(Figs. 1, 2)
Material examined. Holotype: NMCR 27005, female CL 2.7 mm, Philippines, Panglao, St. B 15, Sungcolan, 9 ° 38.8 ' N, 123 ° 49.2 ' E, 2–4 m, reef wall, dead coral, 16 Jun. 2004.
Paratypes: ZRC 2006.0140, 1 female CL 2.6 mm, Philippines, Panglao, St. B 12, Diljo Point, 9 ° 35.6 ' N, 123 ° 43.2 ' E, 24–27 m, reef slope, 14 Jun. 2004. ZRC 2006.0141, 1 male CL 1.7 mm, Philippines, Panglao, St. B 16, Bingag, 9 ° 37.6 ' N, 123 ° 47.3 ' E, 20 m, coral rubble on sand and gravel, 17 Jun. 2004. ZRC 2006.0142, 2 males CL 1.8, 2.6 mm, 1 ovig. female CL 1.8 mm, Philippines, Panglao, St. B 39, Pontod Lagoon, 9 ° 32.8 ' N, 123 ° 42.1 ' E, 17–25 m, wall reef with small caves, 2 Jul. 2004. MNHN-Ga 5743, 1 ovig. female CL 2.4 mm, Loyalty Islands, Lifou Island, Santal Bay, St. 1459, Ngoni, 20 °47.0’ S, 167 °03.0’ E, 55–80 m, dredge, 13 Nov. 2000.
Description. Carapace (Fig. 1 A, B) transverse rounded hexagon in dorsal view, 1.1–1.2 times as broad as long, broadest across median branchial margin. Dorsal surface moderately convex, entirely covered with small pits and very short striae. Protogastric ridges rounded. Cervical grooves moderately demarcated. Hepatic margins roundly produced. Branchial margins strongly convex, median part somewhat constricted, with rounded crest; posterior margin and adjacent region with long, transverse and oblique ridges.
Rostrum (Fig. 1 C) comparatively narrow, somewhat produced beyond eyes, transverse or weakly convex in dorsal view, with row of plumose setae along anterior margin; trilobate in frontal view, median lobe strongly bent ventrally, narrowly rounded, more prominent than laterals, without dorsal, median longitudinal groove; lateral lobes rounded. Orbits (Fig. 1 A, B, F) shallow; supra-orbital margins oblique; outer orbital angles rounded.
Pterygostomian flaps (Fig. 1 F) entire, with crenulate crest along anterior dorsal margin and clearly visible in dorsal view; undulated ridge present along midline; anterior apex subacute.
Third thoracic sternite (Fig. 1 D) very low, somewhat depressed medially; anterior margin concave medially in ventral view but slightly convex in anterior view, with row of setae; lateral lobes narrow, each with rounded apex. Fourth thoracic sternite with transverse series of setae along concave, anterior margin; median margin incompletely separated from third sternite.
Telson(Fig. 1 E) composed of 7 plates; proximolateral plates much smaller than others.
Ocular peduncles (Fig. 1 A, B, F) small, smooth; dorsal extension on to cornea rounded.
Basal article of antennular peduncles (Fig. 1 G) unarmed; anterior surface somewhat concave; anterior, dorsal and ventral margins minutely tuberculate, anterior margin weakly bilobed; ventral surface with transverse ridge on median lateral part.
Antennal peduncles (Fig. 1 A, B, F) 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 rounded. Second and third articles roundly rectangular, third article elongate. Fourth article short.
Third maxilliped (Fig. 1 H) with coxa bearing small rounded projection on distoflexor margin; distomedian projection rounded, not articulated. Basis articulating with ischium, rounded subtriangular. Ischium broad, ovate, with short striae bearing sparse setae on ventral surface; no distinct ridge along extensor margin; anterior extensor projection broad, roundly subtriangular. 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 short rugae on extensor surface. Propodus elongate, slightly tapering distally, with short rugae along extensor margin. Dactylus elongate, ovate. Merus to dactylus with long setae on flexor margin, setae on dactylus reaching base of third maxilliped. Exopod with proximal article small, rounded; distal article laminate, robust, over-reaching distal margin of merus, without distal flagellum; proximal part inflated, distal part narrow.
Chelipeds (Fig. 2 A–E) generally similar in shape in males and females, somewhat unequal, subcylindrical; dorsal surface covered with short, oblique and transverse striae and small pits; ventral surface also with numerous small pits. Larger cheliped (Fig. 2 A–C) with ischium bearing few short plumose setae on dorsal surface; flexor margin weakly crenulate, with row of plumose setae. Merus with rounded transverse crest submedially on dorsal surface; dorsoflexor margin with narrowly rounded lobe distally; ventral distoflexor margin broadly rounded, with minute tubercles. Carpus 1.7 –2.0 times as long as broad; dorso-extensor margin rounded; dorsoflexor margin slightly crenulate, with 2 or 3 subtriangular teeth (if 3 teeth present, 1 tooth much smaller than other 2 and situated proximally) and row of plumose setae on median part; dorsodistal margin with rounded lobe on extensor part; ventral flexor margin slightly concave. Chela moderately narrow, elongate, 1.5–1.6 times as long as carpus, 2.6–2.9 times as long as high, lying on extensor side; dorsal surface adjacent to extensor margin and of fixed finger with plumose setae; extensor margin crested, weakly concave on base of fixed finger, with small, blunt and subacute tubercles (proximal 2 or 3 larger, spiniform) and numerous plumose setae on distal 0.7. Palm with dorsal surface convex, no distinct dorsomedian longitudinal ridge present; dorsoflexor margin without longitudinal rugose ridge; dorsoflexor distal part with low but broad projection extending onto dactylus; ventral surface inflated, without longitudinal crest on midline, distal flexor part with several plumose setae. Fixed finger with weakly curved distal claw; cutting edge with row of small teeth; ventral surface with row of plumose setae along cutting edge. Dactylus 0.4–0.5 length of chela, opening at oblique angle, with strongly curved distal claw; dorsal surface with longitudinal ridge of small tubercles and plumose setae along flexor margin; cutting edge with small teeth; ventral surface with plumose setae on cutting margin and adjacent part.
Smaller cheliped (Fig. 2 D, E) almost identical to larger, except for: chela narrower; dactylus opening at stronger angle, with more strongly crested, longitudinal ridge of small tubercles along flexor margin.
Ambulatory legs (Fig. 2 F–I) relatively short, subcylindrical, decreasing in length posteriorly, with scattered, plumose setae marginally, setae numerous on meri; decalcified part absent on mesial surfaces of ischia and meri. Ischia unarmed (first leg) or 1 or 2 spines (second and third legs) on flexor margin. Meri somewhat compressed latero-mesially, elongate oval, tapering distally; extensor margin weakly crenulated; distoflexor margins of lateral and mesial surfaces without lobes or spines; flexor surface concave, lateral flexor margin crenulate, mesial flexor margin with spines and protuberances, spines of second and third legs strong. Carpi elongate, with small pits and very short striae on surfaces; disto-extensor and distoflexor corners unarmed, narrowly rounded. Propodi 1.4–1.6 times as long as dactyli, 2.6 –3.0 times as long as high; lateral and mesial surfaces with scattered, small pits; extensor margin nearly smooth; flexor margin with 5 or 6 corneous spines, distal pair of spines subequal in size. Dactyli each terminating in curved, bifurcate claws, flexor claw subequal in length to but stouter than extensor; flexor margin with 2 or 3 small corneous spines on slightly produced bases, spines much smaller than terminal claw.
Fifth pereopods slender, chelate; propodus with numerous short simple setae and 10–14 hooked setae.
Male with pair of developed pleopods modified as gonopods on second abdominal segment (Fig. 1 I); protopod naked; endopod spoon-shaped, elongate oval, with marginal setae except for proximal part; exopod very small, ovate, with terminal seta; pleopods on third to fifth abdominal segments reduced to inconspicuous rudiments. Female with pair of well developed pleopods on each of third to fifth abdominal segments; pair of small, indistinct rudiments on second abdominal segment.
Coloration. Not recorded.
Etymology. The specific name is from the Latin, spina (spine), referring to the meri of the ambulatory legs armed with strong spines; used as a noun in apposition.
Distribution. Philippines and Loyalty Islands, at depths of 2–80 m; no associations were recorded in the collection data.
Remarks. The new species resembles species of the P. s i n e n s i s group defined by Johnson (1958), but differs in having the carpus of the cheliped with teeth on the dorsoflexor margin and the dactylus of the ambulatory leg with a flexor claw being stouter than, but subequal in length to the extensor claw. The known species of the P. sinensis group have the carpus of the cheliped being unarmed on the dorsoflexor margin and the ambulatory dactylus with the extensor claw being much smaller than the flexor claw. In species of the genus Polyonyx, the possession of the teeth on the carpus of the cheliped is known as a character of juvenile and young specimens (Haig 1964, 1966; for P. obesulus Miers, 1884). The type specimens of P. s p i n a n. sp. are very small (CL 1.7–2.7 mm). However, the presence of males with well developed gonopods and ovigerous females indicates that they are mature. The armature of the carpus of the cheliped can be treated as a reliable specific character for the adults of the new species.
Polyonyx spina is most closely allied to P. pedalis Nobili, 1905, of the P. s i n e n s i s group in having spines on the flexor margin of the merus of each the ambulatory legs. This character is known only in the two species among the Indo-West Pacific species of the genus. Polyonyx confinis Haig, 1960, P. g i b b e s i Haig, 1956, and P. nitidus Lockington, 1878, known from the eastern Pacific and western Atlantic, also have spines on the flexor margin of at least the merus of the third ambulatory leg. However, these three species are readily distinguished from the two Indo-West Pacific species by the possession of a subrectangular, proximal angle on the dorsoflexor margin of the carpus of the cheliped. Polyonyx pedalis and P. s p i n a lack such the structure.
To supplement the sole published figure of P. pedalis by Nobili (1906, pl. 8, fig. 3) and to facilitate comparisons with the new species, selected parts of a specimen of P. pedalis are illustrated (Fig. 3). Although the type material of P. pedalis was not available for study, the specimens examined from New Caledonia agree well with Nobili’s illustration in the characteristic shapes of the carapace and larger cheliped and the rostrum being produced forward in dorsal view. Polyonyx spina is distinguished from P. pedalis by the configuration of the carapace, thoracic sternum, and ambulatory legs, in addition to the distinctions mentioned above. The branchial margins of the carapace are convex in P. s p i n a, whereas they are subparallel in P. p e d a l i s. The third thoracic sternite is distinctly concave on the anteromedian margin in the new species, instead of being broadly convex in P. pedalis. The spines on the flexor margins of the meri of the second and third ambulatory legs are much fewer in number but much stronger in the new species than P. pedalis. Polyonyx spina also differs from P. pedalis in having a pair of gonopods on the second abdominal segment in males, which are absent in the latter species.
Polyonyx spina lacks a distal flagellum on the exopod of the third maxilliped. This character is also known in Heteropolyonyx biforma Osawa, 2001.