Hexaplax 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 : 464

Hexaplax Doflein, 1904

Hexaplax Doflein, 1904: 122 ; Manning & Holthuis, 1981: 171.

Diagnosis. Carapace broader than long, surface very finely granular; regions indistinct, with shallow median H-shaped depression. Anterolateral margin arcuate; pterygostomial region with row of short striae forming long oblique stridulatory ridge. Eye large, movable; cornea dilated. Third maxilliped wide, leaving gap when closed; ischium longer than merus, dilated distally, mesial margin rounded; carpus, propodus and dactylus cylindrical; dactylus longer than propodus; exopod narrow, flagellum present. Chelipeds stout, subequal or unequal, P2–P4 long, slender. Thoracic sternites 1 and 2 fused; sternites 3 and 4 fused; sternite 8 exposed, triangular, reaching proximal half of male abdominal somite 2. Sternal abdominal cavity reaching distal margin of thoracic sternite 4. Male thoracic sternal groove absent. Male abdomen relatively narrow; somite 1 hidden under carapace; male abdominal somites 2 and 6 free; somites 3–5 fused (but sutures may be visible); distal margin of telson rounded. G1 straight to sinuous, apice concealed under abdomen. Female abdomen with 6 free somites and telson.

Species included. Hexaplax megalops Doflein, 1904 (type species by original designation), Hexaplax aurantium , new species, and Hexaplax saudade , new species.

Remarks. Hexaplax is characterised by the large hammershaped cornea, and long, slender P2–P4, and has long believed to be monotypic. Examination of recent collections from Taiwan, the Philippines and Papua New Guinea show that specimens from the Pacific belong to two new species. It is a deep-water genus, with all three species collected from 400 to 500 m.

The record of “ Hexaplax sp. ” by Rathbun (1910: 349, textfig. 37) from the Gulf of Thailand is problematic. Her figure of the carapace ( Rathbun, 1910: text-fig. 37a) superficially resembles Hexaplax megalops (or one of the other two species) and Rathbun commented that it had reniform eyes. However, it was a very small male only 1.8 mm in carapace width and in poor condition. Most surprisingly, it was obtained in only six fathoms (ca. 11 m) of water that is much shallower than what is known for members of this genus which are almost always from the deep sea, in excess of 200 m. The third maxilliped she figured ( Rathbun, 1910: 349, text-fig. 37b), however, suggests that her specimen is not a Hexaplax species , being elongated with a rectangular ischium and the dactylus is shorter than the propodus. In Hexaplax , the ischium is much shorter and broader, and the dactylus is much longer than the propodus ( Figs. 62A , 64A , 66A ). This specimen will need to be re-examined to ascertain what genus it belongs to.

Figures

Fig. 62. Hexaplax megalops Doflein, 1904. A–D, male (18.7 × 12.1 mm) (MNHN-IU-2011-2251); E, female (17.7 × 12.2 mm) (MNHN- IU-2011-2251). A, left third maxilliped; B, male abdomen and telson; C, D, G1; E, female abdomen and telson. Scale bars = 1.0 mm.

Fig. 64. Hexaplax aurantium, new species. A–D, holotype male (15.3 × 10.8 mm) (MNHN-IU-2011-2158); E, paratype female (13.2 × 9.5 mm). A, left third maxilliped; B, male abdomen and telson; C, D, G1; E, female abdomen and telson. Scale bars = 1.0 mm.

Fig. 66. Hexaplax saudade, new species. A–D, holotype male (16.4 × 12.1 mm) (NMCR); E, paratype female (13.1 × 9.7 mm). A, left third maxilliped; B, male abdomen and telson; C, D, G1; E, female abdomen and telson. Scale bars = 1.0 mm.