Polyonyx biunguiculatus Dana 1852
Creators: name: Osawa, Masayuki
Publisher: name: Zenodo
Year: 2007
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Decapoda
Porcellanidae
Polyonyx
Polyonyx biunguiculatus

description: Polyonyx biunguiculatus (Dana, 1852)
(Figs. 9, 10)
Porcellana biunguiculatus Dana, 1852: 411; 1855: pl. 26, fig, 1a–d (type locality: Loyalty Islands, restricted by present neotype selection).
Polyonyx parabiunguiculatus Yang 1996: 262, 268, fig. 5 (type locality: Nansha Islands, South China Sea) (new synonymy).
? Polyonyx biunguiculatus.— Haig, 1983: 286; 1992: 320, fig. 16.
Not Polyonyx biunguiculatus.— Miyake, 1942: 371, figs. 30–32.
? Not Polyonyx biunguiculatus.— Haig, 1964: 377; 1965: 112; 1979: 130, figs. 16–19.— Yang & Naiyanetr 1997: 7, fig. 4A–E.
Material examined. Type material: Lifou Island. Santal Bay. Atelier LIFOU. St. 1462 (St. 19), Arète, Aimé Martin (= Acadro), 20°47.1’S, 167°03.2’E, 70–120 m, dredge, 21 Nov. 2000, male CL 2.8 mm, neotype (MNHN-Ga 6466).
New Caledonia. Koumac, Grand récif, lagoon, outer reef slope, 12 m, 7 Oct. 1993, 1 female CL 2.5 mm. Loyalty Islands. Lifou Island. Santal Bay. Atelier LIFOU. St. 1459 (St. 6), Ngoni, 20°47.0’S, 167°03.0’E, 55–80 m, dredge, 13 Nov. 2000, 1 male CL 2.7 mm.
Description. Carapace (Fig. 9 A) transversely subrectangular, 1.2 times as broad as long, broadest on anterior branchial margin. Dorsal surface convex, nearly smooth, with scattered, very short striae and small pits. Rostrum (Fig. 9 B) broad, bent ventrally, trilobate in frontal view; median lobe triangular, exceeding obtuse lateral lobes, with median longitudinal groove on dorsal surface. Orbits shallow, supra-orbital margins oblique; outer orbital angles rounded. Protogastric ridges and cervical grooves faintly demarcated. Branchial margins somewhat constricted medially; posterior margin and adjacent region with long transverse rugae. Cardiac region smooth.
Third thoracic sternite (Fig. 9 C) with anterior margin trilobate, bearing sparse setae; median lobe broad, low subtriangular; lateral lobes distinctly exceeding median lobe, each with rounded apex. Fourth thoracic sternite with transverse series of short setae along weakly concave, anterior margin.
Ocular peduncles (Fig. 9 A, D) moderately large; dorsal extension onto cornea weakly produced.
Basal article of antennular peduncles (Fig. 9 D) unarmed; anterior surface concave, weakly rugose; anterior, dorsal, and ventral margins weakly tuberculate; ventral surface with short transverse ridge on median lateral part.
Antennal peduncles (Fig. 9 A, D) short, slender, nearly smooth, unarmed. First article largest, strongly produced forward in lateral view, broadly in contact with lower orbital margin, with longitudinal ridge along ventral margin; lateral surface concave; anterior margin narrowly rounded. Second, third, and fourth articles roundly subrectangular, third article elongate, fourth article short.
Third maxilliped (Fig. 9 E) with coxa bearing small ronded projection on distoflexor margin; distomedian projection not articulated. Basis articulating with ischium, rounded subtriangular. Ischium broad, ovate, with few delicate striae on ventral surface, with weak longitudinal ridge along extensor margin; anterior extensor projection rounded. Merus with laminate, broad, rounded subrectangular lobe on ventroflexor margin; ventral surface weakly striate. Carpus with subtriangular projection on median part of flexor margin and row of short rugae on extensor surface. Propodus relatively short, somewhat tapering distally. Dactylus short, rounded subtriangular. Merus to dactylus with long setae on flexor margin. Exopod with proximal article small; distal article laminate, robust, nearly reaching distal margin of merus, with distal flagellum (not illustrated), proximal part inflated.
Chelipeds (Fig. 10 A–H) unequal, subcylindrical, inflated. Larger cheliped (Fig. 10 A–C, F, G) with merus provided with rounded transverse crest submedially on rugose dorsal surface; dorsoflexor margin with round lobe distally. Carpus 2.2 (female)–3.0 (male) times as long as broad; dorso-extensor margin rounded; dorsoflexor margin weakly concave, smooth or weakly tuberculate, with rounded proximal angle; dorsodistal margin nearly transverse. Chela relatively broad, ovate, 1.4–1.5 times as long as carpus, 2.3–2.4 times as long as high, lying on extensor side; dactylus opening at oblique angle; extensor margin smooth, slightly concave at base of fixed finger; dorsal surface generally smooth, with longitudinal blunt ridge, short rugae, and scattered short setae along extensor margin. Palm with dorsal surface convex, no dorsomedian longitudinal ridge present; dorsoflexor margin with faintly or weakly developed, longitudinal rugose ridge; dorsoflexor distal part with low projection extending onto dactylus; ventral surface with rounded longitudinal ridge proximally along midline, distal flexor margin with few short simple setae. Fixed finger with weakly or moderately curved distal claw; dorsal surface with scattered short simple setae; cutting edge with row of small, rounded and subtriangular teeth (female) or unarmed except for large proximal tooth with marginal tubercles (male); ventral surface with few short simple setae along cutting edge. Dactylus 0.4–0.5 length of chela, with strongly curved distal claw; dorsal surface nearly smooth; dorso-extensor margin rounded (male) or with rugose ridge (female); cutting edge concave, with row of small, rounded and subtriangular teeth (female) or unarmed except for broad proximal tooth (male); ventral surface with few short simple setae along cutting edge.
Smaller cheliped (Fig. 10 D, E, H) genarally similar to larger cheliped in shape, except for: chela with small protuberances and subacute tubercles on distal 1/2–2/3 of extensor margin and adjacent dorsal surface; cutting edges of fingers nearly transverse, each with row of smaller teeth; dactylus opening at more stronger angle, with more strongly crested, longitudinal ridge along flexor margin.
Ambulatory legs (Fig. 9 F–I) stout, subcylindrical, with few scattered, simple setae marginally, most numerous on propodi; lateral surfaces nearly smooth. Meri somewhat compressed lateromesially, elongated ovate, decreasing in size posteriorly; extensor margin unarmed but slightly crenulated, weakly convex; distoflexor margins of lateral and mesial surfaces without lobes or spines. Carpi moderately elongate; lateral surface with longitudinal rows of short rugae on extensor half; disto-extensor and distoflexor corners unarmed, narrowly rounded. Propodi 1.6 times as long as dactyli, 3.4 times as long as high; extensor margin slightly crenulated; flexor margin with 4 corneous spines, proximal spine smaller than others, distal paired spines subequal in size. Dactyli each terminating in weakly curved, bifurcate claw, flexor claw stouter but shoter than extensor claw; flexor margin with 2 small corneous spines.
Male with pair of short but developed pleopods modified as gonopods on second abdominal segment.
Remarks. Dana (1852) did not specify the type locality of Porcellana biunguiculata and the type specimen is apparently lost (see Deiss & Manning 1981). Therefore, the male specimen from the Loyalty Islands (CL 2.8 mm, MNHN-Ga 6466) is herein selected as the neotype of Polyonyx biunguiculata to fix its identity.
The specimens examined agree well with the original description and figures of P. biunguiculatus by Dana (1852, 1855), especially on account of the larger chela being rather smooth on the dorsal surface (Fig. 10 A, B, F, G). Although Dana (1852) did not indicate the sex of the specimen, it seems to be a male because of the possession of a large proximal tooth on the cutting edge of the fixed finger (Fig. 10 B). The female lacks such a large tooth (Fig. 10 G). The smooth condition of the larger chela in males is also illustrated for Hong Kong material of P. biunguiculatus (Haig 1992, fig. 16C), whereas the chela is figured as rather tuberculate on the fixed finger in the material from the Moluccas and Gulf of Thailand (Haig 1979: fig. 19; Yang & Naiyanetr 1997: fig. 4D).
Haig (1964) showed that P. biunguiculatus could be distinguished from P. obesulus by the dactyli of the ambulatory legs each with two small spines on the flexor margin and the absence of pleopods in males, based on an examination of a good series of P. biunguiculatus from the Indo-Malayan area. The absence of pleopods in males was also confirmed in the Western Australian and Moluccas material (Haig 1965, 1979). However, Haig (1983) showed that this character was less reliable than she had supposed since the all males from the “Reves 2” collection from the Seychelles have well-developed pleopods.
The smooth larger chela and presence of male pleopods are species-specific characters for the true P. biunguiculatus, and the specimens with tuberculate chelae and no pleopods in males represent a different, undescribed species. It is likely that the material from the Seychelles and Hong Kong reported by Haig (1983, 1992) belongs to true P. biunguiculatus, whereas the specimens from the the Indo-Malayan area, Moluccas, Gulf of Thailand, and Western Australia reported by Haig (1964, 1965, 1979) and Yang & Naiyanetr (1997) are an undescribed species; although the true identities of these specimens need re-examination. On the other hand, re-examination of the specimens from Palau (ZLKU 10318, 11 specimens) reported by Miyake (1942) showed that they had small tubercles on the base of the fixed finger of each cheliped and no pleopods in males, and thus belonged to the undescribed species.
Baba (1989) recorded P. biunguiculatus from the Ryukyu Islands and noted that the coloration of the specimens is different from that provided by Miyake (1942). His specimens were totally reddish with irregular reticulation of white, and had the carapace with a rough longitudinal stripe of white color, whereas Miyake (1942) noted the dorsal surface (of the carapace) to be light orange yellow, the ventral (surface) whitish. New material with information of coloration would help to distinguish the P. biunguiculatus species complex.
Yang (1996) described a new species, P. parabiunguiculatus, from the Nansha Islands, South China Sea. This species was reported to differ from “ P. biunguiculatus ” in the median lobe of the rostrum being narrower and more prominent, merus of the cheliped with a well developed, flexor distal lobe, and presence of pleopods in males. In addition to the smooth dorsal surface of the larger chela, these characters agree with those of the male specimens examined including the neotype of P. biunguiculatus. Therefore, P. parabiunguiculatus can be treated as a junior synonym of P. biunguiculatus.
The specimens examined of P. biunguiculatus have a rounded, longitudinal ridge on the proximal ventral surface of the palm of the cheliped (Fig. 10 C), which is a similar structure observed in the material of P. triunguiculatus Zehntner, 1894. However, P. triunguiculatus is distinguished from P. biunguiculatus by the longitudinal ridge being better developed and more strongly crested as well as the median lobe of the rostrum being narrower and more strongly produced and carpi of the chelipeds being rather transverse or convex on the dorsoflexor margin (the margin is weakly concave in P. biunguiculatus).
In the female specimen examined, the dactylus of the right second ambulatory leg is largely deformed and does not show a bifurcate form as seen in other ambulatory dactyli (Fig. 9 I).
Distribution. Presently known with certainty only from New Caledonia (type locality), Loyalty Islands, and Nansha Islands in South China Sea; at depths of 12– 120 m.
description: Published as part of Osawa, Masayuki, 2007, Porcellanidae (Crustacea: Decapoda: Anomura) from New Caledonia and the Loyalty Islands, pp. 1-49 in Zootaxa 1548 on pages 31-36, DOI: 10.5281/zenodo.178040