Hexapodidae Miers, 1886

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 : 397

Family Hexapodidae Miers, 1886

Remarks. Following suggestions by Monod (1956), Manning & Holthuis (1981) distinguished the various hexapodid genera using a combination of characters such as the structure of the eyes, shape of the third maxilliped (especially the shape of the propodus), presence or absence of a flagellum on the exopod of the third maxilliped, relative length of the second, third, and fourth pereopods, structure of the male thoracic sternal groove, shape and configuration of the male abdomen, and structure of the G1. To separate Hexapus De Haan, 1833 , and Hexapinus Manning & Holthuis, 1981 , Manning & Holthuis (1981: 169) argued that Hexapus has broad and deep male thoracic sternal grooves that extend anterolaterally from the sternoabdominal cavity to the bases of the third maxillipeds; while those in Hexapinus are not as long or broad, being only a triangular projection and not reaching the bases of the third maxillipeds. The present study shows that the shape and extent of the male thoracic sternal groove cannot always be used to separate the two genera. This character varies with the size of the individual and there are intermediate conditions between species in other hexapodid genera (see Mariaplax chenae , new species). These generic groupings are better defined by other characters. As such, both genera need to be rediagnosed.

In this paper, separation of the genera in the family Hexapodidae essentially follows the system suggested by Manning & Holthuis (1981) except for relying less on the shape and extent of the male thoracic sternal groove. One new character added is the shape and proportions of the ischium and merus of the third maxillipeds. At the species level, the structure of the adult G1 is important and it can sometimes be the only reliable means of separating species that are otherwise very close (e.g., see Mariaplax granuliferus and M. daviei , new species). If only juveniles or females are known, species identifications may be difficult.

That been said, the proportions of the carapace and pereopods as well as structure of the male abdomens are usually useful in discriminating species. While most hexapodid taxa are known from only few specimens, we are fortunate in this study to be able to study abundant material of four species ( Hexapinus simplex , new species, Mariaplax chenae , new species, Hexaplax megalops , and Hexaplax aurantium , new species). They demonstrate that the characters of the carapace, third maxillipeds, and pereopods do not vary substantially. The good series of specimens of both sexes and various sizes also allows us to ascertain variation in key characters as well as changes in the G1 structure associated with growth.

Comparative material. Tritoplax stebbingi (Barnard, 1950) : lectotype male (14.1 × 9.5 mm), 1 female (12.2 × 8.2 mm) ( NHM), Agulhas Bay (St. Sebastian Bay to Algoa Bay), South Africa, coll. 19 November 1946 ( Fig. 7 ) .

Figures

Fig. 7. Tritoplax stebbingi (Barnard, 1950). A, B, lectotype, male (14.1 × 9.5 mm); C, lectotype, female (12.2 × 8.2 mm). A, overall view of carapace, chelipeds and P2–P4; B, C, ventral view of carapace.