Polyonyx deezi Osawa & Sato 2022, n. sp.
Creators: name: Osawa, Masayuki
name: Sato, Taigi
Publisher: name: Zenodo
Year: 2022
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Decapoda
Porcellanidae
Polyonyx
Polyonyx deezi

description: Polyonyx deezi n. sp.
[New Japanese name: Kawari-yadori-kanidamashi]
(Figs. 1–4)
Type material. Holotype: RUMF-ZC 7601, male (cl 2.3 mm, cw 3.0 mm), Sakimotobu, Motobu, Okinawa Island, 8–12 m, coral sand, associated with Chaetopterus sp., 21 February 2021, collected by T. Sato. Paratype: RUMF-ZC 7602, 1 male (cl 2.2 mm, cw 2.8 mm), same data as in holotype.
Diagnosis. Numerous granules and short granular ridges on dorsal surfaces of carapace and chelipeds. Carapace with median branchial margins being bluntly angular and produced laterally; gastric, branchial, cardiac regions with 1, 2, 1 distinct protuberances, respectively. Rostrum produced anteriorly in dorsal view, trilobed, median lobe roundly triangular; anterior margin with row of sparse short setae. Third thoracic sternite slightly concave on anteromedian margin. Telson distal plates each distinctly longer than broad. Antennal first article approximately 0.8 length of combined second to fourth articles. Third maxilliped merus with roundly subrectangular lobe on ventrolateral margin. Chelipeds somewhat unequal in size; meri each with broad, rounded lobe distally on dorsoanterior margin; carpi and palms each with at least 1 distinct protuberance on dorsal surface; carpi comparatively short, 1.3–1.4 times as long as broad, dorso-anterior margin gently convex, without distinct row of setae; chelae with dense, soft plumose setae on dorso-anterior surface; palms inflated dorsoventrally; dactylus of larger cheliped short, 0.3–0.4 length of chela. Ambulatory legs moderately short, numerous short ridges on lateral surfaces of meri and carpi; meri wide, 1.5–1.8 times as long as high in mesial view, strongly convex on dorsal margins; ventral surfaces concave distally to receive ventral margins of carpi; propodi comparatively short, 2.8–3.1 times as long as high, ventral margins each with 1 or 2 corneous spines in addition to 2 corneous spines at terminal end; dactyli each with dorsal claw much smaller than ventral claw, ventral margins with 2 teeth each bearing small corneous spine. Male with pair of gonopods on second abdominal somite.
Description. Carapace (Fig. 1A, B) hexagonal in general outline, 1.2–1.3 times as broad as long, broadest on median branchial margins. Dorsal surface slightly convex, covered with numerous granules and short granular ridges and sparse short setae; 5 transverse protuberances present (1 on mesogastric region, 2 mesially on branchial region, and 1 cardiac region, respectively). Protogastric ridges well demarcated, elevated. Cervical grooves shallow. Hepatic margins convex. Branchial margins divergent on anterior half and convergent on posterior half, with row of short and moderately long plumose setae; median margin bluntly angular, produced laterally; anterior half margin carinate, thin, with shallow sulcus mesially; posterior half gently convex. Posterior margin with elevated ridge of granules. Rostrum (Fig. 1A, C) moderately broad, reaching beyond anterior margin of eyes, produced anteriorly in dorsal view, ventrally bent toward anterior tip, trilobate in anterior view; median lobe triangular, with rounded apex, slightly overreaching rounded lateral lobes, with shallow median longitudinal groove on dorsal surface; anterior margin with row of sparse short setae. Orbits moderately shallow; supra-orbital margins slightly concave; lateral orbital angles rounded.
Pterygostomial flaps (Fig. 1B) entire; surface with short ridges on dorsal half and long undulate ridge on longitudinal midline; scattered plumose setae, numerous ventrally; dorso-anterior surface somewhat concave, rounded anteriorly.
Third thoracic sternite (Fig. 1D) 3.5 times as broad as long measured on midline; surface somewhat convex medially, with few granules anterolaterally; anterior margin slightly concave on median part, with row of sparse setae; lateral lobes narrow, produced, each with rounded apex. Fourth thoracic sternite with anterior margin moderately concave; surface depressed medially, with 2 transverse rows of small tubercles, anterior row bearing setae.
Telson (Fig. 1E) slightly broader than long, composed of 7 plates; lateroproximal plates much smaller than others; distal plates each distinctly longer than broad.
Ocular peduncles (Fig. 1A, G, H) 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 row of small tubercles on anterior margin and transversely oblique row of small tubercles laterally; ventro-anterior face slightly concave, with many granules.
Antennal peduncle (Fig. 1A, G, H) slender. First article broad, largest, approximately 0.8 length of combined second to fourth articles, produced forward in lateral view, broadly in contact with lower orbital margin; surface shallowly concave, with numerous granules and blunt ridge along ventral margin; anterior margin tapering, terminating in rounded apex. Second and third articles roundly rectangular, with granules and short granular ridges marginally; third article elongate. Fourth article short, rounded, nearly smooth.
Third maxilliped (Fig. 1I, J) 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 sparse granules and weak longitudinal ridge along dorsal margin; dorsodistal projection blunt; long, soft plumose setae on ventrodistal surface. Merus with laminate, roundly subrectangular lobe on ventrolateral margin; lateral surface with scattered granules and short ridges. Carpus with small subtriangular projection on median part of ventral margin and with row of short blunt ridges on dorsal surface. Propodus moderately elongate, with row of short blunt ridges on dorsal surface. Dactylus short, subtriangular, nearly smooth. Merus to dactylus with long plumose setae on ventral margins. Exopod with proximal article small, rounded; distal article laminate, robust, reaching distal two-thirds of merus, constricted on median part, with row of short ridges dorsally; distal flagellum present.
Chelipeds (first pereopods; Figs. 2A–F, 3A, B) somewhat unequal in size, left thicker than right; dorsal and ventral surfaces of merus to dactylus covered with numerous granules and short granular ridges and with sparse very short setae. Larger cheliped (Fig. 2A–F) with ischium slightly crenulated but without spines on ventro-anterior margin; scattered, soft plumose setae on dorsal surface and anterior margin. Merus with blunt transverse crest submedially on dorsal surface; dorso-anterior margin with broad, rounded lobe distally; ventro-anterior margin crenulate, anterodistal corner with small tubercle; sparse, soft plumose setae on dorsoproximal part and ventroanterior distal corner. Carpus 1.3–1.4 times as long as broad, broadest on median part; dorsal surface slightly convex, with transverse protuberance on median part; dorso-anterior margin with elevated lobe, anterior margin of lobe gently convex, slightly crenulated, bearing few short setae, proximal corner of lobe roundly angled; dorsodistal margin posteriorly with roundly produced lobe bearing tuft of plumose setae; posterior margin rounded; ventro-anterior margin concave on median or distal one third part; ventrodistal margin with row of plumose setae. Chela 1.8–1.9 times as long as carpus, relatively narrow, 3.0–3.2 times as long as broad, lying on anterior side; anterior margin delimited by row of small, blunt and subacute denticles; dorso-anterior surface with dense, short and long, soft plumose setae. Palm somewhat inflated; dorsal surface convex, with 2 elongate protuberances on midline, proximal protuberance much lower than distal protuberance or obsolete; dorsoposterior margin with weak, longitudinal blunt ridge; ventral surface convex. Fixed finger with weakly curved, blunt distal claw; ventral surface with tuft of short plumose setae on base of fixed finger; cutting edge with row of blunt teeth, median tooth larger. Dactylus 0.3–0.4 length of chela, opening at oblique angle, with curved, blunt distal claw; dorsal surface with longitudinal ridge composed of small blunt and subacute denticles posteriorly on midline, dorso-anterior surface proximally with soft plumose setae; posterior margin rounded, with short blunt carina proximally; cutting edge with 2 large blunt teeth each on subproximal and subdistal parts and small blunt teeth; ventral surface proximally with sparse, short plumose setae adjacent to cutting edge.
Smaller cheliped (Fig. 3A, B) generally similar to larger cheliped except for: chela proportionally broader, 2.8–3.1 times as long as broad; distal claws of fixed finger and dactylus narrower; cutting edges of fixed finger and dactylus each with entire row of much smaller blunt and subacute teeth; dactylus proportionally longer, 0.4–0.5 length of chela.
Ambulatory legs (second to fourth pereopods, P2–P4; Fig. 3C–H) moderately short, compressed laterally, decreasing in size posteriorly (P2 largest); P2 merus 1.1 and 1.3–1.4 times respective P3 and P4 meri lengths; P2 propodus 1.1 and 1.2–1.3 times respective P3 and P4 propodi lengths; dorsal and ventral margins with long, soft plumose setae on meri to propodi; latero-distal margins of propodi and dorsal and ventral margins of dactyli with sparse stiff setae. Ischia without weakly calcified parts on each mesial surface. Meri 1.8 (P2), 1.6 (P3), and 1.5 (P4) times as long as high in mesial view, highest on each submedian or median part; dorsal margin strongly convex, bluntly cristate; lateral surface with numerous transverse granular ridges bearing sparse short setae; mesial surfaces covered with granules and granular ridges, weakly calcified on dorsoproximal part; ventrodistal margins of lateral and mesial surfaces rounded; ventral surface concave distally to receive ventral margin of carpus, ventromesial margin thin, gently convex. Carpi moderately elongate; lateral surface with irregular rows of short ridges bearing sparse short setae; dorsodistal and ventrodistal margins rounded. Propodi 3.0–3.1 (P2), 2.8 (P3 and P4) times as long as high, broadest on each subproximal part; lateral surface with short transverse ridges bearing sparse short setae; dorsal margin weak crenulated; ventral margin slightly concave on median part, with 1 or 2 corneous spines, subdistal spine always present, subproximal spine present or absent; ventrodistal 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, dorsal claw much smaller than ventral claw; ventral margin with 2 low teeth, distal tooth larger than proximal tooth, each tooth with small corneous spine.
Fifth pereopod slender, chelate; propodus with numerous short simple setae and 2 subdistal hooked setae.
Male with pair of gonopods on second abdominal somite; endopod elongate, oval, somewhat twisted medially, rounded on distal margin; exopod small, rounded.
Coloration in life. Body and pereopods white in ground color. Carapace with numerous but scattered, small brown spots; some dark brown spots on anterior lateral margins and posterior margin. Basal article of antennular peduncle white; penultimate and ultimate peduncular articles and flagella translucent white. Antennal peduncle white, flagella translucent, pale brown. Third maxilliped also white in general. Merus and carpus of each cheliped with pale brown tint and with some brown blotches and small dark brown spots on dorsal surface; palm with 3 or 4, small dark brown blotches on proximal half of dorsal surface; dactylus with large dark brown blotch on proximal half posteriorly. Ambulatory legs with dark brown markings; meri each with blotch dorsally on midlength in P2 and P3 (size of blotch much smaller in P3) or no blotch (P4); carpi each with transverse blotch on half of dorsal surface; propodi each with ring (P2 and P3) or dorsal blotch (P4) on midlength. All setae whitish. See Fig. 4.
Distribution. So far only known from the type locality, Okinawa Island, Ryukyu Islands, southwestern Japan.
Habitat. The two type specimens were extracted from sand bottom at a depth of 8–12 m, coral reef environment, using a commercial suction pump (yabby pump). They were found inside tubes of an unidentified species of the polychaete genus Chaetopterus Cuvier, 1830 (family Chaetopteridae). The worm specimens are not preserved. Although the tubes obtained were broken and incomplete, they were externally covered with sand and small shell fragment and their bare surfaces have a stiff parchment-like appearance. The largest widths of the tubes were approximately 10–20 mm. No females of the present new species were obtained.
Etymology. The species name is derived from the Ryukyu language deezi which means “very”, in reference to the clearly discriminable morphology of the new species in the genus Polyonyx.
Remarks. Polyonyx deezi n. sp. is immediately distinguished from all other congeners by the median branchial margins of the carapace being bluntly angular and produced laterally and the dorsal surfaces of the carapace and of carpi and palms of the chelipeds with distinct protuberances. In other congeners, the branchial margins of the carapace are evenly convex or nearly straight and subparallel, and somewhat constricted on the median part, and no distinct protuberances on the carapace and chelipeds.
The new species belongs to the P. sinensis group, an informal species group of the genus defined by Johnson (1958), primarily characterized by having the carapace without spines on the lateral margins, the chela with various degrees of setation on the dorso-anterior surface and the ambulatory dactyli each with the dorsal claw much smaller than the ventral claw. The species group includes 18 species from the Indo-West Pacific (cf. Osawa 2018; Osawa et al. 2018; Werding & Hiller 2019): P. angustus Osawa, 2018; P. boucheti Osawa, 2007; P. haigae McNeill, 1968; P. heok Osawa & Ng, 2016; P. kumejima Osawa & Ng, 2016; P. loimicola Sankolli, 1965; P. maccullochi Haig, 1965; P. pedalis Nobili, 1906; P. pilosibrachium Osawa, Naruse & Ng, 2018; P. planus Osawa & Ng, 2016; P. sasekumari Osawa, Naruse & Ng, 2018; P. sinensis Stimpson, 1858; P. socialis Werding & Hiller, 2019; P. transversus (Haswell, 1882); P. thai Werding, 2001; P. tulearis Werding, 2001; P. utinomii Miyake, 1943; and P. vermicola Ng & Sasekumar, 1993. Among the species group, P. deezi n. sp. may be closest to P. socialis known from Vietnam in the comparatively short carpi of the chelipeds and broad meri of the ambulatory legs in their proportions. Nevertheless, the new species differs from P. socialis in the dorsal surface of the carapace covered with granules instead of shallow transversal striae, the anterior margin of the median lobe of the third thoracic sternite being concave instead of convex, the carpi and palms of the chelipeds and meri of the ambulatory legs with numerous granules and granular ridges on the dorsal or lateral surfaces, and the dorso-anterior marginal lobe of the carpus of each cheliped being more weakly developed, in addition to the distinction mentioned above. Coloration is also different between P. deezi n. sp. and P. socialis; the dorsal surfaces of the carapace and chelipeds are generally white in the new species, rather than brown in P. socialis.
On the Vietnamese coast, Britayev et al. (2017) reported that P. socialis (as an undescribed species at that time) occurred in heterosexual pairs in the tube of a possibly undescribed species of Chaetopterus and frequently shared the polychaete tube with a heterosexual pair of significantly larger congeneric porcellanid P. heok and the tergipedid nudibranch Phestilla sp. or rarely with larger pinnotherid crab Tetrias sp. instead of P. heok. Werding & Hiller (2019) remarked that the extremely broadened carpi of the chelipeds and meri of the ambulatory legs of P. socialis are distinctive characters within the genus Polyonyx, and these characters are most likely adaptations for living tightly attached to the walls of the worm tube without being perceived as an obstacle for the larger crabs inhabiting the same tube. Polyonyx deezi n. sp. has similarly broadened carpi of the chelipeds and meri of the ambulatory legs, but no females nor other symbiont animals were obtained from the same tubes of host polychaetes. The Chaetopterus worms as candidates of the hosts of P. deezi n. sp. have not been preserved, but they are likely relatively small-sized species considering from the size of the broken tubes collected (10–20 mm in the largest widths of the tubes), and the space occupied by P. deezi n. sp. inside the tube can be very narrow. It is difficult at present to persuasively explain the morphological significance of the broadened chelipeds and ambulatory legs in P. deezi n. sp. as well as the presence of distinct protuberances on the dorsal surfaces of carapace and chelipeds. In Japanese waters including the Ryukyu Islands, unlike observations of the Vietnamese material by Britayev et al. (2017), P. heok is found in heterosexual pairs or singly in parchment tubes of C. pacificus Nishi, 2001, which usually adhere to the underside of rocks partially buried in sand or gravel bottom of embayed coasts (cf. Osawa et al. 2018); other symbionts, at least porcellanid or brachyuran crabs, from the same host tubes were never recorded. At Iriomote Island of the Ryukyus, Naruse et al. (2017) recorded an example of co-existence of an ovigerous female of P. pilosibrachium (as P. cf. utinomii) and an ovigerous female of hexapodid brachyuran Hexapinus simplex Rahayu & Ng, 2014 from the same tube of Chaetopterus cautus Marenzeller, 1879.
The occurrence of P. deezi n. sp. from coral reefs as a rather exposed environment may be unusual in species of the P. sinensis group because many species of the group have been recorded from estuaries or coastal embayments. Only P. angustus, P. boucheti, P. kumejima, and P. planus were obtained from substrates of coral rubble or sand, although their commensal host animals were not always recorded (Osawa & Ng 2016; Osawa 2018; Osawa et al. 2018).
Osawa et al. (2018) presented an identification key to the Indo-West Pacific species of the Polyonyx sinensis group known at the time. The identification key is amended with the addition of two species described since Osawa et al. (2018), P. angustus and P. socialis, and the present new species, as shown below.
description: Published as part of Osawa, Masayuki & Sato, Taigi, 2022, A distinctive new species of the genus Polyonyx Stimpson, 1858 (Crustacea Decapoda: Anomura: Porcellanidae) from Okinawa, southwestern Japan, pp. 587-597 in Zootaxa 5091 (4) on pages 588-594, DOI: 10.11646/zootaxa.5091.4.6, http://zenodo.org/record/5864259