Hexapinus latipes De Haan 1835
Creators: name: Ng, Peter K. L.
name: Rahayu, Dwi Listyo
Publisher: name: Zenodo
Year: 2015
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Decapoda
Hexapodidae
Hexapinus
Hexapinus latipes

description: Hexapinus latipes (De Haan, 1835)
(Figs. 2–6)
Hexapus sexpes —De Man 1888: 322, pl. 13 figs. 3, 3a, 3b (not Cancer sexpes Fabricius, 1798) Hexapus latipes De Haan, 1835: pl. D.
Hexapinus latipes — Rahayu & Ng 2014: 409, figs. 10–13.
See Rahayu & Ng (2014: 409) for a complete synonymy.
Material examined. Lectotype female (10.0 × 6.7 mm) (RMNH 31783d), Japan. Others: Japan: 1 female (10.2 × 7.6 mm) (NSMT-Cr 5694), Kushimoto, Kii Peninsula, Wakayama Prefecture, 33 ° 27.2'N 135 ° 45.4'E – 33 ° 27.2'N 135 ° 45.6'E, 19–27 m, dredge, coll. M. Takeda, 17 July 1978; 1 female (30.4 × 17.9 mm) (KPM NH 7123), Enoshima Island, T. Sakai Collection; 1 female (28.7 × 16.7 mm) (KPM NH 7460) Minabe, Wakayama Prefecture, T. Sakai Collection; 1 female (30.4 × 17.5 mm) (KPM NH 6331), Mikawa-Ishiki, Aichi Prefecture, T. Sakai Collection. Indonesia: 1 male (20.9 × 12.5 mm), 1 female (18.9 × 12.1 mm) (SMF ZMG 160), South China Sea at Moluccas Seas, Ambon, 13°42.03’S 128°9.867’E, coll. J.G.F. Brock, 7 September 1885.
Diagnosis. Carapace subquadrate, much wider than long, width 1.3–1.7 times length, dorsal surfaces almost entirely covered with distinct pits (Fig. 2); anterolateral margin lined with distinct rounded granules (Fig. 2); suborbital, subhepatic, and upper part of pterygostomial regions covered with flattened granules, and punctate (Fig. 3 A, B); pterygostomial region with oblique striae, dense setae under striae, adjacent to Milne Edwards opening (Fig. 3 B, C); outer surfaces of third maxillipeds covered with numerous small granules; ischium expanded proximolaterally, distomesially, subequal in length to merus medially, with gently convex mesial margin (Figs. 3 B, C, 4A–C); outer surfaces of chelipeds granulated, adult major male chela with cluster of granules expanded inward forming triangular structure in upper view, followed downward by median tubercles (Fig. 4 E, F); P2–P4 short, stout, outer surfaces with numerous distinct small granules, short setae (Figs. 2, 4 G–I); male thoracic sternum with sternites 1, 2 fused, forming subtriangular structure, each with deep transverse groove that does not extend to lateral margins, posteromedian part of sternite 3 with 2 granulated projections that bracket telson (Fig. 5 A, D); sternite 3 delimited from sternite 4 by deep tranverse groove that forms thoracic sternal groove, relatively shallower along margins (Fig. 5 A, D); male abdomen (Fig. 5 A–C) with somites 1, 2 free; somites 3–5 fused but median parts of sutures still visible, somite 6 longer than broad, shorter than fused somites 3–5, longer than telson, telson pentagonal (Fig. 5 A, B); G1 slender, curved along distal third, distalmost part gradually tapering to subtruncate tip (Fig. 6A–D).
Description. Carapace subquadrate, much wider than long, width 1.3–1.7 times length, prominently pitted on almost entire dorsal surface, regions indistinct, median H-shaped depression distinct (Fig. 2). Anterolateral margin arcuate, lined with distinct rounded granules; merging with distinctly diverging sinuous posterolateral margin sinuous, surface with numerous granules, widest point convex; posterolateral corner with round prominence over base of coxa of P4; lateral branchial surfaces produced, visible from dorsal view (Fig. 2). Front narrow, deflexed, truncate, divided into 2 lobes with almost straight margins, projecting just beyond lower edge of orbits (Figs. 2, 3 A, B). Orbit distinct, transversely ovate; eye small, movable, cornea small, as wide as peduncle (Fig. 3 A, B). Epistome longitudinally narrow; posterior margin concave with broadly triangular median projection (Fig. 3 A, B). Suborbital, pterygostomial, subhepatic regions covered with flattened granules, punctate (Fig. 3 A, B). Pterygostomial region with oblique striae, dense setae under striae adjacent to Milne Edwards opening (Fig. 3 B, C). Third maxillipeds broad, outer surfaces covered with numerous small granules; ischium laterally expanded, subequal in length to merus, expanded distally, with gently convex mesial margin, merus wider than long, carpus, propodus, dactylus cylindrical; dactylus longer than propodus, combined length of dactylus, propodus, carpus subequal to length of merus, ischium; exopod relatively broad, about 0.4 times width of ischium, flagellum well developed (Figs. 3 B, C, 4A–C).
Chelipeds stout, unequal; outer surfaces covered with numerous small rounded granules (Fig. 4 D). Carpus ovate, without setae on upper outer surface, inner angle low, granular, outer face uneven or with small granules; merus short, upper outer surface covered with granules, fringe of setae dorsomesially (Fig. 2 B, C). Major chela with gaping fingers when closed; palm slightly longer than wide, inner surface with subdorsal expanded triangular granular structure, median tubercle (Fig. 4 E, F); dactylus relatively short, dorsal margin with small granules, tufts of setae proximally; cutting edge of dactylus with 2 large proximal teeth, rest of edge crenulated; propodal finger smooth, cutting edges with low teeth (Fig. 4 D). Minor chela similar but palm more slender, ventral margin with median tufts of short setae; fingers strongly gaping when closed; dorsal margin with rounded granules, setae (Fig. 4 D).
P2–P4 short, stout; P3 longest (Figs. 2, 4 G–I). P2 shortest, stoutest; merus almost 3 times length of carpus, outer surface with numerous distinct small granules, short setae, upper, lower margins with short setae, unarmed otherwise; dactylus slender, gently upcurved, fringe of setae on upper, lower margins, slightly longer than propodus (Fig. 4 G). P3 relatively stouter, longer than P2; surfaces relatively more granular; merus almost 3 times length of carpus, outer surface with numerous distinct, small granules, setae, upper, lower margins with setae, unarmed otherwise; carpus as long as propodus, outer surface with granules, denser on upper half, surface with short setae; outer surface of propodus granular, lower margin with setae, crenulations, spines present distally; relatively strongest among pereopods (Fig. 4 H). P4 most slender; merus more than twice length of carpus, outer surface with small granules, short setae, upper, lower margins with tufts of dense short setae, unarmed otherwise; carpus as long as propodus, outer surface with granules, denser on upper half, surface with short setae; outer surface of propodus with low granules, lower margin with setae, crenulations, spines present distally; dactylus slender, gently upcurved, fringe of setae on upper, lower margins, slightly longer than propodus (Fig. 4 I).
Male thoracic sternum broad; sternites 1, 2 fused, forming subtriangular structure, each with deep transverse groove that does not extend to lateral margins, separated from sternite 3 by distinct ridge, adjacent deep transverse groove; sternite 3 longitudinally narrow, posteromedian part with 2 granular projections that bracket telson; sternite 3 delimited from sternite 4 by deep tranverse groove that forms thoracic sternal groove, relatively shallower along margins; sternites 4–7 well developed, separated by distinct sutures, surfaces distinctly punctate; sternite 8 just exposed when abdomen closed, visible as small triangular plate between abdomen, sternite 7, about half length as male abdominal somite 1 (Figs. 5 C). Sternoabdominal cavity elongated, reaching to just below base of third maxilliped, posterior margin of thoracic sternite 3 (Figs. 3 C, 5A). Abdominal press button locking mechanism of male distinct, present as large rounded tubercle on anterior edge of sternite 5, overlapping posterior part of sternite 4 (Fig. 5 D). Male abdomen relatively long, slender, surfaces granular; somites 1, 2 free; somites 3–5 fused but median parts of sutures still visible as shallow grooves, lateral margins sinuous; somite 6 longer than broad, shorter than fused somites 3–5, longer than telson, lateral margins convex; telson pentagonal, with rounded distal margin, angular anterolateral edge (Fig. 5 A–C).
G1 simple, slender, curved along distal third, lying in oblique groove on anterior part of sternum, distal onefifth not concealed under abdomen; distalmost part gradually tapering to subtruncate tip; lateral margins, distal surfaces lined with long plumose setae which partially obscure surfaces (Fig. 6A–D). G2 short, about quarter of G1 length; distal part spatuliform (Fig. 6E, F).
Females. Chelipeds approximately symmetrical, inner surface subdorsal margin has raised patch of granules but no obvious projection. Female thoracic sternum relatively broad (Fig. 5 E), sternites 1, 2 fused, each with transverse groove, separated from sternite 3 by distinct ridge, transverse groove; sternite 3 separated from sternite 4 by ridge laterally, thoracic sternal groove short, shallow, extends obliquely from sternoabdominal cavity (Fig. 5 E, H); sternites 4–7 distinct, separated by distinct sutures; sternite 8 just visible as small triangular plate when abdomen closed, about half length of abdominal somite 1 (Fig. 5 G). Vulvae large, without opercular cover, on anterior half on sternite 6, opening subovate (Fig. 5 H). Sternoabdominal cavity reaches distal part of sternite 4. Female abdomen subovate, with 6 free somites; somite 6 longest, trapezoidal; telson subtriangular with rounded tip (Fig. 5 E–G,).
Variation. As in Hexapinus simplex, the ambulatory meri of the male (Figs. 2 B, 4G–I) is proportionately longer than those of females (Fig. 2 A, C). There is also a distinct tendency for specimens to become proportionately broader as they increase in size. The largest females from Japan have carapace width to length ratios of 1.71–1.73, with the intermediate sized ones from Ambon and Japan 1.56–1.67. The smallest specimens (including the lectotype) are 1.34–1.49. The form of the merus of the third maxilliped also varies, with smaller specimens having a slightly more quadrate structure (Fig. 4 B, C) while in the largest ones, they are more subovate (Fig. 4 A).
Remarks. This is a poorly known species and hitherto known only from females. Rahayu & Ng (2014: 409) commented that: “De Man’s (1888: 322, pl. 13, fig. 3) female specimen from Ambon is also likely to be a species of Hexapinus although he does not show the maxillipeds; it resembles H. simplex described from nearby Lombok”. De Man (1888: 322) actually had two specimens (a male and a female) and figured the male. Examination of these two specimens, which originally belonged to the Museum of Zoology, Göttingen University but are now conserved in SMF, shows that they belong to H. latipes instead.
Rahayu & Ng (2014: 423) noted that Hexapinus simplex can be separated from Hexapinus latipes by its relatively wider carapace, the dorsal surface of the carapace is less prominently pitted, and the ischium of the third maxilliped is relatively shorter, with the mesial margin more strongly convex. The form of the carapace surface and proportions of the third maxilliped are obviously different in the present specimens of both species, although the proportions of the carapace is not an easy character to use as it can vary. The pits on the carapace of Hexapinus latipes are very pronounced and all over the dorsal surface. This is evident even on the small holotype female (Rahayu & Ng, 2014: fig. 10A). In Hexapinus simplex, while there are also pits, they are less distinct, shallower, more scattered even when more prominent and not over the entire surface (Rahayu & Ng 2014: fig. 18C).
Another useful character is the form of the chela. In adult male Hexapinus latipes, the inner surface of the major palm has a well-developed dorsal, subdorsal granular projection (Figs. 2 B, 4E, F). It is also visible on the smaller chela; and in female chelae, but is relatively shorter (Fig. 2 A, C). This character is visible even in the holotype female and Japanese female specimens (Rahayu & Ng 2014: figs. 10A, 11A, 12A). In Hexapinus simplex, the inner surface of the palm is almost smooth and not raised (Rahayu & Ng, 2014: fig. 18). The G1 of Hexapinus latipes is similar to that of Hexapinus simplex, but the median part is relatively stouter and towards the distal third, it bends outwards, with the distal part gradually tapering to a subtruncate tip (Fig. 6A–D). In Hexapinus simplex, the median part of the G1 is more slender, the entire G1 gently curves outwards and the distal part tapers down suddenly to a slender rounded tip (Rahayu & Ng 2014: fig. 21A, B).
Also noteworthy, as discussed by Rahayu & Ng (2014), Hexapinus latipes grows to much larger adult sizes. A female measuring 10.2 by 7.6 mm from Japan was still immature, with the abdomen narrow and the gonopods not evident (Rahayu & Ng 2014: 414, fig. 11). The largest specimen of Hexapinus latipes, a mature female, measures 30.4 mm in carapace width (Fig. 2 A; Rahayu & Ng 2014: 412). Specimens of Hexapinus simplex measuring about 6.0 to 7.0 mm in carapace width are already mature, with the males having well-developed gonopods and the females having an ovate abdomen with setose pleopods (Rahayu & Ng 2014: 418). The present adult specimens of Hexapinus latipes from Ambon are mature, with the male measuring 20.9 mm and the female 18.9 mm in carapace width, respectively.
The form of the merus of the third maxilliped appears to vary, being more quadrate in smaller specimens (Fig. 4 B, C) but more ovate in large ones (Fig. 4 A); as has been noted by Rahayu & Ng (2014: fig. 13C–E). The mesial margin of the ischium, however, is much less convex (Fig. 4 A–C) compared to that of Hexapinus simplex (Rahayu & Ng 2014: fig. 20G).
description: Published as part of Ng, Peter K. L. & Rahayu, Dwi Listyo, 2015, Notes on Hexapodidae (Crustacea, Brachyura) from Indonesia and Malaysia, with description of a new species of Mariaplax Rahayu & Ng, 2014, from Lombok, Indonesia, pp. 125-137 in Zootaxa 3981 (1) on pages 126-134, DOI: 10.11646/zootaxa.3981.1.7, http://zenodo.org/record/241815