Metacrangon bythos, Komai, 2012

Komai, Tomoyuki, 2012, 3468, Zootaxa 3468, pp. 1-77 : 12-16

Metacrangon bythos n. sp.

( Figs 2–4, 39)

Material examined. Holotype: RV Tansei-maru, KT 98-14 cruise, stn 15, NE of Miyake Island , Izu Islands, Japan, 34°15.61’N, 140°14.81’E, 1988–2007 m, coarse volcanic sand and sponges, 1 September 1998, beam trawl, coll. T. Komai, female (cl 11.1 mm), CBM-ZC 6405.

Paratypes: RV Tansei-maru, KT 98-14 cruise, stn 14-1, NE of Miyake Island , Izu Islands, Japan, 34°21.29’N, 140°14.47’E, 2484–2518 m, coarse volcanic sand, 1 September 1998, beam trawl, coll . T. Komai , 1 female (cl 9.0 mm), 2 males (cl 5.2, 6.5 mm), CBM-ZC 6402 .

Description. Female. Rostrum ( Figs 2A, B, E, 3A) narrowly triangular in dorsal view, directed forward, exceeding beyond antennal teeth (about 0.2 times as long as carapace); tip acute both in dorsal and lateral views; dorsal surface nearly flat; ventral surface nearly flat. Carapace ( Figs 2A, B, E, 3A) widened posteriorly in spawning molt, as long as wide postorbitally; middorsal carina extending nearly to posterodorsal margin; anterior middorsal tooth elongate, narrow styliform, arising slightly posterior to rostral base, slightly ascending (angle against horizontal plane of carapace 15°), overlapping rostrum, tip slightly to distinctly overreaching rostral tip; posterior tooth small (holotype) or moderately large (paratype), arising at about 0.6 of carapace length; minute denticle present between 2 middorsal teeth; submedian teeth present; antennal tooth acuminate, ascending in similar degree to anterior middorsal tooth; postorbital angle rounded; orbital cleft present; anterolateral margin between antennal and branchiostegal teeth slightly sinuous, with minute denticle inferior to base of antennal tooth; branchiostegal tooth moderately strong, directed slightly laterally in dorsal view, slightly ascending in lateral view, reaching dorsolateral distal angle of antennal basicerite; branchiostegal ridge curving, reaching near to level of posterior end of hepatic groove; branchial carina distinct; branchial region smooth.

Abdomen ( Fig. 2C, D) fairly sculptured; first somite with trace of middorsal carina, second to fifth somites each with rather broad, clearly delimited middorsal carina. Tergites of first to fourth somites slightly elevated along posterior margins. Pleura of first to fifth somites each with shallow depressions laterally and accompanied blunt ridges or elevations; first pleuron with blunt ventral tooth, second and third pleura with obtuse ventral and anteroventral tooth respectively, fourth pleuron with rounded anteroventral and posteroventral margins, fifth with sharp posteroventral tooth. Sixth somite with clearly delimited submedian carinae; posterodorsal margin faintly concave; pleuron shallowly depressed; posterolateral projection terminating in acute tooth. Telson abnormal in holotype ( Fig. 2C), broken in paratype.

Eye ( Fig. 2E) moderately stout, 1.3 times longer than wide; cornea as wide as eyestalk, corneal width about 0.13 of carapace length; eyestalk slightly constricted, bearing blunt or subacute tubercle.

Antennular peduncle ( Figs 2E, 3A) relatively slender, slightly overreaching midlength of antennal scale. First segment with short, blunt distolateral process directed anteriorly; stylocerite terminating in subacute spine far falling short of tip of distolateral process of first segment, lateral margin gently convex. Second segment with short, blunt distolateral process. Outer flagellum subequal in length to peduncle, overreaching distal margin of lamella of antennal scale by half length, consisting of 13–15 articles.

Antenna ( Figs 2E, 3A) with basicerite bearing dorsolateral distal angle produced in small subacute lobe, ventrodistal tooth slender, acute. Antennal scale 0.5 times as long as carapace and 3.1–3.2 times longer than wide; lateral margin faintly sinuous or faintly concave; distolateral tooth slightly overreaching distal margin of lamella; lamella tapering to rounded distal margin; notch separating distolateral tooth and distal lamella moderately deep.

Third maxilliped relatively slender; ultimate segment gradually tapering distally in distal half, about 6.3 times longer than wide; penultimate segment about 2.2 times longer than wide ( Fig. 3B).

First pereopod missing in both holotype and paratype. Second and third pereopods typical of genus ( Fig. 3E, F); third pereopod just reaching distal margin of antennal scale by tip of dactylus. Fourth pereopod ( Fig. 3G) slightly falling short of distal margin of antennal scale; dactylus ( Fig. 3H) subspatulate, with fringe of numerous stiff setae on margins. Fifth pereopod ( Fig. 3I) with merus-ischium combined about 0.8 times as long as that of fourth pereopod; dactylus ( Fig. 3J) subspatulate, 0.7–0.8 times as long as propodus, subequal in length to that of fourth pereopod.

Uropodal exopod with spinule just mesial to base of acute posterolateral tooth ( Fig. 3K).

Male. Armature on carapace ( Fig. 4A) and abdominal sculpture stronger than in female. Rostrum ( Fig. 4A, B) 0.25 times as long as carapace, slightly ascending (angle against horizontal plane of carapace about 15°). Carapace ( Fig. 4A) with anterior middorsal tooth greatly elongate, obliquely erect (angle against horizontal plane of carapace about 40°); posterior tooth strong, hooked, arising at about 0.60 of carapace length; antennal and branchiostegal teeth ascending in similar degree to rostrum. Corneal width 0.15 of carapace length. Second segment of antennular peduncle bearing acutely pointed distolateral process; antennular flagellum distinctly longer than peduncle, overreaching distal margin of antennal scale by about 0.8 length, consisting of 18 articles (articles nearly as long as or slightly wider than long except for first article) ( Fig. 4B). First pereopod ( Fig. 3C, D) relatively slender for genus; palm not particularly narrowed or widened, 3.8 times longer than wide, lateral and mesial margins nearly straight; carpus without distolateral tooth; merus with small dorsodistal tooth, ventral margin very slightly convex, sharply carinate.

Coloration. In life, body and appendages generally white; cornea of eye light gray, with reflecting pigment.

Size. Largest male cl 6.5 mm; largest female (spawning molt) cl 11.1 mm.

Distribution. Izu Islands, Japan ( Fig. 39); 1988–2518 m.

Remarks. This new species is similar to M. agassizi , M. ochotensis , M. punctata n. sp. and M. similis in having distinct middorsal carina on the second to fifth abdominal somites. Metacrangon bythos n. sp. is distinguished from M. agassizi , M. punctata and M. similis by the shorter stylocerite of the antennular peduncle, falling far short of the distolateral process on the first segment of the antennular peduncle. In the latter three species, the stylocerite reaches or overreaches the distolateral process of the first segment of the antennular peduncle. The proportionally narrower antennal scale distinguishes M. bythos from M. agassizi and M. similis (the antennal scale is 3.1–3.2 times longer than wide in M. bythos , 2.1–2.4 times in M. agassizi , and 2.5–2.9 in M. similis ). Metacrangon ochotensis resembles M. bythos in the short antennular stylocerite, but as mentioned above, M. ochotensis is characterized by the strong sculpture on the abdomen and the shape of the antennal scale.

Metacrangon sp. reported by Komai & Komatsu (2009) from off Sohma, Fukushima Prefecture, at depths of 1466–1511 m is very similar to M. bythos n. sp. Nevertheless, the two specimens referred to the unidentified taxon differ from M. bythos in the less produced distolateral process on the first segment of the antennular peduncle and the distolateral tooth of the antennal scale falling short of the lamella. These specimens are small (male cl 5.5 mm; female cl 5.3 mm) and damaged, and therefore positive identification is postponed until additional material becomes available.

Etymology. The specific name is from the Greek for “from the deep”, and refers to the deep habitat of the new species, relative to that of most of its congeners.