Leptomithrax sinensis Rathbun, 1916

Wong, Kingsley J. H., Ng, Peter K. L. & Jeng, Ming-Shiou, 2018, From an Old Eroded Carapace: Rediscovery of the Majid Crab Leptomithrax sinensis Rathbun, 1916 (Crustacea, Brachyura, Majidae) from Taiwan and Japan, Zoological Studies 57 (49), pp. 1-11 : 2-3

Leptomithrax sinensis Rathbun, 1916

( Figs. 1A ; 2 A-E; 3A-F; 4A-J)

Leptomithrax sinensis Rathbun, 1916: 555 . - Bennett 1964: 46 [list]. - Sakai 1965: 86. - Griffin 1976: 199. - Griffin and Tranter 1986: 208 [key], 209. - McLay et al. 1995: tab. 1 [list]. - Ng et al. 2008: 117 [list]. - Ng and Richer de Forges 2015: 206 [list]. - Richer de Forges and Ng 2015: 127, fig. 3A-C.

Material examined: Holotype (25.3 × 32.0 mm, carapace only) ( USNM 48219 ), station 5311, South China Sea, “near southern Luzon”, 21°33'N 116°15'E, 88 fathoms, Philippines, coll. RV Albatross, 4 November 1908.

Others: 1 male (32.1 × 40.6 mm) ( ZRC 2018.0726), Cape Muroto, Shikoku, Japan, 8 July 1988; 1 female (32.4 × 41.2 mm) ( ASIZCR 101304), off Kueishan Island, 24°43.608'N 122°12.199'E, 134 m, coll. Fishing Vessel De-Cheng 136, M.-L. Chang, 23 Sep 2016; 1 male (35.2 × 43.1 mm) ( ASIZCR 101305), off Kueishan Island, 24°43.309'N, 122°11.625'E, 149 m, coll. Fishing Vessel De-Cheng 136, M.-L. Chang, 23 May 2018.

Comparative material examined: Leptomithrax bifidus ( Ortmann, 1893) : Lectotype (here designated): 1 male (25.8 × 32.3 mm) ( MZS Cru3193) (= larger male of MZS 755 recorded in Komai 1999), Sagami Bay, coll. L. Döderlein, 1881. Paralectotypes (here designated): 1 male (17.9 × 22.0 mm) ( MZS Cru0755), details same as lectotype; 1 female (11.8 × 15.2 mm) ( MZS Cru4246), details same as lectotype; 1 male (30.0 × 36.0 mm) ( MZS Cru2406), details same as lectotype; 1 female (26.8 × 32.4 mm) ( MZS Cru4326), details same as lectotype. Others: 1 male (27.5 × 32.5 mm) ( NHM 1961.11.13.28-29), Seto, Japan, coll. Gordon, I. & Harada, 1950s; 1 male (29.4 × 36.5 mm; dried) ( KPM NH4024), Kii, Nagashima, Japan, T. Sakai Collection, coll. March 1969; 1 male (18.0 × 22.6 mm) ( ZRC 2014.0354), Cape Muroto, Shikoku, Japan, 8 July 1988.

Description: Carapace pyriform, regions well defined, distinctly elevated along median line, surface covered with rugose tubercles, larger ones with granular surface. Pseudorostral spines slender, elongated (from 0.24-0.37 CL), cross-section rounded, moderately diverging, lateral margin slightly concave ( Figs. 1A ; 2 A- D; 4A). Supraocular eave broad, posterior lobe triangular, with acute tip, leaving distinct gap from acutely triangular intercalated spine with sharp tip. Postorbital lobe closely appressed to intercalated spine, broad, anteriorly produced, tapering, tip appearing truncated (type) or weakly bifid, posterior portion crested, produced laterally as faint convex lobe. Gastric region with 2 prominent granulated tubercles, never spiniform ( Fig. 3A, B ); cardiac region with 2 distinct tubercles, directed obliquely laterally, with 2 shorter ones on posterior margin. Hepatic lobe as 2 stout, robust conical spines, anterior one more distinct. Lateral margin with 3 spines, branchial region with 1 robust, acute spine ( Figs. 1A ; 2 A-D; 3A-B; 4A).

Basal antennal article broad, completely fused with carapace, mesial distal spine more produced than interantennular spine, external one more produced, acute, directed anterolaterally, visible in dorsal view, followed posteriorly by strongly convex lobe (margin of orbit), basal part forming a narrow U-shaped hiatus against ventral margin of postorbital lobe ( Figs. 3C ; 4B ). Antennal peduncle positioned far from orbit ( Fig. 2E ). Epistome broad rectangular, smooth; posterior margin forming median triangular lobe with shallow median cleft ( Figs. 3C ; 4B ).

Third maxilliped elongated, swollen portions glabrous, prominently sculptured, suture between ischium and merus distinctly sinuous, forming a boss-like elevation; ischium longer than broad, surface with deep, distinct Y-shaped groove covered with setae, lateral margin with triangular tooth, inner margin with 7-9 distinct rounded teeth; merus broader than long, swollen along strongly concave proximal margin, subparallel with slightly produced anteroexternal margin, anterior extremity with distinct tooth. Exopod relatively broad with longitudinal median sulcus ( Figs. 3D ; 4C ).

Male chelipeds robust, sub-symmetrical, about as long as carapace ( Fig. 2A ); merus and carpus dorsally irregularly decorated by low granulated tubercles, forming rugose surface, merus with distinct curved spine distally along dorsal margin, ventral margin with 3 low tubercles, carpus with 1 conical tubercle on proximal margin ( Figs. 2A ; 4D ). Chela slightly inflated, without granules or tubercles, surfaces minutely pitted, palm distinctly longer than fingers, ventral margin of pollex confluent with that of palm ( Fig. 4E ). Cheliped of female proportionately more slender, surfaces of merus and carpus likewise granular, palm not inflated ( Fig. 2C ).

Thoracic sternites of male distinctly excavated medially: third and fourth sternites medially divided by a well-defined longitudinal carina, each portion subdivided into 2 by faint oblique ridge, that proximal to base of cheliped shallower; fifth to seventh sternites prominently excavated on anterior half, with posterior part distinctly raised, surfaces granular ( Fig. 3F ). Pleon of both sexes with 6 articulated somites and telson ( Figs. 3F ; 4 F-G); male pleon relatively broad transversely, heavily sculptured, elevated along the median, telson broad, rounded trapezoidal, third somite broadest, with a distinct glabrous ovoid elevation on each side ( Figs. 3F ; 4F ); that of female broad, subcircular, entirely covering thoracic sternum, somites and telson moderately sculptured, elevated along median, fifth somite broadest ( Fig. 4G ). G1 slender, tapering, external margin lined with setae, curving laterally on distal quarter ( Fig. 4H ), subdistal portion armed with minute conical spines on mesial surface, tip extended, rounded, directed anterolaterally ( Figs. 3E ; 4 H-J). G2 about 0.3 length of G1, basal segment relatively elongate, distal part spatuliform without flagellum ( Fig. 4K ).

Life coloration: Dorsal surface of carapace and ambulatory legs overall dull orange, chela and ambulatory dactyli amber, chelae fingers cream ( Fig. 2D ).

Habitat: The holotype carapace was collected from a substrate of coarse sand and shells ( US Bureau of Fisheries 1910; Rathbun 1916). The collection sites of both Taiwanese specimens are located in the seas off Yilan, northeastern Taiwan, some 13 km approximately east of Kueishan Island, within the “Zone B” legally designated for precious coral harvesting ( Huang and Ou 2010; Ministry of Justice, Taiwan 2014), where depths are greater than 130 m with the bottoms typically rocky with low sedimentation. Weighted fishing nets are dragged across the rugged hard substrate, and organisms, such as pieces of targeted precious coral (Family Corallidae ), and “by-catch” including corals of fan-shaped octocorallians, occasionally some scleractinians, athecate hydroids, or antipatharians, and crustaceans are entangled within (see Jeng 2014). For the Japanese specimen from Shikoku in Japan (probably collected by tangle nets), the same lot also contained a smaller male specimen of L. bifidus ( ZRC 2014.0354), indicating that the two species may occur sympatrically.

Figures

Fig. 1. Leptomithrax sinensis: A, overall habitus. L. bifidus: B, overall habitus; C: thoracic sternum and pleon; D: G1, in-situ. A, L. sinensis, holotype male (25.3 × 32.0 mm) (USNM 48219), northern South China Sea; B-D, L. bifidus, lectotype male (25.8 × 32.3 mm) (MZS Cru3193), Sagami Bay, Japan. Photo credits: B by T. Komai; C, D by M. Meister.

Fig. 2. Leptomithrax sinensis: A, C: overall habitus; B: carapace, appendages omitted; D: overall habitus, live coloration; E: right basal antennal articles and position of flagellum: ps, pseudorostral spine; af, antennal flagellum; mds and eds, mesial and external distal spines of antennal article. L. bifidus: F, G: overall habitus. A, D, E, L. sinensis, male (35.2 × 43.1 mm) (ASIZCR 101305), off NE Taiwan; B, L. sinensis, male (32.1 × 40.6 mm) (ZRC 2018.0726), Cape Muroto, Shikoku, Japan; C, L. sinensis, female (32.4 × 42.1 mm) (ASIZCR 101304), off NE Taiwan; F, L. bifidus, male (27.5 × 32.5 mm) (NHM 1961.11.13.28-29), Seto, Japan; G, L. bifidus, male (18.0 × 22.6 mm) (ZRC 2014.0354), Cape Muroto, Shikoku, Japan.

Fig. 3. Leptomithrax sinensis: A, B: lateral view of carapace; C: basal antennal article and orbit, ventral view; D: third maxillipeds, right denuded; E: G1, in-situ; F: male thoracic sternite and pleon. L. bifidus: G: lateral view of carapace; H: basal antennal article and orbit, ventral view; I: left third maxilliped; J: G1, in-situ; K: male thoracic sternite and pleon; L: right chela. A, L. sinensis, holotype (25.3 × 32.0 mm) (USNM 48219), northern South China Sea; B, C, L. sinensis, male (32.1 × 40.6 mm) (ZRC 2018.0726); D-F, L. sinensis, male (35.2 × 43.1 mm) (ASIZCR 101305); L. bifidus, G: male (18.0 × 22.6 mm) (ZRC 2014.0354), Cape Muroto, Shikoku, Japan; H-L, L. bifidus, male (27.5 × 32.5 mm) (NHM 1961.11.13.28-29), Seto, Japan.

Fig. 4. Leptomithrax sinensis: A: carapace; B: basal antennal article and orbit, ventral view; C: right third maxilliped; D: right cheliped merus and carpus, lateral view; E: right chela; F: male pleon; G: female pleon; H: right G1, mesial view; I: tip of right G1, mesial view; J: tip of right G1, lateral view; K: left G2, ventral view. L. bifidus: L: left G1, mesial view; M: tip of left G1, mesial view; N: tip of left G1, lateral view; O: left G2, ventral view. A-F, H-J, L. sinensis, male (35.2 × 43.1 mm) (ASIZCR 101305), off NE Taiwan; G, L. sinensis, female (32.4 × 42.1 mm) (ASIZCR 101304), off NE Taiwan; K, L. sinensis, male (32.1 × 40.6 mm) (ZRC 2018.0726), Cape Muroto, Shikoku, Japan; L-O, L. bifidus, male (18.0 × 22.6 mm) (ZRC 2014.0354), Cape Muroto, Shikoku, Japan. Structures A, C, F, G denuded.