Hexapinus Manning & Holthuis, 1981

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 : 407-409

publication ID	https://doi.org/ 10.5281/zenodo.5353945
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scientific name	Hexapinus Manning & Holthuis, 1981
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Genus Hexapinus Manning & Holthuis, 1981 View in CoL

Hexapinus Manning & Holthuis, 1981: 169 View in CoL .

Diagnosis. Carapace broader than long; regions not demarcated except for poorly defined cardiac region; anterolateral margin arcuate, posterolateral margin sinuous. Eye movable, small, stout, transverse; with small cornea. Third maxillipeds broad, completely covering buccal cavity; merus and ischium subequal, ischium subauriculiform, expanded distally, mesial margin strongly convex; carpus, propodus and dactylus slender, subcylindrical; dactylus same length as, or longer than propodus; exopod broad, or narrow compared to ischium width, flagellum well developed. Pterygostomial region with row of oblique striae. Chelipeds equal or subequal. P2–P4 very short; merus short without median longitudinal groove on outer surface; dactylus almost as long as or slightly longer than propodus. Male thoracic sternum broad; sternites 1 and 2 fused, separated from sternite 3 by distinct ridge; sternite 3 delimited from sternite 4 laterally; thoracic sternal groove short, extends obliquely from sternoabdominal cavity, sometimes to middle of sternite 4; sternites 4–7 well developed, separated by distinct sutures; sternite 8 exposed, triangular or quadrangular, half or same length as male abdominal somite 1. Sternoabdominal cavity elongated, reaching base of thoracic sternite 3. Male abdomen relatively long, slender, extending beyond bases of third maxilliped, moderately narrow; somites 1 and 2 free, somites 3–5 fused, narrow; somite 6 slightly shorter than fused somites 3–5; telson subpentagonal, with rounded distal margin. G1 simple, slightly curved, lying in oblique groove on anterior part of sternum, distal one-fifth not concealed under abdomen. Female abdomen with 6 free somites and rounded or subtriangular telson.

Species included. Hexapus latipes De Haan, 1835 (type species by original designation), Hexapus edwardsi Serène & Soh, 1976 , Hexapinus ceres , new species, Hexapinus latus , new species, and Hexapinus simplex , new species.

Remarks. The type species of Hexapinus , Hexapus latipes De Haan, 1835 , is poorly known. The series of specimens available, including the dried type, does not include any adult males and so male diagnostic characters are not available. The type specimen is dried and almost certainly a young female, with the telson missing. The genus characters of the male abdomen, G1 and female abdomen are therefore based mainly on H. simplex , new species.

As redefined here, the most characteristic feature of Hexapinus s. str. is the third maxilliped possessing a short and distally expanded ischium with a strongly convex mesial margin ( Figs. 13C–E View Fig , 15B View Fig , 17B View Fig , 20G View Fig ). No other hexapodid genus has such a third maxilliped. Some species of Mariaplax , new genus, have third maxillipeds that approach the condition in Hexapinus (e.g., M. mica , new species, and M. narusei , new species, Figs. 38B View Fig , 40D View Fig ), but none are as short and laterally expanded. The male abdomen of Hexapinus is also distinctively more slender and elongated ( Fig. 21E–G View Fig ) compared to Hexapus ( Figs. 5F View Fig , 9E View Fig ) or Mariaplax ( Fig. 25F View Fig ). Compared to Hexapus , the P2–P4 of Hexapinus are relatively shorter and stouter (cf. Figs. 1A View Fig , 18A, C View Fig ). Mariaplax species are distinctive because their P2–P4 are usually prominently longer and more slender, with almost always with a distinct longitudinal groove on the outer surface ( Fig. 24A View Fig ).

Manning & Holthuis (1981) included Hexapus buchanani Monod, 1956 , and H. granuliferus Campbell & Stephenson, 1970 , in Hexapinus . However, the shapes of the third maxillipeds of H. buchanani ( Fig. 67B View Fig , 68G View Fig ), and H. granuliferus ( Fig. 36E View Fig ; see Campbell & Stephenson, 1970: 286, fig. 49) are quite different from Hexapinus as defined at present. The shapes of the third maxilliped of H. granuliferus conform to what is here described as Mariaplax , new genus, and this species is transferred there. Hexapus buchanani Monod, 1956 , was collected from West Africa and it differs from other hexapodids in a suite of characters: the carapace is ovate ( Figs. 67A View Fig , 68E View Fig ), the meri of P2–P4 do not have a longitudinal groove on the outer surface ( Figs. 67A View Fig , 68E View Fig ), the abdomen is proportionately more elongate ( Fig. 68F View Fig ), and it has a relatively much straighter G1 ( Fig. 68H, I View Fig ). As such we establish Theoxapus , new genus, for this species (see later).

Tesch’s (1918: 240, pl. 17, fig. 1) specimens from the Indonesian Kei islands probably belong to a species of Hexapinus . The third maxilliped figured, with the expanded ischium ( Tesch, 1918: pl. 17 fig. 1a) is indicative. It is different from H. simplex , new species, here described from nearby Lombok Island as it has a relatively short and wide male abdominal somite 6. In the form of the abdomen, it resembles what is known for H. latipes ( Figs. 10B View Fig , 13G View Fig ). The same is true for Guinot (1979: 115, 145, Figs. 32 View Fig , 33A, B, C–E View Fig , 37F View Fig ) who used Tesch’s specimen for her study. Sakai’s (1939: 577, text fig. 68, pl. CII, fig. 4) female specimen from Kii Peninsula in Japan seems closest to H. simplex with regard to its relatively smoother carapace. Sakai (1976: 554) had access to more Japanese material, and he appears to have material of both species; one from Sagami Bay closely resembling H. latipes in its heavily pitted carapace ( Sakai, 1976: pl. 196, fig. 1) with the other smoother animal from Yoron Island likely to be H. simplex ( Sakai, 1976: pl. 196, fig. 2). Odawara’s (1965: 47, fig. a–d) Japanese records could be either as well but as his figures clearly depict heavily pitted specimens, this suggests they are probably H. latipes . Serène & Soh’s (1976: 24, fig. 24, pl. 7, fig. A) “ Hexapus ? sexpes ” from the Andaman Sea is almost certainly a species of Hexapinus but we cannot identify it for the moment with any known taxon. De Man’s (1888: 322, pl. 13, fig. 3) female specimen from Ambon is also likely to be a species of Hexapinus although he does not show the maxillipeds; it resembles H. simplex described from nearby Lombok. All these specimens will need to be re-examined to ascertain their precise identities.

Miyake’s (1983: 151, pl. 51–1) “ Hexapus anfractus ” from Japan is probably a species of Hexapinus with regard to its broad carapace and short pereopods. However, a reexamination of his specimen(s) will be required to determine its precise identity.

Serène & Soh’s (1976: 25, fig. 27, pl. 7, fig. D) species, Hexapus edwardsi , described from a subadult male from the Andaman Sea, is also probably a species of Hexapinus as redefined here, especially considering the shapes of the third maxilliped and male abdomen ( Serène & Soh, 1976: 25, fig. 27, plate 7, fig. D). It is here transferred to Hexapinus .