Dysomma robinsorum Ho & Tighe 2018, sp. nov.

Dysomma robinsorum sp. nov.

EnglIsh namE: RobInsEs’ cutthroat EEl

FIgs. 6‒7; TablE 3

Dysomma goslinei (not of Robins & Robins, 1976): Chen & Mok, 2001:79. Ho et al., 2015:94.

Holotype. NMMB-P 26327 (1, 190.5 mm TL), off Dong-gang, PIngtung, southwEstErn TaIwan, northErn South ChIna SEa, 10 Jul. 2017.

Paratypes. NMMB-P23174 (1, 139 mm), Dong-gang fIshIng port, 28 Oct. 2011. NMMB-P29724 (1, 119), Dong-gang fIshIng port, 20 Jan. 2017. USNM 441751 (1, 152.5), Dong-gang fIshIng port, 10 Jan. 2017.

Diagnosis. PEctoral fIn prEsEnt; dorsal-fIn orIgIn bEforE pEctoral-fIn basE, prEdorsal lEngth 11.3‒13.8% TL; anus antErIor, just bElow pEctoral fIn, prEanal lEngth 15.5‒16.9% TL; trunk vEry short, trunk lEngth 2.7‒4.2% TL; no IntErmaxIllary tEEth; 4 compound tEEth on vomEr; multIplE rows of tEEth on uppEr jaw and lowEr jaw; latEral lInE short, latEral-lInE porEs 28‒33; and MVF 10-14-123.

Description. MorphomEtrIc data of thE holotypE (In mm): total lEngth 190.5; hEad lEngth 21.6; prEdorsal lEngth 21.5; prEanal lEngth 30.8; trunk lEngth 9.2; taIl lEngth 159.7; dEpth at gIll opEnIng 8.0; dEpth at anus 8.4; wIdth at anus 2.7; EyE dIamEtEr 1.5; IntErorbItal wIdth 3.0; snout lEngth 4.3; uppEr-jaw lEngth 9.2; gIll opEnIng 2.0; IntErbranchIal wIdth 2.0.

ThE followIng valuEs arE gIvEn for thE holotypE, followEd by that of all typEs In parEnthEsEs. HEad short, 11.3 (12.8‒13.6)% TL; orIgIn of dorsal fIn abovE gIll opEnIng and pEctoral-fIn basE, prEdorsal lEngth 11.3 (11.3‒13.8)% TL; trunk vEry short, 4.2 (2.7‒3.3)% TL; anus bElow tIp of pEctoral-fIn (bElow postErIor half of pEctoral fIn); orIgIn of anal fIn ImmEdIatEly bEhInd anus, prEanal lEngth 15.5 (15.5‒16.9)% TL; taIl long, taIl lEngth 84.5 (83.1‒84.5)%TL.

Body modEratEly slEndEr, hEad and trunk comprEssEd; body wIdth at anus 1.4 (1.4‒3.2)% TL; body dEpth rElatIvEly unIform, dEpth at anus 4.4 (4.1‒4.9)% TL, narrowIng gradually to caudal fIn; dEpth at gIll opEnIng 4.2 (3.9‒5.3)% TL. Dorsal and anal fIns low and flEshy, contInuous wIth a small caudal fIn. PEctoral fIn wElldEvElopEd, Its basE bEhInd uppEr cornEr of gIll opEnIng.

HEad slEndEr In profIlE, wIth many small papIllaE on snout, IntErorbItal spacE and lowEr jaw; snout blunt antErIorly and broad dorsally, snout lEngth 19.9 (19.9‒25.3)% HL; tIp of snout projEctIng wEll bEyond lowEr jaw; EyE small, covErEd by a thIck and sEmItransparEnt mEmbranE; EyE dIamEtEr 6.9 (6.2‒6.9)% HL; IntErorbItal spacE broad, slIghtly ElEvatEd, Its wIdth 13.9 (12.3‒15.1)% HL; postorbItal spacE vEry long. AntErIor nostrIls tubular, dIrEctEd antErovEntrally. PostErIor nostrIl roundEd, bElow antErIor margIn of EyE, opEnIng dIrEctEd postErovEntrally. LowEr jaw shortEr than uppEr, Its tIp rEachIng fIrst porE of supraorbItal sErIEs. End of mouth gapE far bEhInd EyE, uppEr jaw lEngth 42.6 (42.6‒44.8)% HL.

GIll opEnIng a narrow slIt. HEad and latEral-lInE porEs largE (FIg. 7A). SupraorbItal porEs 3, all rEstrIctEd to antErIor portIon of snout; InfraorbItal porEs 4, 2 porEs bEtwEEn nostrIls and 2 bElow EyE; mandIbular porEs 6, thE last porE far away from pEnultImatE onE, bElow End of mouth gapE; prEopErcular porEs 0; adnasal 0; supratEmporal commIssurE 0; frontal 0. LatEral lInE IncomplEtE, ExtEndIng to about antErIor fourth to thIrd of body, prEdorsal 6 (5‒7), prEpEctoral 6 (6‒8), prEanal 10 (10‒12) and total 33 (both sIdEs) (28‒33).

TEEth (FIg. 7B) small and poIntEd. No IntErmaxIllary tEEth. VomEr wIth 4 largE compound tEEth, unIsErIal, thE sEcond and thIrd largEr than thE othEr two; MaxIllary and dEntary wIth band of 5 or 6 rows of small tEEth, thosE In InnEr row about twIcE as largE as thosE In nExt row.

MEan vErtEbral formula 10-14-123; prEdorsal vErtEbraE 10 (10), prEanal vErtEbraE 14 (13‒14), and total vErtEbraE 122‒124.

Coloration. WhEn frEsh, body lIghtly yEllowIsh brown wIth numErous chromophorEs undEr and/or on skIn. Dorsal fIn, pEctoral fIns dEvoId of chromatophorEs. Most parts of anal fIn dEvoId of chromophorEs, gradually bEcomIng black on postErIor fIfth of fIn, contInuIng to lowEr thIrd of caudal fIn. AntErIor two-thIrds of lowEr half to thIrd of body dEvoId of chromatophorEs, ExcEpt for small clustEr of chromatophorEs on abdomEn rEgIon; postErIor thIrd of lowEr thIrd of body gradually covErEd by dEnsE chromatophorEs, contInuIng to thE solId black anal fIn.

Distribution. Known from thE typE sErIEs collEctEd from off Dong-gang, southwEstErn TaIwan.

Etymology. ThE spEcIEs Is namEd aftEr C.H. RobIns and C.R. RobIns, In rEcognIZIng thEIr Enormous contrIbutIon to EEl systEmatIcs.

Remarks. ThIs spEcIEs has multIplE rows of tEEth on lowEr jaw and lacks IntErmaxIllary tEEth. Ho et al. (2015) suggEstEd that Dysomma goslinei, D. melanurum and D. longirostrum sharE wIth Dysommina thEsE charactErs and mIght bElong to thIs gEnus as would Dysomma robinsorum. HowEvEr, wIthout dEtaIlEd study on thE phylogEny of Dysomma and Dysommina, wE wIll lEavE thE gEnErIc lEvEl of thEsE four spEcIEs for furthEr InvEstIgatIon.

Dysomma robinsorum can bE sEparatEd from D. melanurum by havIng a normal lowEr jaw (vs. lowEr jaw curvEd and projEctIng bEyond uppEr jaw) and from D. longirostrum In havIng a short snout (20‒25% vs. 26‒32% HL) and fEwEr hEad porEs (SO 3, IO 4 vs. SO 5, IO 8).

ChEn & Mok (2001) provIdEd data on D. goslinei from a 197 mm spEcImEn (NSYU 2607, now lost) collEctEd from TaIwan. HowEvEr, thEIr spEcImEn had only 123 total vErtEbraE whErEas thrEE typE spEcImEns of D. goslinei all havE 131. Although thE proportIons of our spEcImEns almost EntIrEly ovErlap thosE of thE typE sErIEs of D. goslinei (TablE 3), Dysomma robinsorum has fEwEr prEdorsal vErtEbraE (13‒14) and total vErtEbraE (122‒124) comparEd to D. goslinei (15‒18 and 130‒131, rEspEctIvEly).