Mariaplax secus, Rahayu & Ng, 2014

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 : 449-453

Mariaplax secus , new species

( Figs. 1I , 46 , 47 )

Material examined. Holotype: male (4.7 × 3.0 mm) ( MNHN IU-2013-916 ), station DW3953, 05°14'S 145°49'E, Bismarck Sea , Madang, south of Cape Kussoron, 130–140 m, Papua New Guinea, coll. Expedition PAPUA NUIGINI, 2012, 7 November 2012.

Diagnosis. Carapace subquadrate, about 1.6 times as broad as long, dorsal surface strongly granulated; regions indistinct, with inconspicuous H-shaped shallow depression medially ( Figs. 46A , 47A ). Eye small, very slightly movable, cornea black, wider than granular peduncle. Pterygostomial region covered with granules, with short row of oblique striae. Third maxillipeds ( Figs. 46B , 47C ) broad, covering buccal cavity; ischium longer than merus, slightly expanded subproximally, with oblique mesial margin, combined length of dactylus, propodus and carpus almost as long as that of merus and ischium, dactylus as long as propodus; exopod relatively narrow, about 0.4 width of ischium, flagellum well developed. Chelipeds asymmetrical, covered by closely spaced tubercles and very short setae ( Fig. 46D ); major chela with broad gaping finger when closed; dactylus and fixed finger with longitudinal ridge at midline distally; palm slightly shorter than broad; minor chela with broad gap when fingers closed; dactylus and fixed finger with median longitudinal ridge distally. P2–P4 ( Fig. 46A ) relatively long, slender, covered by dense granules; P3 longest, lateral surface of merus with shallow longitudinal groove; dactylus of P4 upcurved, ventral margin with few granules proximally; merus of P4 long, about 4.3 times as long as broad. Male thoracic sternum broad ( Figs. 46B , 47B ), surface prominently granular; sternites 1 and 2 fused, separated from sternite 3 by distinct furrow; sternite 3 and 4 fused, thoracic sternal groove deep, relatively short, extending obliquely from edge of sternoabdominal cavity ( Figs. 46B , 47B ); sternite 8 exposed, quadrangular, slightly longer than male abdominal somite 1; sternoabdominal cavity reaching to middle of sternite 4 ( Fig. 47B ), distal margin sinuous. Male abdomen broad, short ( Figs. 46B , 47D ), not covering sternoabdominal cavity, reaching only to fifth sternite; surface granular; somites 1 and 2 free, somites 3–6 fused, lateral margin slightly arched; telson subtriangular ( Figs. 47D ), distal margin rounded. G1 ( Fig. 47E, F ) bent ca. 90° at midlength, directed anterolaterally, tuft of long setae on mesial surface near midlength, distal part tapered, not concealed under abdomen, exposed just before thoracic sternal groove. Female unknown.

Colour. Dirty white in life ( Fig. 1I ).

Etymology. From the Latin secus , for different; alluding to the unusual shape of the male abdomen. The name is used as a noun in apposition.

Remarks. This is the second species in Mariaplax to have the G1 bent at a 90° angle, forming a L-shaped structure; the other species being M. cyrtophallus , new species. The differences between these two species are as follow: the carapace is more rectangular, being 1.6 times as broad as long in M. secus ( Fig. 46A ) (1.5 times as broad as long in M. cyrtophallus , Fig. 28A ); the ischium of the third maxilliped is as long as broad with the dactylus the same length as the propodus in M. secus ( Fig. 47C ) (slightly longer than broad with the dactylus much longer than the propodus in M. cyrtophallus , Fig. 29E ); the thoracic sternum is proportionately less broad in M. secus compared to that of M. cyrtophallus ( Fig. 47B versus Fig. 29B ); the distal part of sternoabdominal cavity is relatively long and narrow, with distal margin sinuous in M. secus ( Figs. 46B , 47B ) (short, broad with rounded distal margin in M. cyrtophallus , Fig. 29B ); the lateral surface of the P3 merus has a distinct shallow longitudinal groove in M. secus ( Fig. 46A ) (an indistinct groove, obscured by tubercles, in M. cyrtophallus , Fig. 29I ); the P4 merus is relatively much longer, being 4.3 times as long as broad in M. secus ( Fig. 46A ) (versus 3.4 times as long as broad in M. cyrtophallus , Fig. 29I ); and the G1 has a tuft of long medial setae in M. secus ( Fig. 47E, F ) (few setae along dorsal and ventral margin in M. cyrtophallus , Fig. 29J, K ).

The male abdomen in M. secus is unusual, being broad and very short, with somite 6 apparently fused to somites 3–5. It is completely different from all know hexapodids. However, this is likely to be associated with some parasitic infection (e.g., a rhizocephalan as it appears unusually swollen), or the result of past damage and anomalous regrowth. As such, two of the other observed differences between M. secus and M. cyrtophallus : the differently structured male sternoabdominal cavities and G1s, should be used with caution. It is possible they have also been affected by the above factors. In any case, the differences in the carapace, third maxillipeds, thoracic sternum and P2–P4 argue for them to be recognised as separate species.

Interestingly, both M. cyrtophallus and M. secus are found in the island of Indonesian Papua – Papua New Guinea, the latter from the east, the former from the west.

Type locality. Madang, south of Cape Kussoron , Bismarck Sea, Papua New Guinea .

Distribution. Madang, Papua New Guinea. Subtidal, 130– 140 m.

Figures

Fig. 1. Colours in life. A, Hexapinus simplex, new species, paratype male (8.7 × 5.0 mm) (MZB), Lombok, Indonesia; B, Hexapinus simplex, new species, male (9.3 × 5.8 mm) (ZRC), Lembeh Straits, Sulawesi Indonesia; C, Hexapinus simplex, new species, male (12.6 × 8.4 mm) (RUMF-ZC-2110), Tekebu, Amami-Ohshima Island, Japan; D, Hexapinus simplex, new species, male (12.6 × 8.4 mm) (ZRC 2013.1688), Funaura, Iriomote Island, Japan; E, Hexapinus simplex, new species, female (8.3 × 6.0 mm) (ZRC 2013.1687), Raffles Reserve near Raffles Lighthouse, Singapore; F, Hexapinus simplex, new species, male (11.2 × 7.0 mm) (ZRC), Pulau Senang, Singapore; G, Mariaplax galaxeae, new species, holotype female (4.2 × 3.2 mm) (ZRC 2013.1690), Singapore Strait; H, Mariaplax chenae, new species, East China Sea (courtsey W.-L. Liu, specimen not examined); I, Mariaplax secus, new species, holotype male (4.7 × 3.0 mm) (MNHN IU-2013-916), Madang, Papua New Guinea; J, Mariaplax mica, new species, holotype female (6.7 × 4.5 mm) (NMCR 39069), Bohol Sea, Philippines.

Fig. 46. Mariaplax secus, new species, holotype male (4.7 × 3.0 mm) (MNHN IU-2013-916). A, overall view of carapace, chelipeds and P2–P4; B, ventral view of cephalothorax; C, front view of cephalothorax; D, major chela and minor chela.

Fig. 47. Mariaplax secus, new species, holotype male (4.7 × 3.0 mm) (MNHN IU-2013-916). A, dorsolateral margin of carapace; B, thoracic sternites 1–6, showing sternoabdominal cavity, thoracic sternal groove and telson; C, left third maxilliped; D, male abdomen and telson; E, F, G1. Scale bars: A, B, D = 1.0 mm; C, E, F = 0.5 mm.

Fig. 28. Mariaplax cyrtophallus, new species, holotype male (6.5 × 3.8 mm) (MZB).A, dorsal view carapace; B, ventral view of cephalothorax.

Fig. 29. Mariaplax cyrtophallus, new species, holotype male (6.5 × 3.8 mm) (MZB). A, dorso-lateral margin of carapace; B, thoracic sternites 1–5, showing sternoabdominal cavity and thoracic sternal groove; C, thoracic sternites 1–5, showing thoracic sternal groove and telson; D, anterior view of cephalothorax; E, left third maxilliped; F, male abdomen and telson, G, major chela; H, minor chela; I, left P4; J, K, left G1. Tubercles on thoracic sternum omitted. Scale bars = 1.0 mm.