Rizalthus, Mendoza & Ng, 2008

Mendoza, Jose Christopher E. & Ng, Peter K. L., 2008, New Genera And Species Of Euxanthine Crabs (Crustacea: Decapoda: Brachyura: Xanthidae) From The Bohol Sea, The Philippines, Raffles Bulletin of Zoology 56 (2), pp. 385-404 : 386-387

Rizalthus , new genus

Diagnosis. – Carapace much broader than long, regions welldefined; epigastric, protogastric, mesogastric and metagastric regions raised, together forming a dorsally convex anterior portion on carapace; all dorsal regions of carapace with large granules, surrounded by short setae basally. Front about 0.4 times carapace width, bilobed; lobes separated by a broad V-shaped cleft, continuous with a deep median fissure on frontal region; each lobe broadly triangular. Anterolateral margin without clearly demarcated teeth or lobes, lined with conical granules, appearing unevenly serrated, feebly cristate, anterior-most part not meeting orbital margin but descending towards anterolateral corner of buccal cavity; anterior two-thirds strongly arcuate, posterior third slightly convex, junction with posterolateral margin marked by prominent, dorsally directed, granular tooth; posterior margin uniformly straight. Last ambulatory leg coapted against concavity in posterolateral margin. Eyestalks with large, conical granules on distal end, bordering the corneas. Antennules folding transversely. Basal antennal segment large, granulose, subrectangular, occupying entire space between antennular fossa and internal orbital angle, filling orbital hiatus; flagellum long, reaching outer edge of orbit. Epistome with small granules near anterior edge, posterior margin slightly sweeping outwards, central region slightly undulate, concave. Endostome without oblique or longitudinal ridges. Outer surface of third maxilliped granulose, appearing eroded; merus subquadrate, median length about half that of ischium; ischium subrectangular, with deep, median longitudinal sulcus bordered by granules; exopod granulose, tapering toward distal end which almost reaches anterior edge of merus, flagellum long. Thoracic sternum eroded, with several distinct depressions on anterior region and on either side of abdomen; sternites 1 and 2 completely fused, separated from sternite 3 by deep transverse suture; sternites 3 and 4 partially fused, with partial suture seen near anterior edge of cheliped coxae; median suture present within a median depression anterior to telson; tubercle for abdominal locking mechanism on sternite 5 slightly nearer to suture with sternite 4. Chelipeds similar, subequal; carpus with prominent lateral truncatiform wedge-like expansion. Ambulatory legs relatively short, granulose, edges with conical granules and setae. Male abdomen very granulose, appearing eroded; telson subtriangular with rounded tip, about half the length of segment 6; segments 3–5 completely fused, lateral margins concave. G1 long, slender, mostly straight, curving outward and tapering to a fine point distally; terminally located opening unadorned by flaps or folds; several long, thick and stiff plumose setae located subdistally. G2 short, about one-third length of G1.

Comparative material. – Olenothus uogi Ng, 2002 , holotype male (16.5 × 10.8 mm) ( UF 2096 ), on wall, 2 m, at night, Tepungan Channel, Guam, coll. by L. Kirkendale, 17 Nov.1998; paratype female (14.5 × 9.3 mm) ( UF 2097 ), on wall, 1–2 m, coll. by L. Kirkendale, 17 Jul.1999 ; Euxanthus huonii (Hombron & Jacquinot, 1846) , 1 male (53.7 × 36.3 mm) ( ZRC 1965.11.9.46), Linderman Island , Whitsunday Passage, Queensland, Australia, coll. by M. Ward, Dec.1934 ; Euxanthus exsculptus ( Herbst, 1790) , 1 female (51.4 × 34.2 mm) ( ZRC 1973.10.30.37), Puerto Galera , Mindoro, Philippines, coll. by V. P. Marula, 22 Jun.1971 ; Hypocolpus kurodai Takeda, 1980 , 1 male (21.8 × 13.7 mm) (NMCR- 6609), Kasili , Santa Cruz, Marinduque, Philippines, coll. by J. Cabrera, R. Garcia & R. Velarde, 21 Aug.1979 ; H. haanii Rathbun, 1909 , 2 females (30.9 × 22.3 mm, 41.2 × 32.1 mm) ( ZRC 1965 . 7.6.17–18), 12 fathoms, 05°09.2’S 106°57.25’E, Java Sea, coll. by M. W. F. Tweedie, 6 Sep.1938 .

Type Species. – Rizalthus anconis , new species, by present designation.

Etymology. – This genus is named in honour of Dr. Jose P. Rizal, national hero of the Philippines, who was also an avid naturalist. The genus name is an arbitrary combination of the surname “ Rizal ” and “-thus”, a common suffix of xanthid genera. Gender masculine.

Remarks. – Rizalthus , new genus, clearly belongs to the subfamily Euxanthinae because the anterolateral margin of the carapace does not clearly meet the orbit but, instead, continues down to the subhepatic region and terminates at or near the buccal frame. Other features that contribute to its inclusion are chelipeds and ambulatory legs that can be coapted against the carapace (see Serène, 1984). Rizalthus is morphologically most similar to the monotypic genus Olenothus Ng, 2002 by virtue of the following features: 1) the lobes of the bifid front are broadly triangular; 2) the carapace outline is strongly arcuate anterolaterally and concave posterolaterally; 3) the antennal flagellum is relatively long; and 4) the carpus of the cheliped has an extensive lateral expansion. However, it differs significantly by having the following features: 1) the carapace has well-defined regions, and is highly invested with large granules like the rest of the body ( Figs. 1 , 2 ) (vs. carapace regions not well-defined, finely granulose in Olenothus ); 2) the anterior portion of the carapace, particularly the proto- and epigastric regions, is distinctly elevated over the rest of the carapace ( Fig. 1A, B ) (vs. not distinctly elevated in Olenothus ); 3) the epistome is broader and less arched, the central region is more or less straight ( Fig. 1B ) (vs. narrower, strongly arched, with central convexity in Olenothus ); 4) the basal antennal article is relatively narrower and the large tubercle near the internal orbital angle is absent or obscured by many granules ( Fig. 2A ) (vs. broad, inflated basal antennal article, and tubercle present in Olenothus ); 5) the posterior two-thirds of the exopod of the third maxilliped is not expanded laterally ( Fig. 2B ) (vs. distinctly expanded laterally in Olenothus ); 6) in the male abdomen, segment 6 is distinctly longer than the telson and the lateral margins of the fused segments 3–5 are strongly concave ( Fig. 2C ) (vs. telson and penultimate segment subequal in length, lateral margins of fused segments 3–5 undulate, not concave in Olenothus ); and 7) the distal end of the G1 is straight and tapering, with a terminal opening, and with several long, stiff subterminal setae ( Fig. 2F, G, H ) (vs. hooked with triangular flap, subterminal opening, and only two long, stiff subterminal setae in Olenothus ). Furthermore, there are visible traces of the suture between thoracic sternites 3 and 4 (vs. completely absent in Olenothus ) and the tubercle for the abdominal locking mechanism is not adjacent to the suture between sternites 4 and 5 (vs. adjacent to suture between sternites 4 and 5 in Olenothus ) (cf. Ng, 2002).

In addition to the material we have in our possession (see Comparative Material), Guinot-Dumortier (1960) and Serène (1984) provide excellent figures and descriptions of most of the known species of Euxanthus Dana, 1851 , and Hypocolpus Rathbun, 1897 , particularly the type species E. huonii (Hombron & Jacquinot, 1846) and H. diverticulatus ( Strahl, 1861) , respectively, from which we can now make accurate comparisons. We also took into consideration species belonging to these two genera that have been described afterward (cf. Guinot, 1971; Crosnier, 1991; 1997). Rizalthus shares morphological affinities with such genera, particularly in the general outline and sculpturing of the carapace, and in the general form of the third maxillipeds. Rizalthus also shares the lateral expansions of the cheliped carpus with at least one species, Hypocolpus kurodai Takeda, 1980 . However, Rizalthus can be easily distinguished from Euxanthus by 1) the wide V-shaped gap separating the two triangular lobes of the front ( Fig. 1A ) (vs. narrow fissure in Euxanthus , frontal lobes are more rounded); 2) the straight central region of the epistome ( Fig. 1B ) (vs. with pointed projection/convexity); 3) the absence of clearly demarcated teeth on the carapace anterolateral margin ( Fig. 1A ) (vs. with 4–6 teeth); 4) the presence of large conical tubercles on the distal end of the eyestalks, bordering the rim of the corneas ( Fig. 2A ) (vs. absent or minute); 5) the absence of the tubercle near internal orbital angle ( Figs. 1B , 2A ) (vs. present); 6) the near-absence of the suture between sternites 3 and 4 ( Fig. 1C ) (vs. distinct and entire); 7) the presence of lateral expansions on the carpus of the chelipeds ( Fig. 1A, D ) (vs. absent); 8) the less distinctly triangular and more rounded telson ( Fig. 2C ) (vs. distinctly triangular); and 9) the tapering terminal end of the G1 with several stiff and long subterminal setae ( Fig. 2F–H ) (vs. relatively blunt, with triangular flap, and fewer long and stiff setae) (cf. Guinot-Dumortier, 1960; Serène, 1984). Rizalthus can also be easily distinguished from Hypocolpus by 1) the absence of subhepatic cavities ( Fig. 1B ) (vs. present); 2) the lack of a cristate anterolateral carapace margin ( Fig. 1A ) (vs. present); 3) the wide V-shaped gap between the lobes of the front ( Fig. 1A ) (vs. narrow fissure between lobes); 4) the flat central region of the epistome ( Fig. 1B ) (vs. convex); 4) the more rounded telson ( Fig. 2C ) (vs. more triangular); and 5) the tapering terminal end of the G1 ( Fig. 2F–H ) (vs. relatively blunt, with triangular flap) (cf. Guinot-Dumortier, 1960; Serène, 1984).

The lateral expansion on the cheliped carpus of Rizalthus is most similar to those seen in the monotypic genus Pleurocolpus Crosnier, 1995 in that the apices of these extensions are more acute than in Olenothus uogi Ng, 2002 , and Hypocolpus kurodai Takeda, 1980 , which tend to be broader. There is also some similarity in the form of the G1. However, Rizalthus is easily distinguished from Pleurocolpus by the bilobed front ( Fig. 1A ) (vs. quadrilobate in Pleurocolpus ); the relatively shorter antennal flagellum ( Fig. 2A ) (vs. long, going beyond orbit); the absence of a respiratory cavity formed by the carapace and pereiopods ( Fig. 1A ) (vs. present and diagnostic in Pleurocolpus ); the absence of a large, laterally directed triangular tooth at the junction of the anterolateral and posterolateral carapace margins ( Fig. 1A ) (vs. present); the feeble median longitudinal suture on the male thoracic sternum just anterior to the sternal cavity ( Fig. 1C ) (vs. deep and extensive fissure); and the more slender G1, with several stiff and long subterminal setae ( Fig. 2F–H ) (vs. stouter and without subterminal setae) (cf. Crosnier, 1995).

The combination of characters seen in Rizalthus make its placement within the four genera mentioned rather difficult and thus the establishment of a new genus is warranted.

Figures

Fig. 1. Rizalthus anconis, new genus, new species, male holotype (NMCR-27507) (12.5 × 7.9 mm): A, dorsal view; B, frontal view; C, ventral view; D, left chela, outer surface.

Fig. 2. Rizalthus anconis, new genus, new species, male holotype (NMCR-27507) (12.5 × 7.9 mm): A, frontal view, left side, showing antenna, antennule and orbit; B, left third maxilliped, external view; C, abdomen and telson; D, right third ambulatory leg; E, right fourth ambulatory leg; F, left G1, external view; G, distal end of left G1, external view; H, distal end of left G1, internal view; I, left G2, external view. Scale bars: A–E, = 1.0 mm; F–I = 0.5 mm.