Mariaplax, 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 : 423-424

Mariaplax , new genus

Diagnosis. Carapace broader than long, surface granular, region indistinct, H-shaped medial depression shallow. Anterolateral margin arcuate; pterygostomial region with rows of long and short, broken or entire, oblique striae and scattered tubercles. Eyes small, slightly movable, corneas subglobular. Third maxilliped broad, completely covering buccal cavity; ischium longer than merus, mesial margin straight, oblique, or very slightly rounded; carpus, propodus and dactylus cylindrical, dactylus longer than propodus; exopod narrow compared to width of ischium, flagellum present. Chelipeds equal or subequal. P2–P4 relatively long; merus slender, usually with distinct median groove on lateral surface on some of legs; dactylus straight or upcurved, as long as propodus. Thoracic sternites 1 and 2 fused, sternite 3 partly or completely separated from sternite 4 by tuberculate ridge, sternite 8 exposed, triangular or subrectangular, as long as or half length of first abdominal somite. Male sternoabdominal cavity reaches distal margin of thoracic sternite 4; thoracic sternal groove oblique, short or long, straight or curved, wide or narrow, sometimes reaches base of cheliped. Male abdomen relatively narrow; somites 1, 2 and 6 free, somites 3–5 fused; telson shorter than somite 6, subtriangular to subpentagonal, distal margin rounded. G1 curved or twisted, more or less S-shaped, apice exposed on thoracic sternal groove, not concealed under abdomen. Female abdomen with 6 free somites and telson.

Etymology. We dedicate this new genus to Mary J. Rathbun who described Lambdophallus anfractus , the type species. The name “ Mariaplax ” is derived from “Maria”, the Latin of her first name Mary, and “-plax”, a typical suffix for many crab genera. Gender of genus feminine.

Species included. Lambdophallus anfractus Rathbun, 1909 (type species by present designation), M. chenae , new species,

M. cyrtophallus , new species, M. daviei , new species, M. galaxeae , new species, Hexapus granuliferus Campbell & Stephenson, 1970 , M. mica , new species, M. narusei , new species, M. ourabay , new species, M. propinqua , new species, M. secus , new species, M. sinensis , new species, Hexapus stephenseni Serène & Soh, 1976 , and M. streptophallus , new species.

Remarks. Rathbun (1909) assigned her new species to Lambdophallus without any comments. Manning & Holthuis (1981) transferred Lambdophallus anfractus to Hexapus on the basis of the G1 shape, and the shape and extension of the male thoracic sternal groove. However, as discussed for Mariaplax chenae , new species (see later), the form of the male thoracic sternal groove sometimes varies with the size of the specimen.

Lambdophallus anfractus shares a unique suite of third maxilliped, ambulatory leg, male abdominal and G1 characters with Hexapus granuliferus Campbell & Stephenson, 1970 , and H. stephenseni Serène & Soh, 1976 , as well as 11 new species from the Indo-West Pacific. These characters argue for establishing a new genus Mariaplax , for these species.

Mariaplax , new genus, differs markedly from Lambdophallus in that it has slightly movable eyes, with globular corneas (versus immovable eyes with small corneas in Lambdophallus ); the male telson is shorter than the sixth abdominal somite ( Fig. 23C ) (versus the telson is longer than the sixth abdominal somite in Lambdophallus, Alcock, 1902 : pl. 62, fig. 1); and the G1 is usually twisted and S-shaped in Mariaplax (see Fig. 23F, G ) (with the exception of M. cyrtophallus , new species and M. secus , new species) (versus sharply bent at a 90° angle and L-shaped in Lambdophallus, Alcock, 1902 : pl. 62, fig. 1).

Mariaplax differs from Hexapus s. str. as follows: the third maxilliped of Mariaplax is broad, completely covering the buccal cavity, and the ischium is as broad as the merus ( Fig. 23D ) (the third maxilliped is longitudinally narrow, showing most of the underlying mouthparts, with the ischium much narrower than the merus in Hexapus , Fig. 4E ); P2–P4 is relatively longer, with the merus usually possessing a shallow or deep longitudinal groove on the outer surface in Mariaplax (the groove on outer surface of P2–P4 is absent in Hexapus ).

Hexapinus s. str. differs from Mariaplax in possessing a subauriculiform ischium of the third maxillipeds ( Fig. 13C ) (subtruncate or subquadrate ischium in Mariaplax , Fig. 23D ), a male thoracic sternite 4 which is transversely narrower but longitudinally broader ( Fig. 10B ) (transversely broader but longitudinally narrower in Mariaplax , Fig. 23B, C ), and the short P2–P4 with almost evenly convex lateral surface ( Figs. 11A , 12A ) (long P2–P4 with longitudinal groove on the outer surface of the merus in Mariaplax , Fig. 22A, C ).

As discussed earlier, Hexapus granuliferus Campbell & Stephenson, 1970 , originally placed in Hexapinus by Manning & Holthuis (1981), and Hexapus stephenseni ( Serène &Soh,

1976), which was placed in Tritoplax by Huang et al. (2002) and Ng et al. (2008), should be transferred to Mariaplax .

Figures

Fig. 23. Mariaplax anfracta (Rathbun, 1909). A–G, paratype male (5.0 × 3.2 mm) (USNM 39752); H, female (4.1 × 2.8 mm) (ZRC 1965.11.24.6). A, dorso-lateral margin of carapace; B, thoracic sternites 1–4, showing sternoabdominal cavity and thoracic sternal groove; C, thoracic sternites3 and 4, showing thoracic sternal groove and telson; D, right third maxilliped; E, major chela; F, G, G1; H, female abdomen and telson. Scale bars = 1.0 mm.

Fig. 4. Hexapus sexpes (Fabricius, 1798), male (8.5 × 5.4 mm) (ZRC 2012.1014). A, ventral view of cephalothorax; B, front, showing orbit and eye; C, chelae; D, third maxilliped and abdomen.

Fig. 13. Hexapinus latipes (De Haan, 1835). A, B, E, G, female (10.2 × 7.6 mm) (NSMT-Cr 5694); C, holotype female (10.6 × 6.4 mm) (RMNH 31783d; D, F, female (30.4 × 17.5 mm) (KPM NH 6331). A, overall view of carapace; B, thoracic sternites 1–5, showing thoracic sternal groove and sternoabdominal cavity; C–E, left third maxilliped; F, G, female abdomen and telson. Scale bars: A, B, D–G = 1.0 mm; C not to scale.

Fig. 10. Hexapinus latipes (De Haan, 1835), lectotype female (10.0 × 6.7 mm) (RMNH 31783d). A, overall view of carapace, chelipeds and P2–P4; B, ventral surface of cephalothorax; C, frontal view of cephalothorax; D, card with mouthparts.

Fig. 11. Hexapinus latipes (De Haan, 1835), female (10.2 × 7.6 mm) (NSMT-Cr 5694). A, overall view of carapace, chelipeds and P2–P4; B, ventral surface of cephalothorax; C, chelae.

Fig. 12. Hexapinus latipes (De Haan, 1835), 1 female (30.4 × 17.5 mm) (KPM NH 6331). A, overall view of carapace, chelipeds and P2–P4; B, ventral surface of cephalothorax; C, frontal view of cephalothorax.

Fig. 22. Mariaplax anfracta (Rathbun, 1909). A, B, paratype female (6.1 × 4.0 mm) (USNM 39752); C, paratype male (5.0 × 3.2 mm) (USNM 39752). A, C, overall views of carapace, chelipeds and P2–P4; B, frontal view of cephalothorax.