Uroptychus megistos, 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 : 304-308

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Uroptychus megistos

n. sp.

Uroptychus megistos n. sp.

Figures 143 View FIGURE 143 , 144 View FIGURE 144

TYPE MATERIAL — Holotype: Vanuatu. MUSORSTOM 8 Stn CP983, 19°21.61’S, 169°27.76’E, 480- 475 m, 23.IX.1994, ov. ♀ 5.2 mm ( MNHN-IU-2014-16726 ) GoogleMaps . Paratypes: Collected with holotype, 1 ♂ 4.3 mm (MNHN-IU-2014-16727). Solomon Islands. SALOMON 1 Stn DW1826, 9°56.4’S, 161°04.0’E, 418-432 m, 4.X.2001, 1 ♂ 6.0 mm, 1 ov. ♀ 6.6 mm (MNHN-IU-2014-16728).

ETYMOLOGY„ From the Greek megistos (largest, greatest), alluding to the ultimate of the flexor marginal spines of P 2-4 dactyli, that is the largest among the others, by which the species is distinguished from U. duplex n. sp. and U. zigzag n. sp.

DISTRIBUTION„ Vanuatu and Solomon Islands; 418- 480 m.

SIZE„ Males, 4.3, 6.0 mm; ovigerous females, 5.2, 6.6 mm.

DESCRIPTION„ Medium-sized species. Carapace: Posteriorly broadened, 1.3-1.4 × broader than long (0.7-0.8 × as long as broad); greatest breadth 2.1 × distance between anterolateral spines. Dorsal surface covered with short fine setae, smoothly convex from anterior to posterior, without any depression; small scattered spines on hepatic region and transverse row of very tiny spines on anterior gastric region (both barely discernible in holotype). Lateral margins convexly divergent posteriorly, with row of spines of irregular sizes: first anterolateral spine strong, overreaching lateral orbital spine; second and third very small (third obsolete on right side in holotype) placed on hepatic region; fourth situated at anterior end of anterior branchial margin, larger than second and third but much smaller than first, followed by a few small spines; about 10 short spines on posterior branchial margin, posteriorly diminishing and tending to be laciniate. Rostrum triangular, with interior angle of 28°, nearly horizontal; dorsal surface flattish, with fine setae; length less than half that of remaining carapace, breadth one-third carapace breadth at posterior carapace margin. Lateral orbital spine very small, slightly anterior to level of and well separated from anterolateral spine. Pterygostomian flap anteriorly somewhat angular, produced to small spine, anterior surface with several spinules.

Sternum: Excavated sternum blunt subtriangular or nearly transverse on anterior margin, surface with longitudinal ridge in midline. Sternal plastron about twice broader than long, lateral extremities convexly divergent posteriorly. Sternite 3 weakly depressed, anterior margin deeply excavated, with semicircular or narrow median notch, lacking submedian spines. Sternite 4 with smooth anterolateral margin anteriorly blunt angular, twice as long as posterolateral margin. Anterolateral margin of sternite 5 strongly convex, 1.5 × longer than posterolateral margin of sternite 4.

Abdomen: Smooth, sparingly with short fine setae. Somite 2 tergite 2.6-2.9 × broader than long; pleural lateral margins concavely divergent, rounded on anterolateral corner, angular at posterolateral terminus. Pleuron of somite 3 with bluntly angular lateral end. Telson more than one-third as long as broad, less than half as long; posterior plate slightly longer than anterior plate, posterior margin nearly transverse, slightly or deeply concave.

Eye: Elongate (1.6 × longer than broad), slightly overreaching midlength of rostrum, proximally somewhat broader. Cornea not dilated, half as long as remaining eyestalk.

Antennule and antenna: Ultimate article of antennule 3.0-3.3 × longer than high. Antennal peduncle overreaching cornea. Article 2 distolaterally angular, without distinct spine. Antennal scale 1.7-1.8 × broader than article 5, bearing long setae distally, varying from slightly falling short of to slightly overreaching tip of article 5. Distal 2 articles each with distomesial spine accompanying plumose setae; article 5 1.3-1.4 × longer than article 4, breadth 0.6-0.7 × height of ultimate article of antennule. Flagellum consisting of 12-18 segments, fully or barely reaching distal end of P 1 merus.

Mxp: Mxp1 with bases broadly separated. Mxp3 basis lacking denticles on mesial ridge. Ischium rather thick, with tuft of plumose setae lateral to slightly rounded distal end of flexor margin; crista dentata with numerous (more than 40) very small denticles. Merus 2.0 × as long as ischium, setose (relatively long plumose setae along flexor margin); distolateral spine distinct; flexor margin sharply ridged, with 2-4 denticles on distal third; mesial face flattish. Distolateral spine of carpus obsolete.

P 1: 4.8-5.9 × (males), 4.0-4.9 × (females) longer than carapace, covered with soft, fine plumose setae. Ischium dorsally with dorsoventrally flattened triangular spine, ventromesially with row of 4-7 spinules including very small subterminal spine. Merus and carpus with short denticulate ridges supporting setae. Merus 1.0-1.2 × length of carapace; mesial face with several small denticle-like processes or spines on proximal half, ventral surface with distomesial and distolateral processes, both blunt; distal margin with papilla-like, very small spines. Carpus 1.3-1.4 × longer than merus, dorsally with a few tubercles near proximal end and a few obsolescent papilla-like tubercles on distal margin, ventrally with distomesial and distolateral processes, both blunt and short. Palm 3.3-3.4 × longer than broad, slightly longer (1.1 x) than carpus; surface smooth but a few small tubercles near proximal part of dorsal side. Fingers directed somewhat laterally, proportionately broad, distally incurved, crossing when closed; opposable margin with median eminence on fixed finger, rounded process on movable finger; length 0.4 × that of palm.

P 2-4: setose like P 1, broad relative to length, moderately depressed. Meri successively shorter posteriorly ( P 3 merus 0.9 × length of P 2 merus, P 4 merus 0.7-0.8 × length of P 3 merus), breadths subequal on P 2 and P 3, P 4 merus 0.9 × breadth of P 3; length-breadth ratio, 3.4-3.6 on P 2, 2.7-2.9 on P 3, 2.1-2.5 on P 4; P 2 merus 0.8 × length of carapace, about as long as P 2 propodus; P 3 merus 0.9 × length of P 3 propodus; P 4 merus 0.7-0.8 × length of P 4 propodus; dorsal margin with obsolescent eminences on proximal portion. Carpi successively shorter posteriorly ( P 3 carpus 0.9 × length of P 2 carpus, P 4 carpus 0.8-0.9 × length of P 3 carpus); carpus-propodus length ratio, 0.5-0.6 on P 2, 0.5 on P 3, 0.4-0.5 on P 4. Propodi longer on P 3 than on P 2 and P 4, slightly longer on P 2 than on P 4; flexor margin distinctly convex on distal half, ending in pair of spines preceded by row of 5-7 movable spines along distal third on P 2, 4-6 spines on P 3, 3-5 spines on P 4. Dactyli subequal on P 2-4 or shorter on P 2 than on P 3 and P 4; dactylus-carpus length ratio, 0.7 on P 2, 0.8 on P 3, 0.9-1.0 on P 4; dactylus-propodus length ratio 0.3-0.4 on P 2, 0.4 on P 3 and P 4; flexor margin gently curving, with 6 or 7 strong triangular spines somewhat obliquely directed, loosely arranged and proximally diminishing, ultimate strongest.

Eggs. About 30 eggs carried; size, 1.07 mm × 1.24 mm - 1.08 mm × 1.24 mm.

REMARKS — Slight morphological differences are observed between Vanuatu and Solomon Islands specimens. The P 4 merus is shorter in the Vanuatu specimens ( P 4 merus-P3 merus length ratio, 0.7 versus 0.8). Also, the P 2-4 propodi in the Vanuatu specimens are broader, with the length-breadth ratio 3.4-3.7 versus 4.2-4.3 on P 2, 3.9-4.0 versus 4.3-4.4 on P 3, 3.6-4.3 versus 4.5-4.7 on P 4.

The new species resembles U. paracrassior Ahyong & Poore, 2004 from off southern Queensland in having the spinous carapace lateral margin and in having the distally broadened P 2-4 propodi and in the spination of the P 2-4 dactyli, especially the ultimate being subequal to or slightly larger than the penultimate. Uroptychus megistos can be separated from that species by: sternite 3 without submedian spines, the P 2-4 dactyli with 5 instead of 9-11 flexor marginal spines, the pterygostomian flap that is unarmed instead of bearing 2 spines between the anterior terminal spine and the anterior end of the linea anomurica, and the antennal article 2 with a very short instead of strong distolateral spine. In addition, the lateral orbital spine is tiny in U. megistos , whereas relatively large in U. paracrassior .

Uroptychus megistos also resembles U. duplex n. sp., U. macrolepis n. sp. and U. zigzag n. sp. in having the spinous carapace lateral margin and in having the distally convex flexor margins of P 2-4 propodi. However, these three species have a different spination of the P 2-4 dactyli, the ultimate spine being smaller than instead of equally strong as the penultimate as in U. megistos . Uroptychus megistos is differentiated from U. duplex by the anterior margin of sternite 3 that is deeply excavated with a median notch, instead of being transverse on the median third, without median notch; the P 2-4 dactyli bear fewer flexor spines (7 or 8 instead of 12 or 13), and the antennal articles 4 and 5 each bear a distoventral spine instead of being unarmed. Uroptychus macrolepis has elongate eyes, sternite 3 without a distinct median notch on the anterior margin, the flexor spines of the P 2-4 dactyli more numerous (11-13), the ultimate of these being much more slender than the penultimate, and the distal two articles of the antennal peduncle unarmed, all features that separate the species from U. megistos .

The relationships with U. zigzag are discussed under the remarks of that species (see below).