Metacrangon trigonorostris ( Yokoya, 1933 )

Komai, Tomoyuki, 2012, 3468, Zootaxa 3468, pp. 1-77 : 21-25

Metacrangon trigonorostris ( Yokoya, 1933)

( Figs 8–10, 40)

Crangon trigonorostris Yokoya, 1933: 37 , text-fig. 13 [type-locality: south of Sioya-zaki, Fukushima Prefecture, 390 m].

Metacrangon ? trigonorostris . — Zarenkov 1965: 1764.

Metacrangon trigonorostris . — Miyake 1982: 188 (list); Komai 1994: 97 (list); Komai & Komatsu 2009: 520; De Grave & Fransen 2011: 458 (list).

Argis trigonorostris . — Burukovsky 2003: 44, figs 1, 2.

Material examined. RV Soyo-maru , 1992 cruise, stn 5, E of Shimokita Peninsula, Aomori Prefecture, 41°39.4’N, 142°10.2’E, 1150–1170 m, 30 June 1992, benthos net, coll. H. Saito, 1 male (cl 6.0 mm), 1 female (cl 8.7 mm), CBM-ZC 10667 ; RV Tanshu-maru , 2002 cruise, stn D900, S of Konbu-mori, Kushiro, eastern Hokkaido, 42°37.7’N, 144°22.5’E, 902–906 m, 3 August 2002, sledge net, coll . T. Omura , 1 female (cl 6.2 mm), CBM-ZC 9545; stn H700, SE of Cape Erimo, eastern Hokkaido, 41°50.32’N, 143°50.00’E, 723–725 m, 2 August 2002, sledge net, coll . T. Omura, 1 female (cl 9.0 mm), CBM-ZC 9546 . RV Wakataka-maru, 2005 cruise, stn DE380D, E of Shizugawa, Miyagi Prefecture, 38°39.1’N, 142°02.2’E to 38°38.6’N, 142°02.1’E, 373–375 m, 19 November 2005, dredge, coll. H. Komatsu, 2 females (cl 4.5, 4.6 mm), NSMT-Cr 16896; stn DE410, similar locality, 38°39.3’N, 142°02.6’E to 38°40.9’N, 142°03.5’E, 404–407 m, 20 November 2005, trawl, 1 ovigerous female (cl 12.1 mm), NSMT-Cr 16894; stn E425, E of Kinkazan, Miyagi Prefecture, 38°24.1’N, 142°03.0’E to 38°00.2’N, 142°02.6’E, 424–425 m, 25 October 2005, 1 female (cl 10.4 mm), NSMT-Cr 16893; 2006 cruise, stn E480, E of Kinkazan, 38°22.7’N, 142°05.2’E to 38°21.2’N, 142°05.9’E, 480–484 m, 3 November 2006, trawl, coll. H. Komatsu, 2 ovigerous females (cl 9.8, 10.5 mm), NSMT-Cr; stn E900, similar locality, 38°29.8’N, 142°21.6’E to 38°29.1’N, 142°21.5’E, 905–908 m, 2 November 2006, 1 ovigerous female (cl 8.8 mm), NSMT-Cr 17343; stn F410, Kashima Sea, E of Sohma, 37°43.0’N, 141°53.9’E to 37°44.6’N, 141°53.6’E, 411 m, 30 October 2006, 2 females (cl 9.4, 9.7 mm), NSMT-Cr 17341; stn F425, similar locality, 37°44.0’N, 141°54.8’E to 37°42.3’N, 141°55.1’E, 425– 424 m, 30 October 2006, 3 females (cl 9.3–9.7 mm), 1 ovigerous female (cl 10.6 mm), NSMT-Cr 17342; 2007 cruise, stn D650, E of Kesen’numa, 39°02.3’N, 142°14.7’E to 39°03.3’N, 142°14.9’E, 640–661 m, 5 October 2007, trawl, coll. H. Komatsu, 2 ovigerous females (cl 10.3, 10.6 mm), NSMT-Cr 19157; stn F410, Kashima Sea, E of Sohma , 37°42.9’N, 141°53.9’E to 37°44.6’N, 141°53.6’E, 412–441 m, 30 October 2007, 4 females (cl 8.5–11.0 mm), NSMT-Cr 19158 .

Type material. Holotype: RV Soyo-maru, stn 73, south of Shioya-zaki, Fukushima Prefecture, Kashima Sea , 40°57.99’N, 141°36.25’E, 390 m, 27 July 1926, Kamiya’s dredge, 1 female (size not specifically indicated). Presumed no longer extant.

Redescription. Rostrum ( Figs 8A, B, E, 9A) roundly triangular in dorsal view, somewhat ascending (angle against horizontal plane of carapace about 40°), slightly overreaching antennal teeth (less than 0.2 times as long as carapace); dorsal surface slightly elevated in midline; ventral carina thick, high, rostral tip in lateral view bluntly pointed. Carapace ( Figs 8A, B, E, 9A) widened posteriorly in spawning molt, slightly longer than wide postorbitally; middorsal carina extending anteriorly to proximal half of rostrum and posteriorly to posterior 0.2 of carapace, only with one middorsal teeth arising from slightly posterior to midlength of carapace; no minute denticle between rostral base and middorsal tooth; submedian teeth absent, but trace of swelling sometimes present; antennal tooth directed forward, subacute or blunt; branchiostegal tooth moderately strong, somewhat compressed vertically, blunt or subacute, directed forward in dorsal and lateral views; postorbital angle slightly produced; anterolateral margin between antennal and branchiostegal teeth usually with obtuse denticle; branchiostegal carina distinct, curving, reaching level of hepatic tooth; branchial carina absent.

Abdomen ( Fig. 8C, D) barely sculptured. First to third somites rounded dorsally; fourth and fifth somites with trace of middorsal carina. Lateral ridges on third to fifth somites obsolescent. Pleura of anterior three somites rounded, fourth pleuron with rounded anteroventral angle. Sixth somite with submedian carinae parallel, separated by shallow median groove; posterodorsal margin faintly concave; posterolateral projection triangular, subacutely pointed. Telson ( Fig. 8C) with 2 pairs of dorsolateral spines, first pair located distinctly posterior to midlength.

Eye ( Fig. 8E) stout, approximately as long as wide; cornea as wide as eyestalk, its width slightly more than 0.1 of carapace length; eyestalk not constricted, dorsodistal protrusion not reaching distal corneal margin, bearing rounded dorsal tubercle.

Antennular peduncle ( Figs 8E, 9A) overreaching midlength of antennal scale. First segment without distolateral process; stylocerite very short, not reaching midlength of first segment, terminating in subacute tooth, lateral margin angular. Second segment subcylindrical, not widened distally, with slightly produced distolateral angle. Outer flagellum overreaching lamella of antennal scale by 0.3–0.5 length, consisting of 7–9 articles.

Antenna ( Fig. 8E) with basicerite having dorsolateral distal angle produced in rounded lobe, ventrolateral tooth acute. Carpocerite reaching distal 0.3 of antennal scale. Antennal scale ( Fig. 8E) short, 0.5–0.6 times as long as carapace and about 2.8 times longer than wide; lateral margin slightly concave; distolateral tooth rather broadly triangular, reaching or slightly overreaching distal margin of lamella; lamella gently rounded.

Third maxilliped ( Fig. 9B) relatively slender for genus; ultimate segment gradually tapering distally, about 6.7 times longer than wide; penultimate segment about 2.6 times longer than wide.

First pereopod ( Fig. 9C, D) with palm about 2.6 times longer than wide, not markedly widened proximally or distally, lateral margin faintly sinuous, mesial margin nearly straight; carpus with distolateral tooth slightly smaller than distoventral tooth; merus with small dorsodistal tooth, ventral margin sinuous, carinate. Second and third pereopods typical of genus ( Fig. 9E, F); third pereopod falling short of distal margin of antennal scale. Fourth pereopod ( Fig. 9G) relatively slender, reaching midlength of antennal scale by tip of dactylus; propodus-dactylus articulation showing slight degree of rotation; dactylus ( Fig. 9H) spatulate with distinctly convex dorsal and slightly convex ventral margins, 3.5–4.0 times longer than broad. Fifth pereopod ( Fig. 9I) with dactylus subspatulate, 0.7 times as long as propodus ( Fig. 9J); merus-ischium combined about 0.8 times as long as that of fourth pereopod.

Uropodal exopod with laterally pointed posterolateral tooth and minute spinule mesial to posterolateral tooth ( Fig. 9K).

Male. Rostrum more produced ( Fig. 10A); branchiostegal tooth acute ( Fig. 10A); outer antennular flagellum slightly longer than peduncle, overreaching antennal scale by 0.7 length, consisting of 13–14 articles ( Fig. 10A).

Distribution. Restricted to the Pacific coast of northern Japan, from Kushiro, eastern Hokkaido to Kashima Sea ( Fig. 40); 390–1150 m.

Coloration. Not known.

Size. Male cl 6.0 mm (only one male is known); largest female cl 12.1 mm, ovigerous females cl 9.8–12.1 mm.

Remarks. This species was originally described on the basis of a single specimen (indicated as “1 probably female”) from south of Shioya-zaki, Fukushima Prefecture, at a depth of 390 m ( Yokoya 1933). The holotype could not be located, and it is presumably no longer extant. Burukovsky (2003) reported two specimens (one female and one male) of this species from off Kuji, Iwate Prefecture, Japan, and he transferred this taxon to Argis . As noted before, Burukovsky’s (2003) action cannot be justified. Komai and Komatsu (2009) reported the occurrence of this species from the area off Miyagi Prefecture to Kashima Sea, Tohoku District, Japan, although they did not provide morphological details.

Metacrangon trigonorostris appears closest to M. monodon and M. nipponensis in the ventrally carinate rostrum and the possession of only one middorsal tooth on the carapace, but it is readily distinguished from the latter two species by the submedian teeth on the carapace greatly reduced to rudimentary elevations or absent. Furthermore, from M. nipponensis , M. trigonorostris differs in the lack of prominent distolateral processes on the first segment of the antennular peduncle, the broadly triangular distolateral tooth of the antennal scale not overreaching the distal lamella and the relatively slender distal two segments of the third maxilliped (the ultimate segment is 6.0–7.0 times longer than wide in M. trigonorostris versus about 5.0 times in M. nipponensis ; the penultimate segment is 2.5–3.0 times longer than wide versus about 2.0 times).