Polyonyx angustus Osawa 2018, n. sp.
Creators: name: Osawa, Masayuki
Publisher: name: Zenodo
Year: 2018
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Decapoda
Porcellanidae
Polyonyx
Polyonyx angustus

description: Polyonyx angustus n. sp.
(Figs. 1–3)
Polyonyx utinomii. —Osawa 2001: 513 (part).— Osawa & POupin 2013: 4, fig. 3 (nOt P. utinomii Miyake, 1943).
Type materal. Holotype. PANGLAO 2004 Marine Biodiversity Project, Philippines: NMCR, male (cl 3.0 mm), Pontod Lagoon, Panglao Island, sta. B39, 9°32.8'N, 123°42.1'E, 17–25 m, reef wall with small caves, brushing, 3 July 2004.
Paratypes. PANGLAO 2004 Marine Biodiversity Project, Philippines: ZRC 2018.1015, 1 female (cl 2.9 mm), Bingag, Panglao Island, sta. B16, 9°37.6'N, 123°47.3'E, 20 m, coral rubble on sand and gravel, brushing, 17 June 2004; ZRC 2018.1016, 2 males (cl 3.0, 3.2 mm), 1 ovigerous female (cl 3.4 mm), collection data as in holotype.
Non-types. Maldives: CBM-ZC 5157, 1 male (cl 1.7 mm), 1 ovigerous female (cl 2.1 mm), Vadoo, outer side of eastern reef, 20 m, in tube of Chaetopterus sp., collected by K. Nomura, 22 April 1996.
Description. Carapace (Fig. 1A) transversely oval in general outline, 1.1–1.2 times as broad as long (except for paratype parasitized by bopyrid isopod: ZRC 2018.1016, male, cl 3.2 mm), broadest on median branchial margins. Dorsal surface nearly flattish from side to side, somewhat convex on gastric and branchial regions, entirely covered with short to long transverse striae, those on gastric, hepatic, and branchial regions anteriorly bearing sparse minute setae. Protogastric ridges moderately demarcated. Cervical grooves shallow. Hepatic margins weakly convex. Branchial margins strongly convex, weakly constricted on each median part; anterior 0.6 margins thin, with sparse short setae; posterior margins and adjacent regions with long, transverse ridges. Rostrum (Fig. 1B) moderately broad, distinctly reaching beyond anterior margin of eyes, produced in dorsal view, ventrally bent toward anterior tip; trilobate in anterior view, median lobe rounded, overreaching rounded lateral lobes, with shallow 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 some short and long longitudinal ridges; dorso-anterior margin terminating acutely (Figs. 1C).
Third thoracic sternite (Fig. 1D) 4.1–4.6 times as broad as long measured on midline, with surface somewhat convex medially; anterior margin slightly concave on median margin, with sparse long setae; lateral lobes narrow, produced, each with rounded apex. Fourth thoracic sternite with anterior margin moderately concave; surface depressed medially, with transverse series of sparse setae anteriorly.
Telson (Fig. 1E) as broad as long or slightly broader than long, composed of 7 plates; proximolateral plates much smaller than others; distal plates each distinctly longer than broad.
Ocular peduncles (Fig. 1G) moderately small, short; dorsal extension onto cornea low, rounded.
Basal article of antennular peduncle (Fig. 1F) approximately as long as broad; anterior margin slightly convex, somewhat undulate; ventral surface with transverse ridge anteriorly and short row of granules medially; ventroanterior face slightly concave.
Antennal peduncle (Fig. 1G) slender, with sparse, faint short striae on surfaces. First article largest, approximately 0.7 length of combined second to fourth articles, produced forward in lateral view, broadly in contact with lower orbital margin; surface shallowly concave, with blunt ridge along ventral margin; anterior margin tapering, terminating in subacute apex (Fig. 1H). Second and third articles roundly rectangular, third article elongate. Fourth article short, rounded.
Third maxilliped (Fig. 1I) with coxa bearing blunt projection on ventrodistal margin; distomedian projection with blunt transverse ridge, not articulated. Basis articulating with ischium, subtriangular. Ischium broad, with ventral margin strongly convex; lateral surface with short, transverse granular ridges and longitudinal ridge along dorsal margin; dorsodistal projection blunt. Merus with laminate, roundly subrectangular lobe on ventrolateral margin; lateral surface with short transverse ridges and longitudinal ridge along dorsal margin. Carpus with small subtriangular projection on median part of ventral margin and 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, constricted on median part, with row of short ridges dorsally; distal flagellum present.
Chelipeds (first pereopods; Figs. 2A–I, 3A–D) 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); sparse, short simple setae present on anterior and posterior margins of merus and carpus and on dorsal and ventral surfaces of fixed finger and dactylus (setae at posterodistal corner of merus and on dorsal and ventral surfaces of fixed finger and dactylus occasionally longer than others); dense, long simple setae present on anterior margin of chela (more numerous on smaller cheliped). Larger cheliped (Fig. 2A–I) with ischium slightly crenulated but without spines on ventro-anterior margin. Merus with blunt transverse crest submedially on dorsal surface; dorso-anterior margin with broad, rounded or somewhat angular lobe distally; ventral surface with short transverse striae; ventro-anterior margin crenulate, anterodistal corner with small denticles. Carpus 1.6–1.8 times as long as broad, broadest on median part; dorsal surface slightly convex; dorso-anterior margin with elevated lobe, anterior margin of lobe somewhat undulate distally, slightly crenulated, proximal corner of lobe roundly angled; dorsodistal margin with low, rounded lobe on posterior part; posterior margin rounded; ventral surface with short transverse striae; ventroanterior margin nearly transverse or slightly undulate on distal part. Chela 1.7–1.8 times as long as carpus, relatively narrow, 2.9–3.0 times as long as broad, lying on anterior side; anterior margin thin, slightly undulate, with row of small rounded denticles on distal two thirds. Palm somewhat inflated; dorsal surface convex; dorsoposterior margin with longitudinal blunt ridge distally; ventral surface convex, nearly smooth. Fixed finger with weakly curved distal claw; dorsal surface less striate than on that of palm; cutting edge with large subtriangular tooth on median part and row of small blunt teeth. Dactylus 0.3 length of chela, opening at oblique angle, with curved distal claw; dorsal surface with longitudinal ridge composed of small rounded denticles on midline; posterior margin with blunt carina of denticles proximally; cutting edge somewhat concave, with large rounded tooth proximally and row of small rounded teeth distally.
Smaller cheliped (Fig. 3A–D) generally similar to larger cheliped except for: chela proportionally narrower, 3.2–3.3 times as long as broad, anterior margin with row of stronger denticles, cutting edges with entire row of small rounded teeth; dactylus proportionally longer, 0.4 length of chela, with more strongly developed longitudinal ridge composed of stronger denticles on dorsomidline, cutting edge nearly straight, with entire row of small blunt teeth.
Ambulatory legs (second to fourth pereopods, Fig. 3E–H) moderately short, decreasing in size posteriorly (second largest), compressed laterally, with scattered, short and moderately long simple setae marginally; lateral and mesial surfaces nearly smooth. Ischia without weakly calcified parts on mesial surfaces. Meri 2.6–2.9 (second), 2.5–2.7 (third), and 2.5–2.8 (fourth) times as long as high in lateral view, highest on proximal third (second) and submedian or median (third and fourth) part; dorsal margins slightly crenulated, gently convex; mesial surfaces without weakly calcified parts; ventrodistal margins of lateral and mesial surfaces rounded. Carpi slender (second and third) or moderately elongate (fourth); dorsodistal and ventrodistal corners subacute. Propodi 4.3 (second), 4.0 (third), and 4.0–4.1 (fourth) times as long as high, broadest on proximal third (second and third) or nearly equalhigh along entire length except for narrowed proximal and distal parts (fourth); dorsal margins minutely crenulated; ventral margins each slightly concave on median part, with 1 subdistal corneous spine; distoventral margin with 2 corneous spines subequal in size. Dactyli 0.3–0.4 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 teeth, distal tooth larger than proximal tooth, each tooth usually with minute corneous 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, oval, 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. Unknown.
Distribution. Philippines and Maldives; 17–25 m depth.
Habitat. Coral reefs, coral rubble on sand and gravel. The type specimens from the Philippines were obtained by brushing of the substrates, but no host records are present. The non-type specimens from the Maldives were collected from a tube of unidentified species of the genus Chaetopterus Cuvier, 1830 (Polychaeta, Chaetopteridae).
Etymology. The specific name, angustus, is Latin meaning “narrow”, referring to the relatively narrow carapace of the new species in the Polyonyx sinensis group.
Remarks. Polyonyx angustus n. sp. resembles P. boucheti Osawa, 2007, P. pilosibrachium Osawa, Naruse & Ng, 2018, and P. utinomii Miyake, 1943. Shared characters of the four species include: branchial margins of carapace being strongly convex, carapace and carpi of chelipeds with relatively long transverse striae on dorsal surfaces, merus of each cheliped with a broad lobe on dorso-anterior margin, and meri of the ambulatory legs being unarmed on the ventral margins. However, the new species is distinguishable from the other three species by the comparatively narrower carapace (1.1–1.2 versus 1.3–1.4 times as broad as long), and by the rostrum distinctly, rather than not or slightly, reaching beyond the anterior margin of the eyes. Additionally, the anteromedian margin of the third thoracic sternite is slightly concave in the type material of P. angustus n. sp., instead of weakly to distinctly convex in P. boucheti, P. pilosibrachium, and P. utinomii (cf. Osawa 2001, 2007; Osawa et al. 2018).
Polyonyx angustus n. sp. is more closely allied to P. pilosibrachium and P. utinomii than P. boucheti in having the median lobe of the rostrum being distinctly produced anteriorly in frontal view (in P. boucheti, it is faintly developed). Nevertheless, the shape of the median lobe the rostrum distinguishes P. angustus n. sp. from P. pilosibrachium and P. utinomii; it is low and rounded in the new species, rather than triangular in the other two species (cf. Osawa 2001, 2007; Osawa et al. 2018).
Many species of the P. sinensis group have been reported to live in association with tube-dwelling polychaetes of the family Chaetopteridae (most commonly recorded), Sabellidae, or Terebellidae, and to inhabit the intertidal to subtidal flats of mud and sand in the estuarine or basin environment (e.g., Sankolli 1965; Osawa & Ng 2016; Osawa et al. 2018). Polyonyx angustus n. sp. and P. boucheti may be unusual in the species group in occurring in the subtidal, coral reef environment (cf. Osawa & Poupin 2013; Osawa et al. 2018; present study).
Re-examination of the specimens from the Maldives reported as P. utinomii by Osawa (2001) and Osawa & Poupin (2013: fig. 3A, B) showed that they are referable to the present new species instead, because of the characteristic shapes of the carapace and rostrum, although the anteromedian margin of the third thoracic sternite is transverse or faintly convex rather than concave unlike the type material from the Philippines. This difference can be attributed to the size of Maldives specimens, being much smaller than that of the type material (carapace length, 1.7–2.1 versus 2.9–3.4 mm). Polyonyx angustus n. sp. is known at present from the Philippines and the Maldives in the northern central Indian Ocean, whereas P. utinomii is distributed only in Japanese waters ranging from the central Pacific coast of Honshu to the western coast of Kyushu (cf. Osawa 2001; Watanabe 2013). Polyonyx pilosibrachium, the supposed sister species closely related to P. utinomii, has been recorded from the Ryukyu Islands, southwestern Japan (cf. Osawa et al. 2018).
A paratype (ZRC 2018.1016, male, cl 3.2 mm) is parasitized by a bopyrid isopod in the right gill chamber; the carapace has a strongly deformed right branchial region and is abnormally broader than that of the other specimens examined.
description: Published as part of Osawa, Masayuki, 2018, A new species of the Polyonyx sinensis group (Crustacea: Decapoda: Anomura: Porcellanidae) from the Central Philippines, pp. 393-400 in Zootaxa 4486 (3) on pages 394-399, DOI: 10.11646/zootaxa.4486.3.10, http://zenodo.org/record/1437058