Uroptychus medius, Komai & Tsuchida & Fujiwara, 2023

Komai, Tomoyuki, Tsuchida, Shinji & Fujiwara, Yoshihiro, 2023, Squat lobsters of the superfamily Chirostyloidea (Decapoda: Anomura) from seamounts on the Nishi-Shichito and Mariana ridges, North-West Pacific off Japan, with descriptions of two new species, Zootaxa 5293 (1), pp. 45-73 : 48-55

publication ID	https://doi.org/ 10.11646/zootaxa.5293.1.2
publication LSID	lsid:zoobank.org:pub:727F3FD8-4E36-4F19-AAB2-060121DB9F59
DOI	https://doi.org/10.5281/zenodo.7959799
persistent identifier	https://treatment.plazi.org/id/5119C7C0-9CB3-4D03-98AF-5926A878D17C
taxon LSID	lsid:zoobank.org:act:5119C7C0-9CB3-4D03-98AF-5926A878D17C
treatment provided by	Plazi
scientific name	Uroptychus medius
status	sp. nov.

Treatment

Uroptychus medius n. sp.

[New Japanese name: Ritto-kumo-ebi]

( Figs. 1–4 View FIGURE 1 View FIGURE 2 View FIGURE 3 View FIGURE 4 )

Type material. Holotype: JAMSTEC 106694, ovigerous female (cl 9.1 mm, DNA voucher), R / V Kaimei , KM20- 10 C, KM-ROV dive #131, Ritto Seamount, West Mariana Ridge, 21°43.01’N, 142°04.53’E, 2001 m, associated with chrysogorgiid octocoral, 6 December 2020, suction sampler. GoogleMaps

Diagnosis. Carapace as long as broad; surface unarmed, smooth; lateral margins unarmed except for anterolateral spine, smooth; posterolateral corner with short, blunt ridge; anterolateral spine tiny, slightly overreaching blunt lateral orbital angle. Rostrum basal width greater than half distance measured between carapace anterolateral spines. Pterygostomial flap smooth on surface, anteriorly produced to small spine. Excavated sternum with small median spine-like process on anterior margin. Sternite 3 with anterior margin deeply excavated, with median notch flanked by tiny submedian spines. Sternite 4 smooth on surface; anterolateral margin irregular, anteriorly not reaching end of sternite 3, subequal in length to posterolateral margin. Sternite 5 with distinctly convex anterolateral margin. Ocular peduncle stout (1.3 times longer than broad); cornea not pigmented. Antennal peduncle article 2 unarmed on distolateral angle; antennal scaphocerite slightly overreaching distal margin of article 4; article 5 2.0 times as long as article 4. Cheliped about 5 times as long as carapace; ischium unarmed on ventromesial margin; merus with small dorsodistal spine; palm mesial face rounded, not ridged. Pereopods 2–4 meri subequal in width, pereopod 4 merus 0.8 and 0.9 length of pereopods 2–3 meri, respectively. Pereopods 2–4 carpi distinctly longer than dactyli; pereopod 4 carpus shorter than pereopod 3 carpus. Pereopods 2–4 propodi each with single terminal spine followed by 6 spines along flexor margin in distal 0.5–0.6. Pereopods 2–4 dactyli strongly curved, approximately half-length of propodi, not truncate distally, with row of obliquely directed spines, increasing in size distally, on flexor margin, penultimate spine located at midway between terminal spine and antepenultimate spine.

Description. Ovigerous female holotype. Medium-sized species (cl 9.1 mm in ovigerous female). Carapace ( Figs. 1A View FIGURE 1 , 2A, B View FIGURE 2 ) as long as wide; width at posterolateral angles about twice of distance between anterolateral spines. Dorsal surface smooth, glabrous, unarmed, slightly convex from anterior to posterior, with trace of cervical groove anterior to midlength. Lateral margins regularly divergent posteriorly without constriction, bluntly ridged along posterior part of branchial margin; anterolateral spines small, slightly overreaching lateral orbital angle; no other spines or tubercles present on branchial margin. Pterygostomial flap anteriorly produced to small spine, smooth on surface, somewhat inflated and then clearly visible in dorsal view; anterior height subequal to posterior height.

Rostrum ( Fig. 2A, B View FIGURE 2 ) narrow triangular, very slightly ascending, 0.4 times as long as carapace, basal width 0.4 of maximal carapace width across posterolateral angles; dorsal surface flattish; lateral margins concave proximally; ventral surface almost flat.

Thoracic sternum with excavated anterior part anteriorly ending in small spine between bases of maxilliped 1, surface not ridged in midline. Sternal plastron (sternites 3–7; Fig. 3A View FIGURE 3 ) slightly wider than long; sternites successively wider posteriorly. Sternite 3 depressed well, concave anterior margin with 2 small submedian spines separated by V-shaped median notch and with additional minute denticle on either side; anterolateral angles faintly denticulate. Sternite 4 with transverse ridge preceded by strong depression; anterolateral margins slightly convex, each bearing irregular denticles, anteriorly produced, but not in form of spine; posterolateral margin subequal in length to anterolateral margin. Anterolateral margins of sternite 5 strongly convex, slightly shorter than posterolateral margin of sternite 4.

Pleon ( Figs. 1A View FIGURE 1 , 2A View FIGURE 2 , 3B View FIGURE 3 ) smooth and glabrous. Pleomere 1 with faint transverse ridge. Pleomere 2 3.4 times wider than long; pleuron tapering posteroventrally to blunt point, anterior margin concave. Pleura of pleomeres 3–5 each tapering ventrally to blunt tip. Telson ( Fig. 3B View FIGURE 3 ) 1.3 times as long as wide, divided into two plates by deep lateral notches; anterior plate 1.2 times wider than posterior plate; posterior plate 1.5 longer than anterior plate, lateral margins broadly rounded, posterior margin roundly truncate.

Ocular peduncle ( Figs. 1A View FIGURE 1 , 2A, B View FIGURE 2 ) wide relative to length (1.3 times longer than wide), broadened distally, overreaching midlength of rostrum. Cornea sub-globular, devoid of dark pigment, slightly dilated, more than half-length of remaining ocular peduncle.

Antennular peduncle ( Fig. 2A, B View FIGURE 2 ) with ultimate article about twice as long as high; upper flagellum distinctly longer than ultimate peduncular article.

Antennal peduncle ( Fig. 3C View FIGURE 3 ) distinctly overreaching distal corneal margin. Article 2 unarmed on distolateral angle. Scaphocerite 1.3 times wider than article 5, reaching proximal 0.1 of article 5, terminating in acute tip. Distal 2 articles unarmed; article 5 2.2 times as long as article 4, width half height of ultimate article of antennular peduncle. Flagellum not reaching distal end of cheliped merus.

Maxillipeds 1 with bases close but not contiguous to each other.

Maxilliped 3 ( Fig. 3D, F View FIGURE 3 ) barely setose on lateral surface. Basis with 1 minute denticle on mesial margin. Ischium with about 10 minute denticles on poorly developed crista dentata ( Fig. 3E View FIGURE 3 ); flexor margin distally not rounded; articulation to merus strongly oblique. Merus and carpus unarmed; merus subequal in length to ischium, not particularly flattened, moderately thick, rounded along flexor margin. Propodus elongate, 2.0 times as long as carpus, with convex flexor margin with peak proximal to midlength. Dactylus elongate suboval, 0.6 times as long as propodus.

Cheliped (pereopod 1) ( Figs. 1A View FIGURE 1 , 2C–F View FIGURE 2 ) about 5 times as long as carapace; nearly smooth and barely setose except for fingers. Ischium with small dorsodistal spine, otherwise unarmed. Merus 1.1 times as long as carapace; dorsal surface with moderately large, dorsodistally directed spine at mesiodistal angle; lateral, mesial and ventral surfaces with short, scale-like, ridges. Carpus 1.3 times as long as merus, 5.2 times as long as distal width; no conspicuous armature; surfaces with obsolete, short transverse ridges. Palm 3.4 times longer than wide, 1.3 times length carpus. Fingers moderately slender, slightly curving ventrally, distally barely crossing, bearing scattered tufts of long setae and row of short setae obscuring distal half of occlusal margins of fingers; occlusal margins each with 2 prominent, blunt teeth in proximal half, fitting to each other into concavities between teeth, distal half narrowly gaping; dactylus 0.64 as long as palm.

Pereopods 2–4 ( Figs. 1A View FIGURE 1 , 4A–F View FIGURE 4 ) relatively slender, well compressed laterally, bearing long setae numerous on distal two articles, partially obscuring armature, becoming shorter toward posterior, but similar in general structure and ornamentation; ischia short, articulation to merus strongly oblique.

Pereopod 2 ( Fig. 4A, B View FIGURE 4 ) merus 1.1 times as long as carapace, 7 times as long as wide, linear, surfaces smooth. Carpus slightly widened distally, unarmed, with few long setae on extensor margin. Propodus 0.6 times as long as merus, subequal in length to carpus, widest at about midlength; extensor margin straight; flexor margin slightly convex, armed with comb-like row of 8 long, equidistant, movable spines in distal 0.6, distalmost one single (not paired), somewhat remote from juncture with dactylus. Dactylus noticeably curving at proximal one-third, 0.6 times as long as propodus, tapering toward distal at base of penultimate spine on flexor margin, terminating in strong spine; flexor margin with 7 spines inclined but not oriented parallel to flexor margin, penultimate spine broadly equidistant between ultimate and antepenultimate spines; other flexor spines, including antepenultimate spine, equidistantly narrowly spaced and successively diminishing toward proximal.

Pereopod 3 ( Fig. 4C, D View FIGURE 4 ) merus 0.9 times as long as and subequal in width to pereopod 2 merus. Carpus slightly shorter than pereopod 2 carpus. Propodus subequal in length to pereopod 2 propodus; flexor margin with row of 7 long movable spines in distal 0.6, distalmost one single (not paired), somewhat remote from juncture with dactylus. Dactylus flexor margin with 7 spines, similarly arranged as in pereopod 2 dactylus.

Pereopod 4 ( Fig. 4E, F View FIGURE 4 ) merus 0.8 times as long as pereopod 2 merus, 0.9 time as long as pereopod 4 merus. Carpus 0.8 times as long as pereopod 2 carpus, 0.9 times as long as pereopod 3 carpus. Propodus subequal in length to those of pereopods 2 and 3; flexor margin armed with row of 7 spines in distal half. Dactylus flexor margin with 7 spines, similarly arranged as in pereopod 2 dactylus.

Egg size at stage of near hatching about 2.5× 4.1 mm.

Colouration in life. Carapace generally pink; pleomeres 1–3 and chelipeds orange, fingers pinkish; ocular peduncles dark pink, corneas opaque; pereopods 2–4 pale orange ( Fig. 1A View FIGURE 1 ).

Distribution. Presently known only from Ritto Seamount in the West Mariana Ridge, 2001 m deep.

Ecology. The holotype was associated with a colony of chrysogorgiid octocoral ( Fig. 1B View FIGURE 1 ).

Remarks. Morphologically, the present new species appears close to Uroptychus remotispinatus Baba &Tirmizi, 1979 and U. vandamae Baba, 1988 in the dorsally smooth and glabrous carapace with a tiny anterolateral spine, the single, unpaired ultimate flexor spine on the pereopods 2–4 propodi, and the penultimate flexor spine on the pereopods 2–4 dactyli clearly separated from other proximal flexor spines. Nevertheless, the new species is readily distinguished from the latter two species in the different position of the penultimate flexor spine on the pereopod 2–4 dactyli, which is located midway between the ultimate spine and the antepenultimate spine ( Fig. 4B, D, F View FIGURE 4 ). In the latter two species, the penultimate spine is located close to the base of the ultimate spine ( Baba & Tirmizi 1979: fig. 1e; Baba 1988: fig. 21e; Baba 2018: figs. 222I–J, 275E–H; Schnabel 2020: fig. 151H). Differences are also seen in the spination of the pereopods 2–4 propodi: the flexor marginal spines are equidistantly arranged in U. medius n. sp. and U. vandamae , but the distalmost spine is clearly more distant from the juncture with the dactylus in U. vandamae ; in U. remotispinatus , the distalmost and the distal second spines are widely separated from each other. Furthermore, the pterygostomial flap of the carapace is substantially inflated and thus is clearly visible in dorsal view in U. medius n. sp. ( Fig. 2A View FIGURE 2 ). In contrast, the pterygostomial flap is not so inflated and is as to be hardly visible in dorsal view in U. remotispinatus (cf. Baba & Tirmizi 1979: figs. 1a, 2a; Schnabel 2020: fig. 151A) and U. vandamae (cf. Baba 1988: fig. 21a). The merus of the cheliped is armed with a small submarginal spine adjacent to the dorsomesial distal angle in U. medius n. sp. ( Fig. 1C, D View FIGURE 1 ), whereas such a spine is absent in U. remotispinatus (cf. Baba & Tirmizi 1979: fig. 1a; Schnabel 2020: fig. 151H) and U. vandamae (cf. Baba 1988: fig. 21a). Uroptychus vandamae further differs from the new species in having dactylar flexor spines of the pereopods 2–4 oriented parallel to the flexor margin and in having the distalmost propodal spine distinctly separated from other spines (Baba 2018: figs. 275E–H). The shape of the thoracic sternite 4 is also different between the new species and U. vandamae ; in the new species, the anterolateral and posterolateral margins are subequal in length and the anterolateral margin is not produced anteriorly as spines ( Fig. 3A View FIGURE 3 ), whereas in U. vandamae , the anterolateral margin is distinctly longer than the posterolateral margin and the anterolateral angle is produced to spines (Baba 2018: fig. 275C).

A partial fragment of the 16S rRNA gene (511 bp) was sequenced for the holotype of the present new species, although sequencing of the COI was not successful. Sequences of the16S rRNA gene are available for 17 species of Uroptychus in the GenBank database ( Table 1 View TABLE 1 ), although no sequence data is available for U. remotispinatus and U. vandamae , mentioned above. The interspecific genetic divergences among species of Uroptychus available for comparison range from 1.7 to 17.7%, and that between U. medius n. sp. and other species (5.1 to 14.7%) included in the intrageneric range ( Table 2 View TABLE 2 ). Among the 17 species, Uroptychus thermalis Baba & de Saint Laurent, 1992 is closest to the present new species. Nevertheless, U. thermalis differs from the new species in many characters, including the carapace dorsal surface bearing scattered short granular ridges ( Baba & de Saint Laurent 1992: fig. 2a, b), the flexor spines on the pereopods 2–4 separated into two groups (Baba 2018: figs. 262, 263) and the absence of spines on the prehensile margins of the pereopods 2–4 propodi ( Baba & de Saint Laurent 1992: fig. 2g).