Sextonia caecus Labay 2017, sp. nov.
Creators: name: Labay, Vjacheslav S.
Publisher: name: Zenodo
Year: 2017
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Malacostraca
Amphipoda
Liljeborgiidae
Sextonia
Sextonia caecus

description: Sextonia caecus sp. nov.
Figures 1 –11
Type material. Holotype male, 8.2 mm, 47114/Cr-2214, Okhotsk Sea, Far East of Russia (57°53'92''N 144°41'82''E, 125 m), silt and pebble, 0 3 August 2013. Paratypes: 1 female, 7 mm, 47115/Cr-2215, Okhotsk Sea, Far East of Russia (58°48'50''N 146°20'01.6''E, 133 m), silt and gravel, 31 July 2013; 1 male, 7.3 mm, 41804/Cr-1509 with same data as holotype; 1 male, 47118/Cr-2218, Okhotsk Sea, Far East of Russia (58°36'711''N 142°52'432''E, 116 m), silt, pebble and gravel, 0 7 August 2013; 2 females, 47119/Cr-2219, Okhotsk Sea, Far East of Russia (57°53'281''N 142°54'458''E, 160 m), silt, 0 8 August 2013; 1 male, 47120/Cr-2220, Okhotsk Sea, Far East of Russia (58°33'109''N 144°15'106''E, 138 m), silt, pebble and gravel, 0 5 August 2013; 5 females, 47121/Cr-2221, Okhotsk Sea, Far East of Russia (57°53'918''N 144°41'82''E, 159 m), silt, 10 August 2013; 2 males, 47117/ Cr-2217, Okhotsk Sea, Far East of Russia (57°53'918''N 144°41'82''E, 159 m), silt, 10 August 2013; 1 female, 47123/Cr-2223, Okhotsk Sea, Far East of Russia (58°56'313''N 145°00'938''E, 116 m), gravel and silt, 0 4 August 2013; 2 females, 47124/Cr-2224, Okhotsk Sea, Far East of Russia (58°12'273''N 145°19'965''E, 152 m), silt, 0 2 August 2013; 1 female, 47125/Cr-2225, Okhotsk Sea, Far East of Russia (58°51'426''N 145°37'713''E, 125 m), silt and pebble, 0 3 August 2013.
Additional material. 1 female, 7.3 mm, 47116/Cr-2216, Okhotsk Sea, Far East of Russia (51°16'18'' N 144°12' 77''E, 174 m), silt, I.N. Moukhametov & V.N. Chastikov, 13 November 2013; 1 female, 47126/Cr-2226, Okhotsk Sea, Far East of Russia (51°16'18'' N 144°12' 76''E, 180 m), silt, 13 November 2013; 1 male, 47127/Cr-2227, Okhotsk Sea, Far East of Russia (57°31'897'' N 143°01' 765''E, 142 m), sand, pebble and gravel, 12 August 2013; 1 juvenile, 47128/Cr-2228, Okhotsk Sea, Far East of Russia (56°40'575'' N 143°50' 535''E, 245 m), silt, 13 August 2013; 1 male, 47129/Cr-2229, Okhotsk Sea, Far East of Russia (56°22'964'' N 143°50' 862''E, 309 m), silt, 13 August 2013; 2 females, 47130/Cr-2230, Okhotsk Sea, Far East of Russia (57°32'685'' N 143°47' 802''E, 182 m), silt, 11 August 2013; 3 juveniles, 47131/Cr-2231, Okhotsk Sea, Far East of Russia (57°32'685'' N 143°47' 802''E, 182 m), silt, 11 August 2013; 4 females and 1 juvenile, 47132/Cr-2232, Okhotsk Sea, Far East of Russia (51°21'47'' N 144°16'53'' E, 200 m), silt, 11 September 2016.
Type locality. The northern part of Okhotsk Sea, Far East of Russia (57°53'92'' N 144°41' 82'' E, 125 m).
Diagnosis. Eyes absent. Pleon segments 2 and 3 with 10–12 posterodorsal teeth. Urosomite 1 with 7 posterodorsal teeth. Urosomite 2 with 5 posterodorsal teeth. Antenna 1: ratio length of accessory flagellum / length of major flagellum: 0.60. Mandible: palp with long isolate B3-setae near anterior border. Maxilla 1: outer plate with 8 cuspidate setae; inner plate with 1 short and 3 long setae. Maxilliped: article two of palp with outer distal long and strong setae. Gnathopod 1: propodus with several group of setae on the posterior margin; palm of female with 4 rows of articulated structures: outer side with sparse setae; medial side with row of narrow hooked cuspidate setae and a row of bigger and stout bilobed spineformed spinulose setae and a row of spinulose long and stout setae. Gnathopod 2: carpus long, in female strongly setose on posterior margin only. Pereopods 3–4: dactylus with vestigial unguis and specific paddle-shaped seta. Uropod 3: inner ramus with rare cuspidate setae on both sides and with the short proximal cuspidate seta arising from ventral surface.
Description. Male (8.2 mm). Live body color is almost colourless, uniformly pale yellow. Thoracic segments smooth dorsally (Fig. 2a, b). Pleon segment 1 with one small tooth posterodorsally. Urosome segment 1 with 7 teeth (median tooth the longest) and wirh two simple cuspidate setae; urosome segment 2 with 5 posterodorsal teeth and with two simple cuspidate setae.
Head: (Fig. 2a, c) slightly longer than pereonites 1 and 2 together, with short pointed rostrum; eyes absent; anterior head lobe large with an obtuse angle.
Antenna 1: (Fig. 2d) short, nearly 20% of body length, slender; flagellum exceeding 0.5 times the length of peduncle, bearing 10 articles, few very thin and short setae scattered along flagellum; peduncle article 1 double width but 1.2 times shorter length to article 2, with a few simple setae at the distal corner of posterior margin, article 3 0.3 times the length of article 2; accessory flagellum long, 6–7-articulate.
Antenna 2: (Fig. 2e) flagellum almost 0.43 times as long as peduncle, 9–10 articles, with few thin setae on each article; peduncle articles 5 1.3–1.4 times as long as article 4, articles 1, 2, 3 together shorter than article 4 or article 5 each; articles 3 with 2 stout cuspidate setae at the posterodistal corner.
Mouthparts.
Labrum: (Fig. 2f) frontal margin notched (notch asymmetrical).
Mandible: (Fig. 3a, b) incisor crenulate and stretching in a rounded tip; left lacinia mobilis with 5 teeth of which the 2 median ones are small triangular and the external ones are large triangular; right lacinia mobilis with 6 teeth; raker setae stout subplumose, numerous (8+), shorter than incisor in length; molar process large, fully triturative, with only 1 posterior molar seta; palp 3-articulate, article 1 of palp with 4 posterodistal setae, with posterior border slightly concave, forming a geniculate articulation with article two, palp segment 2 with a dence row of simpl setae (D-2 setae), palp segment 3 slightly curved, with posterior row of specific D3-setae (which are not of increasing length towards tip), most of these spinulose setae have 2 anterior strong spinules, with group of distal E3-setae (the longest of them is plumose), with 3 isolated anterior B3-setae, the percentage of total length of the articles of the palp 24% (article 1), 43% (article 2) and 33% (article 3).
Labium: (Fig. 3c) inner lobes small but well defined, rounded; outer lobes rounded at distal margin.
Maxilla 1: (Fig. 3d) inner plate subquadrate, truncated apically; outer plate elongate, 2 times as long as inner plate, apically with 2 rows of 8 apically bifurcate strong setae; palp 2-articulate, article 2 3.3 times as long as article 1, article 2 apical margin with a row of 6 simple setae and 4 well-developed facial setae, without setae on anterior margin.
Maxilla 2: (Fig. 3e) inner plate shorter than outer plate, facial plumose setae of inner plate reduced (6), distal, closely submarginal, tip covered with numerous simple setae; outer plate with numerous simple setae on the tip.
Maxilliped: (Fig. 3f) long; inner plates small, fully cleft, distal margin with a row of 4 short conate setae (Watling type II.A4) (Watling 1989), inner margin with 4 long plumose setae; outer plates almost reaching of 1/3 as long as palp article 2, with inner margin distinctly crenulate, with 10 to 12 well-developed slender and well-spaced simple setae on mediofacial border, with 3 apical strong and long cuspidate setae, and with 2 strong plumose setae on anterolateral border, with numerous tiny setae along lateral margin; palp 4-articulate, the percentage of total length of articles of the palp are 15% (article 1), 39% (article 2), 27% (article 3), 19% (article 4), article 1 of palp without setae, article 2 with numerouse long setae along inner margine, with single seta at the laterodistal corner; article 3 with 2 rows of long simple setae along distal half of inner margin, with a crown of long setae along distal margin, with single long seta near the middle of lateral border; article 4 (dactylus) slender, with long unguis.
Gnathopod 1 (pereopod 1): (Fig. 4a, b) coxa broad, extended distally, with or without small denticle on the rounded anterior corner, without anterior notch, with posterior tooth; basis straight with a row of long simple setae at the middle part of anterior margin and with few long setae at the proximal part of posterior margin; merus with 4 tufts of distally plumose setae along posterior margin, with distal tooth; carpus with tiny posterodistal process, without small posterodistal notch, with 1 posterodistal tuft of 3–4 setae; propodus suboval, slightly broadest distally, 1.8 as long as wide, with 7–8 groups of long setae along posterior margin, anterior margin without setae and with few tiny simple setae at the anterior distal corner, palm oblique and convex, 1.4 times as long as posterior margin, palmar corner with a cluster of 3–5 strong stout setae (spines) associated with a few setae, palmar margin with 2 rows of articulated structures: medial side with row of narrow strong cuspidate setae and a row of bigger and stout spineformed setae interspersed with stout plumose setae; dactylus as long as palm, toothless, without medial setae.
Gnathopod 2 (pereopod 2): (Fig. 4c, d) coxa 1.5 as long as wide, rounded anterodistally, with nearly straight posterior margin, with posterior tooth, anterior and ventral margins with a row of tiny setae; basis slightly expands distally, 1.5 as long as coxa, with a row of long simple setae along anterior margin and with long simple setae along medial part of posterior margin; merus subrectangular, with a cluster of 4–5 setae at the posterior distal corner; carpus long, 1.3 as long as propodus, anterior margin with 4–5 groups of rare setae, anterodistal corner with a transverse row of 5–6 setae, posterior margin with numerouse setae; propodus 2.34 times as long as wide, with 6 well-developed anterior and anterodistal transverse groups of setae, with 9 transverse groups of posterior and posterodistal setae; palm short, 0.45 as long as posterior margin, oblique, palmar corner with 2 strong stout setae (spines), palmar margin with two rows of stout bifurcate setae, 3 anterior setae in each row are plumose; dactylus as long as palm, slightly curved, with one seta at the border of 1/6 of anterior margin.
Pereopod 3: (Fig. 4e) coxa slightly longer than coxa 2, 1.8 as long as wide, rounded distally, without anteroventral notch or tooth, with posterior tooth, posterodistal margin of coxa with 8 setae forming a row; leg rather slender; basis linear with rare long setae along anterior and posterior margins; merus long, with single simple seta at the border of 2/5 of posterior margin and with a cluster of few small setae at the anterior distal corner; carpus 0.8 as long as merus, with two simple setae at the border of 3/5 of posterior margin and with a cluster of few setae at the posterior distal corner; propodus linear, more narrow and 1.36 as long as carpus, with 3 bunches of simple setae along middle part of posterior margin, with groups of setae at anterior and posterior distal corners; dactylus long (0.85 as long as propodus).
Pereopod 4: (Fig. 4f) coxa 1.4 times as long as coxa 3, broad (1.1 times as long as wide), posteriorly excavated, with anterior and posterior margins straight and slightly diverging downwards, with ventral margin scarcely convex, with 4 normally developed serrations on posterior border; leg slender; basis linear with rare long setae along anterior and posterior margins; merus long, with single simple seta at the border of 3/5 of posterior margin and with a cluster of few small setae at the anterior distal corner; carpus 0.77 as long as merus, with a cluster of few setae at the anterior distal corner; propodus linear, more narrow and 1.4 as long as carpus, with 3 bunches of simple setae along posterior margin, with few small setae at the anterior distal corner; dactylus long (0.85 as long as propodus).
Pereopods 5–7 elongated from pereopod 5 to pereopod 7.
Pereopod 5: (Fig. 5a, d, e) coxa with oblique rounded front margin, with rounded hind margin and with rounded anterior lobe pulled back and down, with 1 minute setule on posteroventral corner; basis regular, with posterior wing, width 2/3 as long, anteriorly strongly and regularly convex, posterior distal corner rounded, hind margin slightly convex, crenulated, anterior margin with 10–11 small spines; ischium subquadratic, with 2 short spines on anterodistal corner; merus narrow, with 5–6 groups of short stout setae (“spinules”) along anterior margin, and with a cluster of 2–3 strong setae at anterior and posterior distal corners each; carpus 0.61 as long as merus, with 2 groups of 3 strong setae at the anterior margin and with a single clusters of 5 strong setae at anterior and posterior distal corners each; propodus linear, more narrow and 1.07 times as long as carpus, with 2 tiny simple setae along anterior margin, with a bunch of long setae at the distal margin; dactylus small (1/3 of propodus), weakly curved, slender, with protrusion (with 2 tiny setules) just before minute terminal unguis.
Pereopod 6: (Fig. 5d, f) coxa with subquadrate hind margin and with short rounded anterior lobe, front margin with two cuspidate setae in the ventral part; with 1 minute setule on posteroventral corner; basis regular, with posterior wing, width 0.6 as long, slightly narrowed distally, anteriorly strongly and regularly convex, posterior distal corner rounded, hind margin straight, crenulated, anterior margin with 6–7 long stout cuspidate setae; ischium subquadratic, with 2 short spines on anterodistal corner; merus narrow, with 3 groups of 2 short spinules along anterior margin, with 2 stout spinules along posterior margin, and with a cluster of 2–3 strong setae at anterior and posterior distal corners each; carpus 0.81 as long as merus, with 2 groups of 2–3 strong setae at the anterior margin and with a single cluster of 3 strong setae at anterior and posterior distal corners each; propodus linear, more narrow and as long as carpus, with one group of 2 spinules at the distal part of anterior margin, with a group of 2 spinules at anterior and posterior distal corners each, with a bunch of long setae at the distal margin; dactylus small (1/3 of propodus), weakly curved, slender, with protrusion (with 2 tiny setules) just before minute terminal unguis.
Pereopod 7: (Fig. 5c, g) coxa small, without distinct anterior lobe and with oblique rounded posterior margin, posterior margin with the single tiny seta near the base; basis regular, with narrow posterior wing, width 0.58 as long, slightly narrowed distally, anteriorly regularly convex, posterior distal corner rounded, hind margin straight, crenulated, anterior margin with 5 long stout cuspidate setae; ischium subquadratic, with one short spine on anterodistal corner; merus slightly broadened, with 2 groups of 2 spines along anterior margin, with 3 groups of 2– 3 spines along posterior margin, and with a cluster of 3–5 strong setae at anterior and posterior distal corners each; carpus longer than merus, 1.16 as long as merus, with 2 groups of 3 strong setae at the anterior margin and with a single clusters of 3 strong setae at anterior and posterior distal corners each; propodus narrow lanceolate, more narrow and as long as carpus, with 4 groups of 1–2 spinules along anterior margin, with group of 3 spinules and with 2 simple setae at the distal margin; dactylus small (0.17 as long as propodus), weakly curved, slender.
Coxal gills 2–6. Coxal gill 6 smaller than gill 5.
Epimeral plates: (Fig. 6a, b, c) hind corner acuminate; plate 1 rounded triangular, posterior margin without notches; plate 2 ventral margin convex, anterior part of ventral margin with a row of 3 strong simple setae; plate 3 hind corner produced, acute, ventral margin convex, anterior part of ventral margin with a row of 3 strong simple setae.
Pleopods: (Fig. 6d–i) normal, no sexual dimorphism; peduncle 3 with a transverse row of long setae at the middle of inner margin, peduncles with two specific toothed coupling setae on the inner corner; peduncle 1 without strong simple setae on the inner corner, peduncle 2 with one strong simple seta and peduncle 3 with two strong simple setae on the inner corner; inner ramus shorter than outer ramus, coupling basis of inner ramus with 4–5 specific stout two-pointed plumose setae along inner margin: pleopod 1 (5), pleopod 2 (5+1 simple), pleopod 3 (4+1 simple).
Uropod 1: (Fig. 6j) peduncle without ventrofacial setae, with 6 dorsolateral spines: 4 normally developed regularly spaced spines followed by a pair of strong spines consisting of a small one and a long distal spine, with 13 well-developed and regularly spaced dorsomedial spines; outer ramus with 4 outer and 4 medial mediumsized spines, with 4–5 spines on tip; inner ramus with 6 spines on outer border, with 8 spines on medial border, with small proximal ventral seta, with 2 spines on tip.
Uropod 2: (Fig. 6k) peduncle slightly shorter than inner ramus; inner ramus longer than outer; distal half of peduncle with 4 long dorsolateral spineformed setae and 1 distal dorsomedial spine at the base of distal procession; outer ramus with 4 small spines and with one medial spine, with 4 spines on tip; inner ramus with 4 spines on outer border and with 6 spines on medial border, with 4 spines on tip.
Uropod 3: (Fig. 6l) sexually dimorphic, long, 2 times as long as uropod 2 and subequal to uropod 1; peduncle 1/3 length of outer ramus; outer ramus 2-segmented, article one subequal to inner ramous in length, article one with 7 groups of 2 to 4 spines on outer margin and with 2 small spines along distal part of medial margin, article two 0.08 x as long as article one; inner ramus is long lanceolate, with 6 groups of 2–3 various spines along inner margin and with single spine on the outer margin near the tip, with large ventral spine.
Telson: (Fig. 6m) cleft to 0.9 of its length; lobes parallel, tips of lobes with a recess, apically with two short strong setae and with one long strong seta each.
Female (7 mm). Similar to male. Pleon segment 1 without teeth posterodorsally. Pleon segments 2 and 3 with 11–12 equal small teeth posterodorsally each (Fig. 9c).
Head: similar to as in male.
Antenna 1: similar to as in male, slender; flagellum exceeding 0.63 times the length of the peduncle.
Antenna 2: similar same to as in male; flagellum almost 0.5 times as long as peduncle, 12 articles.
Mouthparts: same as in male.
Mandible: (Fig. 7a, b, c) left lacinia mobilis with 4 teeth.
Maxilliped: (Fig. 7d) outer plates almost reaching of 2/5 length of palp article 2, with 2 apical strong and long cuspidate setae; article 3 of palp with a group of 5 long setae near the middle of lateral border.
Gnathopod 1: (Fig. 8a, b) with sexual dimorphism; basis with numerouse long setae along posterior margin; merus with 3 tufts of simple setae along posterior margin; propodus suboval, 2 times as long as wide, palm oblique, 1.8 times as long as posterior margin, with a curved crest in the front 7/9 of the palmar margin, with 7 groups of long setae along posterior margin, anterior margin without setae, palmar corner with a cluster of 4 strong setae (spines) associated with a few thin setae, palmar crest with 4 rows of articulated structures: outer side with sparse setae (isolate or forming loose groups); medial side with row of narrow hooked cuspidate setae and a row of bigger and stout bilobed spineformed spinulose setae (each have 1 strong spinule on the tip) and with a row of spinulose long and stout setae (most of these spinulose setae have 2–4 anterior and 1 posterior strong spinules).
Gnathopod 2: (Fig. 8c); basis long, 1.2 times as long as coxa; carpus without setae along anterior margin; propodus with 7 groups of posterior and posterodistal setae; palm oblique, convex, short, 0.5 times as long as posterior margin, palmar margin with two rows of simple short setae.
Pereopod 3: same as in male, coxa 1.8 times as long as wide, rounded anterodistally, with oblique ½ of posteror margin.
Pereopod 4: same as in male.
Pereopod 5: (Figs. 9a, b) basis regular, anterior margin with 7 small stout setae (spines); merus with 3 groups of spinules along anterior margin; carpus with single group of 2 strong setae at the anterior margin; propodus with two groups of 2–3 short stout setae at the distal part of anterior margin.
Pereopods 6–7: same as in male.
Coxal gills 2–6 same as in male.
Epimeral plates: (Fig. 9d, e, f) same as in male; plates 1 and 2, anterior part of ventral margin with a row of 1– 2 strong simple setae.
Pleopods: same as in male; coupling basis of inner ramus with 2–4 specific stout two-pointed plumose setae along inner margin: pleopod 1 (4+1 simple), pleopod 2 (3), pleopod 3 (2+2 simple).
Uropod 1: (Fig. 9g) outer ramus with 3 outer and 3 medial mediumsized spines; inner ramus with 5 spines on tip.
Uropod 2: (Fig. 9h) peduncle with one distal dorsomedial spine at the base of distal procession only.
Uropod 3: (Fig. 9i) sexually dimorphic, short, 1.25 times as long as uropod 2 and 0.75 times as long as uropod 1; peduncle 1/2 length of outer ramus; outer ramus 2-segmented, article one slightly shorter than inner ramous in length, with 5 groups of 1 to 2 spines along outer margin and with 2 spines along distal part of medial margin, article two 0.125 x as long as article one; inner ramus is narrow lanceolate, with 5 spines along medial margin and with single spine on the outer margin near the tip, with small ventral spine.
Telson: similar to that in male.
Sexual dimorphism: Only in structure and palmar armament of pereopod 1 and in the length and structure of uropod 3.
Etymology. Species name caecus is based on a Latin word caecus (blind). The name is feminine in gender.
Ecology. S. caecus was found in the depth interval 109–309 m on gravel-silt, silt-gravel and silt bottoms. Females with oostegites detected on the shelf of northeastern Sakhalin in September 2016 and November 2013.
Distribution. The northern part of Okhotsk Sea, the shelf of north-eastern Sakhalin Island (Fig. 1).
Remarks. S. caecus differs from S. longirostris by the absence of eyes, in the armament of dorsal surface of pleosome and urosome and in the shape of propodus of pereopod 1. In S. caecus pleon segments 2–3 with 10–12 teeth posterodorsally; in S. longirostris pleon segments 1–3 without posterodorsal teeth. In S. caecus urosomite 1 with 7 posterodorsal teeth; urosomite 2 with 5 posterodorsal teeth. Urosomite 1 of S. longirostris with 3 posterodorsal teeth (median tooth the longest); urosomite 2 with 3 posterodorsal teeth (median tooth the longest). In S. caecus the palmar margin of propodus of pereopod 1 (gnathopod 1) oblique, as long as posterior margin; in S. longirostris the palmar margin of propodus of pereopod 1 vertical, much longer than posterior margin.
d’Udekem d’Acoz (2010) noted in S. longirostris a spoon-like process ‘hooding’ a reduced unguis on the dactylus of pereopods 3–4 analogous as in Oedicerotidae. But in S. caecus on exemplars with a broken unguis of dactylus of pereopods 3-4 is clearly visible the specific paddle-shaped seta covering the nail (Fig. 7e, f).
description: Published as part of Labay, Vjacheslav S., 2017, A new species of Sextonia Chevreux, 1920 (Crustacea: Amphipoda: Liljeborgiidae) from the Okhotsk Sea, pp. 506-520 in Zootaxa 4353 (3) on pages 507-519, DOI: 10.11646/zootaxa.4353.3.6, http://zenodo.org/record/1065824