Shinobium, Naruse & Ng, 2020

Naruse, Tohru & Ng, Peter K. L., 2020, Revision of the sesarmid crab genera Labuanium Serène and Soh, 1970, Scandarma Schubart, Liu and Cuesta, 2003 and Namlacium Serène and Soh, 1970 (Crustacea: Decapoda: Brachyura: Sesarmidae), with descriptions of four new genera and two new species, Journal of Natural History (J. Nat. Hist.) 54 (7 - 8), pp. 445-532 : 466-469

Genus Shinobium gen. nov.

Type species

Sesarma trapezoideum H. Milne Edwards, 1837, by present designation. Gender neuter.

Included species

Shinobium trapezoideum (H. Milne Edwards, 1837).

Diagnosis

Carapace trapezoidal, longer than wide; divergent posteriorly, widest at bases of P3, with 1 distinct epibranchial tooth behind external orbital angle, posterolateral regions setose; dorsal surface slightly convex, regions well defined. Front deflexed at anterior margin of postfrontal lobes at more than 90°, distally sloping ventroposteriorly; frontal margin bilobed with rounded median concavity, each lobe recurved, directed anteriorly, median concave part attached to narrow, long, clearly exposed antennular septum. Two pairs of postfrontal lobes present, lateral lobes slightly exceeding mesial lobes anteriorly, lateral lobes only slightly narrower than mesial lobes, all lobes projected anteriorly, overhanging onto front. Orbit, in dorsal view, L-shaped, median part of supraorbital margin almost horizontal; inner orbital tooth triangular, long, directed anteriorly. External orbital angle with 1 longitudinal ridge on ventral surface. Epistome posterior margin with 3 triangular lobes, all lobes directed anteriorly. Antenna entering orbit through wide gap between inner orbital tooth and front. Mxp3 exopod with distinct flagellum. Chela palm with many irregular rows of granules on upper surface both in male and female; outer surface granulated but without prominent protuberance; inner surface without transverse rows of granules; narrow rim extending along the occlusal margin of immovable finger to dactylar articulation on both outer and inner surfaces, these narrow rims of both outer and inner surfaces not interrupted near dactylar articulation. Upper-inner margin of movable finger with 1 dense, regular row of horizontal, oblong, tiny granules over most of length in male, row shorter with smaller granules in female. Ambulatory legs (P2–5) long, dorso-ventrally flattened; distal anterior corner of each merus forming falcate lamella, followed proximally by subdistal tooth; carpi and propodi distinctly narrower than respective meri; dactyli very short, shorter than half length of respective propodi. Male thoracic sternum transversely narrow; sternite 8 clearly exposed. Male pleon relatively narrow, bell shaped, lateral margin weakly concave, telson not reaching proximal half of bases of cheliped coxae. G1 narrow, very long, constricted at distal two-fifths, reaching distal fifth to distal end of thoracic sternite 5; beak-like and corneous process narrow. Vulvae opening on distal half of sternite 6, anterior margin adjacent to thoracic sternal suture 5/6, ellipsoidal; 3 rounded sternal vulval covers developed from posterolateral margin, middle largest, connected with lower cover by narrow rim.

Etymology

The generic name ‘ Shinobium ’ is derived from an arbitrary combination of ‘ Shinobi ’ and the suffix for the Labuanium ‘ -ium ’. ‘ Shinobi ’ is an alias of ‘Ninja’ in Japanese, alluding to the swift and cryptic behaviour of the type species. Gender neuter.

Remarks

Shinobium is morphologically most similar to Labuanium in its proportionally longer carapace, but the former can be distinguished from the latter by many characters of the carapace, male thoracic sternum, cheliped and ambulatory legs ( Table 1 ). Shinobium is characterised by a distinctly trapezoidal carapace ( Figure 8 (a)), whereas the carapace of Labuanium is longitudinally rectangular ( Figures 1 (a), 2(a)). The male pleon of Shinobium is proportionally shorter and there is a substantial distance between the anterior end of the sternopleonal cavity and the border between thoracic sternites 3 and 4 ( Figure 10 (b)). In Labuanium , the distance is distinctly shorter ( Figure 1 (b)). Rim-like structures of the cheliped palm are also present in Shinobium ( Figure 9 (b)), but it is entire throughout its length without interruption, unlike Labuanium ( Figure 3 (b)). In addition, the anterodistal corner of the ambulatory meri in Shinobium has a falcate lamella ( Figure 9 (d)), but that of Labuanium is simply pointed ( Figure 3 (d)). The ambulatory dactyli of Shinobium are also proportionally longer than those of Labuanium ( Figures 8 (a), 10 vs Figures 1 , 2 (a)). The shape of the G1 also differs significantly between the two genera; Shinobium has a medially narrowed shaft of the G1 with a bulb-like subdistal part ( Figure 11 (b–e)), whereas that of Labuanium has a more or less straight shaft ( Figure 4 (b,c)).

When De Man (1892) described Sesarma weberi De Man, 1892 (now placed in Sesarmops, sensu Serène and Soh 1970 ), he compared it with Se. trapezoideum . Ng et al. (2008a, p. 224) commented that ‘ Sesarma trapezoidea H. Milne Edwards, 1837 and Sesarma weberi De Man, 1892 should be placed in a same genus as they share a suite of cheliped, gonopodal and larval features’. These characters are the presence of a dense row of tiny tubercles on the upper margin of chelal movable fingers ( Figures 9 (c), 13(b)), trapezoidal carapace shape as well as the general form of the antennule and antenna ( Figures 8 (a), 12, 13(a, b, d), 14(b–d)) ( Ng et al. 2008a; Jeng et al. 2003. Sesarmops weberi also differs from all known congeners in having a dense row of tiny tubercles on the upper margin of the movable finger of the cheliped in both the male and the female ( Figure 13 (b)). Sesarmops weberi is similar to Shinobium in its relatively trapezoidal carapace ( Figure 12 (a)), setose posterolateral regions of the carapace, and weakly lamellar anterodistal end of ambulatory meri ( Figure 12 (a)). Nevertheless, Sesarmops weberi is not assigned to Shinobium because of its stout and almost straight G1 with distal corneous beak-like process that is directed laterally ( Figure 14 (b–d)) (vs medially narrowed shaft of G1 with bulb-like subdistal part in Shinobium , Figure 11 (b–e)), lateral and median lobes of epistome directed anteroventrally and ventrally, respectively ( Figure 12 (b)) (vs both lateral and median lobes directed anteriorly, Figure 8 (b)), the rim along the occlusal margin of immovable finger to dactylar articulation is disconnected only at the inner surface of male major chela ( Figure 13 (d)) (vs rim not disconnected on both inner and outer surfaces of both male and female chelae, Figure 9 (b)), and the male pleonal somite 6 is rounded laterally and has subequal proportions ( Figure 14 (a)) (vs distolaterally angulated, proportionally longer in Shinobium , Figure 10 (b)). These differences indicate that Sesarmops weberi is not congeneric with Shinobium . An ongoing revision of the genus Sesarmops by the second author and C.D. Schubart will clarify the generic position of ‘ Sesarmops ’ weberi and allied species at a later date.

Figures

Figure 1. Labuanium politum (De Man, 1887). Lectotype, NHM 1886.52, male, 18.7 × 17.1 mm (Mergui Archipelago). a, habitus, dorsal view; b, habitus, ventral view.

Figure 2. Labuanium politum (De Man, 1887). ZRC 2003.0381, male, 40.1 × 36.2 mm (Bohol, Philippines). a, habitus, dorsal view; b, anterior part of cephalothorax, anteroventral view.

Figure 3. Labuanium politum (De Man, 1887). ZRC 2003.0381, male, 40.1 × 36.2 mm (Bohol, Philippines). a–c, left chela; a, outer view; b, inner view; c, upper view; d, meri of right P4 and P5. Arrow denotes rim.

Figure 4. Labuanium politum (De Man, 1887). ZRC 1965.7.29.58, male 40.8 × 36.2 mm (Borneo, Malaysia). a, pleon and telson; b, left G1, ventral view; c, distal part of left G1, dorsal view. Scale bars: a= 5 mm; b, c= 1 mm.

Figure 8. Shinobium trapezoideum (H. Milne Edwards, 1837). ZRC 2018.0051, male, 29.5 × 26.8 mm (Vanuatu). a, habitus, dorsal view; b, anterior part of cephalothorax, anteroventral view.

Figure 9. Shinobium trapezoideum (H. Milne Edwards, 1837). ZRC 2018.0051, male, 29.5 × 26.8 mm (Vanuatu). a–c, left chela; a, outer view; b, inner view; c, upper view; d, meri of right P4 and P5.

Figure 10. Shinobium trapezoideum (H. Milne Edwards, 1837) (Holotype of Sesarma trapezoideum longitarsis var. longitarsis De Man, 1889). SMF 1980, male, 31.4 × 28.9 mm (Fiji). a, habitus, dorsal view; b, habitus, ventral view.

Figure 11. Shinobium trapezoideum (H. Milne Edwards, 1837). ZRC 2018.0051, male, 29.5 × 26.8 mm (Vanuatu). a, pleon; b–e, left G1; b, ventral view; c, dorsal view; d, distal part, ventral view, denuded; e, distal part, dorsal view, denuded. Scale bars: a= 5 mm; b–e = 1 mm.

Figure 12. Sesarmops weberi (De Man, 1892). Lectotype, RMNH D2615, male, 14.5 × 15.0 mm (Flores, Indonesia). a, habitus, dorsal view; b, cephalothorax, anterior view.

Figure 13. Sesarmops weberi (De Man, 1892). a–c, lectotype, RMNH D2615, male, 14.5 × 15.0 mm (Flores, Indonesia); d, ZRC 2018.0771, male, 16.2 × 16.3 (Cebu, Philippines). a, major chela, outer view; b, major chela, upper view; c, cephalothorax, ventral view; d, major chela, inner view.

Figure 14. Sesarmops weberi (De Man, 1892). Lectotype, RMNH D2615, male, 14.5 × 15.0 mm (Flores, Indonesia). a, pleonal somites 3–6 and telson; b–d, left G1; b, ventral view; c, distal part, ventral view; d, distal part, dorsal view. Scale bars: a= 5 mm; b–d = 1 mm.