Ocypode mortoni George, 1982

Wong, Kingsley J. H., Shih, Hsi-Te & Chan, Benny K. K., 2012, The ghost crab Ocypode mortoni George, 1982 (Crustacea: Decapoda: Ocypodidae): redescription, distribution at its type locality, and the phylogeny of East Asian Ocypode species, Zootaxa 3550, pp. 71-87 : 75-82

Ocypode mortoni George, 1982

( Figs 3–6 )

Ocypode mortoni George, 1982: 187 , figs. 1C, 2C, pls. 3. ― Dai & Yang 1991: 457, fig. 203B. ― K. Sakai 2000: 1159, figs. 2a–b, 3a–e.

Ocypode macrocera (not O. macrocera H. Milne Edwards, 1837 ) ― Dai et al. 1985: 370, figs. 1–7, pls. 1–2.

Material examined. 63 2ƤƤ (CEL-Ocy-HK-003), Sai Wan, Hong Kong, 20 Jun. 2009, coll. K. Wong et al.; 33 1Ƥ (CEL-Ocy-HK-021), Bui O, Hong Kong, 21 Jun. 2009, coll. H.-N. Chen et al.; 1Ƥ (CEL-Ocy-HK-017), Fan Lau, Hong Kong, 23 Jun. 2009, coll. K. Wong; 131Ƥ (ZRC-2012.0141), Sai Wan, Hong Kong, 20 Jun. 2009, coll. K. Wong et al.; 13 (NMNS-6879-001), Sai Wan, Hong Kong, 20 Jun. 2009, coll. K. Wong et al.

Diagnosis. Carapace subquadrate, regions defined by grooves; cornea with ocular stylets on the distal end, never longer than cornea, even in adult males ( Figs. 3 a, 5a–b). Outer surface of major cheliped palm often lemonyellow in color ( Fig. 5 b); stridulating ridge on inner surface of major cheliped consist of 44–65 evenly-spaced striation ( Fig. 3 c, i); base of immovable finger broad, compressed; finger tips of minor cheliped rounded ( Fig. 3 b–c).

Description. Carapace subquadrate, broader than long, longitudinally convex, anterior 1/3 deflected, raised around branchial regions; surface evenly covered by beaded tubercles; regions markedly divided by grooves, those around cardiac region H-shaped, deeper; antero-lateral teeth absent; external orbital tooth forming right angle; anterior 1/3 of lateral margins roughly parallel, diverging posteriorly ( Figs. 3 a, 6a). Cornea on elongated stalks, bilobed, distal end extended with ocular stylets about half of cornea length in mature males ( Figs. 3 a, 6a). Small, acute spine adjacent to antennule, suborbital ridge composed of 30–35 elongated granules; pterygostomian region sparsely covered by flattened granules ( Fig. 6 b). External maxillipeds flat, ischium longer than merus, both roughly quadrate ( Fig. 6 b), inner margins lined with long, light-colored stiff setae; exopod narrow, ill-developed, reaching 1/4 of merus length.

Chelipeds asymmetrical; major merus triangular in latitudinal cross-section; lower fringe of inner surface leaflike, margin strongly serrated; carpus as long as broad, inner angle armed with strong tooth, outer-distal margin serrated; outer surface of palm covered with beaded tubercles of varies sizes, margins strongly serrated ( Fig. 3 b); inner surface with latitudinal stridulating ridge composed of 44–65 evenly-spaced striations ( Fig. 3 c, i); immovable finger triangular, high at base, which compressed and leaf-like, ridge of rounded tubercles lined subparallel to cutting edge, movable finger lined with ridge of large rounded tubercles; fingers shorter than palm, tips pointed ( Fig. 3 b–c, i, 5b). Minor cheliped carpus slightly longer than broad, inner angle armed with strong tooth, fingers lined with ridge of rounded granules along respective median lines, tips rounded ( Figs. 3 d, 5b). Ambulatory legs slender, 2nd and 3rd subequal in total length, longest, 4th shortest; meri compressed, robust, distal end of anterior margin weakly armed with tooth ( Fig. 6 a). Propodi and dactyli of the first three pairs lined with rolls of soft plumose setae along median line on anterior surface; dactyli pointed, curved inwards for the first three pairs and outwards for the 4th ( Fig. 6 a).

Urocardiac ossicle constricted beneath the T-shaped bifurcation ( Fig. 3 h), and mildly expanded on the posterior half; zygocardiac ossicle bearing 16 comb-like teeth ( Fig. 3 g).

Ventral surface of carapace glabrous except for tuft of soft setae between bases of 2nd and 3rd ambulatory legs. Male abdomen elongated; telson rounded, proximal margin convex; 6th somite as long as broad, lateral margins convex; 5th somite broader than long, roughly rectangular ( Fig. 3 e). Male G1 slightly sinuous when viewed ventrally, not apparently tapering; distal part strongly curved outwards, tip divided into two chitinous beak-like lobes, upper lobe larger; neck of the distal curve fringed brating short setae ( Fig. 4 a–b, d). Female abdomen broad, telson as semicircle, narrow, proximal margin convex, 6th somite broader than long, lateral margins convex; preceding somites much broader than long, roughly rectangular ( Fig. 3 f).

Coloration. Dorsal surface of carapace dark brownish with light-colored thin sinuate lines ( Fig. 5 a); pterygostomian region and external maxillipeds creamish with microscopic dark dots; orbits grayish, ocular stylets yellowish ( Fig. 5 b); ambulatory legs creamish with random faint blotches; outer surface of major cheliped distinctly yellow ( Fig. 5 b; might still be observable after alcohol preservation). Juveniles displaying a more conspicuous pattern with dark, sinuous markings on a pale yellowing background, on both dorsal surface of carapace and ambulatory leg meri ( Fig. 5 c). Cheliped palm also yellowish, with whitish fingers.

Distribution. Hong Kong (type locality), Guangdong, Guangxi, Hainan Island ( Dai et al. 1985; Dai & Yang 1991), and the Pacific coast of Shikoku, Japan ( K. Sakai 2000). See also “Remarks” below.

Remarks. As suggested in previous works, O. mortoni morphologically resembles O. fabricii and O. nobilii ( George 1982) , which all develop short ocular stylets above the cornea. Ocypode mortoni can be distinguished from the others based on the number of striations along the stridulating ridge on the inner surface of the major cheliped palm: up to 54 for O. mortoni , 120 for O. nobilii and 108–141 in O. fabricii (see George & Knott 1965; George 1982). In our specimens, there were 44–65 striations on the stridulating ridges which conforms to O. mortoni (see Fig. 3 c, i). G1 morphology of our specimens, with two chitinous beak-like structures at the distal tip, also matches that of the original description (see Fig. 4 ).

Shen (1940) recorded O. ceratophthalmus , O. cordimanus and O. stimpsoni in his review of the brachyura of Hong Kong, based on his own collections, materials from other institutes and records in the earlier literature ( Stimpson 1858, 1907; Balss 1922; Gee 1926). The presence of O. ceratophthalmus has long been validated (see Morton & Morton 1983), however the records of O. cordimanus has been shown to represent O. sinensis ( Huang et al. 1998) . The last species O. stimpsoni , appears to be a temperate species ( Dai et al. 1985), and reported from Japan ( T. Sakai 1939), Korea ( Kim 1973), North China ( Shen 1932) and Taiwan ( Huang et al. 1992). Records from the coasts of South China (e.g. Shen 1936, 1940), thus, represents the southern limits for O. stimpsoni (see also Dai et al. 1986; Dai 1991; Dai & Yang 1991). However, studies on the genus from China ( Dai et al. 1985) recognized material from Hong Kong as O. mortoni (then as O. macrocera ) instead of O. stimpsoni .

Our molecular phylogenetic analyses ( Fig. 9 ), show O. stimpsoni and O. mortoni to be closely related, which is not surprising as they are very similar morphologically except for the presence of ocular stylets in O. mortoni adults. It is very difficult to separate juveniles or younger individuals of the two species. As for the Hong Kong records of “ O. stimpsoni ” by Shen (1940), all four sites where he collected material (Shatin, Ngau Chi Wan, Repulse Bay and Stanley) have been altered or urbanized over the last 70 years, and no further records have been found since then (see also George 1982; Morton & Morton 1983). Extensive searches performed in this study also failed to acquire any material of this species. While it is tempting to regard the earlier records of O. stimpsoni as probable misidentifications, the past presence and subsequent local extinction is nevertheless still possible. As Hong Kong is within the range of the known southern distribution of O. stimpsoni we tentatively continue to include it as known from there, though with some hesitation.

For records in the East Asian region, aside from southern China (including Hong Kong), Ocypode mortoni has only been recorded on the Pacific coast of Shikoku, Japan (see K. Sakai 2000). The illustrations of the Japanese material agrees with our specimens, in terms of the number of striae along the stridulatory ridge (Shikoku specimens: 60 in male and 52 in female; Hong Kong specimens: 44–65), and in the form of the zygocardiac ossicle ( Fig. 3 h). However, the distal tip of the G 1 in material from Hong Kong, when viewed ventrally, ends with a pronounced chitinous bi-lobed, pointed, beak-like structure, with the upper lobe larger than the lower; and fine setae on the “neck” of the curvature beneath (see Fig. 4 a–b, d). Both characters are somewhat wanting in Sakai’s (2000) illustrations (fig. 3d in K. Sakai 2000). The importance of these slight variations is uncertain, and we would opt to consider both the Hong Kong and Japanese records conspecific, at least for now. While O. mortoni could be expected to occur in Taiwan and adjacent areas, no specimens have been collected despite numerous efforts to find it, and none are present in museum collections. The biogeography of the species remains unclear, and needs further investigation based on more extensive targeted collecting.

Distribution pattern of Ocypode spp. in Hong Kong. From the seven sandy shores surveyed, Ocypode ceratophthalmus was collected in all sites while O. mortoni was only limited at Sai Wan, Tong Fuk and Bui O ( Fig. 1 ). Ocypode mortoni was found at the same tidal levels as O. ceratophthalmus (2.0– 2.3 m above C.D.) but not on higher shores, where the zone is solely dominated by O. sinensis .

Vertical distribution pattern at Sai Wan and relative abundance of O. mortoni and O. ceratophthalmus . At Sai Wan, along the transect close to the stream (T1), average density of O. ceratophthalmus reached 2.2 individuals m -2, while for O. mortoni only occurred at 0.4 individuals m -2. On the transect further beyond the stream (T2), O ceratophthalmus maintained an average density of 1.8 individuals m -2, and O. mortoni at 0.8 individuals m - 2 ( Fig. 7 ). In Fan Lau and Bui O, the relative abundance of O. ceratophthalmus to O. mortoni was 96:4%, and 83:17% respectively ( Fig. 7 ).

Molecular analysis. From 28 individuals of Ocypode spp., 24 haplotypes (see Table 1) of COI sequences, each of 636 basepairs were obtained. The alignment are AT-rich (in average A: 27.6%, T: 34.3%, G: 17.0%, C: 21.2%), within which 156 sites are variable, and 130 of these are parsimoniously informative.

Pairwise nucleotide divergences (K2P distance) and the mean number of differences of the COI sequences between five East Asian Ocypode species are displayed in Table 2. Apart from O. cordimanus which involved a single haplotype, the remaining 4 species show a certain degree of intraspecific variation. The percentage of bp differences and K2P can be as much as 1.57% (10/636) and 1.60% (for O. mortoni ), respectively.

A phylogenetic ( BI) tree of COI of East Asian Ocypode species is shown in Figure 9 , with the additional support values of ML and MP analyses beside the nodes. Results show all the studied Ocypode species formed well-supported, distinct clades at the species level. Ocypode mortoni and O. stimpsoni are genetically the closest related (average K2P distance: 6.25%, see Table 2). Of the 24 haplotype analysed, the East Asian Ocypode can be divided into two main clades with robust clade support: one contains the species with a stridulating ridge on the inner surface of major cheliped, i.e. O. mortoni , O. stimpsoni and O. ceratophthalmus ; while the other clade contains species that lack ridges on the inner side of the chelae: O. cordimanus and O. sinensis . Within the first clade, O. ceratophthalmus is sister to the O. mortoni and O. stimpsoni subclade.

Figures

FIGURE 1. Map of Hong Kong showing sandy shore study sites. Squares indicate the sites visited by George (1982) and circles indicate the sites visited in the present study. Closed circle / square indicates the presence of O. mortoni. Empty circle / square indicated absence of O. mortoni.

FIGURE 3. Morphology of O. mortoni: a, dorsal surface of carapace; b, outer surface of major cheliped palm; c; inner surface of major cheliped palm, showing stridulating ridge; d, outer surface of minor cheliped; e, male abdomen; f, female abdomen; g, zygocardiac ossicle; h, urocardiac ossicle; i, inner surface of major cheliped palm of the type specimen, redrawn from George (1982).

FIGURE 4. Morphology of male first pleopod (G 1) of O. mortoni: a, ventral view of left G 1; b, tip of left G 1, ventral view; c, dorsal view of right G 1; d, G 1 of type specimen, redrawn from George (1982).

FIGURE 5. Life coloration of O. mortoni: a, dorsal view, adult; b, dorsal view, juvenile. c, frontal view, adult. Scale bars represent 1 cm.

FIGURE 6. Ocypode mortoni: ♂ from Sai Wan, carapace 24.5 mm x 21.2 mm (CEL-Ocy-HK- 003): a, dorsal view; b, frontal view.

FIGURE 7. Density of O. mortoni and O. ceratophthalmus along T 1 and T 2 in Sai Wan (refer to Y-axis on left hand side); and the total number of crabs collected in the transects at Fan Lau and Bui O along the 30 meter-transects (refer to the Y axis on the right hand side).

FIGURE 9. Bayesian Inference (BI) tree showing phylogenetic relationships of East Asian Ocypode spp., based on COI gene. Additional support values of BI, maximum likelihood (ML) maximum parsimony (MP) analyses printed at the nodes. For details of haplotypes see Table 1, with species and localities behind. Black bars represent “ ridge-bearing ” species, and gray bars represent “ ridge-less ” species (see Discussion).