Uroptychus posticus, Baba, 2018

Baba, Keiji, 2018, Chirostylidae of the Western and Central Pacific: Uroptychus and a new genus (Crustacea: Decapoda: Anomura), Tropical Deep-Sea Benthos (Mémoires du Muséum national d'Histoire naturelle 212), pp. 1-612 : 419-422

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Uroptychus posticus

n. sp.

Uroptychus posticus n. sp.

Figures 208 View FIGURE 208 , 209 View FIGURE 209

TYPE MATERIAL — Holotype: Vanuatu. MUSORSTOM 8 Stn CP 974, 19°21.51’S, 169°28.26’E, 492-520 m, 22.IX.1994, ov. ♀ 5.1 mm (MNHN-IU-2014-16865) GoogleMaps . Paratype: Vanuatu. MUSORSTOM 8 Stn CP1089, 15°08.82’S, 167°17.23’E, 494- 516 m, 6.X.1994, 1 ♀ 4.6 mm ( MNHN-IU-2014-16866 ). GoogleMaps

ETYMOLOGY„ From the Latin posticus (that which is behind), alluding to a small spine that is located behind the rostrum.

DISTRIBUTION„ Vanuatu; 492- 520 m.

DESCRIPTION„ Medium-sized species. Carapace: 1.2 × broader than long; greatest breadth 1.7 × distance between anterolateral spines. Dorsal surface convex from anterior to posterior and from side to side, sparingly covered with short fine setae; small spine in midline directly behind rostrum (flanked by small spine behind each eye in paratype). Lateral margins somewhat convexly divergent posteriorly, with 8 or 9 spines; first anterolateral, well developed, slightly overreaching lateral orbital spine (with accompanying spine directly behind it in holotype); second situated at anterior end of branchial margin, rather remote from first, somewhat dorsal in position, accompanying 1 small spine dorsomesial to it (and another small spine ventral to it in holotype); remaining spines placed at same level in profile; third as large as first, fourth larger than others, with 0-3 accompanying small spines; remaining spines posteriorly diminishing; posterior quarter of lateral margin ridged. Rostrum narrow triangular, with interior angle of 17°, somewhat deflected dorsally, dorsally flattish, laterally with small denticle distally; length half that of remaining carapace, breadth much more than half carapace breadth at posterior carapace margin. Lateral orbital spine relatively large but smaller than anterolateral spine, situated directly mesial to and moderately remote from that spine. Pterygostomian flap anteriorly angular, ending in small spine followed by 1 or 2 very small spines along upper margin (discernible under high magnification); smooth on surface.

Sternum: Excavated sternum anteriorly broad triangular, surface with weak longitudinal ridge in midline. Sternal plastron 1.2 × broader than long; lateral extremities slightly convexly divergent. Sternite 3 well depressed, anterior margin of broad V-shape without submedian spines (in holotype) or with very small median notch flanked by obsolescent spine (in paratype), laterally angular. Sternite 4 with scattered tubercles on surface, relatively short; anterolateral margin convex and irregular, length slightly less than twice (1.8 x) that of posterolateral margin. Sternite 5 as long as sternite 4, anterolateral margin strongly convex, subequal to or slightly longer than posterolateral margin of sternite 4.

Abdomen: Nearly glabrous on tergites, bearing fine setae on pleura. Somite 1 gently convex from anterior to posterior, without transverse ridge. Somite 2 tergite 2.4-2.5 × broader than long; pleuron anterolaterally rounded, posterolaterally angular, lateral margin concavely strongly divergent. Pleuron of somite 3 tapering to blunt tip. Telson half as long as broad; posterior plate 1.5 × longer than anterior plate, posterior margin moderately concave or distinctly emarginate.

Eye: Overreaching midlength of rostrum, 2 × longer than broad, feebly narrowed distally. Cornea about half as long as remaining eyestalk.

Antennule and antenna: Ultimate article of antennular peduncle 3 × longer than high. Antennal peduncle overreaching cornea. Article 2 with distinct lateral spine. Antennal scale 1.8 × as broad as article 5, somewhat overreaching article 5, laterally with small spine in holotype, smooth in paratype. Distal 2 articles each with very small distomesial spine; article 5 1.3 × longer than article 4, breadth 0.8 × height of ultimate article of antennule. Flagellum of 13-16 segments nearly reaching distal end of P 1 merus.

Mxp: Mxp1 with bases nearly contiguous. Mxp3 basis without distinct denticles on mesial ridge. Ischium with more than 30 small denticles on crista dentata; flexor margin distally not rounded. Merus 1.9 × longer than ischium, moderately thick mesiolaterally, bearing small distolateral spine; flexor margin proximally with rounded ridge, distally with keeled ridge, bearing 2 or 3 small spines on distal third. Carpus with small distolateral spine.

P 1: Massive, with soft fine setae; length 4.4-4.8 × that of carapace. Ischium with short, basally broad, depressed spine; ventromesial margin with row of small spines, lacking distinct subterminal spine. Merus slightly shorter than carapace; mesial face denticulate, with several spines distinct in 2 rows (paratype) or obsolescent (holotype) on proximal half; dorsal surface with row of small spines distinct in paratype, obsolete in holotype; distal margin with short blunt spines; greatest breadth equal to distance between anterolateral spines of carapace. Carpus 1.3 × longer than merus. Palm slightly depressed, 2.6-2.8 × longer than broad, 1.2-1.5 × longer than carpus. Fingers broad relative to length, slightly gaping, inclined somewhat laterally, strongly incurved distally, crossing when closed; movable finger with low process on proximal third, length 0.4-0.5 × that of palm; fixed finger straight or somewhat concave on opposable margin, without distinct concavity to accommodate opposite process of movable finger when closed.

P 2-4: Relatively short. Meri successively shorter posteriorly ( P 3 merus 0.9 × length of P 2 merus, P 4 merus 0.8 × length of P 3 merus), equally broad on P 2-4; length-breadth ratio, 3.4-3.7 on P 2, 3.1-3.3 on P 3, 2.8-3.2 on P 4; slightly compressed mesiolaterally, slightly granulate on surface, flexor face rounded; dorsal margin with 4 or 5 very small spines on P 2 and P 3, few in paratype and obsolete in holotype on P 4, other than very small distal spine; ventrolaterally with terminal spine present on P 2 and P 3, absent on P 4; P 2 merus 0.8 × length of carapace, subequal to or very slightly longer than P 2 propodus; P 3 merus 0.9 × length of P 3 propodus; P 4 merus 0.8 × length of P 4 propodus. Carpi successively shorter posteriorly ( P 3 carpus 0.9 × length of P 2 carpus, P 4 carpus 0.9 × length of P 3 carpus), distinctly longer than dactyli (carpus-dactylus length ratio, 1.5- 1.6 on P 2, 1.4-1.5 on P 3, 1.2-1.5 on P 4); carpus-propodus length ratio, 0.56-0.59 on P 2, 0.51-0.55 on P 3, 0.46-0.49 on P 4. Propodi subequal on P 3 and P 4, longest on P 2; flexor margin somewhat convex around distal fifth, bearing pair of distal spines preceded by 6 spines roughly in zigzag arrangement (mesial and lateral to midline of flexor face) and a few others in midline, unarmed on proximal half. Dactyli shorter than carpi (dactylus-carpus length ratio, 0.6-0.7 on P 2 and P 3, 0.7-0.9 on P 4), 0.4 × length of propodi on P 2-4, strongly curving; flexor margin with row of 12-14 relatively large spines, ultimate spine slender, penultimate largest among others, remaining spines somewhat inclined and diminishing toward proximal end of article.

Eggs. About 20 eggs carried, measuring 1.04 mm × 1.28 mm.

PARASITES — The female paratype bears a rhizocephalan externa on the abdomen.

REMARKS — The species is characterized by the elongate eyes, the presence of a spine directly behind the rostrum, the spinose lateral margins of the carapace, and the zigzag arrangement of flexor spines on the P 2-4 propodi.

Uroptychus posticus mostly closely resembles U. exilis n. sp. (see above, for their similarities). However, U. exilis has no spine directly behind the rostrum and anterolateral spine, the anterior margin of sternite 3 is shallowly V-shaped, and the P 2-4 dactyli are longer than the carpi.

The spination of the carapace including the relatively large lateral orbital spine, the shapes of the antenna, Mxp3 and P 1, and the spination of the distal two articles of P 2-4 in this new species are similar to those of U. paracrassior Ahyong & Poore, 2004 from southern Queensland. Uroptychus posticus is distinguished from that species by having a spine directly behind the rostrum, the ventromesial subterminal spine of the P 1 ischium vestigial instead of well developed, the ultimate spine of the P 2-4 dactyli much more slender than instead of subequal to the penultimate spine, and the pterygostomian flap with an anteriorly produced spine only, not successively followed by two additional spines as in U. paracrassior .

This species resembles U. lanatus n. sp. (see above) in the Mxp3 ischium that is not rounded on the distal end of the flexor margin, and in having the massive P 1 and the long antennal scale. However, U. lanatus is largely different in having no spine behind the rostrum, in having the lateral spines of the carapace much smaller instead of relatively large, in having the lateral orbital spine small instead of relative large compared to the anterolateral spine, and in having the P 1 merus mesially unarmed instead of bearing a row of spines.

As mentioned under the remarks of U. modicus n. sp. (see above), U. posticus also resembles that species, from which it is distinguished by having more numerous, smaller spines (7-8 instead of 4 spines) on the branchial margin, by having a median spine only instead of a transverse row of spines on the epigastric region, and by having less numerous flexor marginal spines (11-13 instead of 18-22 spines, proximal to the slender ultimate spine) on the P 2-4 dactyli.