Metacrangon tsugaruensis, Komai, 2012

Komai, Tomoyuki, 2012, 3468, Zootaxa 3468, pp. 1-77 : 60-63

Metacrangon tsugaruensis n. sp.

( Figs 31, 32, 42)

Material examined. Holotype: RV Soyo-maru , 1992 cruise, stn 6-2, Tsugaru Strait, northern Japan, 41°32.5’N, 141°04.2’E, 408–417 m, 1 July 1992, dredge, coll. H. Saito, ovigerous female (cl 8.2 mm), CBM-ZC 10672.

Description. Female (holotype). Rostrum ( Figs 31A, B, E, 32D) triangular in dorsal view, moderately broad, slightly ascending (angle against horizontal plane of carapace 15°), overreaching antennal teeth (slightly less than 0.2 times as long as carapace); tip rounded in dorsal view; dorsal surface shallowly concave; ventral carina high. Carapace ( Figs 31A, B, E, 32A) widened posteriorly, as long as wide postorbitally; middorsal carina extending from rostral base to posterior 0.2 of carapace, bearing two slightly unequal teeth; anterior middorsal tooth moderately large, epigastric in position (arising at about 0.1 of carapace length), laterally compressed, acuminate; posterior tooth smaller than anterior tooth, arising slightly posterior to midlength of carapace length; no denticle present between two middorsal teeth; submedian teeth present; antennal tooth moderately large, directed forward to slightly ascending, subacute; postorbital angle slightly produced; anterolateral margin between antennal and branchiostegal teeth concave, with tiny acute denticle; branchiostegal tooth moderately strong, ascending in similar degree to rostrum, overreaching dorsolateral distal angle of antennal basicerite; branchiostegal carina curving in lateral view, reaching level of posterior end of hepatic groove; branchial carina absent.

Abdomen ( Fig. 31C, D) slightly sculptured; first and second somites rounded dorsally; third somite with trace of middorsal carina; fourth and fifth somites each with rather broad, distinct middorsal carina, not reaching posterodorsal margin of respective somite. Second to fifth tergites not elevated along posterior margins. Pleura of anterior three somites rounded, fourth pleuron with rounded anteroventral angle. Sixth somite with distinct, parallel submedian carinae; posterodorsal margin faintly bilobed; pleuron shallowly depressed; posterolateral process abruptly terminating in acute tooth. Telson ( Fig. 31C) with 2 pairs of dorsolateral spines, first pair located slightly posterior to midlength.

Eye ( Fig. 31E) stout, approximately as long as wide; cornea as wide as eyestalk, corneal width less than 0.2 of carapace length; eyestalk with dorsodistal protrusion not reaching distal corneal margin, bearing rounded dorsal tubercle.

Antennular peduncle ( Figs 31E, 32A) moderately stout, overreaching midlength of antennal scale. First segment with prominent, bluntly distolateral process; stylocerite terminating in subacute tooth, lateral margin obtusely angular. Second segment with prominent, blunt distolateral process. Outer flagellum distinctly shorter than peduncle, overreaching distal margin of lamella of antennal scale by 0.4 length, consisting of 8–10 articles.

Antenna ( Figs 31E, 32A) with basicerite having dorsolateral distal angle produced in bluntly pointed lobe, ventrodistal tooth acute. Antennal scale ( Fig. 32B) 0.4 times as long as carapace and 2.3 times longer than wide; lateral margin slightly concave; distolateral tooth moderately broad, reaching distal margin of roundly triangular distal lamella.

Third maxilliped moderately stout; ultimate segment gradually tapering distally in distal half, about 4.1 times longer than wide; penultimate segment about 2.0 times longer than wide ( Fig. 32C).

First pereopod ( Fig. 32D, E) moderately stout; palm 2.6 times longer than wide, not markedly widened proximally or distally, lateral and mesial margins slightly sinuous; carpus with ventrolateral tooth; merus with small dorsodistal tooth, ventral margin sinuous, bluntly carinate. Second and third pereopods typical of genus ( Fig. 32F, G); third pereopod overreaching distal margin of antennal scale by half length of dactylus. Fourth pereopod ( Fig. 32H) moderately stout, overreaching distal margin of antennal scale by length of dactylus; dactylus spatulate, but abnormal ( Fig. 32I). Fifth pereopod ( Fig. 32J) with dactylus subspatulate, slightly curved, 0.7 times as long as propodus, longer than that of fourth pereopod ( Fig. 32K); merus-ischium combined about 0.7 times as long as that of fourth pereopod.

Uropodal exopod truncate with minute lateral point; no spinule mesial to posterolateral tooth ( Fig. 32L).

Eggs 1.6 x 1.5 mm; not counted.

Coloration. Unknown.

Size. Ovigerous female cl 8.2 mm.

Distribution. Known only from the type locality, Tsugaru Strait, northern Japan ( Fig. 42); 408– 417 m.

Remarks. This new species is morphologically similar to M. asiaticus , M. proxima , and M. variablis . Nevertheless, M. tsugaruensis n. sp. is distinguished from the latter three species in the lack of a distinct middorsal carina on the second and third abdominal somites. Metacrangon asiaticus differs further from M. tsugaruensis in the straight, rather than sinuous, mesial margin of the palm of the first pereopod. Metacrangon proxima differs from M. tsugaruensis in the narrower rostrum (cf. Fig. 37F and Fig. 31E), and the relatively slender antennular peduncle (cf. Fig. 37F versus Fig. 31E), the ultimate segment of the third maxilliped (6.5–7.0 times longer than wide versus 4.1 times) and the palm of the first pereopod (2.9–3.4 times longer than wide versus 2.6 times) with a straight mesial margin.

Etymology. Named after the type locality of this new species.