Mariaplax cyrtophallus, 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 : 431-434

Mariaplax cyrtophallus , new species

( Figs. 28 , 29 )

Material examined. Holotype: male (6.5 × 4.2 mm) ( MZB), off Otakwa , Timika, Papua, Indonesia, near 05º01'32"S 137º10'56.7"E, coll. A. Haris, 7 November 2000.

Diagnosis. Carapace subquadrate, about 1.5 times as broad as long, dorsal surface strongly granulated; regions indistinct, with H-shaped shallow depression medially ( Figs. 28A , 29A ). Eye small, slightly movable, cornea black, wider than granular peduncle. Pterygostomial region ( Figs. 28B , 29D ) covered by granules, with row of 5 oblique striae. Third maxillipeds broad ( Figs. 28B , 29E ), completely covering buccal cavity; ischium slightly 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; exopod relatively narrow, about 0.4 width of ischium, flagellum well developed. Chelipeds asymmetrical, covered by closely spaced tubercles ( Fig. 29G, H ); major chela ( Fig. 29G ) with slightly gaping finger when closed; dactylus and fixed finger with longitudinal ridge at midline distally; palm slightly shorter than broad; minor chela ( Fig. 29H ) with narrow gap when fingers closed; dactylus and fixed finger with median longitudinal ridge distally. P2–P4 ( Fig. 29I ) relatively long, slender, covered by dense granules; P3 longest ( Fig. 29I ); merus of P4 relatively long, about 3.4 times as long as broad; dactylus of P4 gently upcurved ( Fig. 29J ); merus without discernible distinct median groove on lateral surface ( Fig. 29J ). Male thoracic sternum broad ( Fig. 29B, C ), surface prominently granular; sternites 1 and 2 fused, separated from sternite 3 by distinct furrow; sternite 3 separated from sternite 4 laterally, medially appear fused, thoracic sternal groove deep, transverse, extending laterally from edge of sternoabdominal cavity, parallel to frontal margin, not reaching base of chelipeds ( Figs. 28B , 29C ); sternite 8 exposed, quadrangular, as long as male abdominal somite 1; sternoabdominal cavity not reaching middle to sternite 4 ( Fig. 29C ). Male abdomen relatively narrow ( Fig. 29F ), surface granular; somites 1 and 2 free, somites 3–5 fused, lateral margin slightly arched; somite 6 shorter than length of somites 3–5, median part of lateral margin expanded into blunt triangular projection; telson subpentagonal ( Figs. 28B , 29F ), slightly shorter than somite 6, distal margin rounded, sparsely setose. G1 ( Fig. 29J, K ) bent 90° at midlength, directed anterolaterally, distal part tapered, not concealed under abdomen, exposed on thoracic sternal groove. Female unknown.

Colour. Dirty white when freshly preserved.

Etymology. From the Latin cyrto, bend, and phallus, for penis; alluding to the bent structure of the G1. The name is used as a noun in apposition.

Remarks. Several features of the present new species suggest its affinity to Lambdophallus , notably the transverse sternal groove extends a good distance laterally and is parallel to frontal margin, the G1 bent at a 90° angle, and the shape of the third maxilliped that is slightly expanded subproximally and has straight mesial margin. However, Lambdophallus has immovable and very short eyes peduncle, the third maxillipeds was rather narrow and leave wide gap between them, the male thoracic sternal groove starts above the articulation of the sixth abdominal somite and the telson was longer than the sixth somite (Alcock, 1902: pl. 62, fig. 1). These characters indicate that we cannot place this new species in Lambdophallus . Although the longitudinal groove on the outer surface of the merus of P2–P4 is indiscernible, obscured by tubercles, other characters such as the movable eyes peduncle, the male thoracic sternal groove that starts almost at subdistalmargin of the telson, the telson that was shorter than the sixth abdominal somite, and the oblique mesial margin of ischium of the third maxilliped clearly place this species in Mariaplax . The structure of the male thoracic sternal groove ( Fig. 29B, C ) and shape of the G1 of M. cyrtophallus , new species ( Fig. 29I, J ), however, easily distinguishes it from all congeners.

Unusual among members of the genus is that M. cyrtophallus does not have a discernible median groove on the lateral surface of the ambulatory merus. This is distinct in all other congeners, although it may be shallow at times. In M. cyrtophallus , the surface of the merus is covered with dense granules and no groove can be seen. In all other aspects, however, M. cyrtophallus is a typical Mariaplax species. It is apparently closest to M. secus , new species, from the other side of the island of New Guinea, which also has a similar strongly bent G1; but in M. secus , the lateral surface of the ambulatory merus has a distinct longitudinal groove (see discussion for that species).

Distribution. Timika, Papua, Indonesia. Intertidal, 15– 30 m.

Figures

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.