Amblyops magnus Birstein & Tchindonova 1958

Amblyops magnus Birstein & Tchindonova, 1958

(Figs 1–7)

Birstein & Tchindonova 1958: 319–321, Fig. 36 (the Kurile-Kamchatka Trench, 49°23' N, 158°30' E, 7120–7260 m); Mauchline & Murano 1977: 46 (in checklist); Belyaev 1989: 189 (in checklist); Müller 1993: 75 (in checklist); Fukuoka 2009: 421–422, Fig. 9 (the Japan Trench, 37°00.6'–36°56.3' N 142°57.4'–142°58.9' E, 5000–5268 m, 41°22.1'–41°19.7' N 144°29.1'–144°24.6' E, 4480–4820 m); Murano 2012: 82–84 (key).

Material examined. 1 adult female (carapace length 13 mm, total length ca. 35 mm), IDSSE Lab-2016- 0 616.001, Genbank accession number MF 062486, R/V 'Xiangyanghong 9', St. JL-Dive116-S11, the Mariana Trench, 10°57' N 141°56.3' E, 6555 m, collected by baited trap, coll. Y. Wang, 16 June 2016.

Material for comparison. 1 adult female (total length 27.6 mm), NSMT-Cr 19673, R/V ‘ Soyo-maru’, St. S 007-04, off Onahama, Tohoku, Japan, 36°54.0'– 36°55.5' N 142°56.6' – 142°57.3' E, 5219–5268 m, collected by benthos net, 7 August 2007; 1 immature female (total length 20.6 mm), NSMT-Cr 19674, R/V ‘ Soyo-maru’, St. SO-SE Erimo, SE Erimo, Hokkaido, Japan, 41°22.1' –41°19.7' N 144°29.1'– 144°24.6' E, 4480–4820 m, collected by benthos net, 25 June 1992 (Fukuoka 2009).

Description. (The present specimen, adult female, IDSSE Lab-2016-0616.001) Large-sized mysid shrimp with subcylindrical body form.

Carapace (Figs 1a, b, 5a) of thin texture, soft, glabrous on surface; anterior margin evenly rounded, without distinct rostral projection, leaving most of eyeplates exposed; cervical groove and transverse groove behind eyeplates distinct; anterolateral corner slightly pointed in lateral view; posterior margin emarginate, exposing last two thoracic somites dorsally.

Eyes (Figs 1a–d, 5a) plate-like without visual elements, separated from each other, almost reaching to distal margin of first segment of antennular peduncle; lateral margin about 3 times as long as mesial margin, maximum width about 1.9 times as length of lateral margin; sub-equilateral triangular process situated at middle of anterior margin, with discernible tiny cavity at tip; margins and dorsal surface inconspicuously spinulose, ventrolateral surface densely furnished with minute spinules.

Antennular peduncle (Figs 1b, e, 5a) robust; first segment wider than long, with distolateral corner prolonged and tipped with several long setae; second segment rather short, dorsal surface and anteromesial margin with several setae, inner margin armed with one plumose seta; third segment sub-rectangular, almost as long as wide, mesial margin armed with many (more than 15) long plumose setae.

Antennal scale (Figs 1b, f–i, 5a) rather long, extending beyond distal end of antennular peduncle by about 2/3 of its length, about 2.8 times as long as maximum width at about proximal 1/3, gradually narrowing from widest part to distal end, with distinct submedian longitudinal vein throughout whole length; lateral margin naked, slightly concave at middle part, terminating in denticle extending slightly beyond apex of blade; terminal denticle without subsidiary spinule near basal end of its inner margin; distal suture feeble, terminal lobe inconspicuous and very small. Antennal peduncle (Fig. 1f, j, k) 4-segmented, almost as long as antennular peduncle; second segment short and narrow, fused with third segment on ventral side; third and fourth segments almost equal in length, both with setae at distodorsal corner and on lateral margin. Antennal sympod (Fig. 1b, f) armed with stout tooth at anterolateral corner.

Mandible (Fig. 2a) robust, palp 3-segmented; second segment broad, leaf-like, lateral margin armed densely with long setae, mesial margin with two rows of shorter setae; third segment about half as long as second, about 3.5 times as long as broad, ending with one prominent spine, inner margin armed densely with bristle on distal 2/3, mesial margin with two rows of long setae; pars incisiva with 4 (left) or 5 (right) strong teeth; lacinia mobilis with 2 large teeth and 3 small teeth (left) or rudimentary (right); pars centralis with row of spinose teeth; pars molaris with strongly grinding surface, margin with numerous tiny teeth, distal end with a brush of fine setae. Maxillule (Fig. 2b), basis with 14 strong spines on distal margin and four setae on ventral surface; coxa with about 12 long plumose setae on distal margin and eight setae on ventral surface; praecoxa with dense setae on lateral marigin. Maxilla (Fig. 2c), exopod about 2.5 times as long as broad, outer margin fringed with about 25 long plumose setae; endopod with second segment 2.2 times longer than broad, outer margin fringed with about 30 long setae; coxa and basis with outer margins bearing dense long setae. Labrum (Fig. 2d) asymmetry, width about 1.6 times length, anterior margin rounded, posterior margin with many minute spinules. Labium (Fig. 2e) symmetrical, anterior margin rounded, with numerous fine setae on mesial surfaces.

First thoracopodal with short lanceolate epipodite, endopod (Fig. 3a, b) robust, dactylus about half as long as carpopropodus, terminating in strong claw 0.8 times as long as dactylus. Second thoracopodal endopod (Fig. 3c, d) rather slender, merus longest, 5 times as long as wide; carpopropodus about 0.9 times as long as merus, 6 times as long as wide; dactylus about half as long as carpopropodus, tapering, with numerous stout setae on surface, terminal claw slender, about 0.4 times as long as dactylus. Third thoracopodal endopod (Fig. 3e) slender, ischium with lateral margin armed with several setae, merus almost as long as carpopropodus, about 10 times as long as wide; carpopropodus 3-subsegmented, about 12 times as long as wide, distal and middle subsegments subequal in length, about 0.3 times as long as proximal one and longer than combined length of dactylus and terminal claw; dactylus small, about 0.7 times as long as terminal claw. Fourth and fifth thoracopods missing. Sixth thoracopodal endopod (Fig. 3f) similar to that of third thoracopod in general, but more robust and hirsute, with oval, rudimentary oostegite furnished with long setae on outer margins. Seventh thoracopodal endopod (Fig. 3g) longest, with welldeveloped oostegite; ischium 1/3 as long as merus, 2.5 times as long as wide; merus slightly shorter than carpopropodus, about 9 times as long as wide; carpopropodus 3-subsegmented, about 17 times as long as wide, distal and middle subsegments subequal in length, about 0.2 times as long as proximal one and longer than combined length of dactylus and terminal claw; dactylus about 0.4 times as long as terminal claw. Eighth thoracopodal endopod (Fig. 3h) generally similar to that of seventh thoracopod, but slightly slender and shorter, with broad oostegite. Exopod of thoracopods (Fig. 3a, c, e–h) with 15 or 16-segmented flagellum, basal plate with pointed outer distal corner.

First, second, fourth and fifth abdominal somites subequal in length, third somite about 1.3 times longer than second, six somite about 1.6 times longer than fifth (Fig. 4a); first and second somites both with posterolateral margins rounded; third to fifth somites with posterolateral margin straight, fourth and fifth somites with tiny notch on posterolateral margins; six somite with posterolateral margin ending in a large lobe with acute tip.

All female pleopods uniramous, unsegmented.

Uropod long (Figs 4b, c, 5b); endopod tapering distally, armed with four slender, acute spines on ventral surface in statocyst region; exopod with distal margin truncated, overreaching end of endopod by about 1/3 of its length.

Telson (Figs 4b, c, 5b) broken off from about proximal 1/3, with distinct longitudinal groove in midline on dorsal surface, lateral margins of remaining part without spines.

(Materials for comparison, adult female, NSMT-Cr 19673; immature female, NSMT-Cr 19674) Generally agree with the description of the holotype and the present specimen. Both with telson complete (Fig. 6; see Fukuoka 2009, Fig. 9e), narrowly trapezoidal, about 1.4 times as long as sixth abdominal somite, 2.1 times as long as maximum width near base, sharply constricted at about proximal 1/4, then gradually narrowing towards truncate apex; lateral margin armed with 19 or 20 spines on posterior 3/5, increasing in length posteriorly; posterior margin armed with three pairs spines, innermost pair very small, middle pair longest, about 1/9 of telson length, outer pair slightly shorter than middle pair; pair of plumose setae present, about half length of middle spine pair, situated on dorsal surface between minute innermost spines.

Coloration. Body and thoracopods generally translucent ivory, mouthpart reddish.

Remarks. Despite the telson being incomplete, the present specimen is almost in perfect accordance with the original description of Amblyops magnus Birstein & Tchindonova, 1958. There are however some notable differences, which are as follows:

1. There is a tiny terminal lobe on the antennal scale in the present specimen, while it is absent in the holotype and the comparison materials of A. magnus (Fig. 7a, b). However, the distal suture is feeble in the present specimen, making the terminal lobe inconspicuous. We presume that this character is correlated with the preservation conditions of the individuals, or that an intraspecific variation has been revealed in the limited number of individuals that have been collected and examined.

2. The anterior margin of the carapace is much more rounded and covers almost the entire mesial margins of eyeplates in comparison materials (see Fukuoka 2009, Fig. 9a), while it is evenly rounded and only covers a small part of the eyeplates in the present specimen.

3. The antennal scale is much more slender (length-width ratio 3.8, based on single complete antennal scale) and the lateral margin is slightly arched in comparison materials, while it is stouter (length-width ratio 2.8–3.0) and the lateral margin is slightly concave at middle part in the present specimen and the holotype (see Birstein and Tchindonova, 1958, Fig. 36).

4. The terminal denticle of the antennal scale is armed with a subsidiary spinule near base of the inner side in comparison material (Fig. 7a, b). In contrast, the terminal denticle lacks a subsidiary spinule in both the present specimen and the holotype.

5. The flagellum of thoracopodal exopods is 9 or 10-segmented in comparison materials, rather than 15 or 16- segmented in the present specimen and the holotype.

6. Endopod of the uropod is armed with six spines on ventral surface in statocyst region in comparison materials, while there are three spines in the holotype and four spines in the present specimen.