Ptychognathus andamanicus Alcock, 1900

Patel, Heris, Mendoza, Jose C. E., Bineesh, K. K., Mitra, Santanu & Trivedi, Jigneshkumar N., 2025, On the taxonomy of Ptychognathus andamanicus Alcock, 1900 (Crustacea: Decapoda: Brachyura: Varunidae) from the Andaman Islands, India, Zootaxa 5584 (3), pp. 429-436 : 430-434

Ptychognathus andamanicus Alcock, 1900

( Figures 1–3 )

Ptychognathus andamanica Alcock, 1900: 404 , 405.— Alcock & McArdle 1903: pl. 65 figs. 3, 3a.— Ng et al. 2008: 229 (list as junior synonym of Ptychognathus riedelii (A. Milne-Edwards, 1868) .

Ptychognathus andamanicus — De Man, 1905: 542 (discussion & key).— Tesch 1918: 92 (discussion).

Ptychognathus riedelii — Ng et al. 2008: 229 (list) [in part].— Trivedi et al. 2018: 71 (list). [Not Ptychognathus riedelii (A. Milne-Edwards, 1868) ].

Material examined. Lectotype (herein designated), female (CW 14.9 mm, CL 13.3 mm) ( ZSI3349-50/10 ), Saddle Hill, North Andaman Island, India, coll. A. R. Anderson, no date (ca. 1890’s) . Other material: 1 male (CW 13.2 mm, CL 11.4 mm) ( LFSC.ZRC-220 ), Mannar Ghat, South Andaman Island, India, coll. K. K. Bineesh, 11 December, 2018 .

Comparative material examined. Ptychognathus riedelii (A. Milne-Edwards, 1868) : 2 males (CW 12.9 mm, CL 11.6 mm; CW 19.3 mm, CL 17.2 mm) 4 females (CW 13.9–16.5 mm, CL 12.1 mm – 14.6 mm) ( ZRC 2011.0650 ), Sungai Jalange at Desa Mallawa , 20 km South of Pare Pare on road to Ujung Pandang (= Makassar ), Kecamatan Malusetassi, Kabupaten Barru, Sulawesi Selatan, Indonesia, 04°09'09"S 119°37'27"E (MK 88–015), coll. M. Kottelat, 15 June 1988 ; 3 males, (CW 14.5–20.2 mm, CL 12.9–18.0 mm) ( ZRC 2013.0302 ), Sungai Bone , Lombongo Village, Gorontalo Province, North Sulawesi, Indonesia, coll. Haryono, 22 November 1993 ; 1 male (CW 20.6 mm, CL 18.4 mm), 1 female (CW 20.2 mm, CL 18.1 mm) ( ZRC 2013.0094 ), Sungai Ifis , Halmahera Island, Indonesia, coll. D. Robb, September 1994 ; 1 male (CW 11.7mm, CL 10.5 mm), 3 females (CW 10.7–19.7 mm, CL 9.8–17.0 mm) ( ZRC 2013.0300 ), Parang River , near Alas / Utan, NW Sumbawa , West Nusa Tengara Province, Indonesia, coll. A.J. Whitten , no date; 1 male (CW 26.8 mm, CL 24.3 mm) ( ZRC 2013.0301 ), Telaga River , Bali Island, Indonesia, coll. D.G.B. Chia & C.P. How, 21 July 1995 .

Description of male. Carapace ( Figs. 1A, C , 3A ) subquadrate, slightly broader than long (CW/CL ratio: 1.16); dorsal surface smooth, glabrous), regions weakly defined, with noticeable groove between epigastric regions, metabranchial region granulated, weakly sloping outwards. Front moderately broad (FW/CW ratio: 0.41), weakly sloping forward, front near orbital regions slightly convex; frontal margin lined with small, rounded granules. Orbits shallow, elongated, slightly sinuous. Supraorbital margins lined with small granules; infraorbital ridge consisting of 16–21 small, rounded granules in both sexes. Anterolateral margin smooth, not granulated, with three well defined teeth including external orbital tooth, separated by shallow notches; first (external orbital) tooth largest, most distinct; third tooth smallest but distinct. Posterolateral margins slightly convergent posteriorly; posterolateral regions regularly furnished with short, soft setae. Surface of pterygostome with sparse soft setae. Epistome broad, median part triangular, posterior edge with tiny granules.

Third maxillipeds ( Figs. 2E , 3B ) broad, external surface glabrous; merus broad, subtrapezoidal, mesial part with oblique shallow depression, anterolateral angle broadly rounded, slightly sloping laterally, anteromesial margin with angular recess onto which palp is articulated; ischium narrow, sub rectangular, with distinct vertical shallow groove on external surface. Exopod broad, distal tip almost reaching anterolateral angle of merus 1.7 times as broad as ischium at level of distal end of ischium, mesial margin more convex than lateral margin; flagellum well developed, length more than twice width of merus.

Thoracic sternum wide in both sexes, with smooth, finely pitted on ventral surface, male sternites 1, 2 fused, separated from sternite 3 by distinct suture, sternite 3, 4 completely fused, sternite 3 separated from sternite 2 by shallow groove, sternites 5–7 almost rectangular, similar in width, sternite 8 smaller compared to 5–7.

Chelipeds ( Figs. 1A, B , 2A–D ) symmetrical, stronger, more robust in male; merus with blunt postero-distal spine, dorsal margins with long soft setae, ventral margins glabrous; surface of carpus glabrous, with several outer distal margin (at articulation with propodus) with conspicuous fringe of long soft setae angle blunt in male ( Figs. 1A , 2B ), outer surface of palm in male finely granulate; inner surface glabrous, slightly convex medially; movable finger approximately as long as palm, cutting edges with 7–9 small blunt teeth subdistally; immovable finger equal to movable finger, cutting edges with 3–5 smaller teeth and 2 large blunt teeth, with single prominent horizontal ridge toward palm, fingers with a small tuft of setae at tips of pollex.

Ambulatory legs slender, P3 and P4 longest ( Fig. 1A, B ); meri with antero-distal spine, with long soft setae on proximal half of anterior margin, proximal part of posterior margins with sparse short setae, ventral surfaces of carpus and propodus almost glabrous, densely setose on anterior surface; propodus as long as dactylus in P2–P4, propodus about 1.2 times length of dactylus in P5, anterior margins of propodus and dactylus with short setae, posterior margins of dactylus with long soft setae and propodus sparsely setose, ventral surfaces of carpus and propodus almost glabrous; P4 relatively long, anterior margins of carpus and propodus with short setae and posterior margins with long soft short setae ( Fig. 2F ); P5 relatively short, anterior margins of carpus and propodus with short setae and posterior margins of distal part of propodus with tuft of short setae ( Fig. 2G ).

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

Male G1 ( Fig. 3C–E ) slender, almost straight, distally curved outwards, tip chitinous, tapering at tip, apical lobe with long, stout setae. G2 shorter than ¼ length of G1 ( Fig. 3F ).

Female morphology. The morphology of the female (viz. lectotype) is mostly similar with that of the male, except in the aspects of the chelipeds, pleon, and external sexual structures. Chelipeds symmetrical, fringe of setae absent on outer distal margin of carpus, inner distal angle with single small blunt spine ( Figs. 1C , 2D ). Female with outer surface of palm almost glabrous and granulated, inner surface glabrous and middle part convex; immovable finger with single prominent horizontal ridge toward palm, fingers with a small tuft of setae at tips of pollex. Pleon wide, with six free somites and telson, pleomere with rounded distal margin, widest at somites 2, 3, decreasing towards telson. Telson broadly triangular, 1.5 time broader than long.

Remarks. The morphological characters of the lectotype female and fresh male show agreement with the description of P. andamanicus provided by Alcock (1900) and the illustration by Alcock & McArdle (1903). Ptychognathus andamanicus most closely resembles P. riedelii in the general shape of the carapace and the leg proportions, and particularly in having no setae on the external surface of chela other than the short stiff setae on the tip of the pollex, which is why earlier workers ( De Man 1905; Tesch 1918) considered them to be conspecific. Although the type of P. riedelii could not be examined for this study, we were able to examine adult male and female specimens from Sulawesi, the type locality, as well as other nearby parts of Indonesia (see Comparative material). Ptychognathus andamanicus can be distinguished from P. riedelii on the basis of the following characters: most significantly in the male, the outer distal margin of the cheliped (P1) carpus is fringed with long soft setae in P. andamanicus ( Fig. 2B ) (versus such setae absent in P. riedelii ; cf. A. Milne-Edwards 1868: pl. 27 fig. 4; Cai & Ng 2001: fig. 16D); the distal tip of the G1 is relatively narrower and less curved in P. andamanicus ( Fig. 3C–E ) (versus stouter and more curved in P. riedelii ; cf. Cai & Ng 2001: fig. 16E, F).

Aside from these morphological differences, it is also important to note that the current known range of P. riedelii could be limited to the type locality, Sulawesi, and the nearby Indonesian islands of Flores, Halmahera, Sumatra (viz. Holthuis 1978; Cai & Ng 2001), Bali and Sumbawa (viz. this study), as well as on the Philippines islands of Mindanao ( Lagare et al. 2020), Cebu and Bohol ( Hsu & Shih 2024), Papua New Guinea (New Britain), Solomon Islands, Vanuatu and Samoa Independent State (Upolu) ( de Mazancourt et al. 2024)—all of which are in the Pacific marine region. On the other hand, P. andamanicus is known from the Andaman Islands, in the eastern Indian Ocean. The Indian record of P. riedelii (viz. Trivedi et al. 2018) is corrected and now refers to P. andamanicus . Some studies on marine crabs have suggested that there is a biogeographic barrier separating the crab faunas of the western Pacific and eastern Indian Oceans (see Lasley et al. 2023; Shahdadi & Mendoza 2023), and this may be the case here, whereby two sister species straddles either side of this Indo-Pacific barrier.

Ptychognathus andamanicus is not to be confused with P. andamanensis Pretzmann, 1984 , which was described from June Creek, Corbyns Cove, South Andaman Island; the latter can easily be distinguished by the setose dorsal surface of the carapace and the presence of a conspicuous patch of setae on the external surface of the palm of the chela (cf. Pretzmann 1984: figs. 9, 10).

Distribution. Ptychognathus andamanicus has so far been reported from its type locality, Saddle Hill North Andaman Island ( Alcock 1900; Trivedi et al. 2018) and in the present study, it is reported from Mannar Ghat, South Andaman Islands, India, extending its distribution range.

Figures

FIGURE 1. A, B, Ptychognathus andamanicus Alcock, 1900. A, B, male (CW 13.2 mm, CL 11.4 mm), LFSC.ZRC-220, Mannar Ghat, South Andaman Islands, India; C, D, lectotype, female (CW 14.9 mm, CL 13.3 mm), ZSI3349-50/10, North Andaman Island, India. A, C, dorsal view; B, D, ventral view. Scale bar = 2 mm.

FIGURE 2. Ptychognathus andamanicus Alcock, 1900. A, B, E, F, G, male (CW 13.2 mm, CL 11.4 mm), LFSC.ZRC-220, Mannar Ghat, South Andaman Islands, India; C, D, lectotype, female (CW 14.9 mm, CL 13.3mm), ZSI 3349-50/10, North Andaman Island, India. A, setae on the tip of pollex of right chela; B, left cheliped, outer view; C, right chela, outer view; D, carpus of left cheliped; E, third maxilliped; F, right P4; G, right P5, outer view. Scale bar = 2 mm.

FIGURE 3. Ptychognathus andamanicus Alcock, 1900, male (CW 13.2 mm, CL 11.4 mm), LFSC.ZRC-220, Mannar Ghat, South Andaman Islands, India. A, male carapace, dorsal view; B, third maxilliped; C, left G1 dorsal view; D, left G1 lateral view; E, left G1 ventral view; F, G2; G, male pleon, dorsal view. Scale bars: B, E, G = 1 mm, F = 0.5 mm.