Geosesarma sinuofrontatum ( Roux, 1933 ) 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 : 513-517

Geosesarma sinuofrontatum ( Roux, 1933) , comb. nov.

( Figures 33 , 34 )

Sesarma (Sesarma) sinuofrontata Roux, 1933, p. 14 , figs 1, 2 [type locality: Triton Bay, Indonesian West Papua].

Labuanium sinuatifrontatum: Serène and Soh 1970, p. 406 ; Ng et al. 2008a, p. 221 (misspelling of Sesarma (Sesarma) sinuofrontata Roux, 1933 ).

? Labuanium aff. sinuatifrontatum: Serène et al. 1974, p. 26 (misspelling of Sesarma (Sesarma) sinuofrontata Roux, 1933 ).

Material examined

Holotype, RBINS, male, 11.6 × 11.6 mm, Tritonbaai (= Triton Bay), Indonesian West Papua, coll. 21 March 1929.

Redescription of holotype

Carapace ( Figure 33 (a)) quadrate, as long as wide, lateral margins almost parallel. Dorsal surface almost flat, glabrous, regions moderately defined. Posterolateral regions sloping posterolaterally. Front deflexed at anterior margin of postfrontal lobes, distally recurved, directed anteriorly, frontal margin bilobed with wide median concavity, overhanging onto antennular septum and fossae. Two pairs of postfrontal lobes present, mesial lobes horizontal, lateral lobes oblique, exceeding mesial lobes anteriorly, anterior margins of all lobes relatively close to, but never reaching to, frontal margin in dorsal view, all lobes followed posteriorly by short horizontal rows of small granules. External orbital angle sharp, directed anteriorly, lateral margin with distinct epibranchial tooth and traces of 1 rudimentary epibranchial tooth. Antennular septum narrow, short, dorsally covered by front. Orbit, in dorsal view, tilted J-shaped, median part of supraorbital margins slightly oblique; infraorbital margin cristate, with sharp triangular inner orbital tooth, directed dorsoanteriorly. Suborbital crista slightly arched, convex posteriorly, granulated, setose. Suborbital, pterygostomial, subhepatic regions with reticulate mat of setae. No longitudinal ridge on ventral surface of external orbital angle.

Epistome relatively long, posterior margin with 3 triangular lobes, lateral lobes distally rounded, recurved, directed ventrally, median lobe directed ventroposteriorly.

Eye ( Figure 33 (a,c)) with stout peduncle, cornea narrower than peduncle. Antennule with wide, laterally long basal article. Antenna with wide, ellipsoidal basal article, distolateral lobe expanded laterally; antenna entering orbit through space between inner orbital tooth and front.

Mxp3 leaving wide gape between mesial margins of ischia and meri; ischium subtriangular, as long as merus; merus ovoid, with oblique depression mesially lined with setae, setose around base of carpus. Exopod reaching distal third of merus, with long flagellum.

Male chelipeds ( Figure 33 (a)) subequal. Cheliped merus with trigonal cross section; upper margin lined with low granules, angled subdistally; lower-inner margin keeled, weakly serrate, subdistally expanded to form rounded lobe; lower-outer margin serrate, with distal sub-pentagonal hinge for carpus; outer surface with short rows of granules; inner surface smooth, with 2 longitudinal rows of soft setae. Carpus with granulated upper surface, inner angle bluntly produced, angle connected downward to low keel. Palm of chela convex, outer surface ( Figure 33 (b)) granulated, distal region of palm (from base of movable finger to immovable finger) without granules, smooth, no median protuberance on proximal area to this region. Upper surface ( Figure 33 (c)) with a few, longitudinal rows of small granules, these rows never throughout the length of upper surface. Inner surface convex, sparsely granulated; 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. Immovable finger almost straight, gradually narrowed towards tip, 3 largest teeth on distal three-fifths, small teeth on proximal two-fifths. Occlusal margins of both fingers sharply pointed but hoof-like on occlusal side. Movable finger gently curved downwards, occlusal margin with 2 largest teeth on distal half, smaller teeth on proximal quarter; inner side of upper margin lined with ca. 8 very low granules on proximal half. Immovable finger with 3 large teeth, 2 largest teeth of movable finger meeting them when fingers closed.

Ambulatory legs ( Figure 33 (a)) moderately long, stout for genus, P4 longest. Small tufts of soft setae on anterior surface of P2, 3 coxae. Meri upper surface wide, compressed dorso-ventrally, oblong, distal corner of anterior margin angled, followed proximally by short tooth, posterior margin extended posteriorly into lamellar form in ventral view. Propodi gradually narrowed distally; inner surfaces with mat of short, soft setae, mat denser in anterior legs, covering distal four-fifths on P2, 3, distal half on P4, limited distal part on P5; outer surface with sparse setae in P2–5. Dactyli gently curved distally, P2–4 with dense mats of soft setae on both outer, inner surfaces, mat on outer surface only in P5.

Male thoracic sternum transversely narrow, thoracic sternites 1–4 fused, 2/3, 3/4 demarcated superficially by anteriorly produced crista, horizontal groove, respectively. Male sternopleonal cavity reaching distal two-fifths of bases of cheliped coxae; margin of sternopleonal cavity on somite 4 rimmed except for posterior end, posterior end of rim thick; lateral slope of sternopleonal cavity posterior to non-rimmed part on sternite 4 depressed, accomodating distal end of G 1 in situ. No sternal button for locking mechanism on sternite 5. Penis sternal.

Male pleonal somites 1, 2 short, wide; somite 3 widest, somites 4–6 with undulate lateral margins; telson rounded distally, slightly longer than somite 6 ( Figure 34 (a)).

G1 ( Figure 34 (b,c)) slender, almost straight proximally, distally bent perpendicularly, distal end sharp in ventral view, spartular in mesiodistal view, opening on subdistal part of dorsal side. G2 ( Figure 34 (d)) short, as long as bent part of G1.

Colouration

Live colouration not known.

Distribution

Known only from the type locality, Triton Bay, Indonesian West Papua.

Remarks

Sesarma (Sesarma) sinuofrontata ( Roux, 1933) is known only from a small male holotype (11.6 × 11.6 mm) collected from Tritonbaai, southern New Guinea (present day Indonesian West Papua). Serène and Soh (1970, p. 406) questionably assigned this species to Labuanium (the specific name was misspelled as ‘ sunuatifrontatum ’), and this generic assignment was followed by Ng et al. (2008a). Roux (1933) already noted that his new species resembled Labuanium politum and Sesarmops weberi in the elongated and flattened carapace, medially depressed frontal margin, mesial postfrontal lobes being wider than lateral lobes, subparallel lateral margins of the carapace, and simply angled distal anterior corners of ambulatory meri. Sesarma sinuofrontatum , however, differs from L. politum at the genus level by the position of the postfrontal margin (far behind frontal margin, Figure 33 (a); vs partially reaching frontal margin in Labuanium , Figure 2 ), the relatively narrow antennular septum that is overhung by the front (vs antennular septum wide, attached to medial concave part of frontal margin and on same level with it in Labuanium , Figure 2 (b)), and the shape of the G1 (slender, gradually curved outward, tapering in sharp point, Figure 34 (b,c); vs stout and straight in Labuanium , Figure 4 (b,c)). Sesarma sinuofrontatum also differs from Sesarmops weberi at the genus level in the shape of the male pleon (rectangular, with lateral margins of somites 3–6 sinuous, Figure 34 (a); vs subtriangular, with lateral margins gradually divergent proximally in Sesarmops , Figure 13 (c), 14(a)) and the shape of the G1 (slender, gradually curved outward, tapering sharply, Figure 34 (b,c); vs stout, almost straight, with laterally directed distal corneous beak-like process in Sesarmops , Figure 14 (b–d)).

As described above, Se. sinuofrontatum has a characteristic G1, with a long terminal process that is tapering and smoothly bent outwards at 90°, which links this species to Geosesarma . The G1 of G. noduliferum ( De Man, 1892) and G. serenei Ng, 1986 , is superficially similar to that of G. sinuofrontatum , but the former two species differ from G. demani in the degree of the curvature of the terminal process of the G1 (weakly bent outwards in the former two species vs bent at 90° in G. sinuofrontatum ) ( Figure 34 (b,c); Ng 1986, fig. 2; Ng et al. 2015b, fig. 2A–C). In any case, G. serenei and its allied species do not have a flagellum on the exopod of the Mxp3 (see Ng 2017). On the basis of the similarity in the G1 structure to those two species, Se. sinuofrontatum is here reassigned to Geosesarma .

Four species of Geosesarma are known from Indonesian West Papua and adjacent islands: G. gordonae (Serène, 1968) (Fakfak, West Papua), G. ianthina Pretzmann, 1985 (Wendessi (Windesi?), West Papua), G. maculatum ( De Man, 1892) (Halmahera) and G. ternatense (Serène, 1968) ( Ternate Islands). Geosesarma sinuofrontatum can be easily distinguished from G. gordonae , G. maculatum and G. ternatense by the shape of the terminal process of the G1 ( Figure 34 (b,c) vs Ng et al. 2004, figs 4, 7 for G. maculatum and G. ternatense ; Serène 1968a, figs 9–11 for G. gordonae and G. maculatum ). The G1 of G. ianthina remains to be described, but the species differs from G. sinuofrontatum in the V-shaped median notch of the frontal margin (vs shallow U-shaped notch) ( Pretzmann 1985, fig. 1; Figure 33 (a)).

Another very different looking sesarmid species, Karstarma waigeo Wowor and Ng, 2009 , with very long ambulatory legs has also been described from the coastal karst forests of Triton Bay ( Wowor and Ng 2009). Karstarma species, however, have very elongate P2–5 with the carapace distinctly trapezoidal ( Davie and Ng 2007).

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 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 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.

Figure 33. Geosesarma sinuofrontatum (Roux, 1933), comb. nov. Holotype, RBINS, male, 11.6 × 11.6 mm (Triton Bay, Indonesian West Papua). a, habitus, dorsal view; b, right chela, outer view; c, right chela, upper view.

Figure 34. Geosesarma sinuofrontatum (Roux, 1933), comb. nov. Holotype, RBINS, male, 11.6 × 11.6 mm (Triton Bay, Indonesian West Papua). a, pleon and telson; b, left G1, ventral view; c, distal part of left G1, ventral view; d, left G2, ventral view. Scale bars: a= 3 mm; b–d = 1 mm.