Polyonyx pilosibrachium, Osawa & Naruse & Ng, 2018

Osawa, Masayuki, Naruse, Tohru & Ng, Peter K. L., 2018, New records of species of the Polyonyx sinensis group (Crustacea: Decapoda: Anomura: Porcellanidae) from Japan, the Philippines, Singapore, and Malaysia, with descriptions of two new species, Zootaxa 4429 (2), pp. 303-323 : 307-312

Polyonyx pilosibrachium n. sp.

( Figs. 1C, D , 3–5 )

Polyonyx cf. utinomii . — Naruse et al. 2017: 2, fig. 2C (not Polyonyx utinomii Miyake, 1943 ).

Type material. Holotype: CBM-ZC 14134, male (cl 3.0 mm), Funaura, Iriomote Island , Ryukyu Islands, Japan, lower intertidal, in tube of Chaetopterus cf. cautus, digging, coll. T. Komai, 24 June 2005.

Paratypes. Japan. Ryukyu Islands. Kume Island, KUMEJIMA 2009 Marine Biodiversity Expedition : RUMF- ZC-4737, 1 male (cl 2.6 mm), 1 female (cl 3.0 mm), sta. 52, 26°20.40'– 26°20.01'N, 126°49.67' –126°49.224'E, 5.1– 4.5 m, dead coral branches and rubbles, dredge, 17 November 2009; RUMF-ZC-4738, 1 male (cl 1.8 mm), sta. 56, 26°19.36'–26°19.29'N, 126°49.20'–126°49.19'E, 7.5– 4.7 m, dead coral blocks and rubbles, dredge, 17 November 2009. Iriomote Island : CBM-ZC 8872, 1 male (cl 2.4 mm), 1 female (cl 2.4 mm), 1 ovigerous female (cl 3.8 mm), Funaura, lower intertidal, in tubes of C. cf. cautus, digging, coll. T. Komai, 24 June 2005 ; RUMF-ZC-4387, 1 male (cl 3.9 mm), Funaura, intertidal, in tube of C. cf. cautus, digging, coll. R. Yoshida, 14 June 2015, 1 male (cl 3.4 mm), 1 ovigerous female (cl 4.1 mm), RUMF-ZC-4386. Funaura, intertidal, in tube of C. cf. cautus, digging, coll. R. Yoshida, 27 October 2015; RUMF-ZC-4385, 1 ovigerous female (cl 3.3 mm), Off Funaura Port , 24°24.41'N 123°49.13'E, 2–6 m, in tube of C. cf. cautus (RUMF-ZO-29), SCUBA diving, coll. T. Naruse & R. Yoshida, 4 August 2016 ; RUMF-ZC-4388, 1 male (cl 3.2 mm), Off Funaura Port , 24°24.41'N 123°49.13'E, 2–6 m, in tube of C. cf. cautus, SCUBA diving, coll. T. Naruse & R. Yoshida, 4 August 2016 ; RUMF-ZC-4393, 1 male (cl 1.8 mm), Midara, intertidal, coll. T. Naruse, 14 August 2014; CBM-ZC 14135, 3 males (cl 1.7–2.1 mm), Shirahama, intertidal, in tubes of C. cf. cautus, digging, coll. T. Komai, 22 June 2005; RUMF-ZC-4389, 1 ovigerous female (cl 3.3 mm), Shirahama, intertidal, mud, sea grass, in tube of C. cf. cautus, coll. M. Osawa, 16 June 2011; RUMF-ZC- 4391, 1 male (cl 2.8 mm), Shirahama, intertidal, coll. T. Naruse, T. Komai & T. Maenosono, 14 May 2014; RUMF- ZC-4392, 1 male (cl 3.1 mm), 1 ovigerous female (cl 3.1 mm), Amitori Bay , near river mouth, intertidal, coll. T. Naruse & T. Maenosono, 16 May 2014 .

Description. Carapace ( Figs. 3A ) transversely ovate in general outline, 1.3–1.4 times as broad as long, broadest on median branchial margins. Dorsal surface nearly flattish from side to side (slightly convex on gastric and branchial regions), entirely covered with short and relatively long, transverse striae anteriorly bearing sparse minute setae. Protogastric ridges and cervical grooves weakly demarcated. Hepatic margins weakly or moderately convex. Branchial margins strongly convex, weakly constricted on each median part; anterior 0.6 margins thin, each with row of plumose setae; posterior margins and adjacent regions with long, transverse ridges. Rostrum ( Fig. 3B ) moderately broad, not produced beyond eyes, weakly produced in dorsal view, ventrally bent toward anterior tip; trilobate in frontal view, median lobe bluntly triangular, overreaching rounded lateral lobes, without median longitudinal groove; dorso-anterior surface with sparse short setae, those of row on anterior margin longer. Orbits shallow; supra-orbital margins slightly concave; lateral orbital angles rounded.

Pterygostomial flaps entire, with short and long, longitudinal ridges; dorso-anterior margin terminating subacutely ( Fig. 3E, F ).

Third thoracic sternite ( Fig. 3C ) 3.5–3.8 times as broad as long measured on midline, with surface somewhat depressed medially; anterior margin trilobite, with sparse long setae; median lobe very broad, convex on anterior margin; lateral lobes narrow, reaching anterior margin of median lobe, each with rounded apex. Fourth thoracic sternite with anterior margin weakly or moderately concave; surface depressed medially, with transverse series of short ridges anteriorly.

Telson ( Fig. 3D ) composed of 7 plates; proximolateral plates much smaller than others; distal plates as long as or slightly longer than broad.

Ocular peduncles ( Fig. 3A, E, F ) moderately small, short; dorsal extension onto cornea rounded.

Basal article of antennular peduncle ( Fig. 3F ) approximately as long as broad; anterior margin slightly convex and undulate; ventral surface with transverse ridge anteriorly and short row of granules medially; ventro-anterior face slightly concave.

Antennal peduncle ( Fig. 3E, F ) slender, with irregular, short striae on surfaces. First article largest, approximately 0.7 times length of combined second to fourth articles, produced forward in lateral view, broadly in contact with lower orbital margin, with blunt ridge along ventral margin; surface shallowly concave; anterior margin tapering, terminating in narrowly rounded apex ( Fig. 3G ). Second and third articles roundly rectangular, third article elongate. Fourth article short, rounded.

Third maxilliped ( Fig. 3H ) with coxa bearing blunt projection on ventrodistal margin; distomedian projection with blunt transverse ridge, not articulated. Basis articulating with ischium, rounded subtriangular. Ischium broad, ovate; lateral surface with undulate ridges and longitudinal ridge along dorsal margin; dorsodistal projection blunt. Merus with laminate, roundly subrectangular lobe on ventrolateral margin; lateral surface with undulate ridges and longitudinal ridge along dorsal margin. Carpus with small subtriangular projection on median part of ventral margin and with longitudinal ridge on dorsal surface. Propodus slender, nearly smooth on lateral surface. Dactylus short, subtriangular. Merus to dactylus with long plumose setae on ventral margins. Exopod with proximal article small, rounded; distal article laminate, robust, reaching midlength of merus, somewhat constrict on median part, with row of transverse ridges dorsally; distal flagellum present.

Chelipeds (first pereopods, Fig. 4A–G ) unequal in size; dorsal surface covered with short and relatively long striae bearing sparse minute setae (striae on meri and carpi generally longer and more distinct); short to long plumose setae (lateral setules minute) present on anterior and posterior margins of merus to chela and also on dorso-anterior and ventroposterior surfaces of chela, ventroposterior setae of chelae sparse. Larger cheliped ( Fig. 4A–C ) with ischium bearing no spines on ventro-anterior margin. Merus with rounded transverse crest submedially on dorsal surface; dorso-anterior margin with broad, rounded lobe distally; ventral surface with transverse striae; ventro-anterior distal corner rounded. Carpus 1.7 times as long as broad, broadest on median part; dorsal surface slightly convex, posterior margin rounded; dorso-anterior margin with elevated lobe, anterior margin of lobe nearly transverse, slightly crenulated or with row of granules; dorsodistal margin with low, rounded lobe on posterior part; ventral surface with transverse striae; ventro-anterior margin nearly transverse. Chela 1.7–1.8 times as long as carpus, relatively narrow, 3.2–3.3 times as long as broad, lying on anterior side; anterior margin thin and with row of small rounded denticles (occasionally obsolete) on distal 0.7. Palm somewhat inflated; dorsal surface convex; dorsoposterior margin with longitudinal blunt ridge distally; ventral surface convex, with short striae along anterior margin. Fixed finger with weakly curved distal claw; dorsal surface less striate than on that of palm, with blunt ridge along midline; cutting edge with large, subtriangular tooth proximally followed by small rounded teeth. Dactylus 0.3 length of chela, opening at oblique angle, with weakly curved distal claw; dorsal surface with longitudinal ridge composed of small tubercles on midline; posterior margin with blunt carina proximally; cutting edge concave, with large, rounded tooth proximally and with small, low, rounded teeth distally.

Smaller cheliped ( Fig. 4D–G ) similar to larger cheliped in general outline, except for: chela proportionally shorter and narrower, 1.5–1.6 times as long as carpus and 3.4–3.7 times as long as broad, with row of small but distinct denticles on almost entire part of anterior margin; cutting edges of fixed finger and dactylus nearly straight, each with row of small rounded teeth; dactylus proportionally longer, 0.4 length of chela, opening at more stronger angle, with more strongly developed, longitudinal ridge composed of stronger tubercles on dorsomidline.

Ambulatory legs (second to fourth pereopods, Fig. 5A–E ) moderately short, decreasing in size posteriorly (second largest), compressed laterally, with scattered, short to long plumose setae (lateral setules minute) marginally; lateral surfaces nearly smooth except for short ridges on laterodorsal parts of meri; mesial surfaces with short ridges on meri and carpi, but nearly smooth on propodi and dactyli. Ischia each weakly calcified on dorsodistal part of mesial surface. Meri 2.5–2.7 (second), 2.3–2.4 (third), and 2.0–2.3 (fourth) times as long as high in lateral view, highest slightly proximal to median part; dorsal margins slightly crenulated, gently convex; mesial surfaces weakly calcified on proximal part; ventrodistal margins of lateral and mesial surfaces rounded. Carpi slender (second and third) or stout (fourth); dorsodistal and ventrodistal corners rounded. Propodi 2.8–3.1 (second), 2.7–2.9 (third), and 2.7–3.0 (fourth) times as long as high, highest at base of proximalmost ventral corneous spine; dorsal margins minutely crenulated; ventral margins each with 2–4 (second, usually 2), 2 or 3 (third, usually 2) and 2 (fourth) corneous spines; distoventral margin with 2 corneous spines subequal in size. Dactyli 0.4–0.5 times as long as propodi, each terminating in curved, sharply pointed, bifurcate claws, ventral claw much larger than dorsal claw; ventral margins each with 2 small corneous spines on slightly elevated bases, distal spine larger or stouter than proximal spine.

Fifth pereopod slender, chelate; propodus with numerous short simple setae and 1 or 2 subdistal hooked setae.

Male with pair of gonopods on second abdominal segment; endopod elongate ovate, rounded on distal margin; exopod small, rounded.

Female with no pleopods on third abdominal segment, but with well developed pleopods on fourth and fifth abdominal segments.

Coloration in life. Carapace pale to reddish or greenish brown, with white, transverse lines or narrow irregular marks on dorsal surface; cardiac region with pair of dark reddish brown spots, whitish on posterior part. Chelipeds pale to greenish brown, with scattered, white and dark brown blotches on dorsal surface. Ambulatory legs white or pale brown; meri, carpi, and propodi each with dark brown blotch on respective median parts. Setae light brown ( Fig. 1C, D ).

Distribution. Presently known only from the Ryukyu Islands (Iriomote Island and Kume Island), southwestern Japan.

Habitat. Although the specimens from Kume Island were collected by dredges at depths of 4.5–7.5 m, their host animals were not recorded. Other specimens examined from Iriomote Island were obtained from parchment tubes of Chaetopterus cf. cautus Marenzeller, 1879 ( Naruse et al. 2017: fig. 2A) in sand/mud substrata of the intertidal flats or depths of 2– 6 m. From intertidal to subtidal depths of 10 m in Iriomote Island, brachyuran crabs such as a hexapodid Hexapinus simplex Rahayu & Ng, 2014 and a species of the pinnotherid genus Tetrias Rathbun, 1898 (cf. Naruse 2013; Rahayu & Ng 2014; Naruse et al. 2017) were also found from the tubes of Chaetopterus cf. cautus. An ovigerous female of Polyonyx pilosibrachium n. sp. (RUMF-ZC-4385) was collected together with an ovigerous female of Hexapinus simplex from a tube of same host worm ( Naruse et al. 2017: fig. 2).

Etymology. The specific name is a combination of the Latin, pilosus (= with soft setae) and brachium (= arm), which refers to the carpus of each cheliped with a row of plumose setae on the dorso-anterior margin; used as a noun in apposition.

Remarks. Polyonyx pilosibrachium n. sp. is morphologically closest to P. utinomii in the carapace with relatively long transverse striae on the dorso-anterior half and strongly convex on the branchial margins, the chelipeds with a broad and rounded lobe on the dorso-anterior margin of each merus and with relatively long transverse striae on the dorsal surface of each carpus. However, the new species is distinguished from P. utinomii by having a distinct row of plumose setae on the dorso-anterior margin of the carpus of each cheliped. In P. utinomii , the carpus of the cheliped lacks or has only a few shorter setae on the dorso-anterior margin. Additionally, the carapace is comparatively broader (approximately 1.4 versus 1.2–1.3 times as broad as long), the rostrum bears longer setae, and the merus of the second pereopod are proportionally more slender (2.5–2.7 versus 2.1 times as long as high) in P. pilosibrachium n. sp. than in P. utinomii (for the characters of P. utinomii , cf. Osawa 2001: figs. 4A, C, D, 6G; Osawa & Poupin 2013: fig. 3A, F; personal observation). In fresh condition, the colored marks on the dorsal surfaces of the carapace and chelipeds are usually darker and more densely distributed in P. pilosibrachium n. sp. than in P. utinomii (cf. Watanabe 2013: unnumbered fig.; present study: Fig. 1C, D ).

Polyonyx boucheti Osawa, 2007 also resembles P. pilosibrachium n. sp. and P. utinomii in having strongly convex branchial margins of the carapace and moderately long transverse striae on the dorsal surfaces of the carapace and carpi of the chelipeds, but their striae are comparatively weaker in P. boucheti than in the latter two species. Additionally, P. boucheti differs from P. pilosibrachium n. sp. and P. utinomii in the median lobe of the rostrum being faintly developed and nearly transverse in dorsal view (in the latter two species, it is broadly triangular and rather distinctly produced anteriorly) and the merus of each cheliped with subrectangular instead of rounded lobe on the dorso-anterior margin (cf. Osawa 2007b; Osawa & Poupin 2013). The coloration in life also distinguishes P. boucheti from P. boucheti and P. utinomii . The carapace has vermiculiform, light brown marks in P. boucheti , but such marks are absent in P. pilosibrachium n. sp. and P. utinomii (cf. Osawa 2001: fig. 7A; Watanabe 2013: unnumbered fig.; Osawa & Poupin 2013: fig. 2; present study: Fig. 1A, C, D ).

Figures

FIGURE 1. Five Polyonyx species. Fresh specimens (A–D, G, H) and preserved specimens (E, F), entire animal, dorsal view. A, Polyonyx boucheti Osawa, 2007, probably female (cl 1.7 mm), Panglao Island, ZRC 2017.204; B, Polyonyx heok, ovigerous female (cl 7.6 mm), Iriomote Island, RUMF-ZC-4736; C, Polyonyx pilosibrachium n. sp., paratype, male (cl 3.1 mm), Iriomote Island, RUMF-ZC-4392; D, Polyonyx pilosibrachium n. sp., paratype, ovigerous female (cl 3.3 mm), Iriomote Island, RUMF- ZC-4389; E, Polyonyx sasekumari n. sp., paratype, male (cl 3.2 mm), Jeram, ZRC 2016.527; F, Polyonyx sasekumari n. sp., paratype, ovigerous female (cl 4.6 mm), Jeram, ZRC 2016.527; G, Polyonyx thai Werding, 2001, male (cl 2.8 mm), Johor Straits, St. DW131, ZRC 2017.202; H, Polyonyx thai Werding, 2001, ovigerous female (cl 3.5 mm), Johor Straits, St. DW131, ZRC 2017.202.

FIGURE 3. Polyonyx pilosibrachium n. sp., holotype, male (cl 3.0 mm), Iriomote Island, CBM-ZC 14134. A, carapace and ocular and antennal peduncles, dorsal view (setae drawn only on anterior part of rostrum); B, rostrum, dorso-anterior view; C, third and fourth thoracic sternites, ventral view (setae not drawn); D, telson, external view; E, left ocular and antennal peduncles and anterior part of pterygostomial flap, dorsolateral view (setae not drawn); F, left ocular and antennal peduncles, basal article of antennular peduncle, and anterior part of pterygostomial flap, ventral view (setae not drawn); G, first article of left antennal peduncle, anterolateral view; H, left third maxilliped, lateral view (setae not drawn). Scale bars = 1.0 mm.

FIGURE 4. Polyonyx pilosibrachium n. sp., holotype, male (cl 3.0 mm), Iriomote Island, CBM-ZC 14134. A, larger cheliped, dorsal view; B, larger cheliped, merus and carpus, ventral view (setae not drawn); C, larger cheliped, chela, dorsal perpendicular view (setae and surface structure not drawn); D, smaller cheliped, dorsal view (surface structure partially drawn, setae not drawn); E, smaller cheliped, chela, dorsal perpendicular view (surface structure partially drawn, setae not drawn); F, smaller cheliped, chela, ventral view (setae not drawn); G, smaller cheliped, distal part of dactylus, posterior view. Scale bars = 1.0 mm.

FIGURE 5. Polyonyx pilosibrachium n. sp., holotype, male (cl 3.0 mm), Iriomote Island, CBM-ZC 14134. A, left second pereopod, lateral view; B, left second pereopod, ischium and merus, mesial view; C, left second pereopod, dactylus, lateral view (setae not drawn); D, left third pereopod, lateral view (setae not drawn); E, right fourth pereopod, lateral view (setae drawn only on lateral surface of merus). Scale bars = 1.0 mm.