Hexapus De Haan, 1833

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 : 400

Hexapus De Haan, 1833

Hexapus De Haan, 1833: 5 .

Diagnosis. Carapace broader than long, surface pitted, granules lining margin; regions indistinct, with shallow median H-shaped depression. Anterolateral margin arcuate; pterygostomial region with rows of long, oblique striae and scattered tubercles. Eyes very small, slightly movable, corneas subglobular. Third maxillipeds narrow, not covering buccal cavity, with gap when closed; ischium narrow, longer than merus, mesial margin rounded subproximally, straight distally, carpus, propodus and dactylus cylindrical, dactylus longer than propodus; exopod relatively broad compared to ischium. Chelipeds unequal or subequal. P2–P4 relatively short; merus relatively short, without median longitudinal groove on outer surface; dactylus as long as or slightly longer than propodus, upcurved or straight. Thoracic sternites 1 and 2 fused; sternite 3 partly or fully separated from sternite 4 by tuberculate ridge; sternite 8 exposed, rectangular, slightly longer than abdominal somite 1. Sternoabdominal cavity reaches base of thoracic sternite 3. Male thoracic sternal groove oblique, short. Male abdomen relatively narrow, somites 1, 2 and 6 free, somites 3–5 fused; telson subtriangular. G1 slender, slightly curved subdistally, distal part gradually tapering, apice not concealed under abdomen, exposed on sternal groove. Female abdomen with 6 free somites and telson.

Species included. Cancer sexpes Fabricius, 1798 (type species by original designation) and H. timika , new species.

Remarks. Manning & Holthuis (1981) separated Hexapus De Haan, 1833 , s. str. from Hexapinus Manning & Holthuis, 1981 , primarily by the extent of the male thoracic sternal groove, being proportionately longer in Hexapus . However, due consideration of the third maxilliped, male abdomen and ambulatory leg structures suggest that the male thoracic sternal groove is not a good character by itself.

The form of the ischium and merus of the third maxilliped

easily distinguishes Hexapus from Hexapinus . The two Hexapus species recognised here have distinctively more elongate third maxillipeds, with the merus subovate and the ischium longer than broad (e.g., Fig. 5E ). In Hexapinus species , the third maxilliped is distinctively shorter, with the merus squarish and the ischium broader than long (e.g., Fig. 13E ). The P2–P4 (notably the meri) of Hexapus are also proportionately more slender and elongated ( Fig. 3A ) compared to those of Hexapinus that are distinctly shorter and stouter ( Fig. 18A, C ). The male abdomen of Hexapus is relatively short and broad ( Fig. 5F ) while in Hexapinus , it is elongated and slender ( Fig. 21E–G ). These characters separate the two genera, with the P2–P4 and third maxilliped also effectively distinguishing females as well. The male abdomen of Hexapus is similar to that of Mariaplax , new genus, but their third maxillipeds are very different, with that of Mariaplax appearing intermediate in condition (e.g., Fig. 38B ) between Hexapus and Hexapinus . Mariaplax can also usually be distinguished from Hexapus and Hexapinus by its differently structured G1, its relatively longer P2–P4 (except M. narusei , new species), with the merus usually possessing a deep longitudinal median groove on the outer surface, sometimes it is not so obvious because of the dense tubercles (P2–P4 relatively shorter and the median groove absent or if present, very shallow).

With regards to the male thoracic sternal groove, Hexapus sexpes has a distinct male sternal groove that extends from the end of the sternoabdominal cavity to the middle part of sternite 4, before the base of the third maxillipeds ( Figs. 4A, D , 5B, C ), but in Hexapus timika , new species, the groove is very short and is just a very short extension from the sternaobdominal cavity ( Figs. 8B , 9C ). Hexapinus simplex , new species, on the other hand, has a male sternal groove that resembles that of Hexapus sexpes ( Figs. 5B , 20B ). Although the condition of the male thoracic sternal groove does not vary substantially in Hexapus and Hexapinus species (even for Hexapinus simplex for which there is a very good series), its value as a taxonomic character in the Hexapodidae is questionable. In the genus Mariaplax (notably for M. chenae , new species, for which there is a good series), the condition of the male thoracic sternal groove varies with size and is intermediate in condition (see remarks for this species).

Figures

Fig. 5. Hexapus sexpes (Fabricius, 1798). A–F, male (8.5 × 5.4 mm) (ZRC 2012.1014); G, female (5.8 × 3.9 mm) (ZRC 1965.11.24.5). A, carapace; B, thoracic sternites 3 and 4, showing thoracic sternal groove and telson; C, thoracic sternites 1–4, showing thoracic sternal groove; D, anterior view of cephalothorax; E, left third maxilliped; F, male abdomen and telson; G, female abdomen and telson. Scale bars = 1.0 mm.

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. 3. Hexapus sexpes (Fabricius, 1798), male (8.5 × 5.4 mm) (ZRC 2012.1014). A, overall view of carapace, chelipeds and P2–P4; B, carapace.

Fig. 18. Hexapinus simplex, new species. A, B, holotype male (7.8 × 5.0 mm) (MZB Cru 3790); C, paratype female (6.9 × 4.5 mm) (MZB 3791). A, C, overall view of carapace, chelipeds and P2–P4; B, carapace.

Fig. 21. Hexapinus simplex, new species. A, B, F, holotype male (7.8 × 5.0 mm) (MZB); C, paratype female (6.9 × 4.5 mm) (ZRC 2013.1689); D, paratype female (4.5 × 2.8 mm) (ZRC 2013.1689); E, paratype male (5.4 × 3.4 mm) (ZRC 2013.1689); G, male (12.7 × 7.5 mm)(ZRC 2012.1016). A, B, G1; C, D, female abdomen and telson; E–G, male abdomen and telson. Scale bars = 1.0 mm.

Fig. 38. Mariaplax mica, new species, holotype female (6.7 × 4.5 mm) (NMCR 39069). A, dorsolateral margin of carapace; B, left third maxilliped; C, 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. 8. Hexapus timika, new species, holotype male (4.6 × 3.0 mm) (MZB Cru 3789). A, overall view of carapace, chelipeds and P2–P4; B, ventral surface of cephalothorax.

Fig. 9. Hexapus timika, new species, holotype male (4.6 × 3.0 mm) (MZB Cru 3789). A, dorso-lateral margin of carapace; B, ventral view of cephalothorax; C, thoracic sternites 1–4, showing thoracic sternal groove and sternoabdominal cavity; D, left third maxilliped; E, male abdomen and telson; F, G, G1. Scale bars: A–E = 1.0 mm; F, G = 0.5 mm.

Fig. 20. Hexapinus simplex, new species, holotype male (7.8 × 5.0 mm) (MZB Cru 3791). A, carapace; B, thoracic sternites 1–5, showing thoracic sternal groove; C, thoracic sternites 1–5, showing thoracic sternal groove and telson; D, anterior view of cephalothorax; E, major chela; F, minor chela; G, left third maxilliped. Scale bars = 1.0 mm.