Metacrangon kaiko, Komai & Chen & Watanabe, 2018

Komai, Tomoyuki, Chen, Chong & Watanabe, Hiromi Kayama, 2018, Two new species of the crangonid genus Metacrangon Zarenkov, 1965 (Crustacea: Decapoda: Caridea) from the Okinawa Trough, Japan, Zootaxa 4410 (1), pp. 97-112 : 103-107

Metacrangon kaiko n. sp.

[New Japanese name: Kaiko-somewake-ebijyako] Figs 3–5

Material examined. Holotype: female (cl 10.8 mm), CBM-ZC 14191, R/V “ Kairei ” cruise KR16-16 (chief scientist Dr. Junichi Miyazaki), ROV “ Kaiko ” dive #716, ‘ Futagoyama Taka’ area (Makabe et al. 2017), NE of Yonaguni Island, Ryukyu Islands, 24°49.1563’N, 123°17.2233’E, depth 2205 m, 20 November 2016, by slurp gun.

Diagnosis. Rostrum 0.20 times carapace length, reaching distal corneal margins, narrowly triangular in dorsal view, directed forward, tip acuminate. Carapace with anterior middorsal spine moderately large, epigastric in position (arising at 0.1 of carapace length), reaching beyond rostral base; posterior middorsal spine cardiac in position (arising at 0.6 of carapace length); submedian spines present; postorbital angle or orbital cleft absent; antennal spine directed forward in dorsal view, ascending in lateral view; branchiostegal spine moderately strong, directed forward in dorsal view. Pleomere 1 without even trace of middorsal carina, pleomeres 2–4 each with low, obsolescent middorsal carina, pleomere 5 with low but clearly delimited middorsal carina; pleura of pleomeres 1–3 without conspicuous tooth or spine marginally; pleomere 6 with blunt but clearly delimited submedian carinae. Telson with posteromedian process strongly produced, acuminate. Eyestalk (including cornea) almost as long as wide; dorsal tubercle relatively large, rounded, papilla-like. Antennular peduncle reaching beyond midlength of antennal scale; article 1 with short, blunt distolateral process; stylocerite falling far short of distolateral process of article 1; article 2 with short, blunt distolateral process. Antennal scale slightly less than half as long as carapace, 2.7 times as long as wide; lateral margin gently concave; distolateral spine overreaching rounded distal lamella. Maxilliped 3 with ultimate article 6.3 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 few setae on extensor surface.

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

Rostrum ( Figs 3A , 4A–C ) narrowly triangular in dorsal view, slightly arcuate and slightly ascending, slightly overreaching distal corneal margins, 0.20 times as long as carapace; tip acute in dorsal view; dorsal surface with faint groove along lateral margin in proximal half; ventral surface medially bluntly carinate, with tuft of plumose setae subterminally. Carapace ( Figs 3A , 4A–C ) 1.1 times as long as wide postorbitally; surface with covering of very short setae; middorsal carina bearing two unequal spines, extending from anterior middorsal spine to posterior 0.2 of carapace length; anterior middorsal spine moderately large, strongly compressed laterally, tooth-like, epigastric in position (arising slightly posterior to rostral base); posterior spine distinctly smaller than anterior spine, cardiac in position (arising at 0.6 of carapace length); no denticle 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 0.3 of carapace length and supported by blunt, sinuous ridge; antennal spine moderately small, directed slightly mesially in dorsal view, somewhat ascending in lateral view (angle against horizontal plane of carapace 45°); orbital cleft absent; anterolateral margin between antennal and branchiostegal spines sinuous, with small acute 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, slightly overreaching dorsolateral distal angle of antennal basicerite; branchiostegal carina 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 ( Figs 3B , 4D, E ) not much sculptured; surface almost glabrous, but with few longish plumose setae dorsally on each somite. Pleomere 1 rounded dorsally, without even trace of middorsal carina; pleomeres 2 and 3 with obsolescent trace of middorsal carina; pleomere 4 and 5 with low, broad, but clearly delimited middorsal carina, none reaching to posterior margin. Tergites of pleomeres 2–5 not elevated along posterior margins. Pleura of anterior three somites rounded, those of pleomere 4 and 5 each with small posteroventral spine; lateral faces of pleura 3–5 each with trace of ridge adjacent to ventral margin. Pleomere 6 with clearly delimited, parallel submedian carinae; posterodorsal margin produced, deeply bilobed; dorsolateral carina distinct; pleuron shallowly depressed; posterolateral process terminating in acute spine. Telson ( Figs 3B , 4F ) 1.7 times as long as pleomere 6, with 2 pairs of tiny dorsolateral spiniform setae, first pair located at midlength; dorsal surface medially grooved; posteromedian process strongly produced, acuminate, flanked by 1 pair of minute spiniform setae and bearing 2 pairs of long, stout plumose setae ( Fig. 4G ).

Thoracic sternites not depressed, sternites 5–8 each with anteriorly directed median teeth diminishing in size and acuteness. Pleonal sternites 1–5 also each with small median spine diminishing in size and acuteness.

Eyestalk ( Figs 3A , 4A–C ) stout, stout, approximately as long as wide, slightly constricted; dorsodistal protrusion not reaching distal corneal margin, bearing relatively large, rounded dorsal tubercle; cornea hemispherical, as wide as eyestalk, corneal width 0.15 of carapace length.

Antennular peduncle ( Fig. 4A, B ) moderately slender, 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. Outer flagellum consisting of 10 articles, distal 4 articles with aesthetascs; inner flagellum setose on outer and inner margins.

Antenna ( Figs 3A , 4A, B ) with basicerite having dorsolateral distal angle produced in bluntly pointed lobe, ventrolateral distal spine well overreaching dorsolateral distal angle. Antennal scale (= scaphocerite) approximately 0.5 times as long as carapace and 2.7 times as long as wide; lateral margin gently concave; distolateral spine moderately broad, slightly overreaching rounded distal margin of lamella.

Maxilliped 3 ( Fig. 5A, B ) overreaching distal margin of antennal scaphocerite by full length of ultimate article, with setation typical of genus. Coxa with large rounded lateral plate. Antepenultimate article slightly sinuous in dorsal view, with 2 subdistal spiniform setae on ventral surface ( Fig. 5C ). Penultimate article (= carpus) 2.4 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, 6.3 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. 5D ) 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. 5F ) about half as long as propodus; chela with elongate palm, dactylus 0.4 times as long as palm.

Pereopod 3 ( Fig. 5G ) overreaching distal margin of scaphocerite by length of dactylus and 0.3 of propodus; length ratio of ischium to dactylus 3: 2.9: 3.9: 2.4: 1.

Pereopod 4 ( Fig. 5H ) relatively slender, reaching midlength of scaphocerite by tip of propodus; long setae on ischium and merus (setae on dorsal margin particularly long). Carpus subcylindrical, without dorsodistal process. Propodus with short to long setae on extensor and flexor margins. Dactyls ( Fig. 5I, J ) narrow, not particularly compressed, approximately 0.6 times as long as propodus; margins not sharply edged; terminating in 2 slender, unequal projection flanking minute spiniform seta ( Fig. 5K ); sparse setae on margins and extensor surface, glabrous on flexor surface.

Pereopod 5 ( Fig. 5L ) slightly shorter and slender than pereopod 4, reaching base of branchiostegal spine of carapace by tip of propodus; setation on articles much less developed than in pereopod 4, though extensor margin of propodus with row of short setae; dactylus ( Fig. 5M, N ) slightly longer and narrower than that of pereopod 4, 0.7 times as long as propodus, similarly setose.

Pleopods without distinguishing features; setose, carrying eggshells after hatching. Uropod with protopod bearing minute spine at posterolateral angle; endopod reaching tip of telson; exopod slightly shorter than endopod, with subacute distolateral spine obscured by setae ( Fig. 4H ).

Colouration in life. Unavailable.

Distribution and habitat. Known only from the type locality, 2205 m deep ( Fig. 6B ). The “Futagoyama Taka” area was evidently under hydrothermal influence judging from the visual dominance of the holobiont deep-sea mussel Bathymodiolus aduloides Hashimoto & Okutani, 1994 and the siboglinid tubeworm Lamellibrachia sp., albeit they only occurred in small patches. However, no high temperature venting was observed and only gas bubbles emitted from seafloor could be seen (Miyazaki et al., 2016; Makabe et al., 2017).

Remarks. In the unique holotype of this taxon, the pereopod 1 were both missing, and consequently, characters derived from this appendage remains unknown.

Etymology. The new species is named after the ROV “Kaiko”, which collected the holotype.

Figures

FIGURE 5. Metacrangon kaiko n. sp., hOLOTyPE, fEmALE (cL 10.8 mm), CBM-ZC 14191. A, rIghT mAXILLIPEd 3, cOXA ANd ANTEPENuLTImATE ArTIcLE, dOrsAL vIEw; B, ThE sAmE, dIsTAL TwO ArTIcLEs, dOrsAL vIEw; C, ThE sAmE, dIsTAL PArT Of ANTEPENuLTImATE ArTIcLE,vENTrAL vIEw (sETAE OmITTEd); D, rIghT PErEOPOd 2, LATErAL vIEw (chELA sTrONgLy fLEXEd mEsIALLy); INsET, chELA; E, cOXA Of rIghT PErEOPOd 2, OuTEr vIEw; F, ThE sAmE, chELA, LATErAL vIEw; G, rIghT PErEOPOd 3, LATErAL vIEw; H, rIghT PErEOPOd 4, LATErAL vIEw;I, ThEsAmE,dAcTyLus,LATErAL vIEw;J,ThE sAmE,EXTENsOr vIEw;K, cLOsE uPOf TIP Of PErEOPOd 4 dAcTyLus,EXTENsOr vIEw; L, rIghT PErEOPOd 5, LATErAL vIEw; M, ThE sAmE, dAcTyLus, LATErAL vIEw; N, ThE sAmE, EXTENsOr vIEw.

FIGURE 6. A, LOcATION whErE ThE hOLOTyPE Of Metacrangon ryukyu n. sp. wAs cOLLEcTEd (IhEyA NOrTh OrIgINAL sITE, IhEyA NOrTh vENTfIELd,OkINAwATrOugh,27°47.444’N,126°53.863’E, 986 mdEEP);B, LOcATIONwhErE ThEhOLOTyPE Of Metacrangon kaikon. sp. wAs cOLLEcTEd (‘FuTAgOyAmA TAkA’ ArEA,NE Of YONAguNI IsLANd,OkINAwATrOugh,24°49.1563’N, 123°17.2233’E, 2205 m) dEEP.

FIGURE3. Metacrangon kaiko n. sp., hOLOTyPE,fEmALE (cL 10.8 mm), CBM-ZC 14190. A, cArAPAcE ANd cEPhALIc APPENdAgEs, LATErAL vIEw (cOvErINg Of shOrT sETAE ON cArAPAcE OmITTEd);B, PLEON,TELsON ANd LEfT urOPOd, LATErAL vIEw.

FIGURE4. Metacrangon kaiko n. sp., hOLOTyPE,fEmALE (cL 10.8 mm), CBM-ZC 14191. A, cArAPAcE ANd cEPhALIc APPENdAgEs, dOrsAL vIEw (cOvErINg Of shOrT sETAE ON cArAPAcE OmITTEd;sETAE ON rIghT sIdE OmITTEd); B, ANTErIOr PArT Of cArAPAcEANd cEPhALIc APPENdAgEs, dOrsAL vIEw (sETAE OmITTEd); C, ANTErIOr PArT Of cArAPAcE ANd LEfT EyE, dOrsOLATErAL vIEw (sETAE OmITTEd); D, PLEOmErEs 1–3, dOrsAL vIEw; E, PLEOmErEs 4–6, dOrsAL vIEw; F, TELsON,dOrsAL vIEw (sETAE ON LEfT sIdE OmITTEd);G, TIP Of TELsON, dOrsAL vIEw; H, POsTErIOr PArT Of LEfT urOPOdAL EXOPOd,dOrsAL vIEw (mArgINAL sETAE ON LAmELLAr PArT OmITTEd).