Eusirus carolinus sp. nov.

Figs 1, 2, 3, 4, 5

Material examined.

Holotype: Western Pacific • 1 brooding female (with 9 intra-marsupial individuals), 12.9 mm; seamount on Caroline Plate; 10 ° 30 ' N, 140 ° 9–10 ' E; depth 520–862 m; 26 Aug. 2017; MBM 286609.

Paratype: Western Pacific • 1 male; 13 mm; seamount on Caroline Plate; 10 ° 30 ' N, 140 ° 9–10 ' E; depth 520–862 m; 26 Aug. 2017; MBM 286608.

Description.

Body. Pleonites 1–3 carinate dorsally; pleonites 1 and 2 with acute distomiddorsal teeth; epimera 1 and 2 postero-distal corner subacute; epimeron 3 postero-distal corner rounded, without serration. Head. Lateral cephalic lobe with apically-oblique rostrum. Eyes large, reniform. Antenna 1 stout, with peduncular articles 1–3 in length ratio of 1: 0.95: 0.36; distal margin of peduncular article 1 with inner surface bearing two distal processes; article 2 distal margin produced as two triangular distal processes in outer and inner surfaces, respectively; primary flagellum 55 - articulate; first article nearly as long as the third peduncular article; accessory flagellum 1 - articulate. Antenna 2 shorter than antenna 1, with peduncular article 4 subequal in length to article 5; flagellum distinctly longer than fifth peduncular article, 29 - articulate.

Mouthparts. Upper lip distally rounded, apex convex, and weakly produced, bearing fine submarginal setae. Right mandible with incisor bearing one blunt anterodistal tooth and not dentate lacinia mobilis; accessory setal row composed of 5 stout setae; molar triturative, columnar; palp 3 - articulate, much longer than mandible body; article 1 shortest; article 2 slightly shorter than article 3, with row of long simple setae; article 3 with ventral margin heavily setose; and outer margin bearing one long (about 3 / 5 length of third palp article) simple seta. Left mandible, incisor with one blunt large anterodistal tooth, lacinia mobilis 8 - dentate; accessory setal row composed of 6 setae; molar and palp similar to those of right mandible. Lower lip with weak inner lobe; outer lobe subovoid, rounded distally; both inner and outer lobes covered with marginal and submarginal setae. Maxilla 1 outer plate with 11 stout multicuspidate setae on apical margin; inner plate subovoid, apex bearing one seta; palp long far beyond outer lobe, 2 - articulate; article 2 with about 15 setae along apex and mediodistal margin. Maxilla 2 with inner plate broader than outer plate, both with fine marginal setae. Maxilliped heavily setose, inner plate short, not reaching to distal margin of first palp article, distally armed with about 10 stout setae; outer plate extending to about 1 / 2 length of article 2 of palp, laterally armed with short robust setae and distally armed with long plumose setae; palp 4 - articulate, slender, article 2 slightly longer and broader than article 3; dactylus shorter than article 3, claw-like, unguis very short, ventral margin bearing 5 long stout setae.

Coxal gills present on pereopods 2–7. Oostegites present on pereopods 2–5.

Gnathopods and pereopods. Gnathopods similar in shape: gnathopod 1 subchelate, eusirid form; coxa 1 broadened distally, slightly expanded anteroventrally; basis steady in width, not distinctly broadened distally; anterior margin flat, bearing several setae; posterior margin with a group of setae distally; ischium short, with rounded lobe anterodistally; carpus lobe linguiform, narrow, covered with long setae anterior margin; outer face with acute pointed process distally and inner face rounded distally; propodus wider than long, subrectangular, shorter than carpus in length of anterior margin, with one group of defining setae; palm lined with numerous crooked setae submarginally; dactylus falcate, fitting palm. Gnathopod 2 similar to gnathopod 1, stout, eusirid form, coxa rectangular, ventral margin convex; basis slightly longer than that of gnathopod 1; ischium, merus, carpus propodus, and dactylus similar in appearance to gnathopod 1. Pereopod 3 slender, coxa rectangular; basis linear; anterior margin with simple setae; merus about twice as long as carpus, distinctly longer than propodus; propodus with posterior margin bearing several small stout setae; dactylus elongate, slightly curved; posterior margin dentate. Pereopod 4 broken, coxa subequal in length to coxa 3, posteriorly emarginated, postero-distal corner subacute, basis linear, anterior margin with simple setae. Pereopod 5 shorter than pereopods 6 and 7; coxa equilobate, both expanded posteroventrally; basis increasing in length from pereopods 5 to 7, with posteroventral lobe, posterior margins lateral border moderately expanded, serrate, anterior margin with short robust setae; merus increasing in length from pereopods 5 to 7, posterodistally strongly produced, margins with stout setae; carpus subequal in length to merus, margins with stout setae; length ratios of merus to propodus 1: 0.77: 1.20; dactylus nearly straight, anterior margin with about distal 1 / 3 length dentate. Pereopods 6 and 7 subequal in length; coxa 6 bilobate; merus longer than that of pereopod 5; carpus distinct shorter than merus; propodus slender; margins with stout setae; length ratios of merus to propodus 1: 0.77: 1.31. Pereopod 7 with posterior margins of basis lateral border moderately expanded and distinctly concave, posterodistal corner subacute; length ratios of merus to propodus 1: 0.68: 1.25.

Uropods and telson. Uropod 1 with peduncle subequal in length to outer ramus, bearing numerous short laterals and long medial robust setae dorsally, with one enlarged stout seta at mesiodistal corner; inner ramus 1.3 times as long as outer ramus, both rami bearing robust setae and lateral and medial margins. Uropod 2 with rami lanceolate; peduncle slightly shorter than outer ramus; lateral margin with two subdistal robust setae dorsally; medial margin with seven robust setae; inner ramus nearly twice as long as outer ramus; both rami with inner and medial margins bearing dense robust setae; outer ramus narrower than inner ramus. Uropod 3 shortest, rami lanceolate, subequal in length; peduncle 0.5 times as long as rami; lateral margin bearing one long distal robust sate dorsally; medial margin bearing three small robust setae dorsally; both rami with lateral and medial margin setose dorsally. Telson long, slightly tapering distally, reaching about half of the length of uropod 3 rami, length about twice its breadth at base, cleft 15 % of length.

Description of intra-marsupial individual. Based on one of the intra-marsupial individuals of the holotype. Rostrum present. Antenna 1 with peduncle articles less setose; primary flagellum only slightly longer than peduncle, 5 - articulate; each article slightly longer than the third peduncular article, less setose; accessory flagellum 1 - articulate. Antenna 2 much shorter than antenna 1, peduncle less setose; articulations of flagellum inconspicuous and less setose. Gnathopods 1–2 similar in shape and size, subchelate, eusirid form; carpus lobe linguiform broader than that of adult, only covered with several setae mediodistally; propodus similar in form to that of the adult, but without setae; anterior margin subequal in length to carpus; palm not bearing crooked setae submarginally; dactylus stout, slightly curved. Pereopods 3 and 4 subequal in length and similar in form, less setose; dactylus nearly straight, slightly shorter than propodus, with posterior margin not dentate. Pereopod 5 shorter than pereopods 6 and 7. Pereopod 6 basis with posterior margin smooth, not serrate, anterior margin without setae; merus with large posterodistal pointed process bearing one seta; length ratios of merus to propodus 1: 0.83: 1.33; dactylus elongate, about half length of propodus, nearly straight, anterior margin not dentate. Pleonites 1–3 not carinate; pleonites 1–2 with small acute mid-dorsal tooth. Uropods 1–3 similar to those of adults but without robust setae on peduncles and rami. Telson long, length 1.6 times its breadth at base, cleft 15 % of length, narrower than that of adult, slightly tapering distally.

Coloration.

Anterior part of body is red with yellow eyes; gnathopods 1–2 are deep red; and antennae 1–2 and posterior part of body are pale red.

Etymology.

The new species is named after its type locality, the Caroline Plate.

Distribution.

Presently known only from a seamount of the Caroline Plate at a depth of 520–862 meters.

Remarks.

Table 1 shows the distribution, depth information, and important characters of Eusirus species. Nine species have been reported from the Pacific, including Eusirus antarcticus Thomson, 1880; E. bathybius Schellenberg, 1955; and E. bulbodigitus Jung, Kim, Soh & Yoon, 2016; E. columbianus Bousfield & Hendrycks, 1995; E. cuspidatus Krøyer, 1845; E. fragilis Birstein & M. Vinogradov, 1960; E. hirayamai Bousfield & Hendrycks, 1995; E. liui Wang, Sha & Ren, 2021; and E. parvus Pirlot, 1934. Eusirus carolinus sp. nov. can be distinguished from E. bathybius, E. fragilis, and E. liui by having large, well-pigmented eyes (Schellenberg 1955; Birstein and Vinogradov 1960; Wang et al. 2021). The new species differs from the remaining six Pacific Eusirus species by presenting the posterior margin of epimeron 3 as smooth, while epimeron 3 presents a serrate posterior margin in the mentioned six species (Krøyer 1845; Stebbing 1906; Pirlot 1934; Gurjanova 1951; Bousfield and Hendrycks 1995; Jung et al. 2016; Othaitz and Sorbe 2020). The body of Eusirus carolinus sp. nov. only has distomiddorsal teeth on pleonites 1–2, while the pereonite 7 of three species, E. cuspidatus, E. hirayamai, and E. parvus, also has a distomiddorsal tooth other than the teeth on pleonites 1–2 (Krøyer 1845; Pirlot 1934; Hirayama 1985). The new species also differs from E. hirayamai by the rami of uropod 3 subequal in length and from E. parvus by having a unilobed coxa 7 (Pirlot 1934; Hirayama 1985; Bousfield and Hendrycks 1995).

* Translation of the original description: eyes absent; pereonites and pleonite 1 with weak longitudinal carina, pleonites 1 and 2 with small distomiddorsal teeth, pleonite 3 and segment 1 of without tooth, posteroventral angle of epimeron 1 produced into small acute tooth; head with anterolateral angle acute, rostrum extending 1 / 3 length of basal article of antenna 1, antennae 1 broken off at peduncular article 1; antenna 2 article 3 four times shorter than article 4 and three times shorter than article 5; upper lip rounded, mandible with stout cylindrical molar process, serrated incisor process, large lacinia mobilis and spine row, palp article 1 much longer than article 2, maxilla 1 inner lobe with 2 very short distal setae, outer lobe with 11 multi-cusped spine-teeth, palp thin, bearing row of fine setae on its end, palp article 1 two times shorter than article 2, maxilla 2 inner lobe rounded, as long as, but two times broader than outer lobe, both lobes equipped with setae distally; maxilliped similar with those of Е. bathybius, but outer lobes slightly broader than inner lobes; coxa 1 strongly expanding distally, with rounded anterior margin; coxa 2 smaller than coxa 1; plate 4 small, with convex anterior and concave posterior margins; plates 5–7 oval, gnathopods 1 and 2 typical for Eusirus, gnathopod 1 significantly shorter than gnathopod 2, carpus with posterior margin bearing a tuft of setae on acute projection distally, propodus of gnathopods 1 and 2 similar in shape and size, width less than 1.5 times exceeding their length, Pereopods 3, 4, and 6 broken off; Pereopod 5 with basis slightly expanding distally; basis of pereopod 7 strongly expanding distally; width almost equal to the length, epimera 1 and 3 rounded, with smooth posterior margin, epimeron 2 with acute posteroventral angle, uropod 1 rami equal in length, uropod 2 exopod slightly shorter than endopod, uropod 3 broken off, telson triangular, width slightly less than length; distal end with an acute-angled notch. Remarks. The species described herein sharply differs from all other species of the genus by the smooth epimer III posterior margin and short triangular telson (Birstein and M. Vinogradov 1960, translated by Dr. Olga A. Golovan).