Cavoportunus, NGUYEN & NG, 2010

CAVOPORTUNUS NGUYEN & NG, 2010 View in CoL

( FIG. 9)

= Cavoportunus Nguyen & Ng, 2010: 41 View in CoL . Type species: Neptunus (Achelous) dubius Laurie, 1906 , by monotypy; gender masculine.

Included species: One.

Cavoportunus dubius (Laurie, 1906)

= Neptunus (Achelous) dubius Laurie, 1906

= Portunus (Cycloachelous) yoronensis Sakai, 1974

Diagnosis (after Nguyen & Ng, 2010): Carapace hexagonal in shape, width ~1.3 × length, dorsal surface glabrous; frontal margin slightly upturned; surface microscopically granular with distinct regions; metagastric ridge distinctly visible, other ridges indistinct, dorsal surface with numerous small granules. Front with four teeth, slightly projecting beyond tip of internal supraorbital angle. Orbit relatively large, with moderate dorsal inclination; inner supraorbital margin rounded; upper border of orbit with single fissure. Infraorbital margin straight, with forward-directed tooth. Anterolateral margin with nine teeth: first tooth largest; second to ninth teeth smaller, subequal in size. Posterolateral junction of carapace rounded. Basal antennal article with lobe-like process. Merus of third maxilliped produced anterolaterally. Cheliped length ~1.8 × width of carapace; manus slightly less massive than merus; fingers relatively long, slender. Adult male with two prominent deep depressions on sternite eight inside sternopleonal cavity, anterior being larger, in which first gonopod sits. First male gonopod short, relatively stout, ‘S’-shaped; basal part relatively slender; subdistal part prominently dilated, lobiform; distal part with distinct folds on tip. Second gonopod elongate, tapering, about four-fifths length of first gonopod. Penis coxal, long, slender, proximally lodged between two raised processes of sternite 8. Female vulva mostly surrounded by thick, whitish, crescent-shaped rim; salient, short sternal cover on outer margin; separate, small, round operculum.

Systematic position: In the present three-marker phylogenetic reconstruction ( Fig. 1), Cavoportunus is resolved as an independent branch within a large clade of IWP genera previously belonging to Portunus (s.l.), in a sister position to two multigenera branches (i.e. the Allomonomia – Incultus – Trionectes – Xiphonectes clade and the Cycloachelous – Eodemus – Monomia clade).

As pointed out by Nguyen & Ng (2010), Cavoportunus is morphologically most similar to Cycloachelous . Both taxa share some important synapomorphies, in particular a tendency to a decreasing carapace widthto-length ratio and approaching a circular carapace shape ( Figs 9A, 10A), the secondary symmetrization and homoiodonty of the chelipeds ( Spiridonov et al., 2014), and a short and stout first gonopod ( Fig. 9E), but which is morphologically complex and exhibits coaptation of the median groove. The deep depression

150 M. KOCH ET AL.

( Fig. 9E) on each side on the eighth thoracic sternite (to accommodate the lateral flange of the gonopod apex) is a unique autapomorphy for Cavoportunus ( Nguyen & Ng, 2010) . Cavoportunus can also be distinguished easily from Cycloachelous by the presence of a single fissure (vs. two shallow ones) on the supraorbital margin of the carapace and by a single distal spine (vs. two spines) on the posterior margin of the cheliped merus ( Figs 9A, 10A).

Remarks: Currently, Cavoportunus is monotypic, containing only the species Cavoportunus dubius . In the present study, the genus is represented by two specimens: a male Cavoportunus aff. dubius (MNHN-IU-2008-12565) from the Marquesas Islands, and an ovigerous female Cavoportunus dubius (MNHN-IU-2014-4099) from the Philippines. The size of the female (CW 15 mm) is close to the upper limit known for Cavoportunus dubius (see Nguyen & Ng, 2010), but except for sexual dimorphism there are no noticeable differences between the specimens. Those two specimens seem genetically different [3.0% (COI) or 5.2% (16S)], hence the former might represent an undescribed species.

S i z e: S m a l l c r a b s; m a x i m u m r e p o r t e d s i z e o f Cavoportunus dubius (female) is 14.3 mm × 18.5 mm ( Nguyen & Ng, 2010).

Ecological notes: The single representative of the genus is a low subtidal species occurring between 20–30 and 245 m ( Spiridonov & Zhadan, 1999; Nguyen & Ng, 2010).

Geographical range: Indo-West Pacific: from the Gulf of Aden ( Neumann & Spiridonov, 1999) and Madagascar (present study, MNHN) to Japan, the Philippines,

New Caledonia and French Polynesia ( Spiridonov & Zhadan, 1999: fig. 1A; Nguyen & Ng, 2010).