Metacrangon robusta ( Kobjakova, 1935 )

Komai, Tomoyuki, 2012, 3468, Zootaxa 3468, pp. 1-77 : 38-41

Metacrangon robusta ( Kobjakova, 1935)

( Figs 19, 20, 40)

Sclerocrangon robustus Kobjakova, 1935: 85 , figs 1, 2 [type locality: Peter the Great Bay, Sea of Japan, 70–1380 m; and St. Jonas Island, Okhotsk Sea, 50–515 m]; Derjugin & Kobjakova 1935: 224; Kobjakova 1936: 190 (list), 201 (list), 224 (key), fig. 36; 1937: 135; 1958: 244; Igarashi 1969: 10, pl. 11, fig. 32, pl. 19, fig. 61a, b.

Nectocrangon robusta . — Vinogradov 1947: 94 (key), 95; 1950: 220 (key), pl. 21, fig. 89.

Metacrangon robusta . — Zarenkov 1965: 1764, fig. 4; Burukovsky 1974, fig. 168; Miyake 1982: 189 (list); Komai et al. 1992: 195 (list); Komai 1994: 83 (list), 97; De Grave & Fransen 2011: 458 (list).

Material examined. Japan. JAMARC survey of the Sea of Okhotsk 1992: stn S37-2, Kitami-Yamato Bank, 44°29.9’N, 144°06.7’E, 225 m, 26 July 1992, coll. D. Tsutsui, 2 females (cl 9.1, 11.0 mm), CBM-ZC 4903. JAMARC survey of the Sea of Okhotsk 1993: stn B13, Kitami-Yamato Bank, 44°30.1’N, 144°14.5’E, 208 m, 6 June 1993, otter trawl, coll. M. Yabe, 4 females (cl 10.6–12.0 mm), CBM-ZC 519; stn C9, Kitami-Yamato Bank, 44°17.0’N, 144°17.8’E, 287 m, 7 June 1993, otter trawl, coll. M. Yabe, 4 ovigerous females (cl 11.3–13.7 mm), CBM-ZC 529 .

Type material. According to Kobjakova (1935), specimens used in the original description of this species came from the Sea of Japan ranging from Peter the Great Bay to 50°N, and the northwestern part of the Sea of Okhotsk off St. Jonas’ Island (= Iony Island ). The number of specimens used in the original description was not specifically indicated; no holotype was designated. Therefore , specimens mentioned in the type description should be syntypes. It is suspected that the syntypes were deposited in the Zoological Institute, St. Petersburg, but no information on them is available .

Redescription. Rostrum ( Figs 19A, B, E, 20A) subtrapezoidal in dorsal view, strongly ascending (angle against horizontal plane of carapace about 70°), overreaching antennal teeth (slightly less than 0.2 times as long as carapace); rostral tip roundly truncate; dorsal surface shallowly concave in anterior half; ventral carina thick and high. Carapace ( Figs 19A, B, E, 20A) widened posteriorly in spawning molt, slightly longer than wide postorbitally; middorsal carina extending from midlength of rostrum to posterior 0.2 of carapace, with 2 small middorsal teeth; anterior tooth located at midlength of rostrum, blunt; posterior tooth sharp, arising at about 0.6 of carapace length; no minute denticle between rostral base and middorsal tooth; submedian teeth present; antennal tooth ascending in similar degree to rostrum, rounded or bluntly pointed; postorbital angle slightly produced; anterolateral margin between antennal and branchiostegal teeth with minute denticle; branchiostegal tooth moderately strong, directed forward in dorsal view, ascending in lesser degree to rostrum in lateral view, overreaching dorsolateral distal angle of antennal basicerite; branchiostegal carina curving, reaching level of posterior end of hepatic groove; branchial ridge absent.

Abdomen ( Fig. 19C, D) barely sculptured. First and second somites rounded dorsally; third and fourth somites with trace of middorsal carina. Fifth somite with distinct middorsal carina and 2 blunt ridges laterally. Pleura of anterior three somites rounded, fourth pleuron with rounded anteroventral angle. Sixth somite with distinct submedian carinae; posterodorsal margin produced, distinctly bilobed; posterolateral process terminating in acute tooth. Telson ( Fig. 19C) with 2 pairs of dorsolateral spines, first pair located at about midlength; posteromedian process flanked by 3 pairs of stout, setulose setae in addition to pair of minute spinules ( Fig. 20L).

Eye ( Fig. 19E) stout, slightly longer than wide; cornea not wider than eyestalk, its width about 0.1 of carapace length; eyestalk notably widened basally, bearing rounded dorsal tubercle.

Antennular peduncle ( Figs 19E, 20A) distinctly overreaching midlength of antennal scale. First segment with prominent, blunt distolateral process, directed forward; stylocerite far falling short of distolateral process of first segment, broad, terminating in short, blunt tooth, lateral margin gently convex to slightly angular. Second segment widened distally, with prominent, blunt distolateral process. Outer flagellum overreaching lamella of antennal scale by 0.5–0.6 length, consisting of 7–9 articles.

Antenna ( Figs 19E, 20A) with basicerite having dorsodistal lateral angle produced in rounded or triangular lobe, ventrolateral tooth acute. Antennal scale short, 0.4 times as long as carapace and about 2.5 times longer than wide; lateral margin concave; distolateral tooth moderately broad, falling short of distal margin of lamella; lamella gently rounded.

Third maxilliped ( Fig. 20B) stout; ultimate segment gradually tapering distally, about 3.7 times longer than wide; penultimate segment about 1.6 times longer than wide.

First pereopod ( Fig. 20C, D) with palm about 2.5–2.9 times longer than wide, slightly narrowing distally, lateral and mesial margins faintly sinuous; carpus with distolateral tooth; merus with small dorsodistal tooth, ventral margin sinuous, crested. Second and third pereopods typical of genus ( Fig. 20E, F); third pereopod overreaching distal margin of antennal scale by length of dactylus. Fourth pereopod ( Fig. 20G) stout, reaching distal margin of antennal scale by tip of dactylus; dactylus ( Fig. 20H) broadly spatulate with strongly convex dorsal and very slightly convex ventral margins, about 2.5 times longer than broad. Fifth pereopod ( Fig. 20I) with dactylus spatulate, 3.3–3.4 times longer than broad, about 0.7 times as long as propodus, shorter than that of fourth pereopod ( Fig. 20J); merus-ischium combined 0.6–0.7 times as long as that of fourth pereopod.

Uropodal exopod with laterally pointed posterolateral tooth; no spinule mesial to posterolateral tooth ( Fig. 20K).

Eggs 2.4 x 2.0 mm; not counted.

Male specimens unavailable.

Coloration. Not known.

Distribution. Sea of Japan, ranging from Peter the Great Bay to 50°N, and Sea of Okhotsk ( Fig. 40); 50–1380 m ( Kobjakova 1935, 1937; Vinogradov 1950; this study).

Size. Largest female (spawning molt) cl 12.3 mm.

Remarks. Among the western Pacific species of the genus, Metacrangon robusta is characteristic in having the anterior middorsal tooth arising at the midlength of the rostrum. A similar position of the anterior middorsal tooth is seen in three species of the M. jacqueti species group, viz., M. hauna , M. knoxi and M. teina . The broadly spatulate dactylus of the fourth pereopod is also characteristic for M. robusta within the genus.