Uroptychus megistos Baba 2018, n. sp.

Uroptychus megistos n. sp.

Figures 143, 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). 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 P2-4 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 P1 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.

P1: 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.

P2-4: setose like P1, broad relative to length, moderately depressed. Meri successively shorter posteriorly (P3 merus 0.9 × length of P2 merus, P4 merus 0.7-0.8 × length of P3 merus), breadths subequal on P2 and P3, P4 merus 0.9 × breadth of P3; length-breadth ratio, 3.4-3.6 on P2, 2.7-2.9 on P3, 2.1-2.5 on P4; P2 merus 0.8 × length of carapace, about as long as P2 propodus; P3 merus 0.9 × length of P3 propodus; P4 merus 0.7-0.8 × length of P4 propodus; dorsal margin with obsolescent eminences on proximal portion. Carpi successively shorter posteriorly (P3 carpus 0.9 × length of P2 carpus, P4 carpus 0.8-0.9 × length of P3 carpus); carpus-propodus length ratio, 0.5-0.6 on P2, 0.5 on P3, 0.4-0.5 on P4. Propodi longer on P3 than on P2 and P4, slightly longer on P2 than on P4; flexor margin distinctly convex on distal half, ending in pair of spines preceded by row of 5-7 movable spines along distal third on P2, 4-6 spines on P3, 3-5 spines on P4. Dactyli subequal on P2-4 or shorter on P2 than on P3 and P4; dactylus-carpus length ratio, 0.7 on P2, 0.8 on P3, 0.9-1.0 on P4; dactylus-propodus length ratio 0.3-0.4 on P2, 0.4 on P3 and P4; 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 P4 merus is shorter in the Vanuatu specimens (P4 merus-P3 merus length ratio, 0.7 versus 0.8). Also, the P2-4 propodi in the Vanuatu specimens are broader, with the length-breadth ratio 3.4-3.7 versus 4.2-4.3 on P2, 3.9-4.0 versus 4.3-4.4 on P3, 3.6-4.3 versus 4.5-4.7 on P4.

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 P2-4 propodi and in the spination of the P2-4 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 P2-4 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 P2-4 propodi. However, these three species have a different spination of the P2-4 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 P2-4 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 P2-4 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).