidence shows material of both O. SET6 and O. SET16 being present in Green Island, Taiwan, indicating at least two Opecarcinus species are found at nearby reefs.

Cryptochirus granulatus Shen, 1936, established based on material from Christmas Island (Indian Ocean), has been poorly understood since its description, and was considered a junior synonym of, as then, Pseudocryptochirus crescentus (Edmondson, 1925) by Utinomi (1944). After examining the male holotype deposited at the Natural History Museum, London, Kropp (1989) resurrected the species, and identified material he amassed in Guam as Opecarcinus granulatus. So far this species had been reported only from these two localities. Considering the form of male G1, that of O. ngankeeae sp. nov. bears striking resemblances with O. granulatus (Shen, 1936), both markedly along the constricted medial third along the mesial margin (Fig. 8J, K and Kropp (1989: fig. 4e), and similarities are also observed in the overall form of carapace and chela. However, in terms of overall morphology, male crabs of O. ngankeeae sp. nov. and O. granulatus can be distinguished by: (1) the carapace of O. ngankeeae sp. nov. is covered with small, low granules along lateral and posterior portions of the posterior half (vs. "smoother", bearing only small, sparsely-spaced small granules (Kropp 1989: fig. 4a); (2) IOA each armed with one prominent spine (vs. devoid of prominent spines; Kropp 1989: fig. 4a); (3) P1 carpus distally armed with prominent spine (vs. not armed); (4) P2 propodus internally smooth (vs. entirely armed with conical tubercles); and (5) P5 carpus extensor margin entire (vs. granulated). Females of O. granulatus from Guam, identified by Kropp (1989), also differ from that of O. ngankeeae sp. nov. by: (1) carapace with anterior 1/3 well distinguished from the latter (vs. not markedly differentiated in O. ngankeeae sp. nov.); (2) thoracic sternum anterior plate lined with single tubercle posterior of granular row (vs. scattered with low granules); (3) P2 merus elongated, 1.7 times as long as broad (vs. 1.5 times); (4) P5 merus subquadrate (vs. P5 merus subtriangular, distally arched along dorsal margin); and (5) MXL devoid of seta on external surface (vs. lined with single elongated seta).

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The coral host of the holotype male of O. granulatus was not recorded by Shen (1936). Kropp (1989) collected his Guam material from Gardineroseris planulata, Leptoseris sp. and Pavona explanulata. For O. ngankeeae sp. nov., Xu et al. 's (2022) recorded hosts include Leptoseris and Pavona species, whereas our material from Taiwan were retrieved from Pavona varians and P. decussata. In this consideration, there is a potential overlap in host records of O. granulatus and O. ngankeeae sp. nov.

As indicated by Kropp (1989), live coloration can be distinctive among various species of Opecarcinus. Opecarcinus granulatus’ colour pattern was described as: "(c)arapace opaque, covered with many small black chromatophores posteriorly, with much larger black chromatophores anteriorly, latter overlain with white, giving a gray hue" and "(m)eri of P-2 to P-5 with fine orange line network. Distal merus, carpus and propodus of P-2 and P-3 grayish with scattered bright blue spots" (Kropp 1989: 106). No references to any apparent oblique brownish bands on branchial regions were made (as in Fig. 3A–D).

Female thoracic sternum in species of Opecarcinus. The form of female thoracic sternum is one of the diagnostic characters that separate species of Opecarcinus (see Kropp 1989). That of O. ngankeeae sp. nov. is distinctive in the numerous sparsely-spaced small conical spines posterior to the transverse granular row on the anterior plate (Fig. 6E). This feature is shared only with O. aurantius, O. sierra and O. cathyae (respectively Kropp (1989: figs. 5d, 13d), and Van der Meij (2014: fig. 5B). Between females, both O. aurantius and O. cathyae can be distinguished by longer ambulatory meri (P2 and P5, respectively 1.7 and 1.9 times and 1.8 and 2.0 times as long as high, in comparison with 1.5 and 1.5 times in O. ngankeeae sp. nov.). Opecarcinus cathyae also has markedly shorter cheliped palms (approximately 1.3 vs. 1.7 times as long as high; Van der Meij (2014: fig. 2A), and O. sierra lacks a prominent distal spine on P1 carpus (Kropp 1989: fig. 13F). Opecarcinus aurantius and O. sierra can be further differentiated from O. ngankeeae sp. nov. by the mesial setation of MXL1: both lacking seta on the external surface (vs. single elongated seta), as well as the number of anterior and posterior simple setae.

Morphological comparisons of all described Indo-Pacific species, based on Kropp (1989) and Van der Meij (2014; for O. cathyae), are provided in Table 2.