Metacrangon ryukyu n. sp.

[New Japanese name: Ryukyu-somewake-ebijyako] Figs 1, 2

Material examined. Holotype: female (cl 7.1 mm), CBM-ZC 14190, R/V “ Natsushima ” cruise NT15-13 (chief scientist Dr. Ken Takai), ROV “ Hyper-Dolphin ” dive #1861, Iheya North original site (Nakamura et al. 2015), Iheya North vent field, Okinawa Trough, 27°47.444’N, 126°53.863’E, 986 m, 2 August 2015, by slurp gun among tubeworms.

Diagnosis. Rostrum 0.17 times carapace length, falling slightly short of distal corneal margins, triangular in dorsal view, slightly ascending, tip rounded. Carapace with anterior middorsal spine moderately large, epigastric in position (arising at 0.1 of carapace length), not reaching rostral base; posterior middorsal spine arising at 0.6 of carapace length; shallow orbital cleft discernible; antennal spine directed forward in dorsal view, weakly ascending in lateral view; branchiostegal spine moderately strong, directed forward in dorsal view. Pleomeres 1–4 without middorsal carina, pleomere 5 with trace of middorsal carina; pleura of pleomeres 1–3 without conspicuous tooth or spine marginally; pleomere 6 with blunt submedian carinae. Telson with posteromedian process moderately produced, subacute. Eyestalk (including cornea) almost as long as wide; dorsal tubercle relatively small, rounded, papilla-like. Antennular peduncle reaching beyond midlength of antennal scale; article 1 short, blunt distolateral process; stylocerite falling far short of distolateral process of article 1; article 2 with short, blunt distolateral process. Antennal scale 0.5 times as long as carapace, 2.5 times as long as wide; lateral margin gently concave; distolateral spine reaching as far as rounded distal lamella. Maxilliped 3 with ultimate article 6.9 times as long as wide. Pereopods 4 and 5 relatively slender; dactyli narrowly subspatulate, margins not forming sharp edges, bearing numerous setae on lateral and mesial margins and extensor surface.

Description. Female (holotype). Body robustly built as in other congeneric species.

Rostrum (Figs 1A–C, 2A) triangular in dorsal view, slightly ascending at base (angle against horizontal plane of carapace 10°), slightly overreaching antennal spines but not reaching distal corneal margins, 0.17 times as long as carapace; tip rounded in dorsal view; dorsal surface with low, blunt median ridge extending from anterior margin of anterior middorsal spine, lateral margins slightly upturned; ventral surface medially bluntly carinate, with tuft of plumose setae subterminally. Carapace (Figs 1A–C, 2A) as approximately long as wide postorbitally, surface with covering of very short setae; middorsal carina extending from rostrum nearly to posterodorsal margin of carapace, bearing two unequal spines; anterior middorsal spine moderately large, strongly compressed laterally, tooth-like, epigastric in position (arising at about 0.1 of carapace length); posterior spine distinctly smaller than anterior spine, cardiac in position (arising at 0.6 of carapace length); minute denticle present between two middorsal spines; gastric region delimited by low, blunt postorbital carinae and posterior transverse ridges, particularly depressed posterior to rostral base, with pair of small submedian spines arising at midlength of carapace and supported by short longitudinal carina; antennal spine moderately small, directed forward in dorsal view, ascending similarly to rostrum in lateral view; postorbital angle and orbital cleft faintly defined; anterolateral margin between antennal and branchiostegal spines sinuous, with minute denticle inferior to base of antennal spine; branchiostegal spine moderately strong, directed forward in dorsal view, ascending in similar degree to antennal spine in lateral view, just reaching dorsolateral distal angle of antennal basicerite; branchiostegal carina not reaching to level of hepatic spine, with row of long plumose setae ventrally; hepatic spine moderate in size, accompanied by shallow hepatic groove inferior to its base; branchial carina absent; pterygostomial margin folded beneath branchiostegal spine, with minute pterygostomial spine.

Pleon (Fig. 1D–F) not much sculptured. Pleomeres 1–4 dorsally rounded, without conspicuous middorsal carina or elevation; pleomere 5 with low, obsolescent middorsal carina. Tergites of pleomeres 2–5 not elevated along posterior margins. Pleura of anterior three somites rounded, pleuron 4 with rounded anteroventral angle and acutely pointed posteroventral angle; pleuron 5 with posteroventral spine. Pleomere 6 with blunt, parallel submedian carinae; posterodorsal margin produced, bilobed; dorsolateral carina distinct; pleuron shallowly depressed; posterolateral process abruptly terminating in acute spine. Telson (Fig. 4F) 1.6 times as long as pleomere 6, with 2 pairs of tiny dorsolateral spiniform setae, first pair located posterior to midlength; dorsal surface medially grooved; posteromedian process moderately produced, subacutely pointed, with 1 pair of minute spiniform setae and 2 pairs of long, stout plumose setae (Fig. 4G).

Thoracic sternites shallowly concave, median spines or teeth completely reduced. Pleonal tergites also without median spine or tooth on each somite.

Eyestalk (Figs 1A–C, 2A) stout, approximately as long as wide, slightly constricted; dorsodistal protrusion not reaching distal corneal margin, bearing small but distinct, rounded dorsal tubercle; cornea hemispherical, as wide as eyestalk, corneal width 0.14 of carapace length.

Antennular peduncle (Figs 1A, B, 2A) moderately slender, slightly overreaching midlength of antennal scale. Article 1 with short, blunt distolateral process; stylocerite falling far short of distal margin of article 1, terminating in spine, lateral margin obtusely angular. Article 2 with short, blunt distolateral process. Article 3 very short. Flagella missing on both sides.

Antenna (Figs 1A, B, 2A) with basicerite having dorsolateral distal angle produced in bluntly pointed lobe, ventrolateral distal spine well overreaching dorsolateral distal angle. Antennal scale (scaphocerite) 0.5 times as long as carapace and 2.5 times as long as wide; lateral margin gently concave; distolateral spine moderately broad, just reaching rounded distal margin of lamella.

Maxilliped 3 (Fig. 2B, C) overreaching distal margin of antennal scaphocerite by full length of ultimate article, with setation typical of genus (omitted in Fig.). Coxa with large rounded lateral plate. Antepenultimate article slightly sinuous in dorsal view, with 2 greatly unequal, subdistal spiniform setae on ventral surface (Fig. 2D). Penultimate article (= carpus) 3 times as long as wide, with 1 long spiniform setae at distomesial angle and dense, stiff grooming setae entirely on mesial face. Ultimate article flattened, spatulate, 6.9 times as long as wide; mesial margin with long spiniform setae in individual or sets of 2 or more, mixed with long stiff setae.

Pereopods 1 missing.

Pereopod 2 (Fig. 2E) carried folded, when extended, just reaching distal margin of scaphocerite; coxa with prominent, sickle-like projection; ischium arcuate, slightly shorter than merus; carpus subequal in length to merus; chela (Fig. 2F) 0.5 times as long as propodus, with elongate palm; dactylus 0.4 times as long as palm.

Pereopod 3 (Fig. 2G) overreaching distal margin of scaphocerite by length of dactylus and 0.3 of propodus; length ratio of ischium to dactylus 2.6: 2.8: 3: 2: 1.

Pereopod 4 (Fig. 2H) moderately stout, reaching midlength of scaphocerite by tip of propodus; long setae on ischium and merus (setae on dorsal margin particularly long). Carpus slightly widened distally, without dorsodistal process. Propodus with short to long setae on extensor and flexor margins. Dactyls (Fig. 2I, J) narrow, not particularly compressed, 0.6 times as long as propodus; margins not sharply edged; terminating in 2 slender, unequal projection flanking minute spiniform seta; numerous setae on extensor surface and margins, fewer setae on flexor surface.

Pereopod 5 (Fig. 2K) slightly shorter and slender than pereopod 4, reaching base of branchiostegal spine of carapace by tip of propodus; setation on ischium and merus also less developed, but propodus and dactylus similarly setose as in pereopod 4 (Fig. 2L, M).

Pleopods without distinguishing features; setose, carrying eggshells after hatching. Uropod with protopod bearing tiny spine at posterolateral angle; endopod reaching tip of telson; exopod slightly shorter than endopod, with 1 spiniform seta just mesial to base of small distolateral spine (Fig. 1H).

Colouration in life. Unavailable.

Distribution and habitat. At present, known only from the type locality, Iheya North original site, Okinawa Trough, 986 m deep (Fig. 6A). The Iheya North vent field is well known for its high-temperature venting (up to 311°C; Kawagucci et al. 2011), but this new species was collected from the very periphery of the vent site where only low diffuse venting occurs. The collecting site was visually dominated by the siboglinid tubeworms Lamellibrachia sagami Kobayashi, Miura & Kojima, 2015 and Alaysia sp., as well as the colloniid snail Cantrainea jamsteci (Okutani & Fujikura, 1990) and the stalked barnacle Ashinkailepas seepiophila Yamaguchi, Newman & Hashimoto, 2004.

Remarks. In the unique holotype of this taxon, antennular flagella and pereopods 1 were missing, and consequently, diagnostic characters derived from these structures remains unknown.

Etymology. Named after the Ryukyu Islands, embracing the type locality of this new species.