Metaplax crenulata (Gerstaecker, 1856)

Metaplax crenulata (Gerstaecker, 1856) View in CoL

( Figures 1 View FIGURE 1 , 2 View FIGURE 2 )

Rhaconotus crenulatus Gerstaecker, 1856: 140 View in CoL , pl. 5, fig. 5 [ type locality: unknown (see Remarks and Discussion)].

Metaplax crenulatus View in CoL – De Man 1888: 156 [Mergui Archipelago]. – De Man, 1889: 439 [Bay of Bengal].

Metaplax crenulata View in CoL – Alcock 1900: 436 [Sunderbunds and Mergui]. – Tesch 1918: 116 (key). – Balss 1922: 153 [ Penang, Malaysia]. – Tweedie 1936: 69 [ Selangor, Malaysia]. – Tan & Ng 1994: 82 [western Malaysia and Singapore]. – Ghosh 1995: 235 [ West Bengal, India]. – Ghosh 1998: 336 [ West Bengal, India]. – Ng & Davie 2002: 379 [ Phuket and western Thailand]. – Chakraborty et al. 2005: 154 (list). – Ng et al. 2008: 226 (list). – Dev Roy & Nandi 2008: 199, tab. 1 [ West Bengal, India]. – Dev Roy & Nandi 2012: 216, tab. 1 [Andaman Islands]. – Trivedi et al. 2018: 75, tab. 1 (list). – Pati et al. 2018: 45, tab. 15 (list). – von Rintelen & Wolff 2025: 12 unnumbered figs.

Metaplax crenulaa [sic] – Ghosh 1995: 231.

Metaplax cernulata [ sic] – Naiyanetr 2007: 112 [ Thailand ( Ranong and Phuket) and Andaman Sea] .

Type material examined. Lectotype (herein designated): 1 male (33.0 × 26.0 mm), ( ZMB 604 View Materials ), no collection data . – Paralectotypes (herein designated): 2 females (23.1 × 19.6, 23.0 × 19.0 mm) ( ZMB 604 View Materials ), no collection data (photo only; from von Rintelen & Wolff 2025) .

Other material examined. 1 male (18.8 × 15.1 mm) (NCHUZOOL 17350), Chalong Bay , Phuket, Thailand, 29 May 2012 ; 1 male (18.3 × 15.0 mm), 1 female (16.1 × 13.2 mm) (NCHUZOOL 17351), Chalong Bay , Phuket, Thailand, 29 May 2012 ; 1 female (18.7 × 15.3 mm) ( ZRC 2002.0327 View Materials ), Tang Khen Bay , Phuket, Thailand, coll. J.C.Y. Lai, 3 September 2001 ; 1 female (27.4 × 22.4 mm) ( ZRC 2019.0591 View Materials ), Selangor, Kuala Langat , Malaysia, coll. Z. Jaafar ; 4 males (14.6 × 12.0, 11.6 × 9.2, 9.8 × 8.3 mm, 1 specimen broken), 2 females (21.3 × 17.2, 14.1 × 11.0 mm) ( ZRC), Semantar , Selangor, Malaysia, coll. J.C.E. Mendoza, 10 February 2009 ; 1 male (36.6 × 28.3 mm), 1 ovigerous female (24.3 × 19.6 mm) ( ZRC 2013.1199 View Materials ), Nibong Tebal , Malaysia, coll. Z. Jaafar, 22 December 2001 ; 2 females (33.1 × 25.0, 32.6 × 25.2 mm) ( ZRC 1989.2081 View Materials - 2082 View Materials ), Mandai mangrove swamp, Singapore, coll. D.H. Murphy, 1985 ; 1 male (25.7 × 19.5 mm), 1 female (30.7 × 24.1 mm) ( ZRC 2009.0828 View Materials ), Pandan mangrove, Singapore, coll. Z. Jaafar, 2 October 2008 ; 2 males (43.7 × 33.0 mm, 34.1 × 27.0 mm), 1 female (23.2 × 18.7 mm) ( ZRC 1997.696 View Materials ), Tanjong Kling , Singapore, coll. I. Polunin, 12 July 1965 .

Diagnosis. Carapace rectangular, convex; surface almost glabrous, region well-defined ( Figs. 1A, F, G View FIGURE 1 , 2A–D View FIGURE 2 ). Front narrow, medially slightly concave ( Figs. 1A View FIGURE 1 , 2A View FIGURE 2 ). Lateral margin nearly parallel, with 5 teeth (including orbital tooth), anterior 3 teeth pronounced, posterior 2 relatively indistinct ( Figs. 1A View FIGURE 1 , 2A View FIGURE 2 ). Suborbital crista sexually dimorphic; in male, composed of 22–27 isomorphic tubercles; in females, composed of 22–23 small isomorphic tubercles ( Figs. 1B–D View FIGURE 1 , 2E–G View FIGURE 2 ). Chelipeds distinctly larger in male ( Figs. 1A, B View FIGURE 1 , 2A, B View FIGURE 2 ); merus relatively long, slightly shorter than palm; dactylus with several blunt teeth on cutting edge ( Figs. 1A View FIGURE 1 , 2A View FIGURE 2 ). Ambulatory legs long, slender ( Figs. 1A, B, E, F View FIGURE 1 , 2A–D View FIGURE 2 ); meri with several distinct spines on anterior and posterior margins; anterior margins of carpi and propodi almost glabrous, with several distinct spines; in male, carpi relatively longer [propodus length/ carpus length <1.9 (P4); <1.6 (P5)], P5 merus and propodus relatively stouter (merus length/width <3.8; propodus length/width <4.8) (Appendix 1B, D–F). G1 slender, distal part tapering, distinctly curved toward ventrolateral side; tip chitinous, blunt, distal part distinctly curved toward ventrolateral side.

Coloration. In dorsal view, the carapace is brown to gray, while the chelae are reddish-brown to light brown with a lighter color near the distal ends ( Fig. 1G View FIGURE 1 ). The fingers are light brown to white, and the distal parts of the ambulatory legs are brown.

Size. Largest male CW 43.7 mm (ZRC 1997.696); largest female CW 33.1 mm (ZRC 1989.2081-2082) (N.K. Ng 2006; this study).

Habitat. At Chalong Bay, Phuket, Thailand, this species was found on muddy flats among mangroves ( Fig. 1H View FIGURE 1 ).

Remarks. The type locality of this species is unknown (Gerstaecker 1856: 143), and two morphologically similar species, consistent with the original description and illustrations, have been found in both the Indian Ocean and the western Pacific. However, after comparing specimens from the Indian Ocean with those from the western Pacific ( Vietnam), the results showed that the morphology (the proportions of the male ambulatory legs) of the specimens from the Indian Ocean more closely matches Gerstaecker's (1856) illustrations. Furthermore, we contacted the Berlin Museum to locate and photograph the syntype of Metaplax crenulata , and we were fortunate to obtain images of these specimen (ZMB 604), allowing us to examine its morphology ( Fig. 2 View FIGURE 2 ). In this male specimen, the suborbital cristae and the proportions of P4 and P5 are also similar to those of the Indian Ocean species (Appendix 1). Based on morphological evidence, our results indicate that the Indian Ocean species should be considered the true Metaplax crenulata , while the Vietnamese species represents a new species, M. pristis sp. nov. (see Discussion for details). For the purpose of nomenclatural stability, it is prudent to select a specimen from the syntype (ZMB 604) of Rhaconotus crenulatus (= Metaplax crenulata ) to be the lectotype. Since the male can be morphologically distinguished from the similar species ( M. pristis sp. nov.), we hereby designate the male specimen as the lectotype, with the remaining two female specimens as paralectotypes.

In addition, following the original publication of Metaplax crenulata , lots of records of this species have been reported, almost exclusively from the Indian Ocean region (as listed in the synonymy list above); by contrast, records from the western Pacific are limited to only two from Vietnam, including Wada (2019) and the present study (see Remarks of M. pristis sp. nov.).

Distribution. West Bengal, Andaman and Nicobar Islands, Mergui Archipelago, western Thailand, western Malaysia and Singapore ( Tan & Ng 1994; Ng & Davie 2002; Trivedi et al. 2018; Sasaki 2023; others see the synonymy list above).