Hexapus sexpes ( Fabricius, 1798 )

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 : 400-405

publication ID	https://doi.org/ 10.5281/zenodo.5353945
publication LSID	lsid:zoobank.org:pub:4CF42744-861A-4635-9703-E6639CEBFAA9
persistent identifier	https://treatment.plazi.org/id/03EB87DA-3B3F-E35F-FC68-F994FD2F9C73
treatment provided by	Tatiana
scientific name	Hexapus sexpes ( Fabricius, 1798 )
status

Treatment

Hexapus sexpes ( Fabricius, 1798) View in CoL

( Figs. 3–6 View Fig View Fig View Fig View Fig )

Cancer sexpes Fabricius, 1798: 334 .

Hexapus (Hexapus) estuarinus Sankarankutty, 1975: 1 View in CoL , figs. 1, 2.

Hexapus estuarinus View in CoL – Ng et al., 2008: 86.

Hexapus sexpes View in CoL – Manning & Holthuis, 1981: 172; Manning, 1982: 158, figs. 1, 2; Ng et al., 2008: 86; Low & Ng, 2012: 60.

Material examined. 1 male (8.5 × 5.4 mm), 1 female (7.1 × 4.5 mm) ( ZRC 2012.1014 View Materials ) , 1 male (7.5 × 5.1 mm), 1 female (6.3 × 4.1 mm) ( CMB-ZC 11354 ), sand flat, Ko Sirae , Phuket, Thailand, coll. T. Komai, 22 December 2009 ; 1 female (5.8 × 3.9 mm) ( ZRC 1965.11.24.5), Penang Strait, Malaysia .

Diagnosis. Carapace subquadrate, about 1.5 times as broad as long; regions indistinct; scattered granules on anterolateral surfaces, dorsal surface pitted. Front deflexed, divided into 2 lobes. Eye small, slightly movable, cornea narrower than peduncle. Pterygostomial region with 4 long oblique striae and scattered tubercles. Third maxillipeds narrow; ischium longer and narrower than merus, mesial margin rounded subproximally, straight distally. Male chelipeds stout, unequal; major chela with gap when fingers closed, with large tooth at cutting edge of dactylus; minor chela with relatively wider gap when fingers closed; cutting edges of dactylus and fixed finger with row of small teeth. Female chelipeds equal. P2–P4 relatively short, stout, P3 longest. Male thoracic sternum broad; short thoracic sternal groove extend obliquely from sternoabdominal cavity to middle of sternite 4; sternoabdominal cavity reaches base of sternite 3. Male abdomen relatively narrow; somites 1 and 2 free, somites 3–5 fused; somite 6 slightly shorter than length of somites 3–5, expanded laterally near base; telson subtriangular with rounded tip. G1 bent 60° medially, directed anterolaterally, with subdistal protuberance, distal part gradually tapered, not concealed under abdomen, exposed on thoracic sternal groove, obscured by setae of telson, lower margin with row of tiny spines. Female abdomen with 6 somites and telson free.

Description. Carapace subquadrate, about 1.5 times as broad as long; regions indistinct, with median H-shaped shallow depression; scattered granules on anterolateral surfaces, dorsal surface pitted ( Figs. 3 View Fig , 5A View Fig ). Anterolateral margin arcuate; posterolateral corner with feeble angle over base of posterior pereopods; lateral margin of carapace wall strongly produced posteriorly. Front deflexed, divided into 2 lobes, not projecting beyond outer edge of orbits ( Fig. 4B View Fig ). Orbit distinct, transverse; eye small, slightly movable, cornea black, narrower than peduncle ( Figs. 4B View Fig , 5D View Fig ). Pterygostomial region with row of long oblique striae and scattered tubercles, oblique row of setae under row of striae adjacent to Milne Edwards’ opening ( Figs. 4A View Fig , 5D View Fig ). Third maxillipeds narrow, not covering buccal cavity, with gap when closed ( Figs. 4D View Fig , 5E View Fig ); ischium longer than merus, mesial margin rounded subproximally, straight distally; merus slightly wider than ischium, some granules dorsomesially; carpus, propodus and dactylus cylindrical; dactylus longer than propodus; combined length of dactylus, propodus and carpus shorter than that of merus and ischium; exopod relatively broad, width of exopod about 0.6 times width of ischium, flagellum well developed.

Male chelipeds stout, unequal ( Fig. 4C View Fig ). Major chela ( Figs. 4C View Fig , 6A View Fig ) with gap when fingers closed, with large tooth at cutting edge of dactylus; dactylus with row of setae on proximal third of upper margin; outer and inner surfaces of dactylus glabrous; palm slightly wider than long, outer surface with tubercles on lower half; upper margin with few tubercles proximally; inner surface with tubercles near upper margin and midlength; carpus with row of tubercles on upper margin, setose on upper inner margin, inner angle blunt, unarmed; merus short, unarmed, fringe of setae dorsomesially. Minor chela ( Figs. 4C View Fig , 6B View Fig ) with relatively wider gap when fingers closed; dactylus with short row of tubercles on upper margin proximally, cutting edges of dactylus and fixed finger with row of small teeth; shallow longitudinal groove on outer surface of dactylus and fixed finger; outer surface of palm covered by large tubercles on lower half, carpus and merus unarmed, fringe of setae on each upper inner margin. Female cheliped equal, narrow gap when fingers closed.

P2–P4 relatively short, stout, P3 longest ( Fig. 3A View Fig ). P2 most slender, dactylus slightly upcurved, fringe of setae on upper and lower margins, slightly longer than propodus, latter covered by short setae on outer surface, longer, denser setae on upper and lower margins; carpus slightly longer than propodus, unarmed, tufts of setae distally; merus almost twice length of carpus, upper margin with tufts of short setae, outer surface with sparse short setae on lower half, lower margin with rows of tubercles obscured by long, dense setae. P3 stouter, longer than P2. P4 ( Fig. 6C View Fig ) stouter than P3. Setation of dactylus, propodus and carpus of P3 and P4 similar to that of P2; merus of P3 and P4 with rows of tubercles on upper and lower margin obscured by dense setae; outer face of P3 covered by short setae, outer face of P4 glabrous, about 2.5 times as long as broad.

Male thoracic sternum broad ( Figs. 4A, D View Fig , 5B, C View Fig ), sternites 1 and 2 fused, separated from sternite 3 by distinct ridge; sternite 3 and 4 separated by distinct ridge laterally, medially appear fused, short thoracic sternal groove ( Figs. 4A, D View Fig , 5B, C View Fig ) extends obliquely from sternoabdominal cavity to middle of sternite 4 and below base of third maxilliped, sternites 4–7 well developed, separated from each other by distinct suture; sternite 8 exposed, rectangular, slightly longer than abdominal somite 1; sternoabdominal cavity reaches base of sternite 3. Male abdomen relatively narrow ( Figs. 4A, D View Fig , 5F View Fig ); somites 1 and 2 free, somites 3–5 fused, lateral margin sinuous; somite 6 slightly shorter than length of somites 3–5, expanded laterally near base; telson subtriangular, distal margin rounded, setose.

G1 ( Fig. 6D, E View Fig ) bent 60° medially, directed anterolaterally, with subdistal protuberance, distal part gradually tapered, lower margin with row of tiny spines; distal part not concealed under abdomen, exposed on thoracic sternal groove, obscured by setae of telson.

Female abdomen with 6 free somites and telson ( Fig. 5G View Fig ).

Colour. In life, the species is dirty white overall.

Remarks. Manning & Holthuis (1981) were uncertain about the identity of H. estuarinus Sankarankutty, 1975 , from India. Later, Manning (1982) considered it to be a junior synonym of H. sexpes , but Ng et al. (2008) provisionally treated it as valid species in view of the differences observed by Sankarankutty (1975). The study of fresh material leads us to concur with Manning (1982) (see also Low & Ng, 2012). The present material from Phuket agrees very well with the redescription of the dried type of H. sexpes by Manning (1982); the only difference being in the shape of the minor chela. According to Manning (1982), the fingers of the minor chela are not gaping, while in the present material, those of the minor cheliped form a large hiatus, meeting only at the tips. The prominently crossed fingers of the minor chela in the type ( Manning, 1982: fig. 1d) may be an artifact due to poor preservation (the specimen was dried). In fresh specimens ( Fig. 4D View Fig ), the tips are only partially crossed.

Being the best known member of the family, “ Hexapus sexpes ” has been reported by many workers over the last 200 years. However, it is often difficult to ascertain what species they actually referred to, especially in the context of the substantial changes in hexapodid taxonomy that have occurred in the last 50 years. The records by Tesch (1918), Sakai (1939, 1976), Serène & Soh (1976) and Guinot (1979) are probably referrable to Hexapinus as good figures are provided (see discussion for the genus). Stebbing’s (1910: 315, pl. 15) “ Hexapus sexpes ” is now Tritoplax stebbingi (Barnards, 1950) (see Manning & Holthuis, 1981). Stephensen’s (1946: 182, fig. 53) “ Hexapus sexpes ” from the Persian Gulf, was described as a new species ( Hexapus stephenseni ) by Serène & Soh (1976). Manning & Holthuis (1981: 180) believed Stephensen’s specimen may be in Tritoplax Manning & Holthuis, 1981 , and Huang et al. (2002) and Ng et al. (2008) followed their suggestion by referring Hexapus stephenseni there. We have re-examined the types of T. stebbingi , the type species of Tritoplax ( Fig. 7 View Fig ) and are now certain Hexapus stebbingi and Hexapus stephenseni cannot be classified in the same genus. The present study refers Hexapus stephenseni to Mariaplax , new genus, instead. Records of “ Hexapus sexpes ” by various authors from the Indo-West Pacific: Zehntner (1894: 159, Ambon, Indonesia); Nobili (1906: 146, Persian Gulf); Balss (1938: 74, Marshall Islands); and Griffin (1972: 85, off Yamba, New South Wales, Australia) (see also Davie, 2002: 232) for the moment must be treated as incerta cedis as the description and/or figures were too brief or absent. Although A. Milne-Edwards (1873: 253, pl. 12, fig. 1, New Caledonia), Guinot (1979: Pl. 24 fig. 6, Persian Gulf) and Guinot & Bouchard (1998: Fig. 17E View Fig , Persian Gulf) gave short descriptions and/or figures of their material, they are insufficient to allow us to determine the precise identities of the genus or species.