Megamoera similidentata Labay, 2013, sp. nov.

Megamoera similidentata sp. nov.

Figures 1–9, 34 a

Diagnosis. Pleon segments 1–3 with central tooth and 0–3 lateral denticles on each side; pleons 2 and 3 with large central tooth and 3 denticles, first of which is the smallest (may be reduced). Urosome segment 1 with central tooth and 1 lateral denticle; urosome 2 with 2 pairs of short teeth and single cuspidate seta. Telson lobes, proximal subapical notch more on outer margin. Anterior head lobe shallow, lower margin with prominent accessory process. Pereopod 1 (male) broadest distally, palm oblique, sharply angled from posterior margin. Pereopod 2 (male), anterior margin of dactyl strongly setose; palm oblique, convex, with low irregular hinge tooth and stout posterodistal tooth. Maxilla 1, palp segment 1 with 7–8 lateral setae. Maxilla 2, inner plate, facial setae reduced, marginal in position. Uropod 3, proximal segment of outer ramous slender, with 5 clusters of marginal stout simple setae.

Type material. Holotype male, 34 mm, X 37855 Cr 1421, the north-east shelf of Sakhalin Island, Okhotsk Sea, Far East of Russia (51°42'N 143°53'E, 87 m), silt and sand, M.G. Rogotnev & P.V. Polupanov, 0 9 July 2010. Paratypes: 1 female, 28 mm, X 37856 Cr 1422; 2 males and 1 female X 37857 Cr 1423 with same data as holotype; 1 male X 37858 Cr 1424, the north-east shelf of Sakhalin Island, Okhotsk Sea, Far East of Russia (52°25'N 143°51'E, 101 m), sand, M.G. Rogotnev.

Type locality. The north-east shelf of Sakhalin Island, Okhotsk Sea, Far East of Russia (51°42'N 143°53'E, 85–90 m).

Description. Male (34 mm). Vital body color is grey-olive with an unclear picture of the diffuse bands and spots. Thoracic segments smooth dorsally, without teeth and humps (Fig. 1 a, b). Pleon segments 1 with centre tooth, without lateral denticles; pleons 2 and 3 with large centre tooth and 3 denticles, first of which is unclear. Urosome segment 1 with central tooth and 1 lateral denticle; urosome 2 with 2 pairs of short teeth and single cuspidate seta.

Head: (Fig. 1 c) slightly shorter than pereonites 1 and 2 together, with an indistinct rostrum; eyes medium oval, anterior head lobe shallow, inferior margin with prominent lobe; inferior antennal sinus with a short notch.

Antenna 1: (Fig. 1 d) 60% of body length, slender; flagellum exceeding 1.3 length of peduncle, 48–49 articles, few very thin and short setae scattered along flagellum; peduncle article 1 double width but slightly shorter length to article 2, article 3 0.25 of article 2 length; accessory flagellum 5-articulate.

Antenna 2: (Fig. 1 e) flagellum almost 1/3 times as long as peduncle, 15–16 articles, with numerous thin setae on each article; peduncle articles 5 shorter than article 4, articles 1 + 2 + 3 shorter than article 5; article 2 with protruding lobe reaching the middle of next article; article 3 with short protruding lobe; article 4 and 5 with numerous evenly spaced tufts of thin setae.

Mouthparts.

Labrum: (Fig. 1 f) rounded, frontal margin slightly concave medially, densely covered by minute setae.

Mandible: (Fig. 2 a, b) incisor crenulate and stretching in a rounded tip; raker setae stout subplumose, numerous (14+), shorter then incisor; molar cylindrical triturative, chewing plate striate; palp 3-articulate, palp segments 2 and 3 strongly setose, 1-st article with protruding lobe, the percentage of total length of the articles of the palp 14% (article 1), 36–43% (article 2) and 43–50% (article 3).

Labium: (Fig. 2 c) inner lobes small but well defined, rounded; outer lobes semi-square at distal margin.

Maxilla 1: (Fig. 2 d, e) inner plate subtriangular, lined with a row of simple setae; outer plate elongate, 1.3 as long as inner plate, apically with 2 rows of 9 apically bifurcate strong setae; palp 2-articulate, article 2 2.3times as long as article 1, article 2 apically with two rows of 12–13 simple setae each.

Maxilla 2: (Fig. 2 f) inner plate slightly shorter than outer plate, facial setae of inner plate reduced (16–20), distal, closely submarginal, tip covered with numerous simple setae; outer plate with same numerous of simple setae, but only on the tip, subapically with a rare row of 10–12 setae.

Maxilliped: (Fig. 3 a) compact; inner plates large, fully cleft, distal margin with a row of 4 teeth, inner and distal margins with long plumose setae; outer plates reaching of ¾ of palp article 2, inner margin lined with a row of short stout teethlike setae longer toward the apex, smoothly into the apical row of long thin plumose setae; palp 4-articulate, the percentage of total length of the articles of the palp are 14% (article 1), 44% (article 2), 24% (article 3), 18 (article 4), article 2 with numerouse setae along distal part of inner margine; tip of article 3 bilobed with a row of long simple setae, densely pubescent along apical impression; article 4 inner margin with a line of serrate setae (Watling type IV.1) (Watling 1989).

Pereopod 1: (Fig. 3 b, c) coxa subrectangular, slightly extended distally; basis straight with a dense row of simple setae along anterior margins and more rare long setae along posterior margin; merus subrectangular, with a 4 tufts of plumose setae along posterior margin and with a cushion of short setae along distal part of posterior margin, distal margin with a row of long simple setae; carpus long, 1,5 times as long as propodus, 12–13 oblique rows of long serrate setae along outer posterior margin, with a row of long setae along anterior and posterior parts of distal margin and tomentose at the anterior-distal corner; propodus suboval, broadest distally, with numerous groups of long serrate setae along outer posterior margin, with 6 tufts of long simple setae and with a dense bunch of long simple setae at the anterior distal corner, palm oblique and minutely serrate with a row of short thin simple setae and rare long setae along palmar margin, sharply angled from posterior margin; dactylus as long as palm, curved, with a row of spines along posterior margin, with nail and with tooth-like protrusion as long as nail above it.

Pereopod 2: (Fig. 3 d, e) coxa rounded subrectangular, slightly narrowing distally; basis straight, longer than coxa, with a few long simple setae along posterior margin and with numerous long setae along anterior margin; merus subrectangular with long acute protrusion at the posterior distal corner, with a few long setae along posterior margin; carpus triangular, 2 times shorter than propodus; propodus subrectangular, palm oblique, convex, with low irregular hinge tooth and stout postero-distal tooth, inner face of anterior margin lined with several groups of serrate setae, posterior margin with numerous tufts of serrate setae, palm with small simple setae, and with a dense cluster of serrate setae; dactylus as long as oblique part of palm, curved to the inside of propodus, inner margin with a rare row of short simple setae, outer margin strongly setose, tip of dactylus with obtuse nail.

Pereopod 3: (Fig. 4 a) coxa similar size as coxa 2, suboval; basis long linear with few minutely setae along anterior margin and with rare long simple setae along posterior margin; merus long with 3 strong simple setae along anterior margin and with a cluster of 2 strong simple setae at the anterior distal corner, posterior margin with few long setae; carpus ¾ as long as merus, with 3 bunches of strong simple setae along posterior margin; propodus linear, more narrow and as long as carpus, with 4 bunches of strong simple setae along posterior margin; dactylus medium (1/2 of propodus), with nail.

Pereopod 4: (Fig. 4 b) coxa as long as coxa 3, subquadrate, slightly broaded distally, posteriorly slightly excavated; leg is similar to that of the pereopod 3.

Pereopods 5–7 elongated from pereopod 5 to pereopod 7.

Pereopod 5: (Fig. 4 c) coxa with rounded front margin, with quadrate hind margin and with anterior lobe pulled back and down; basis regular, with posterior wing, slightly narrowed distally, width 2/3 as long, posterior distal corner rectangular, hind margin crenulated, anterior margin with small stout setae; ischium subquadratic; merus slightly broaded medially, with a single strong stout seta at the middle of posterior margin, and with a cluster of 2 strong stout setae at the posterior distal corner; carpus 0.8 as long as merus, with 2 groups of 2 strong stout setae at the anterior margin and with a single clusters of strong stout setae at the anterior and posterior distal corners each; propodus linear, more narrow and as long as carpus, with 2 bunches of strong simple setae along posterior margin; dactylus small (1/3 of propodus), with nail.

Pereopod 6: (Fig. 4 d) coxa with quadrate hind margin and with anterior lobe pulled back and down; basis regular, with posterior wing, narrowed distally, width 0.56 as long, posterior distal corner acute, hind margin crenulated, anterior margin with small stout setae; ischium subquadratic; merus long, with few short stout simple setae along anterior and posterior margins, and with single clusters of 2-3 strong stout setae at anterior and posterior distal corners each.

Pereopod 7: (Fig. 4 e) coxa small, without distinct anterior lobe and with rounded posterior lobe; basis regular, with posterior wing, narrowed distally, width 0.6 times length, posterior distal corner acute, hind margin crenulated, anterior margin with small setae; ischium subquadratic; merus similar to that of the pereopod 6.

Coxal gills 2–6. Coxal gill 6 slightly smaller than gill 5.

Epimeral plates: (Fig. 5 a, b, c) plates 1 and 2, hind corner acuminate; plate 1 rounded triangular with 1 diagonal ridge, posterior margin slightly convex with 4–5 notches; plate 2 posterior and ventral margins slightly convex, with oblique crest, posterior margin with 6–7 notches, ventral margin with a row of notches and 4–5 small stout setae; plate 3 hind corner moderately produced, acute, posterior margin concave, with 2 notches, ventral margin with a row of notches and 10 small stout setae.

Pleopods: (Fig. 5 d, e, f) normal, no sexual dimorphism; peduncle with two specific toothed coupling spines on the inner corner; coupling basis of inner ramus with 9 of specific stout two-pointed plumose setae along inner margin.

Uropod 1: (Fig. 5 g) peduncle with strong and curved interramal spur; rami subequal in length, slightly shorter than peduncle length; peduncle and rami with tiny simple setae along outer edge, both rami with few tiny simple setae at tip.

Uropod 2: (Fig. 5 h) peduncle shorter than outer ramus (0.73 ramus length); inner ramus shorter than outer, with numerous simple cuspidate setae along outer margin; uropod 2 shorter than uropod 1.

Telson: (Fig. 5 i) fully cleft; lobes slender, diverging distally; proximal notch positioned laterally; apical and subapical simple cuspidate setae long.

Female (28 mm). Vital body color grey-olive with an unclear pattern of diffused bands and spots (Fig. 8 d, e, f). Thoracic segments smooth dorsally, without teeth and humps (Fig. 6 a, b). Pleon segments 1 with large central tooth, with few indistinct lateral denticles on each side; pleons 2 and 3 with central tooth and 3 denticles. Urosome segment 1 with central tooth and 1 lateral tooth; urosome 2 with 2 pairs of teeth and single cuspidate seta.

Head: (Fig. 6 a) similar to as in male.

Antenna 1: (Fig. 6 c) 57% of body length, slender; flagellum exceeding 1.3 length of peduncle, 44 articles, few very thin and short setae scattered along flagellum; peduncle article 1 twice as wide but slightly shorter than article 2; article 3 0.25 times as long as article 2; accessory flagellum 4-articulate.

Antenna 2: (Fig. 6 d) similar same to as in male.

Mouthparts: (Figs. 6 e, f, g, h, i; 7a, b, c, d) same as in male.

Pereopod 1: (Fig. 7 e, f) without sexual dimorphism, same as in male.

Pereopod 2: (Fig. 8 a) with sexual dimorphism; coxa rounded subquadrate, slightly narrowed distally; basis straight, longer than coxa, with a few long simple setae along posterior margin and with numerous long setae along distal part of anterior margin; merus subrectangular, with long acute protrusion at the posterior distal corner, with a few long setae along posterior margin; carpus broadened distally, 1.7 times shorter than propodus, posterior lobe slightly produced with numerous serrate setae; propodus subrectangular, slightly broadened distally, palm oblique, convex, with stout posterodistal tooth, posterior margin with numerous tufts of serrate setae, palm with small simple setae, and with a dense cluster of serrate setae; dactylus as long as oblique part of palm, curved to the inside of propodus, inner margin with a rare row of short simple setae, outer margin strongly setose, tip of dactylus with acute nail.

Pereopods 3 and 4: (Fig. 8 b, c) same as in male.

Pereopods 5–7 elongated from pereopod 5 to pereopod 7.

Pereopod 5: (Fig. 8 d) coxa with rounded front margin, quadrate hind margin and anterior lobe pulled back and down; basis regular, with posterior wing, slightly narrowed distally, width 2/3 of length, posterior distal corner rectangular, hind margin crenulated, anterior margin with small cuspidate setae; ischium subquadratic; merus slightly broadened medially, with two cuspidate setae along posterior margin, and with a cluster of 2 strong stout setae at the posterior distal corner; carpus 0.8 times as long as merus, with 2 groups of 2 strong stout setae at the anterior margin and with a single clusters of strong stout setae at the anterior and posterior distal corners each; propodus linear, more narrow and as long as carpus, with 4 bunches of cuspidate setae along posterior margin; dactylus small (0.25 of propodus), with nail.

Pereopod 6: (Fig. 8 e) coxa with quadrate hind margin and with anterior lobe pulled back and down; basis regular, with posterior wing, narrowed distally, width 0.56 of length, posterior distal corner rectangular, hind margin crenulated, anterior margin with small cuspidate setae; ischium subquadratic; merus long, with 2–3 cuspidate setae along anterior and posterior margins, and with single clusters of 2−3 strong stout setae at anterior and posterior distal corners each; carpus 0.8 as long as merus, with 3 groups of 2 cuspidate setae at the anterior margin, with a single cluster of cuspidate setae at the posterior margin and with single clusters of cuspidate setae at the anterior and posterior distal corners each; propodus linear, more narrow and as long as carpus, with 4 bunches of cuspidate setae along posterior margin; dactylus small (0.25 of propodus length), with nail.

Pereopod 7: (Fig. 8 f) coxa small, without distinct anterior lobe and with rounded posterior lobe; basis regular, with posterior wing, narrowed distally, width 0.6 length, posterior distal corner acute, hind margin crenulated, anterior margin with small cuspidate setae; ischium subquadratic; merus and carpus similar to that of pereopod 6; propodus linear, more narrow and as long as carpus, with 6 bunches of cuspidate setae along posterior margin; dactylus small (0.3 of propodus length), with nail.

Coxal gills 2–6. Coxal gill 6 slightly smaller than gill 5.

Epimeral plates: (Fig. 8 g, h, i) plates 1 and 2, hind corner minutely acuminate; plate 1 rounded triangular with 1 diagonal ridge, posterior margin slightly convex with 4–5 notches; plate 2 posterior and ventral margins slightly convex, with oblique crest, posterior margin with 4–5 notches, ventral margin with a row of notches and 4–5 small cuspidate setae; plate 3 hind corner moderately produced, acute, posterior margin concave, with 4 notches, ventral margin with a row of notches and 6–9 cuspidate setae.

Pleopods (Fig. 9 a–g): normal; peduncle with two specific toothed setae on the inner corner; inner ramus slightly longer than outer, basis of inner ramus with 10 (pleopod 1) or 7 (pleopods 2 and 3) specific stout twopointed plumose setae along inner margin.

Uropod 1: (Fig. 9 h) peduncle with strong and curved interramal spur; inner ramus slightly shorter than outer, subequal to peduncle length; peduncle and rami with numerous tiny simple setae along outer edge, both rami with few tiny simple seta at tip.

Uropod 2: (Fig. 9 i) peduncle shorter than outer ramus (0.75 ramus length); inner ramus shorter than outer, with numerous tiny simple cuspidate setae along outer margin both; uropod 2 shorter than uropod 1.

Uropod 3: (Fig. 9 j) peduncle shorter than peduncle of uropod 1, with two cuspidate setae along anterior margin, with one cuspidate simple seta in the middle of posterior margin and with a row of simple cuspidate setae at the apex; elongate 2-articulate outer ramus (8 times length of inner ramus, longer than urosome), with 6 tufts of small simple stout setae along the anterior margin and with 4 tufts of small simple stout setae along the posterior margin of article 1, article 2 minutely, slightly longer than crowning setae; inner ramous short and ovoid with one cuspidate seta at tip and with single cuspidate seta at the posterior margin.

Telson: (Fig. 9 k) similar to that in male.

Sexual dimorphism: Only in structure of pereopod 2. Pereopod 2 in male bigger than in female.

Variations: The dorsal tooth on epimeral segment 1 may be with or without small indistinct teeth laterally.

Etymology. Species name similidentata is based on a Latin word simili (similar) and the species name dentata, which is explained by similarity of habit of the new species and of M. dentata.

Ecology. M. similidentata was found in the depth interval 85–90 m on the silt-sand bottom in the layer of constant low temperature (-1.2 to +0.5°C).

Distribution. Only type locality. The north-east shelf of Sakhalin Island, Okhotsk Sea.

Remarks. Megamoera similidentata sp. nov. is most similar to M. dentata (Krøyer, 1842), M. rafiae Jarret & Bousfield, 1996, M. bowmani Jarret & Bousfield, 1996, M. glacialis Jarret & Bousfield, 1996 and M. borealis Jarret & Bousfield, 1996. Megamoera similidentata differs from M. dentata in having subacute processes on the low margin of anterior head lobe, in the form of dactylus of pereopod 1 (dactylus with nail and with tooth-like protrusion as long as nail above it). It differs from M. rafiae in the size of central dorsal tooth of pleon segments 1– 3 (more than lateral teeth), central tooth of urosome segment 1 not vertical, in the shape of propodus of pereopods 1 and 2, in the shape and armament of telson. Although the dorsal armament of pleon segments 1 and 2 and of urosome segments 1 and 2 are more similar between M. similidentata and M. bowmani, the absence of the medial dorsal tooth of pleon segment 3 and the shape of telson M. bowmani distinguishes these species. Megamoera similidentata differs from M. glacialis in a number of dorsal lateral teeth of pleon segments 1–3 (3, not 2), in the form of propodus of gnathopod 1 and in the form of telson. It differs from M. borealis in having of subacute processes on the low margin of anterior head lobe, and in a number of dorsal lateral teeth of pleon segments 2–3 (3, not 2).