Theoxapus, 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 : 478-481

Theoxapus , new genus

Diagnosis. Carapace broader than long, slightly convex, surface granular, region indistinct, with H-shaped medial depression. Anterolateral margin rounded; lateral margins straight, parallel. Eyes short, movable, corneas subglobular, visible. Third maxilliped broad, completely covering buccal cavity; ischium notably broader and longer than merus, mesial margin slightly convex, crenulated; merus relatively ovoid, crenulated on mesial and lateral margin; carpus, propodus and dactylus cylindrical, dactylus longer than propodus; exopod narrow compared to width of ischium, slightly sinuous, margins crenulated, flagellum present. Chelipeds unequal, granulated; P2–P4 relatively long, granulated and setose; merus slender, dactylus straight or upcurved, as long as propodus. Thoracic sternum broad, granulated. Male sternoabdominal cavity reaches distal margin of thoracic sternite 4; without sternal groove. Male abdomen relatively narrow; somites 1, 2 and 6 free, somites 3–5 fused; telson slightly shorter than somite 6, ovate, distal margin rounded. G1 slightly sinuous, stout, dense stiff setae on subdistal, obscuring pointed tip. Female unknown.

Etymology. The genus is named after famed French explorer, naturalist and carcinologist Théodore André Monod (1902–2000), whose 1956 book on West African crabs remains a mainstay of researchers. The name is an arbritary

combination of his first name, with the suffix of the genus Hexapus . Gender of genus masculine.

Species included. Hexapus buchanani Monod, 1956 (by monotypy).

Remarks. When Monod (1956: 372) described Hexapus buchanani and placed it in the subgenus Hexapus , he included H. sexpes sensu Balss (1922) . As Balss (1922) indicated that the shape of the third maxilliped is similar to Hexapus sexpes sensu Stebbing (1910: 315) , he therefore placed his species in Hexapus s. str. Hexapus sexpes sensu Stebbing (1910) is now known as Tritoplax stebbingi (Barnard, 1947) (see Manning & Holthuis, 1981: 180). Manning & Holthuis (1981: 171), who did not have specimens, transferred Hexapus (Hexapus) buchanani to Hexapinus Manning & Holthuis, 1981 , without any comments, although it was the only member of the genus occuring in the Atlantic. Considering the present revision of Hexapus s. str. and Hexapinus s. str., Hexapus (Hexapus) buchanani Monod, 1956 , is closest to species in Mariaplax , new genus, in that it has a similarly shaped third maxilliped ( Fig. 68G ). Like many Mariaplax species , Hexapus buchanani also has a prominently granulated carapace and pereopods ( Figs. 67A , 68E ). However, Hexapus buchanani cannot be referred to Mariaplax as it has distinctively more ovate carapace with the lateral margins subparallel ( Figs. 67A , 68E ) (rather than more trapezoidal with the lateral margins diverging), the ambulatory meri do not have a longitudinal groove on the outer surface, not even on P3 ( Figs. 67A , 68E ); there is no trace of a male thoracic sternal groove or depression adjacent to the sternoabdominal cavity ( Fig. 67B ); it has a proportionately more elongate male abdomen ( Fig. 68F ); the telson is elongated and much longer than broad ( Fig. 68F ); and it has a relatively much straighter G1 which has the distal part surrounded by long setae ( Fig. 68H, I ). Externally, especially with regards to the general carapace shape, Hexapus buchanani resembles other West African taxa, notably Parahexapus africanus (Balss, 1922) . However, Hexapus buchanani differs markedly from Parahexapus africanus in that the ischium of the third maxilliped is quadrate ( Fig. 68G ) (elongated and ovate in P. africanus , Fig. 68C ); the exopod of the third maxilliped is proportionately longer, reaching to half the length of the merus ( Fig. 68G ) (relatively shorter, reaching to just beyond the length of the ischium in P. africanus , Fig. 68C ); fused male abdominal somites 3–5 are proportionately longer, and longer than somite 6 ( Fig. 68F ) (fused male abdominal somites 3–5 are proportionately shorter, and subequal in length to somite 6 in P. africanus , Fig. 68B ); and the G1 is stouter with the tip tapered and the distal part covered with long setae ( Fig. 68H, I ) (G1 very slender, with the distal third sharpy tapering and the median surfaces lined with short spines in P. africanus , Fig. 68D ).

These differences are significant are indicate that Hexapus buchanani cannot be transferred to Mariaplax or Parahexapus , or any of the other recognised genera. A new genus, Theoxapus , is therefore established here to accommodate the species.

Figures

Fig. 68. A–D, Parahexapus africanus (Balss, 1922); E–I, Theoxapus buchanani (Monod, 1956).A, E, overall views of carapaces, chelipeds and P2–P4; C, G, left third maxillipeds; B, F, male abdomen and telsons; D, H, G1s; I, distal part of G1. A, C, after Monod, 1956: figs. 494, 496; B, D, after Crosnier, 1967: figs. 30, 33; E–I, after Monod, 1956: figs. 497–500.

Fig. 67. Theoxapus buchanani (Monod, 1956), holotype male (7.0 × 4.9 mm) (NHM 1957.12.4.29). A, overall view of carapace, chelipeds and P2–P4; B, ventral view of cephalothorax; C, frontal view of cephalothorax; D, chelae.