Hexaplax megalops Doflein, 1904

Rahayu, Dwi Listyo & Ng, Peter K. L., 2014, New genera and new species of Hexapodidae (Crustacea, Brachyura) from the Indo-West Pacific and east Atlantic, Raffles Bulletin of Zoology 62, pp. 396-483 : 465-473

publication ID	https://doi.org/ 10.5281/zenodo.5353945
publication LSID	lsid:zoobank.org:pub:4CF42744-861A-4635-9703-E6639CEBFAA9
persistent identifier	https://treatment.plazi.org/id/03EB87DA-3B7E-E31B-FECC-FED5FB2F9DB0
treatment provided by	Tatiana
scientific name	Hexaplax megalops Doflein, 1904
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Hexaplax megalops Doflein, 1904 View in CoL

( Figs. 2E View Fig , 55 View Fig , 56A, B View Fig , 57A View Fig , 58A, B View Fig , 59A–C View Fig , 60A View Fig , 61C View Fig , 62 View Fig )

Hexaplax megalops Doflein, 1904: 122 View in CoL , pl. 31 figs, 3, 4, pl. 50 fig. 7; Ng et al., 2008: 86 (part).

Material examined. Holotype: male (15.0 × 10.0 mm) ( ZMB) (photographs), station 199, 00°15.5'N 98°04'E, 470 m, Nias , Indonesia, coll. German Deep Sea Expedition, RV Valdivia, 1898 – 1899. Others : 13 males (10.8 × 6.6 mm – 18.7 × 10.7 mm), 10 females (13.9 × 9.4 mm – 17.7 × 12.2 mm), 1 ovigerous female (16.9 × 11.2 mm) ( MNHN-IU 2011-2251 ), station CP 3740 09°12'S 152°16'E, 556–645 m, coll. BIOPAPUA Expedition , 10 October 2010; 1 female (17.9 × 12.9 mm) (MNHN-IU-2011-2820), station CP 3739, 09°09'S 152°15'E, Woodlark Island , 503–546 m, coll. BIOPAPUA Expedition , 10 October 2010; 3 males (11.7 × 8.0 mm – 18.0 × 11.7 mm), 5 females (15.7 × 10.5 mm – 17.6 × 11.7 mm) (MNHN-IU-2011-2425), 2 males (14.6 × 9.4 mm, 15.0 × 10.0 mm), 2 females (17.9 × 11.9 mm, 18.1 × 12.3 mm) ( ZRC 2013.1695 View Materials ex MNHN-IU-2011-2425), station CP 3743, 09°11'S 152°16'E, Woodlark Island , 540–585 m, coll. BIOPAPUA Expedition , 10 October 2010; 1 male (12.7 × 8.5 mm), 2 females (15.7 × 10.8 mm, 13.6 × 9.3 mm) ( MNHN-IU 2011-2149 ), station CP 3741, 09°14'S 152°18'E, Woodlark Island , 694–766 m, coll. BIOPAPUA Expedition , 10 October 2010; 1 female (14.3 × 9.6 mm) (MNHN-IU-2011-3275), station CP 3671, 04°04'S 151°56'E, north of Rabaul , 585–601 m, coll. BIOPAPUA Expedition , 29 September 2010; 2 males (13.9 × 9.4 mm, 10.8 × 7.2 mm), 1 female (13.9 × 9.3 mm) (MNHN-IU-2011-1168), 1 male (13.4 × 8.9 mm) (MNHN-IU-2011-3343), station CP 3739, 09°09'S 152°15'E, Woodlark Islands , 503–546 m, coll. BIOPAPUA Expedition , 10 October 2010; 1 female (12.5 × 8.3 mm) (MNHN-IU-2011-987), station CP 3655, 02°15'S 150°16'E, west of New Hanover , 402–440 m, coll. BIOPAPUA Expedition , 28 August 2010; 6 males (13.6 × 9.0 mm – 15.9 × 10.1 mm), 7 females (7.8 × 5.4 mm – 14.5 × 9.8 mm) (MNHN-IU-2011-980), 1 male (13.5 × 9.0 mm) ( MNHN- IU-2011-1026 ), station CP 3654, 02°14'S 150°16'E, west of New Hanover , 490–505 m, coll. BIOPAPUA Expedition , 28 August 2010; 2 males (12.3 × 8.2 mm, 13.0 × 8.4 mm), 3 females (13.7 × 9.3 mm – 18.1 × 12.1 mm) (MNHN-IU-2011-999), station CP 3670, 04°06'S 151°56'E, north of Rabaul, 497–500 m, coll. BIOPAPUA Expedition, 24 September 2010. GoogleMaps

Diagnosis. Carapace subquadrate, about 1.5 times as broad as long, dorsal surface smooth, few fine granules on anterolateral surface; regions indistinct, median H-shaped depression shallow. Anterolateral margin arcuate; posterolateral corner with small angle over base of posterior pereopods. Front not deflexed, divided into 2 lobes, not projecting beyond distal edge of orbits in dorsal view. Orbit distinct, transverse; eye movable, large, corneas dilated, pigmented, much wider than peduncle. Pterygostomial region with long row of about 45 short striae forming oblique stridulatory ridge which reaches posterior margin of epistome; shallow oblique groove above ridge. Third maxillipeds not completely covering buccal cavity, with gap when closed; ischium longer than merus, dilated distally, mesial margin with row of small teeth, rounded; merus squarish, as long as broad; exopod relatively narrow, about 0.3 width of ischium, flagellum well developed. Chelipeds subequal; major chela with gap when fingers closed; with large teeth at cutting edge of dactylus and fixed finger; dactylus, palm and carpus finely granulated on outer surface; minor chela with slight gap when fingers closed; ornamentation similar to that of major chela. P2–P4 long, slender; outer surface finely granulated on upper half, lower half almost smooth; merus of P4 8 times as long as broad, upper margin of merus and carpus finely serrated, outer surface of merus flattened; shallow longitudinal groove on outer surface of propodus. Male thoracic sternum broad, surface finely granular; sternites 1 and 2 fused, separated from sternite 3 by distinct ridge; sternite 3 not separated from sternite 4, sternites 4–7 well developed, separated by distinct sutures. Male abdomen relatively narrow; somite 1 hidden under carapace; somite 2 free, somites 3–5 fused with shallow but distinct suture still visible, lateral margin sinuous; somite 6 wider than long, lateral margin slightly expanded subproximally; telson rounded, as long as somite 6. G1 long, stout, tapering to pointed tip; row of 4 tiny spines subdistally. Female thoracic sternum broad, female abdomen relatively broad; somite 1 hidden under carapace; somites 2–6 free; telson rounded, slightly shorter than somite 6.

Description. Carapace subquadrate, about 1.5 times as broad as long, dorsal surface smooth, few fine granules on anterolateral surface; regions indistinct, median H-shaped depression shallow ( Fig. 55 View Fig ). Anterolateral margin arcuate; posterolateral corner with small angle over base of posterior pereopods. Front not deflexed ( Fig. 56A, B View Fig ), divided into 2 lobes, not projecting beyond distal edge of orbits. Orbit distinct, transverse; eye movable, large, corneas dilated, pigmented, much wider than peduncle ( Figs. 55 View Fig , 56A, B View Fig ). Pterygostomial region with oblique stridulatory ridge consist of long row of 28 short striae with ca. 17 relatively shorter, less produced, reaching distal edge of mouthpart ( Fig. 60A View Fig ); shallow oblique groove above ridge. Third maxillipeds ( Figs. 58A, B View Fig , 62A View Fig ) relatively broad, not completely covering buccal cavity, with gap when closed; ischium longer than broad, longer than merus, dilated distally, mesial margin with row of small teeth, rounded; merus squarish, as long as broad; exopod relatively narrow, about 0.3 width of ischium, flagellum well developed.

Chelipeds subequal ( Fig. 57A, B View Fig ); major chela with gap when fingers closed; with large teeth at cutting edge of dactylus and fixed finger; dactylus, palm and carpus finely granulated on outer surface; outer surface of dactylus with 2 longitudinal ridges, inner surface with longitudinal ridge consist of oblique striae; fixed finger with 2 longitudinal ridges; lower margin with row tiny teeth continuing to lower margin of palm, inner surface of palm with 2 low longitudinal ridges; minor chela with slight gap when fingers closed; ornamentation similar to that of major chela.

P2–P4 long, slender ( Fig. 55 View Fig ); P2 most slender; dactylus straight, slightly shorter or as long as propodus, longitudinal ridge medially on outer surface, propodus and carpus smooth on outer surface; carpus shorter than propodus; merus 2.9 times length of carpus, upper margin with small granules. P3 longer than P2; dactylus straight, shorter than propodus, longitudinal ridge medially on outer surface, propodus smooth with shallow longitudinal groove on outer surface; carpus shorter than propodus, outer surface finely granulated on upper half, upper margin with row of small denticles; merus 3.2 times length of carpus, upper and lower margin with row of small denticles distally, proximally smooth, outer surface with tiny granules on distal upper half. P4 shorter than P3; merus 3.4 times length of carpus, 8.8 times as long as broad, outer surface of merus slightly flattened, ornamentation same as P3 ( Fig. 59A, B, C View Fig ).

Male thoracic sternum broad ( Fig. 58A, B View Fig ), surface finely granular; sternites 1 and 2 fused, separated from sternite 3 by distinct ridge; sternite 3 not separated from sternite 4, sternites 4–7 well developed, separated by distinct sutures; sternite 8 not visible; sternoabdominal cavity reaching middle of sternite 4, no thoracic sternal groove. Male abdomen relatively narrow ( Figs. 58B View Fig , 62B View Fig ); somite 1 hidden under carapace; somite 2 free, somites 3–5 fused with shallow suture visible, lateral margin sinuous; somite 6 wider than long, lateral margin slightly expanded subproximally; telson rounded, as long as somite 6. G1 long, stout, tapering to pointed tip ( Figs. 58A View Fig , 62C, D View Fig ); row of 4 tiny spines subdistally. Female thoracic sternum broad, female abdomen relatively broad ( Fig. 62E View Fig ); somite 1 hidden under carapace; somite 2–6 free; telson rounded, slightly shorter than somite 6.

Colour. In life, the carapace is orangish-yellow, with the margin orange, the eye is cream, the orbit is lined with orange, and there is an orange spot under the cornea. The cheliped is white with an orange spot at the articulation of the dactylus and palm, with the distal margins of the carpus, merus, dactylus and propodus of P2−P4 white, the carpus is white with a tinge of orange, and the merus is orange distally and orangish-white proximally ( Fig. 2E View Fig ).

Remarks. The three species of Hexaplax share the same general shape of the carapace, third maxilliped and P2–P4. It is therefore not surprising that material from the Pacific have been referred to H. megalops without query, especially since few specimens have been collected. The good series of specimens of all three species on hand allows more detailed comparisons to be made. The carapace of H. megalops s. str. is proportionately broader, about 1.5 times as broad as long ( Fig. 55 View Fig ) (carapace slightly narrower, being 1.4 as broad as long in H. aurantium , new species, and H. saudade , new species, Figs. 63 View Fig , 65 View Fig ); the dorsal surface of the carapace is unevenly convex in frontal view, with the branchial regions more inflated than the gastric regions ( Fig. 56A, B View Fig ) (the surface is evenly convex in H. aurantium and H. saudade , Fig. 56C, D View Fig ); the merus of the third maxilliped of H. megalops is longer than broad ( Fig. 62A View Fig ) while in H. aurantium and H. saudade , it is about as long as broad ( Figs. 64A View Fig , 66A View Fig ); the P2–P4 of H. megalops is proportionately the longest among the three species with the merus of P4 is 8.8 times as long as broad ( Fig. 59A–C View Fig ) (in H. aurantium and H. saudade , the merus is 7.6 times and 7 times as long as broad, respectively ( Fig. 59D–H View Fig ). In addition, the outer surface of the carapace, chelipeds and P2–P4 of H. megalops are almost smooth ( Figs. 55 View Fig , 57A, B View Fig , 59A–C View Fig ), while in H. aurantium , these structures are densely granulated ( Figs. 57 C View Fig , 59D–F View Fig , 63 View Fig ), and in H. saudade , they are finely granulated ( Fig. 57D View Fig , 59G, H View Fig , 65 View Fig ). Comparisons of female specimens of H. aurantium and H. saudade of similar sizes also show that the abdomen is relatively broader in H. aurantium , with the thoracic sternum also proportionately broader in H. saudade ( Fig. 64E View Fig versus Fig. 66E View Fig ). The stridulatory striae on the pterygostomial region of the three species are also different. Hexaplax megalops has the most, with ca. 45 closely-spaced striae, including the relatively smaller and less produced ones adjacent to the epistome ( Fig. 60A View Fig ), while H. aurantium and H. saudade have 38 or 39 striae which are spaced relatively further apart ( Fig. 60B, C View Fig ). Doflein’s (1904) figure of the abdomen of the type specimen ( Doflein, 1904: pl. 31 fig. 4) depicted an abdomen with 6 free somites and telson. In the specimens of H. megalops studied here, male abdominal somites 3–5 are fused, but the suture separating them is distinct but shallow ( Fig. 58B View Fig ). It is similar in H. aurantium ( Fig. 58C View Fig ), but in H. saudade , the sutures are not discernible and somites 3–5 appear as one large plate ( Fig. 58D View Fig ). The G1s of the three species are different. In H. megalops , the G1 is relatively longer and stouter, with a row of four small denticles on the mesial margin ( Fig. 62C, D View Fig ); in H. aurantium , the G1 is more slender, without any teeth or denticles ( Fig. 64C, D View Fig ); while in H. saudade , the G1 is more slender, with small denticles on the median part of the mesial margin ( Fig. 66C, D View Fig ; see also Serène, 1964: 270, text fig. 21B).