Mindanium, 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 : 499-500

Genus Mindanium gen. nov.

Type species

Sesarma cruciatum Bürger, 1893 , by present designation. Gender neuter.

Included species

Mindanium cruciatum ( Bürger, 1893) , comb. nov.

Diagnosis

Carapace squarish, as long as wide; lateral margin slightly sinuous, with 2 distinct epibranchial teeth behind external orbital angle; posterolateral margins parallel; dorsal surface slightly convex longitudinally and transversely. Front deflexed at anterior margin of postfrontal lobes by more than 90°, distally sloping ventroposteriorly; frontal margin bilobed with wide median concavity, lobes short, recurved, directed anteriorly, slightly overhanging onto antennular fossae; antennular septum dorsally attached to median concavity of frontal margin. Two pairs of postfrontal lobes aligned anteriorly, reaching beyond frontal margin in dorsal view, overhanging onto front; mesial lobes slightly wider than lateral lobes. Antennular septum wide, short. Orbit, in dorsal view, tilted J-shaped, median part of supraorbital margins oblique; inner orbital tooth short, subtriangular, directed anteriorly. No longitudinal ridge on ventral surface of external orbital angle. Epistome posterior margin with 3 triangular lobes, lateral lobes directed anteroventrally,median lobe directed ventrally. Antenna entering orbit through wide gap between inner orbital tooth and front.Mxp3 exopod with distinct flagellum.Female chela palm without ridge or row of granules on upper surface; outer surface granulated but without prominent protuberance; inner surface without transverse rows of granules; thick rim extending along occlusal margin of immovable finger to dactylar articulation on both outer and inner surfaces, thick rims of both outer and inner surfaces not interrupted near dactylar articulation. Upper-inner margin of movable finger with 1 short sparse row of granules over three-fifths of the length. Ambulatory legs (P2–5) moderately long and stout generally, dorso-ventrally flattened; distal anterior corner of each merus weakly falcate, followed proximally by weak angle; carpi and propodi narrower than meri; dactyli short, about half length of respective propodus.

Etymology

The generic name ‘ Mindanium ’ is derived from an arbitrary combination of Mindanao (type locality of the type species) and the suffix for Labuanium , ‘ -ium ’. Gender neuter.

Remarks

Sesarma cruciatum Bürger, 1893 , was described from a single female holotype from Mindanao. Serène and Soh (1970) transferred this species to Labuanium Serène and Soh 1970 , without explanation. Sesarma cruciatum is characteristic in its squarish carapace outline with parallel posterolateral margins ( Figure 27 (a)). The carapace length of Se. cruciatum is slightly shorter than the carapace width, but its parallel posterolateral margins give the carapace the superficial appearance of being longer. Among the 32 extant genera of the family Sesarmidae ( Ng et al. 2008a; Schubart et al. 2009; Naruse and Ng 2012; Brösing et al. 2014), only Labuanium s.s., Shinobium gen. nov., Circulium gen. nov. and Aratus H. Milne Edwards 1853 are characterised by the carapace being clearly longer than wide. In addition to those genera, three species of American Armases Abele, 1992 – Ar. angustum ( Smith, 1870) , Ar. elegans ( Herklots, 1851) , Ar. roberti (H. Milne Edwards 1853) – have carapaces longer than wide ( Capart 1951; Abele 1992). Sesarma cruciatum clearly differs from Circulium , Shinobium , Aratus and the three Armases species in the outline of the carapace [longitudinally rectangular in M. cruciatum ( Figure 27 (a)), laterally swollen in Circulium ( Figure 5 (a)), trapezoidal with anteriorly convergent lateral margins in Shinobium ( Figures 8 (a), 10(a)), rectangular in Ar. roberti ( Rathbun 1918, pl. 91, fig. 2), and trapezoidal with anteriorly divergent lateral margins in Aratus ( Rathbun 1918, pl. 96), Armases ( Ar. angustum ( Abele 1977, fig. 5) and Ar. elegans ( Capart 1951, fig. 77; Rathbun 1921, pl. 44, fig. 1)]. Sesarma cruciatum is morphologically closest to Labuanium s.s., as the assigment to Labuanium by Serène and Soh (1970) indicates, in the postfrontal lobes produced anteriorly and reaching beyond the frontal margin in dorsal and ventral views ( Figures 27 (a), 28). Sesarma cruciatum , however, can be readily distinguished from Labuanium s.s. by the structure of the front and antennular septum [the narrower antennular septum is dorsally overhung by the median concavity of the frontal margin ( Figure 28 ) vs the wider antennular septum exposed and on the same level as the median concavity of the frontal margin in Labuanium s.s. ( Figure 2 (b)]; proportionally wider Mxp3 with a short-rhomboidal hiatus between them ( Figure 28 ) (vs more slender mxp3 with a long-rhomboidal hiatus between them in Labuanium s.s., Figure 2 (b)) and more slender ambulatory legs with proportionally longer dactyli ( Figure 27 (a) vs Figure 2 (a)). Armases angustum , Ar. elegans and Ar. roberti differ from Se. cruciatum by the postfrontal lobes not produced anteriorly and far from frontal margin in the dorsal view ( Abele 1992, fig. 34; Rathbun 1921, pl. 44, fig. 1, pl. 45, fig. 2; Chace and Hobbs 1969, fig. 60, respectively). The differences observed suggest that Se. cruciatum cannot be placed in any of these known genera. As such, the present study establishes a new monotypic genus, Mindanium gen. nov., for Sesarma cruciatum Bürger, 1893 .

Figures

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 5. Circulium rotundatum (Hess, 1865). Neotype, USNM 1150304, male, 36.3 × 36.3 mm (Tonga). a, habitus, dorsal view; b, anterior part of cephalothorax, anteroventral view.

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 27. Mindanium cruciatum (Bürger, 1893), comb. nov. Holotype, SMF ZMG 623, female 16.3 × 16.6 mm (Mindanao, Philippines). a, habitus, dorsal view; b, right chela, outer view; c, right chela, upper view.

Figure 28. Mindanium cruciatum (Bürger, 1893), comb. nov. Holotype, SMF ZMG 623, female 16.3 × 16.6 mm (Mindanao, Philippines). Anterior part of cephalothorax, anteroventral view.