Hexagonaloides, Komai, Tomoyuki, Higashiji, Takuo & Castro, Peter, 2010

Komai, Tomoyuki, Higashiji, Takuo & Castro, Peter, 2010, A new genus and new species of deep-water trapeziid crab (Crustacea: Decapoda: Brachyura: Trapezioidea) from the Ryukyu Islands, Japan, Zootaxa 2555, pp. 62-68 : 63

Hexagonaloides n. gen.

[new Japanese name: Shinkai-sanngo-gani-zoku] Type species. Hexagonaloides bathyalis n. sp.

Diagnosis. Carapace hexagonal; frontal margin smooth; anterolateral margins each with conspicuous, anterolaterally directed tooth arising at midlength. Orbit not concealing eye when retracted; supraorbital border lacking fissures. Chelipeds unequal; meri long, lacking teeth on anterior margin. Dactyli of second to fifth pereopods (first to fourth ambulatory legs) ending in small acute corneous spine, flexor margin bi-edged, each edge crenulate, with slender corneous spinules. Male abdomen with third to fifth somites incompletely fused (suture present laterally), although completely immobile. Male first gonopod slender, tapering to acute tip; second gonopod about 0.3 length of first gonopod, stout.

Composition. Monotypic.

Remarks. The new genus agrees well with the diagnosis of the Trapeziidae given by Castro et al. (2004). The entire frontal margin of the carapace and the lack of teeth on the anterior margin of the meri of chelipeds place this new genus close to Calocarcinus and Philippicarcinus . Hexagonaloides , however, is readily distinguished from these two genera by the shape of the carapace, the structure of the orbit and the form of the male gonopods. The carapace is hexagonal in the new genus, octagonal with parallel, straight sides along the middle portion in Calocarcinus (Castro et al. 2004: pl. 4C), and transversely ovoid in Philippicarcinus (Castro et al. 2004: pl. 4D). The lateral margin of the carapace bears one tooth in Hexagonaloides and Phlippicarcinus, and two teeth in Calocarcinus . The orbit is open, with the inner suborbital angle not reaching to the lateral angle of the front in the new genus, whereas it is closed with the inner suborbital angle reaching to the lateral angle of the front in the latter two genera. The first gonopod is more slender in Hexagonaloides than in Calocarcinus and Philippicarcinus , but the second gonopod is very short, about 0.3 the length of the first gonopod in Hexagonaloides , rather than notably elongated in Calocarcinus (see Serène 1984: figs. 197– 200) and Philippicarcinus (see Garth & Kim 1983: fig. 14c, d). Furthermore, the chelipeds and ambulatory legs are distinctly more slender in Hexagonaloides than in Calocarcinus and Philippicarcinus .

In the shape of the carapace and the elongated chelipeds and ambulatory legs Hexagonaloides resembles Hexagonalia (see Castro et al. 2004: pl. 4B) and Quadrella (see Castro et al. 2004: pl. 4A), but the new genus is readily distinguished from the latter two genera by its smooth frontal margin and the anteriorly unarmed cheliped meri. In Hexagonalia and Quadrella the frontal margin bears acute teeth and the anterior margin of the cheliped meri is armed with a row of spine-like teeth. Furthermore, Hexagonalia differs from Hexagonaloides in having two teeth on the anterolateral margin of the carapace. Spheromerides (see Castro et al. 2004: pl. 4E) shares a smooth frontal margin with Hexagonaloides , but the possession of two lateral teeth and spinose cheliped meri clearly sets them apart. Finally, Trapezia differs from the new genus in having a trapezoidal carapace and dentate cheliped meri (Castro et al. 2004: pl. 2C, D).

Within the Trapeziidae sensu Castro et al. (2004) , Ng et al. (2008) recognised three subfamilies, Trapeziinae Miers, 1886 , Calocarcininae Števčić, 2005 , and Quadrellinae Števčić, 2005 , citing differences in habitat, carapace and pereopodal features, as well as larval characters. This classification was followed by De Grave et al. (2009). It is not easy to decide on the subfamilial affiliation of the new genus, because its mixture of diagnostic features fits both Calocarcininae and Quadrellinae . We provisionally assign the new genus to Quadrellinae because of the similarities in the carapace shape and the proportionately longer ambulatory legs. Further collections should reveal any possible symbiotic associations, and future investigations on larval development and molecular markers would be helpful in determining its position. Among calocarcinines, Calocarcinus species are symbionts of deep-water ahermatypic corals and other colonial cnidarians, Philippicarcinus appears to be associated with deep-water glass sponges (P. K. L. Ng, personal communication) and Sphenomerides is not known to be associated with any particular hosts. Among quadrellines, species of Quadrella are symbionts of shallow-water antipatharians, gorgonians, alcyonaceans and, in one species, of shallow-water ahermatypic corals, while Hexagonalia species are symbionts of deepwater stylasterid (hydrozoan) corals and gorgonians (Castro et al. 2004; Castro 2005).

Etymology. From the generic name Hexagonalia and the suffix - oides (Latin) (= like, resembling), in reference to the superficial resemblance of the new genus to Hexagonalia . Gender: masculine.