Metaplax pristis, Hsu & Shih & Ngo, 2025

Metaplax pristis View in CoL sp. nov.

( Figures 3–6 View FIGURE 3 View FIGURE 4 View FIGURE 5 View FIGURE 6 )

Material examined. Holotype: 1 male (44.1 × 33.1 mm) (NCHUZOOL 17352), T. T. Can Thanh, Can Gio, Ho Chi Minh City, Vietnam, coll. H.-T. Shih & P.-Y. Hsu, 13 October 2017 . – Paratype: 1 female (43.6 × 32.5 mm) (NCHUZOOL 17353), T. T. Can Thanh, Can Gio, Ho Chi Minh City, Vietnam, coll. H.-T. Shih & P.-Y. Hsu, 13 October 2017 .

Diagnosis. Carapace rectangular, convex; surface almost glabrous, region well-defined ( Figs. 3A, C View FIGURE 3 , 4A, D View FIGURE 4 , 5A View FIGURE 5 ). Front narrow, medially slightly concave ( Fig. 5A View FIGURE 5 ). Lateral margin nearly parallel, with 5 teeth (including orbital tooth), anterior 3 teeth pronounced, posterior 2 relatively indistinct ( Figs. 3A, C View FIGURE 3 , 5A View FIGURE 5 ). Suborbital crista sexually dimorphic; in male, composed of 30–31 isomorphic tubercles; in females, composed of 22–24 small isomorphic tubercles ( Figs. 4C View FIGURE 4 , 5C View FIGURE 5 ). Chelipeds distinctly larger in male ( Fig. 3A–D View FIGURE 3 ); in male, merus relatively long, slightly shorter than palm ( Figs. 3A View FIGURE 3 , 4A View FIGURE 4 , 5D View FIGURE 5 ); dactylus with several blunt teeth on cutting edge ( Fig. 5E View FIGURE 5 ). Ambulatory legs long, slender; meri with several distinct spines on anterior and posterior margins ( Figs. 3A–D View FIGURE 3 , 4F, G View FIGURE 4 ); anterior margins of carpi and propodi almost glabrous, with several distinct spines; in male, carpi relatively shorter (propodus length/carpus length> 1.9 (P4);> 1.6 (P5)), P5 merus and propodus relatively slender (merus length/width> 4.3; propodus length/width> 5.7) (Appendix 1B, D–F). G1 slender, distal part tapering, distinctly curved toward ventrolateral side; tip chitinous, blunt, distal part distinctly curved toward ventrolateral side ( Fig. 6 View FIGURE 6 ).

Description. Carapace rectangular, greatest width across third anterolateral teeth, approximately 1.3 ( holotype male = 1.33; paratype female = 1.34) times broader than long ( Figs. 3A, C View FIGURE 3 , 4A, D View FIGURE 4 , 5A View FIGURE 5 ); lateral margins convex, with 5 teeth (including exorbital tooth), anterior 3 teeth pronounced marked, posterior 2 relatively indistinct; first tooth (exorbital tooth) broad, blunt, pointed anteriorly, second tooth similar to first tooth in size, third and fourth teeth much smaller, fifth reduced to a tiny tooth or notch. Supraorbital margin lined with large rounded granules; outer margins of anterolateral teeth edged with small spines or acute granules; lateral margin edged with row of short setae. Carapace surface almost glabrous, smooth, scattered with several minute, smooth granules, with 3 oblique rows of granules short on posterolateral region. Front approximately 0.2 times maximum carapace width, deflexed, slightly notched medially, broad shallow sulcus running posteriorly. Dorsal carapace regions well indicated, with gastric, cardiac and intestinal regions strongly demarcated. Suborbital crista sexually dimorphic ( Figs. 3B, D View FIGURE 3 , 5B, C View FIGURE 5 ); in male, composed of 30–31 isomorphic rounded tubercles; in females, composed of 22–24 small isomorphic rounded tubercles.

Chelipeds equal, large and robust. Merus slender, markedly more elongated, margins armed with remarkable blunt spines or coarse granules (more blunt on ventral margin), denser on distal anterior margin; posterior surface without chitinous crest ( Figs. 3A–D View FIGURE 3 , 5D View FIGURE 5 ). Carpus with several distinct granules on inner margin, 3–4 granules more produced, almost blunt spines; outer surface almost smooth, without granules ( Fig. 3A View FIGURE 3 ). Palm elongated; outer surface covered with tiny granules minutely, larger on part near upper and lower margins ( Figs. 3A View FIGURE 3 , 5E View FIGURE 5 ); granules becoming slightly more prominent along upper and lower margins and on fingers; inner surface with smaller granules on medial part and part near base of immovable finger, granules on medial part larger. Cutting margins bluntly serrated; male holotype with an obvious basal gape between the fingers. Fixed finger moderately deflexed, 0.7 times as palm in length.

Ambulatory legs elongated and slender, P3 and P4 longer than P2 and P5, P3 longest and approximately 1.6–1.9 times carapace width ( Figs. 3A–D View FIGURE 3 , 5F View FIGURE 5 ). Meri approximately 4.1 times longer than wide in P4 of holotype male (approximately 3.4 times in paratype female) (approximately 4.3 times longer than wide in P5 of holotype male; approximately 3.4 times in paratype female) ( Fig. 5F View FIGURE 5 ); anterior and posterior margins slightly converging mesially and distally, margins with several curved spines, spines on anterior margin longer and more distinct than those on posterior margin, spines longer in P3 and P4; dorsal surface almost glabrous, sparsely covered with small granules, with a line of larger granules on part near anterior margin. Carpi and propodi with several curved spines on anterior margins, posterior margins without spines; carpi almost glabrous, with a row of spines on dorsoanterior surfaces, and with a line of small granules on dorsal surface; carpi of P4 and P5 shorter in males, no distinct difference in females (Appendices 1B, E, 2B, E); propodi with sparse setae on posterior margins, anterior margins almost glabrous; P5 propodi slender in males, no distinct difference in females (Appendices 1F, 2F). Dactyli long, with rows of short setae on surfaces and margins.

Male abdomen with telson abruptly narrowed, longer than wide, gradually tapering, shorter than penultimate segment; sixth segment the longest, approximately 1.2 times wider than long ( Figs. 3B View FIGURE 3 , 4B View FIGURE 4 ). G1 slender, distal part tapering, distinctly curved toward ventrolateral side; tip chitinous, blunt, distal part distinctly curved toward ventrolateral side ( Fig. 6 View FIGURE 6 ).

Etymology. The species is named after Pristis, which is the genus of sawfish. This new species name is derived from the distinctive meri of its ambulatory legs, which have numerous curved spines along the meri margins, resembling the snout of a sawfish.

Coloration. In dorsal view, the carapace is dark brown to gray, while the chelae are brown with a lighter color near the distal ends ( Fig. 4A, D, F, G View FIGURE 4 ). The fingers are light brown to white, and the distal parts of the ambulatory legs are brown. In ventral view, the body is predominantly white ( Fig. 4B, C View FIGURE 4 ), with the upper part near the suborbital cristae appearing dark reddish-brown ( Fig. 4C View FIGURE 4 ). The chelae are dark reddish-brown, while the pleon and ambulatory legs range from dark brown to gray.

Size. Largest male CW 44.1 mm ( holotype); largest female CW 43.6 mm ( paratype).

Habitat. This species inhabits the intertidal zone with a soft muddy substrate, located in front of mangroves ( Fig. 4H View FIGURE 4 ).

Remarks. In Vietnam, apart from the collection records from this study, Wada (2019) also documented " M. crenulata " from Long Thanh, Can Gio ( Wada 2019: e139, fig. 1A). Although the suborbital cristae and the precise proportions of the ambulatory legs cannot be confirmed from the photograph by Wada (2019: fig. 1A), based on the currently known geographic distribution and the proximity of Wada's collection site to the type locality of M. pristis sp. nov. (both at Can Gio, Ho Chi Minh City), the species recorded by Wada (2019) is more likely to be M. pristis sp. nov. However, the material will need to be examined to confirm the identity.

Metaplax pristis sp. nov. is most similar to M. crenulata in morphology but can be distinguished by the male suborbital cristae and male ambulatory legs (Appendices 1, 2). However, no morphological differences have been observed between the females of M. pristis sp. nov. and M. crenulata , and they can only be differentiated by the locality and molecular evidence ( Fig. 7 View FIGURE 7 ; Appendix 2). In male M. pristis sp. nov., the suborbital cristae consist with more granules ( Fig. 5C View FIGURE 5 ) (versus suborbital cristae with less granules in M. crenulata ; Fig. 1D View FIGURE 1 ); the carpi are relatively shorter ( Fig. 5F View FIGURE 5 ) (versus the carpi are relatively longer in M. crenulata ; Fig. 1E View FIGURE 1 ); and the merus and the propodus of P5 are relatively slender ( Fig. 5F View FIGURE 5 ; Appendix 1D, F) (versus the merus and the propodus of P5 are stout in M. crenulata ; Fig. 1E View FIGURE 1 ; Appendix 1D, F).

Among the 10 known species of Metaplax , this new species, Metaplax pristis sp. nov., and M. crenulata is morphologically distinct from other species by their larger body size (versus smaller body size in other known species) and ambulatory legs with numerous spines (versus with less spines or without spines in other species). The margins of the ambulatory legs meri lack distinct spines in most Metaplax species, such as M. elegans , M. gocongensis , M. indica , M. intermedia , M. longipes , M. sheni , and M. tredecim ( De Man 1888: pl. 11, fig. 7; Davie & Nguyen 2003: fig. 1A; Naderloo 2017: fig. 32.2; Shih et al. 2019: figs. 3A, 4A, 5A, 6A). In contrast, M. dentipes , M. distincta , M. crenulata , and M. pristis sp. nov. have distinct spines on the meri margins of the ambulatory legs ( Figs. 1E View FIGURE 1 , 5F View FIGURE 5 ; De Man 1888: pl. 10 fig. 7, pl. 11 fig. 1). Additionally, the anterior margins of the carpi bear several spines only in M. crenulata and M. pristis sp. nov., whereas the other nine Metaplax species lack spines on the carpi margins.

Distribution. At present, only known from southern Vietnam.