Dysomma formosa, 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 formosa sp. nov.

EnglIsh namE: WhItE cutthroat EEl

FIgs. 1‒3 ; TablEs 1‒2

Holotype. NMMB-P 23172 (1, 324), Dong-gang fIshIng port, PIngtung, SW TaIwan, northErn South ChIna SEa, ca. 300 m, 21 Nov. 2015.

Paratypes. 33 spEcImEns, all collEctEd from nEar thE typE localIty. FAKU 145542 (1, 323), 30 Aug. 2017. NMMB-P12065 (1, 236), 18 FEb. 2011. NMMB-P23173 (1, 245), 29 Mar. 2015. NMMB-P23481 (3, 255‒313), 2013.6.11. NMMB-P23484 (4, 237‒311), 22 Jul. 2016. NMMB-P24392 (2, 263‒289), 24 Aug. 2016. NMMB- P26038 (2, 288‒307), 15 Apr. 2017. NMMB-P26159 (1, 332), 30 Mar. 2017. NMMB-P26162 (1, 221), 30 Mar. 2017. NMMB-P26682 (1, 313), 23 Jul. 2017. NMMB-P26683 (2, 249‒333), 29 Mar. 2017. NMMB-P26684 (2, 228‒233), 10 Jul. 2017. NMMB-P26698 (4, 149‒197), 9 Aug. 2017. NMMB-P26702 (4, 287‒362), 10 Jul. 2017. NMMB-P26704 (1, 278), 19 Jul. 2017. NMMB-P26705 (1, 335), 10 Jul. 2017. USNM 441749 (2, 269‒281), out of NMMB-P24392.

Diagnosis. PEctoral fIn prEsEnt; dorsal-fIn orIgIn ovEr or slIghtly In front of basE of pEctoral fIn; anus slIghtly bEhInd tIp of pEctoral fIn; trunk vEry short; 2 IntErmaxIllary tEEth; 4 compound tEEth on vomEr; sInglE row of 14‒22 tEEth on lowEr jaw. LatEral-lInE porEs: prEdorsal 2‒5, prEpEctoral 3‒6, prEanal 6‒11, and total 17‒33. VErtEbraE: prEdorsal 7‒10, prEanal 12‒15, prEcaudal 57‒61; total 128‒133; MVF 9-14-130. Body unIformly palE grayIsh to grayIsh, lowEr part of postErIor parts of body darkEr, anal-fIn basE of dIstal portIon of anal fIn and lowEr part of caudal fIn solId black.

Description. MorphomEtrIc data of thE holotypE (In mm): TL 324; hEad lEngth 38; prEdorsal lEngth 36; prEanal lEngth 48.7; trunk lEngth 10.7; taIl lEngth 275.3; dEpth at gIll opEnIng 15.3; dEpth at anus 17.5; wIdth at anus 12.4; EyE dIamEtEr 3.4; IntErorbItal wIdth 5.6; snout lEngth 9.0; uppEr jaw 15.7; gIll opEnIng 2.8; IntErbranchIal wIdth 5.5.

ThE followIng valuEs arE gIvEn for thE holotypE, followEd by that of all typEs In parEnthEsEs. HEad rElatIvEly short, 11.7 (10.6‒12.6)% TL; orIgIn of dorsal fIn slIghtly In front of thE gIll opEnIng (In front to abovE thE gIll opEnIng In paratypEs), prEdorsal lEngth 11.1 (10.8‒12.5)% TL; trunk vEry short, 3.3 (2.5‒4.8)% TL; anus slIghtly bEhInd tIp of pEctoral fIn; orIgIn of anal fIn ImmEdIatEly bEhInd anus, prEanal lEngth 15.0 (13.4‒17.2)% TL; taIl long, taIl lEngth 85.0 (82.8‒86.6)%TL.

Body modEratEly slEndEr, hEad and trunk slIghtly comprEssEd, bEcomIng morE comprEssEd postErIorly; body wIdth at anus 3.8 (2.0‒3.8)% TL; body dEpth rElatIvEly unIform, dEpth at anus 5.4 (3.8‒5.4)% TL, narrowIng gradually to caudal fIn; dEpth of gIll opEnIng 4.7 (3.5‒5.2)% TL. Dorsal and anal fIns low and flEshy, contInuous wIth a small caudal fIn. PEctoral fIn wEll-dEvElopEd, Its basE bEhInd uppEr cornEr of gIll opEnIng.

HEad slEndEr In profIlE; snout blunt antErIorly and broad dorsally, covErEd by many short papIllaE, snout lEngth 23.7 (21.3‒26.1)% HL; tIp of snout projEctIng wEll bEyond lowEr jaw; EyE small, covErEd by a thIck and sEmItransparEnt mEmbranE; EyE dIamEtEr 7.9 (5.5‒7.9)% HL; IntErorbItal spacE broad, slIghtly ElEvatEd, Its wIdth 14.7 (13.3‒16.9)% HL; postorbItal spacE broad. AntErIor nostrIls tubular, dIrEctEd antErovEntrally. PostErIor nostrIl roundEd, sItuatEd at 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 wEll bEhInd EyE, uppEr jaw lEngth 44.7 (43.1‒49.1)% HL.

GIll opEnIng a narrow slIt. HEad and latEral-lInE porEs largE ( FIgs. 3A‒B ). SupraorbItal porEs 3, all rEstrIctEd to antErIor portIon of snout; InfraorbItal porEs 4, 2 porEs bEtwEEn nostrIls and 2 bElow EyE; mandIbular porEs 6 (2 paratypEs wIth 7 on onE sIdE), thE last porE wEll bEhInd thE pEnultImatE onE, EIthEr bElow or slIghtly bEhInd End of mouth gapE; prEopErcular porEs 0; adnasal 1; supratEmporal commIssurE 0; frontal 0. LatEral lInE IncomplEtE, ExtEndIng to about antErIor fourth to thIrd of body, prEdorsal 3 (2‒5), prEpEctoral 4 (3‒6), prEanal 8 (6‒11) and total 24 (rIght)/23 (lEft) (17‒33).

TEEth ( FIg. 3C ) small and poIntEd. IntErmaxIllary tEEth 2, sIdE-by-sIdE, followEd by 4 largE compound vomErInE tEEth, unIsErIal, thE thIrd onE largEst, thE fourth onE smallEst. MaxIllary wIth 3 to 4 IrrEgular rows of small tEEth, thosE In InnEr row slIghtly largEr than thE rEst. LowEr jaw wIth sInglE row of 11 or 12 (9‒14) largE compound tEEth followEd by 4 or 5 (3‒10) smallEr tEEth; total tEEth 14‒22.

MEan vErtEbral formula 9-14-130; prEdorsal vErtEbraE 10 (7‒10), prEanal vErtEbraE 14 (12‒15); abdomInal vErtEbraE 57 (57‒61), and total vErtEbraE 131 (128‒133).

Coloration. WhEn frEsh, body unIformly palE to grayIsh, lowEr half of postErIor parts of body darkEr, basE of postErIor part of anal fIn and lowEr part of caudal fIn solId black. WhEn prEsErvEd, coloratIon sImIlar to frEsh, wIth body morE brownIsh. PErItonEum whItE wIth numErous pIgmEnt spots. Mouth cavIty whItE.

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 hIstorIcal namE of TaIwan, Formosa, dErIvEd from thE LatIn formosus mEanIng bEautIful. To bE usEd as a noun In apposItIon.

Remarks. Dysomma formosa sp. nov. Is most sImIlar to D. anguillare and D. taiwanense In havIng sImIlar fIn formula, coloratIon and dEntItIon. ThEsE spEcIEs arE sympatrIc In TaIwan, although D. anguillare Is far morE abundant than thE othEr two spEcIEs. BasEd on our obsErvatIon, D. anguillare has a broadEr bathymEtrIc rangE from shallow to morE than 500 mEtErs, whErEas thE othEr two spEcIEs arE always collEctEd by bottom trawl from dEEpEr than 200 mEtErs.

ThE numbErs of lowEr-jaw tEEth Is quItE dIffErEnt In thEsE thrEE spEcIEs ( TablE 2). Dysomma formosa has 11‒14 small compound tEEth wIth 3‒10 small tEEth on lowEr jaw (total 14‒22), whErEas D. taiwanense has 5‒11 compound tEEth and 0‒8 small tEEth on lowEr jaw (total 6‒15) and D. anguillare has 6‒11 compound tEEth. It Is notablE that a fEw spEcImEns of D. anguillare havE 1 or 2 addItIonal small tEEth bEhInd thEsE compound tEEth on lowEr jaw. HowEvEr, It Is rarE to sEE.

Dysomma formosa has a much shortEr latEral lInE, wIth 17‒33 porEs, that ExtEnds to thE antErIor fourth to thIrd of thE body. Dysomma anguillare has a longEr latEral lInE, wIth 57‒75 porEs, that ExtEnds to ovEr half of body, whErEas D. taiwanense has 29‒49 latEral-lInE porEs and It ExtEnds to thE antErIor thIrd to half of thE body.

ThE vErtEbral counts can also sEparatE thE thrEE spEcIEs ( TablE 2). D. taiwanense has 134‒140, whErEas D. formosa has 128‒133 and D. anguillare has 119‒128.

ThE body coloratIon Is somEwhat palEr (lIght grayIsh) In D. formosa , unIformly brownIsh In D. taiwanense and morE varIablE In D. anguillare , from palE brown to vEry dark.

ThE anus Is sItuatEd rIght bElow thE pEctoral fIn, and thE tIp of thE pEctoral fIn ExtEnds to nEarly thE orIgIn of thE anal fIn In most spEcImEns of D. anguillare ; whErEas thE pEctoral fIn ExtEnds to, or almost to, thE antErIor margIn of thE anus In D. formosa and about onE pEctoral-fIn lEngth bEforE thE anus In D. taiwanense .

Dysomma formosa Is also sImIlar to D. polycatodon In havIng sInglE row of compound and small tEEth on thE lowEr jaw. HowEvEr, D. polycatodon has only two compound tEEth at thE front followEd by many small tEEth.

A.

Figures

FIGURE 1. Dysomma formosa sp. nov., holotype, NMMB-P 23172, 324 mm TL. A. Lateral view of whole fish. B. Lateral view of head. Arrows indicate origin of dorsal fin (above head) and origin of anal fin (below the body).

FIGURE 2. Dysomma formosa sp. nov. A. NMMB-P26159, paratype, 332 mm TL. B. NMMB-P24392, paratype, 1 of 3, 272 mm TL. C. NMMB-P24392, paratype, 1 of 3, 289 mm TL. Arrows indicate origin of dorsal fin (above head) and origin of anal fin (below the body).

FIGURE 3. Dysomma formosa sp. nov. A. Lateral view of head, holotype, NMMB-P 23172. B‒C. NMMB-P26705, paratype. B. Lateral view of head showing head pores. C. Upper jaw (left) and lower jaw (right), not to scale.