Dysomma brachygnathos, Ho & Tighe, 2018

Ho, Hsuan-Ching & Tighe, Kenneth A., 2018, Three new species of the cutthroat eel genus Dysomma, with comments on the variation of D. taiwanense (Anguilliformes: Synaphobranchidae), Zootaxa 4454 (1) : -

Dysomma brachygnathos sp. nov.

EnglIsh namE: Short-jaw cutthroat EEl

FIgs. 4‒5 ; TablE 1

Holotype. USNM 444742 (fEmalE, 208 mm TL), Dong-gang fIshIng port, PIngtung, SW TaIwan, northErn South ChIna SEa, 28 Mar 2014.

Paratype. NMMB-P 20199 (malE, 242+ mm TL), Dong-gang fIshIng port, PIngtung, SW TaIwan, northErn South ChIna SEa, 11 Jun 2013.

Diagnosis. PEctoral fIn absEnt; dorsal-fIn orIgIn slIghtly In front of gIll opEnIng; trunk vEry short; lowEr jaw short, not covErIng IntErmaxIllary tEEth or fIrst vomErInE tooth whEn closEd. LatEral-lInE porEs: prEdorsal 3, prEanal 9, and total 23‒32. VErtEbraE: prEdorsal 8, prEanal 15‒16, total 131+‒136. DEntItIon: 2 compound IntErmaxIllary tEEth; 3‒5 compound tEEth on vomEr; sInglE row of compound tEEth on lowEr jaw; maxIllary tEEth In 2‒3 IrrEgular rows. Body unIformly tannIsh to brownIsh, lowEr part of postErIor parts of anal-fIn and lowEr part of caudal fIn darkEr.

Description. MorphomEtrIc data of thE holotypE (In mm): total lEngth 208; hEad lEngth 26; prEdorsal lEngth 23; prEanal lEngth 37; trunk lEngth 11; taIl lEngth 171; dEpth at gIll opEnIng 8.5; dEpth at anus 8.0; wIdth at anus 5.9; EyE dIamEtEr 1.4; IntErorbItal wIdth 4.0; snout lEngth 5.5; uppEr jaw lEngth 9.5; lowEr jaw lEngth 7.0; gIll opEnIng 1.2; IntErbranchIal wIdth 3.3.

ThE followIng valuEs arE gIvEn for thE holotypE, followEd by thosE of thE paratypE In parEnthEsEs. HEad rElatIvEly short, 12.5 (~11.6)% TL; orIgIn of dorsal fIn slIghtly In front of gIll opEnIng (slIghtly bEhInd gIll opEnIng), prEdorsal lEngth 11.0 (~12.0)% TL; trunk rElatIvEly short, 5.3 (~7.0)% TL and 42.3 (~60.7)% HL; prEanal lEngth 17.8 (~18.6)% TL; taIl long, taIl lEngth 82.2 (~81.4)% TL.

Body modEratEly slEndEr, hEad and trunk slIghtly comprEssEd, bEcomIng morE comprEssEd postErIorly. Dorsal and anal fIns low and flEshy, contInuous wIth a small caudal fIn. PEctoral fIn absEnt; gIll opEnIng vEry small and crEscEntIc, sEt low on body

HEad rElatIvEly stout In profIlE; snout blunt antErIorly and broad dorsally, tIp of snout bulbous and covErEd by wIth numErous plIcaE, snout lEngth 21.2 (27.5)% HL; tIp of snout projEctIng wEll bEyond lowEr jaw; EyE small, covErEd by a thIck and sEmItransparEnt mEmbranE; EyE dIamEtEr 5.4 (6.1)% HL; IntErorbItal spacE broad, Its wIdth 15.4 (18.6)% HL; postorbItal spacE broad. AntErIor nostrIls tubular, locatEd just bEhInd thE bulbous snout, dIrEctEd antErovEntrally. PostErIor nostrIl rElatIvEly largE and roundEd, bElow antErIor margIn of EyE, opEnIng dIrEctEd postErovEntrally. LowEr jaw much shortEr than uppEr, Its tIp not covErIng thE IntErmaxIllary tEEth and fIrst vomErInE tooth whEn closEd. End of mouth gapE bEhInd EyE, uppEr jaw lEngth 36.5 (45.7)% HL.

HEad and latEral-lInE porEs small ( FIg. 4C ). SupraorbItal porEs 3, all rEstrIctEd to antErIor portIon of snout; InfraorbItal porEs 4, 2 porEs bEtwEEn nostrIls and 2 bElow EyE; mandIbular porEs 5; prEopErcular porEs 0; adnasal 1; supratEmporal commIssurE 0; frontal 0. LatEral lInE IncomplEtE, ExtEndIng to about antErIor fourth to thIrd of body, prEdorsal 3 (3), prEanal 9 (9) and total 31 (rIght)/32 (lEft) (23/24); 5 (4) bEforE gIll opEnIng.

TEEth ( FIgs. 5A‒B ) rElatIvEly small and poIntEd. IntErmaxIllary tEEth 2, sIdE-by-sIdE, followEd by 5 (3) largE compound vomErInE tEEth, unIsErIal. OnE of thE vomErInE tEEth In thE holotypE appEars to bE a rEplacEmEnt tooth as It doEs not appEar to bE fusEd to thE vomEr. MaxIlla wIth 2 to 3 IrrEgular rows of small tEEth, thosE In InnEr row slIghtly largEr than thE rEst; 21 or 22 (24 or 32) In outEr row and 16 or 19 (20 or 27) In InnEr row. LowEr jaw wIth sInglE row of largE compound tEEth; thE holotypE has 4 largE compound tEEth antErIorly followEd by a gap of about 5 or 6 mIssIng tEEth and thEn 3 or 4 smallEr tEEth postErIorly; thE paratypE has 10‒11 compound tEEth In a contInuous row, dEcrEasIng gradually In sIZE from antErIor to postErIor.

VErtEbral formula 8-16-136 (8-15-131+).

Coloration. WhEn prEsErvEd, body unIformly palE tannIsh to brownIsh, lowEr half of postErIor parts of anal-fIn basE and lowEr part of caudal fIn darkEr. PErItonEum lIght wIth numErous mElanophorEs. Mouth cavIty crEam colorEd. FrEsh coloratIon unknown, but prEsumablE sImIlar to prEsErvEd condItIon.

Distribution. Known from thE typE spEcImEns collEctEd from off Dong-gang, southwEstErn TaIwan (northErn South ChIna SEa) by bottom trawl at dEpths around 200‒300 mEtErs.

Etymology. ThE spEcIfIc namE Is dErIvEd from thE GrEEk, brachys, short and gnathos, jaw, In rEfErEncE to thE rElatIvEly short lowEr jaw found In thIs spEcIEs, to bE trEatEd as a noun In apposItIon.

Remarks. ThE dIffErEncE In dEntItIon bEtwEEn thE holotypE and thE paratypE of Dysomma brachygnathos Is rEmarkablE, and would normally IndIcatE that thEsE spEcImEns rEprEsEnt two dIffErEnt spEcIEs. HowEvEr, thE consIstEncy In all othEr charactErs EspEcIally thE vEry short lowEr jaw IndIcatE that thEsE two spEcImEns arE In fact thE samE spEcIEs and that somE othEr ExplanatIon for thE dEntItIon dIffErEncEs must bE found. It Is possIblE that thE dIffErEncEs arE duE to sExual dImorphIsm sIncE thE holotypE Is a fEmalE and thE paratypE Is a malE. WhEthEr thIs Is thE casE wIll dEpEnd on thE collEctIon of addItIonal spEcImEns of thIs rarE EEl.

BElow wE also provIdE thE casE of D. taiwanense whIch Is hIghly varIablE In thE composItIon of lowEr-jaw tEEth. Although thE tEEth on thE lowEr jaw arE an Important charactEr for IdEntIfyIng Dysomma spEcIEs, such varIatIon should bE consIdErEd whIlE workIng on thIs group.

Figures

FIGURE 4. Dysomma brachygnathos sp. nov. A. USNM 444742, holotype, 208 mm TL. B. NMMB-P20199, paratype, 242+ mm TL. C. Semidiagramatic lateral view of head showing the head pores, USNM 444742, holotype. Arrow indicates original of dorsal fin (DO) and origin of anal fin (AO)

FIGURE 5. X-rays of teeth of Dysomma brachygnathos sp. nov. A. USNM 444742, holotype. B. NMMB-P20199, paratype.