Metaplax tredecim Tweedie, 1950

Shih, Hsi-Te, Hsu, Jhih-Wei, Wong, Kingsley J. H. & Ng, Ngan Kee, 2019, Review of the mudflat varunid crab genus Metaplax (Crustacea, Brachyura, Varunidae) from East Asia and northern Vietnam, ZooKeys 877, pp. 1-29 : 1

Metaplax tredecim Tweedie, 1950 Figures 2 J–L , 6 , 7 M–P

Metaplax tredecim Tweedie, 1950: 354, fig. 6 (type locality: Labuan, Malaysia); Choy and Booth 1994: 243 (Brunei); Davie and Nguyen 2003: 383, fig. 1 d–e (Malaysia: Labuan; Brunei); Ng et al. 2008: 226 (list); Yang et al. 2008: 803 (list; East China and South China seas).

Metaplax longipes : Dai et al. 1986: 508, fig. 288 (1-2), pl. 72(3) (China: Zhejiang, Fujian, Guangdong) (part); Dai and Yang 1991: 556, fig. 288 (1-2), pl. 72(3) (China: Zhejiang, Fujian, Guangdong) (part); Chertoprud et al. 2012: 276, pl. 47F (Vietnam: Nha Phu, Nha Trang, Khanh Hoa) (not M. longipes Stimpson, 1858).

Materials examined.

Paratypes: 2 ♂♂ (15.6-16.2 mm), 1 ♀ (15.7 mm) (ZRC 1964.7.14.4-18), Labuan, Malaysia, coll. G. Nunong, Aug. 1938. Others. Hong Kong: 1 ♂ (14.2 mm) (NCHUZOOL 15468), Starfish Bay, coll. P.-C. Tsai and H. Y. Cheung, 19 July 2015; 3 ♂♂ (16.4-19.4 mm) (NCHUZOOL 15546), Starfish Bay, coll. K. J. H. Wong, 4 June 2019; 1 ♂ (16.1 mm) (NCHUZOOL 15469), Tai Tan, coll. K. J. H. Wong, 15 July 2015; 1 ♂ (18.1 mm) (NCHUZOOL 15470), Ting Kok, coll. K. J. H. Wong, 22 Aug. 2017; 2 ♂♂ (12.8-16.2 mm), 1 ♀ (15.6 mm) (NCHUZOOL 15471), Kei Ling Ha, coll. K. J. H. Wong, 31 Aug. 2011; 3 ♂♂ (16.9-21.0 mm) (NCHUZOOL 15472), Nai Chung, coll. K. J. H. Wong, 23 June 2015; 2 ♂♂ (16.5-21.5 mm) (NCHUZOOL 15473), Luk Keng, coll. C. W. Lau, 22 May 2016; 1 ♂ (18.2 mm) (NCHUZOOL 15705), Mak Pin, Sai Kung, coll. K. J. H. Wong, 7 July 2019. China: 4 ♂♂ (18.0-22.7 mm), 1 ♀ (10.7 mm) (NCHUZOOL 15474), Dongzhai Harbor, Hainan, 23 June 2004. Vietnam: Quang Ninh: 1 ♂ (21.8 mm), 2 ♀♀ (15.7-23.4 mm) (NCHUZOOL 15476), Dong Rui, 29 May 2016; 1 ♂ (22.4 mm) (NCHUZOOL 15477), Dong Rui, coll. H.-T. Shih and P.-Y. Hsu, 9 Oct. 2017; 1 ♀ (10.3 mm) (NCHUZOOL 15478), Dong Rui, coll. H.-T. Shih and P.-Y. Hsu, 9 Oct. 2017; Khanh Hoa: 4 ♂♂ (14.4-18.7 mm), 5 ♀♀ (12.6-16.1 mm) (NCHUZOOL 15498), Nha Trang, coll. I.-H. Chen and K. J. H. Wong, 24 Nov. 2010. Malaysia: Labuan, 1 ♂ (15.4 mm), 1 ♀ (16.2 mm) (NCHUZOOL 15475), coll. H.-T. Shih, 23 July 2010; 1 ♂ (17.1 mm) (NCHUZOOL 15497), coll. H.-T. Shih, 27 July 2010.

Diagnosis.

Carapace ( Figs 2J , 6A, F, G ) subquadrate, 1.35 times broader than long (N = 39, SD = 0.03), regions defined by shallow grooves, slightly inflated, surface pitted; front nearly straight, medially slightly concave; lateral margin mildly convex, posteriorly converging, cut into five teeth, anterior two pronounced, quadrate, posterior two inconspicuous; posterolateral facet slightly depressed, behind second notch decorated with two short oblique granular ridges. Infraorbital ridge ( Figs 2K , 6C ) markedly sexually dimorphic: in males medial four or five roughly same size, decreasing in breadth laterally, laterally of a row of seven or eight small, rounded, isomorphic, tubercles; females with 21-27 small isomorphic tubercles. Chelipeds ( Figs 2L , 6D ) stout, symmetrical, palm 2.3 times longer than broad, length of palm approximately 1.6 times of length of dactyl (N = 12), meri slightly dilated anteriorly, lined with minute denticles along the margin; chela surface finely granulated, along cutting edges both fingers unarmed. Ambulatory legs slender, elongated, meri of P3 and P4 tomentum-covered on distal half, and propodi of P2 to P4 with thick mat of setae. G1 ( Fig. 7 M–P ) elongated, slender, almost straight.

Distribution.

Southeast and East Asia: northern Borneo (Labuan, Malaysia; Brunei), Vietnam (Quang Ninh; Khanh Hoa), and South China (Hong Kong) ( Fig. 1 ).

Habitat.

In Hong Kong, in comparison to M. longipes , M. tredecim tends to occur in habitats of coarser, grittier substrates, with less freshwater input, and frequently on open sandflats rather unsheltered by mangroves.

Remarks.

Identification of the Metaplax tredecim had been confusing, particularly based on the number of teeth along the lateral margin of the carapace. Tweedie (1950: fig. 6) showed merely three conspicuous lobes, the posterior one occupying more than half of carapace length. However, as noted by Davie and Nguyen (2003), members of the genus often have the structures around the posterolateral facet obscured by setae-trapped sediments, and not visible unless carefully denuded. Reexamination of a paratype male (16.2 mm; ZRC 1964.7.14.4-18), after denudation, showed the lateral margin to be interrupted by 4 notches (hence 5 teeth) ( Figs 2J , 6A, E ), the posterior two being inconspicuously defined by the last notch.

As mentioned above, two forms, differing in the number of tubercles or lobes on the infraorbital ridge, were recognized in specimens identified with M. longipes by Dai et al. (1986) and Dai and Yang (1991). We confirmed that specimens from the study area, characterized by the possession of about 17 tubercles or lobes closely represent M. tredecim . There is little doubt that the material studied by Dai et al. (1986) and Dai and Yang (1991) included two species, M. longipes and M. tredecim .

Chertoprud et al. (2012) recorded " M. longipes " from Nha Phu, southeastern Vietnam. However, given the infraorbital ridge with 17 tubercles and the chelipeds with the length of palm/the length of dactyl ratio about 1.5 (estimated from their plate 47F on page 295), this record is suspected to represent M. tredecim instead (see Table 2 ).

Figures

Figure 1. Collection sites (solid symbols) for species of the genus Metaplax used in this study: green rhombus for M. elegans; red circles for M. longipes; blue triangles for M. sheni; and purple squares for M. tredecim. Empty symbols mean the additional records from references (see synonym lists for each species).

Figure 2. Metaplax elegans De Man, 1888 (A-C NCHUZOOL 15496, male, 12.7 mm), M. longipes Stimpson, 1858 (D-F ZRC 2019.0581, male, 14.9 mm), M. sheni Gordon, 1930 (G-I NCHUZOOL 15466, male, 9.9 mm), and M. tredecim Tweedie, 1950 (J-L paratype, ZRC 1964.7.14.4 - 18, 16.2 mm). A, D, G, J, A carapace B, E, H, K left infraorbital ridge C, F, I, L outer view of right cheliped. Scale bars: 1.0 mm.

Figure 6. Metaplax tredecim Tweedie, 1950. A, F Dorsal view B ventral view C left infraorbital ridge D right cheliped E right side of carapace showing the five teeth of lateral margin. A-E paratype male (CW 16.2 mm; ZRC 1964.7.14.4 - 18; Labuan) F male (CW 16.5 mm; NCHUZOOL 15473; Hong Kong) G male (CW 15.4 mm; NCHUZOOL 15475; Labuan, Malaysia) H, I male (18.2 mm; NCHUZOOL 15705; Hong Kong) G-I color in life. Scale bars: 2.0 mm.

Figure 7. Metaplax elegans De Man, 1888 (A-D NCHUZOOL 15496, male, 12.7 mm), right G 1; M. longipes Stimpson, 1858 (E-H ZRC 2019.0581, male, 14.9 mm), right G 1; M. sheni Gordon, 1930 (I-L NCHUZOOL 15466, male, 9.9 mm), right G 1; and M. tredecim Tweedie, 1950 (M-P paratype, ZRC 1964.7.14.4 - 18, 16.2 mm), left G 1. Scale bars: 0.5 mm (A-H, M-P); 0.2 mm (I-L).