Ptychognathus aff. lipkei, N. K. Ng, 2010

Hsu, Jhih-Wei & Shih, Hsi-Te, 2024, A new species of the genus Ptychognathus Stimpson, 1858 (Crustacea: Brachyura: Varunidae) from Vanuatu, Zootaxa 5476 (1), pp. 152-165 : 158-161

Ptychognathus aff. lipkei

( Figs. 1E, F , 3 )

Material examined. 1 male (11.4 × 9.6 mm), ZRC 2023.0250 , st. VM4 , between Rose Point and Nasouli River, Sanma Province, Espiritu Santo Island , Vanuatu, coll. SANTO 2006 Marine Biodiversity Survey, 11 Sep. 2006 . Others : 1 male (12.8 × 10.7 mm), MNHN-B29902 , Bea Jonho Stream, Isabel Island, Solomon Islands, 3 Jan. 2005 .

Diagnosis. Carapace ( Figs. 1E, F , 3A ) subquadrate, 1.19–1.20 times as wide as long; dorsal surface glabrous (except for metabranchial region), postfrontal region indistinct, not separated into any lobes. Front broad; frontal margin concave medially, lined with tiny granules. Anterolateral margins with only 1 distinct, bluntly triangular tooth behind low external orbital tooth. Third maxillipeds ( Fig. 3B ) almost glabrous, exopod 1.25 times as wide as ischium. Chelipeds ( Fig. 1E, F ) symmetrical; inner surface of palm glabrous; in male, proximal half of immovable finger with tuft of long soft setae on outer surface, extending to palm side, pulvinus absent at base of fingers ( Fig. 3C ). P4 ( Fig. 3D ) relatively long, propodus and dactylus covered with dense short setae on anterior margins and dorsal surface, dense short setae and sparse long setae on posterior margins. P5 ( Fig. 3E ) short; propodus covered with dense short setae and sparse long setae on dorsal surface, anterior margin and distal 1/3–1/2 of posterior margin; dactylus with dense short setae on margins. Male pleon ( Fig. 3F ) narrow; distal margin of telson not concave nor with tuft of setae. G1 ( Fig. 3G–I ) slender, distally curved toward dorsal; tip chitinous, with 1 short rounded lobe in lateral view ( Fig. 3I, J ), opened toward lateral.

Description. Carapace ( Figs. 1E, F , 3A ) subquadrate, 1.19–1.20 times (n = 2) as wide as long; dorsal surface flattish, finely punctate, glabrous (except for metabranchial region); regions weakly defined, with noticeable groove between epigastric regions, metabranchial region slightly sloping outwards. Front broad, part near orbital regions slightly convex; frontal margin concave medially in dorsal view, weakly divided into indistinct two lobes, lined with small, rounded granules; postfrontal region indistinct, not separated into any lobes.

Supraorbital margins and margins of anterior part in external orbital tooth lined with small granules.Anterolateral margins almost not granulated (except for anterior part in external orbital tooth), with only 1 distinct tooth behind external orbital tooth; external orbital tooth low, bluntly triangular, slightly sloping forward; first tooth blunter than external orbital tooth. Posterolateral margins slightly convergent posteriorly; posterolateral regions sparsely furnished with short, soft setae. Infraorbital ridge consisting of 22–25 small, rounded granules. Epistome broad, median part triangular.

Third maxillipeds ( Fig. 3B ) broad, external surface almost glabrous; merus with oblique shallow groove along mesial margin of external surface, anterolateral angle broadly rounded, slightly sloping laterally; ischium with indistinct shallow groove slightly mesial to midline of external surface; exopod very broad, convex, 1.25 times broader than ischium.

Chelipeds ( Fig. 1E, F ) symmetrical both in male and female, stronger in male. Merus without spines, dorsal and inner margins with dense soft setae, ventral margins glabrous. Carpus almost glabrous on surface, only with several tiny granules; inner distal angle blunt, without any spines. In male, palm ( Fig. 3C ) finely punctate, without distinct granules on outer surface; inner surface glabrous, slightly convex medially. Movable finger (dactylus) approximately as long as palm, cutting edge with 6 or 7 smaller and 2 or 3 larger teeth, large teeth blunter; immovable finger slightly shorter than movable finger, cutting edge with 4 or 5 large blunt teeth; proximal half of immovable finger with tuft of long soft setae, extending to palm, without pulvinus at base of fingers.

Ambulatory legs ( Figs. 1E, F , 3D, E ) slender, P3 and P4 equal in length, longest. Meri without spines, but with long soft setae on proximal 2/3 of anterior margin; posterior margins with sparse long setae and dense short setae; lateral dorsal surface with dense short setae. Carpi of P2 and P3 with dense short setae on distal 3/4–4/5 anterior margins and dorsal surface; carpi and propodi with dense short setae and sparse long setae on ventral surfaces; propodi sparsely setose on posterior margins; propodi and dactyli with dense short setae on anterior margins. Carpi of P4 and P5 with dense short setae on distal 2/3–3/4 anterior margins, with sparse long setae on posterior margins. P4 ( Fig. 3D ) relatively long; merus ca. 3.6 as long as wide (ZRC 2023.0250); propodus ca. 2.3 as long as wide (ZRC 2023.0250); propodus and dactylus covered with dense short setae on anterior margins and dorsal surface, with dense short setae and sparse long setae on posterior margins; dactylus ca. 1.1 times as long as propodus (ZRC 2023.0250). P5 ( Fig. 3E ) short; merus ca. 2.7 as long as wide (ZRC 2023.0250); propodus ca. 1.3 as long as wide (ZRC 2023.0250), covered with dense short setae (distal part denser) and sparse long setae on anterior margins and dorsal surface near anterior margin, with dense short setae and sparse long setae on distal 1/3–1/2 posterior margin; dactylus ca. 0.7 times as long as propodus (ZRC 2023.0250), with dense short setae on anterior and posterior margins.

Male pleon ( Fig. 3F ) narrow; external surface smooth, without any granules; lateral margins lined with short setae; telson tongue-shaped, longer and narrower than sixth pleonal segment, distal margin of telson not concave, without tuft of setae.

Male G1 ( Fig. 3G–J ) slender, slightly curved, distally curved toward dorsal; tip chitinous, with 1 short rounded lobe in lateral view ( Fig. 3I, J ), opened toward lateral; G2 shorter than 1/4 length of G1.

Size. Largest male 12.8 × 10.7 mm (MNHN-B29902).

Distribution. At present, only known from Vanuatu and Solomon Islands.

Remarks. The morphology of the two male specimens of this species agrees with the description of P. lipkei in N.K. Ng (2010) and Komai et al. (2021) in most characters (e.g., carapace, maxilliped, ambulatory legs, G1), except the immovable fingers of male chelipeds, which covered with tuft of long soft setae on the proximal half ( Fig. 3C ) [vs. immovable finger glabrous in P. lipkei (N.K. Ng 2010: fig. 2G; Komai et al. 2021: fig. 3B)]. Even the apparent difference on male chelipeds, however, this species and P. lipkei form a well-supported clade in COI tree ( Fig. 4 ) with very low distances within the clade ( Table 2). As a result, we tentatively refer to this species as P. aff. lipkei (see Discussion).

Ptychognathus aff. lipkei is also similar to P. glaber Stimpson, 1858 in the anterolateral teeth of carapace and the setae distribution on ambulatory legs, but the proximal half of outer surface of immovable finger is covered with long dense soft setae, which extends to the palm side in P. aff. lipkei ( Fig. 3C ) [vs. immovable finger is entirely glabrous ( Komai et al. 2021: figs. 1, 3B)].

Among other species of Ptychognathus , P. aff. lipkei is remarkably similar to P. ishii in the immovable finger of male cheliped with a tuft of long setae on outer surface and the ambulatory legs covered with dense short setae on the dorsal surface. However, P. aff. lipkei can be distinguished from P. ishii by the long soft setae only distributed on the proximal half of immovable finger ( Fig. 3C ) [vs. long setae present on that part, as well as the movable fingers and the margin of palm ( Sakai 1939: text-fig. 114b)]; and only the parts near the margins of carpi and propodi of ambulatory legs covered with dense short setae and sparse long setae ( Fig. 3D, E ) [vs. all surfaces of the carpi and propodi covered with dense short setae, but without long setae ( Komai et al. 2021: fig. 9A)].

In addition, P. aff. lipkei is also morphologically similar to P. pusillus , P. sakaii , and P. stimpsoni in the chelipedal fingers with dense long setae on outer surface and the male telson without a distinct tuft of soft setae on distal margin, but can be distinguished by the characters of carapace, distribution of long setae on male chelae, and ambulatory legs.

Ptychognathus aff. lipkei can be separated from P. pusillus by the distribution of setae on male chelipedal fingers and ambulatory legs, including the long soft setae present only on the proximal half of immovable fingers ( Fig. 3C ) [vs. setae present on both proximal halves of immovable and movable fingers ( De Man 1905: 539, pl. 17(5)]; and the carpi of ambulatory legs covered with dense short setae on the anterior margins ( Fig. 3D, E ) [vs. anterior margins of the carpi glabrous ( De Man 1895: 99, fig. 22, De Man 1905: 539, pl. 17(1)].

Ptychognathus aff. lipkei and P. sakaii can be differentiated by the characters of carapace, male chelipedal fingers, and ambulatory legs, including the postfrontal region lacking any lobes, and the anterolateral margin excluding the orbital tooth with only one distinct tooth ( Fig. 3A ) [vs. obscure lobes separated by shallow grooves, and the anterolateral margin with two indistinct teeth ( Hsu et al. 2022b: fig. 2A)]; long soft setae present only on the proximal half of immovable finger ( Fig. 3C ) [vs. setae present on both proximal halves of immovable and movable fingers ( Hsu et al. 2022b: fig. 2D)]; and anterior margins of ambulatory carpi not compressed distally ( Fig. 3D, E ) [vs. distinctly compressed ( Hsu et al. 2022b: fig. 2G, H)].

Ptychognathus aff. lipkei is also similar to P. stimpsoni but can be distinguished by the characters of the male chelipedal fingers and ambulatory legs, including only the proximal half part of immovable finger covered with long dense soft setae ( Fig. 3C ) [vs. proximal halves of both movable and immovable fingers with long dense soft setae ( Hsu & Shih 2020: fig. 4E)]; and anterior margins and surfaces of ambulatory carpi and propodi bearing numerous setae ( Fig. 3D, E ) [vs. less setae ( Hsu & Shih 2020: fig. 4G, H)].

Figures

FIGURE 1. A–D, Ptychognathus amikee sp. nov.; A–B, male holotype (15.8 × 13.6 mm, MNHN-B29852); C–D, female paratype (17.4 × 14.6 mm, MNHN-B29853); E–F, Ptychognathus aff. lipkei, male (11.4 × 9.6 mm, ZRC 2023.0250); A, C, E, dorsal view; B, D, F, ventral view.

FIGURE 3. Ptychognathus aff. lipkei. A–J, male (11.4 × 9.6 mm, ZRC 2023.0250). A, carapace; B, left third maxilliped; C, outer view of left male cheliped; D, left P4; E, left P5; F, male pleon; G–J, male right G1 (G, H, dorsal view; I, J, dorsolateral view).

FIGURE 4. A neighbor-joining tree for species of Ptychognathus, based on the cytochrome c oxidase subunit I (COI) gene. Probability values at the nodes represent support values. Only values> 50% are shown. For haplotype names, see Table 1.