Trigonostigma truncata, Hui, 2020

Hui, Tan Heok, 2020, Trigonostigma truncata, a new species of harlequin rasbora from Malay Peninsula (Teleostei: Danionidae), Raffles Bulletin of Zoology 68, pp. 421-433 : 423-427

Trigonostigma truncata , new species

( Figs. 2 , 3 , 4A , 5 )

Rasbora heteromorpha (non-Duncker) – Mohsin & Ambak, 1983: 50 (part); Kottelat et al., 1992: 9, table 1; Kottelat et al., 1993: 63, pl. 18 (part).

Rasbora cf. heteromorpha – Collins et al., 2012: 10, table 3, supplementary data table S1: 16 (part?).

Trigonostigma heteromorpha (non-Duncker) – Kottelat & Witte, 1999: 54 (part), fig. 10; Kottelat, 2013: 170 (part); Panitvong, 2020: 142.

Trigonostigma aff. heteromorpha – Ng et al., 2019: 527 (part).

Material examined. — Holotype: ZRC 61240, 1 ex., 24.2 mm SL; Thailand: Narathiwat Province, stream along road branching West at about 7 km on road from Waeng to Ban Bu Ke Ta; Kottelat M et al., 2 November 1995.

Paratypes: SOUTH THAILAND — CMK 12049, 21 ex. , ZRC 42090, 21 ex. , 9.7–24.2 mm SL; same locality as

holotype. — ZRC 42131, 2 ex., 17.0– 20.3 mm SL; Thailand: Narathiwat Province , stream about 2 km South of Ban Bu Ke Ta on road to Ban Sac; Kottelat M et al., 2 November 1995 . — ZRC 42076, 6 ex., 15.1–21.6 mm SL; Thailand: Narathiwat Province, swamp area about 2 km North of Ban Bu Ke Ta, about 8 km South of Waeng ; Kottelat M et al., 2 November 1995 . PENINSULAR MALAYSIA — CMK 8221 , 50 ex. , ZRC 24807, 36 ex., 8.2–25.1 mm SL; Malaysia: Terengganu, stream at about 6 km on road from Kuala Brang to Kuala Terengganu; Ng PKL et al., 19 March 1992 . — ZRC 40234, 40 ex., 11.9–24.7 mm SL; Malaysia : Terengganu, 5 km Kuala Brang-Kuala Terengganu road; Ng PKL et al., 16 May 1995 . — ZRC 41933, 7 ex., 15.1–21.7 mm SL; Malaysia: Terengganu, Kuala Brang; Ng PKL et al., October 1997 . — ZRC 1966 , 27 ex., 9.5–22.3 mm SL; Malaysia : Terengganu, 17.5 mile Kuala Terengganu-Kuala Brang road; Alfred ER, 8 July 1958 . — ZRC 1714 , 6 ex., 15.3–20.2 mm SL; Malaysia: Terengganu, Kuala Brang; Tweedie M, August 1950 .

Others: ZRC 54736, 4 ex., 29.7–33.7 mm SL; obtained from aquarium trade, September 2015 .

Diagnosis. Trigonostigma truncata , new species, is most similar to T. heteromorpha , in having a large black axine starting from approximately mid-body with dorsal apex near dorsal-fin origin, ventral apex near pelvic-fin origin, and caudal apex near caudal-fin base; this axine being the largest amongst all congeners. Trigonostigma truncata differs from T. heteromorpha in the following characters: a gently sloping lateral head profile to pre-dorsal region (vs. a steep convex lateral profile from posterior of head to predorsal region); sub-superior mouth (vs. terminal mouth); caudal apex of axine not reaching caudal-fin base (vs. reaching and extending to hypural plate); dorsal and ventral apices of axine originate posterior to both dorsal-fin and pelvic-fin origins by up to three scale-widths (vs. dorsal apex starting one scale-width posterior to dorsal-fin origin and ventral apex starting at pelvic-fin origin; see Figs. 4 , 5 ); faint or indistinct brown humeral streak just posterior to opercular opening (vs. a distinct black humeral streak); base of dorsal fin hyaline and its middle reddish-orange in life (vs. anterior two-thirds of fin orange-red); presence of distinct orange-red colour on anal fin in life (vs. faint or absence); having a bluish-lilac sheen on the body in life (vs. reddish or purplish sheen); and having a shallower body (depth at dorsal-fin origin 28.3–34.6% SL [mean 32.4], vs. 32.6–38.2% [mean 35.8]).

Description. See Figs. 2–4 for general appearance. See Table 1 for meristic and morphometric data. Head pointed, with subsuperior mouth, barbels absent. Profile of head and anterior of body gently sloping upwards to dorsal-fin origin, absence of distinct notch between occiput and nape. Orbit relatively large (orbital diameter 33.8–40.4% HL). Body compressed, deepest at dorsal-fin origin (body depth 28.3–34.6% SL) and shallowest at caudal peduncle (caudal peduncle depth 11.0–14.5% SL). Dorsal and pelvic fins situated mid-body (predorsal length 51.2–56.1% SL; prepelvic length 48.9–53.3 % SL), triangular and small (dorsal-fin base length 12.9– 16.9% SL). Caudal fin forked, symmetrical (upper caudal-fin lobe length 33.2–42.7% SL; lower caudal-fin lobe length 36.4–43.6% SL). Anal fin triangular, situated ⅔ down the body (preanal length 64.3–70.7% SL), and small (anal-fin base length 10.6–15.2% SL). Both pelvic and pectoral fins triangular. Lateral scale series with 26–30 scales (mode 27), lateral line incomplete and leading horizontally away from head, perforated lateral line scales 6–8 (mode 7). Both dorsal-fin and pelvic-fin origins on vertical through lateral scale 8–10 (mode 9), anal-fin origin at 14–15 (mode 14). Axine starting at lateral scale series 10½–12½ (mode 11).

Colouration in preservative. See Fig. 2 . Base body colour cream, dorsum dark brown, with dark brown stripe across dorsum. Head region above eye dark brown. Eye with silvery iris. Region posterior to opercle opening with faint brown humeral streak. All fins hyaline, thin black margin on distal half of anterior edge of dorsal and anal fins. Black or dark brown axine on mid-body, anterior margin concave to straight edged, dorsal apex starting about three scales posterior to dorsal-fin origin, ventral apex starting above pelvic-fin origin, caudal apex extending length of caudal peduncle stopping about 1–2 scales anterior to caudal-fin base; axine surrounded with distinct margin of 1–1½ scale width. Region of body anterior to triangular marking with diffused melanophores.

Colouration in life. See Figs. 3 , 4 . Base body colour pale yellowish-brown, dorsum can be a darker shade; sometimes with slight orangish iridescence on anterior half of body and bright orange blotch at caudal-fin base; in fully acclimatised specimens in captivity, a bluish-lilac sheen can be visible on body. Region of head above eye dark brown or brown. Eye with yellowish-orange iris with black dorsal patch, ventral area silver. Region posterior to opercular opening with faint brown humeral streak. Large velvety black axine on midbody, anterior margin concave to straight edged, dorsal apex at about three scales posterior to dorsal-fin origin, ventral apex at pelvic-fin origin, caudal apex extending length of caudal peduncle to about 1–2 scales anterior to caudal-fin base; whole axine surrounded with distinct margin of 1–1½ scale width. Paired fins hyaline. Dorsal fin base hyaline, thin black margin on distal half of anterior edge, middle section orangish-red, distal margin hyaline. Caudal fin yellowish with hyaline distal margin. Anal fin base hyaline, thin black margin on distal half of anterior edge, middle section orangish-red, distal margin hyaline. Supra-anal region to ventral base of caudal peduncle with dark brown streak.

No sexual dimorphism or dichromatism observed. From hobbyists’ observations, males are usually more intensely coloured.

Distribution. Trigonostigma truncata , new species, is currently known from the east coast of the Malay Peninsula, from south of the Isthmus of Kra in Narathiwat province of southern Thailand to the Malaysian State of Terengganu ( Fig. 6 ). The map in Fig. 6 also shows the distribution of all five species for comparison.

Field notes. This species is found in lowland freshwater acid swamp-forest stream habitats, usually flowing into riverine habitats. Syntopic species collected from Mae Nam Tod Deng swamp forest in South Thailand include the following: Notopterus notopterus (Notopteridae) , Boraras urophthalmoides , Trigonopoma gracile (Danionidae) , Kryptopterusminor ( Siluridae ), Clariasmeladerma ( Clariidae ), Monopterus javanensis (Synbranchidae) , Chaudhuria sp. ( Chaudhuriidae ), Oryzias minutulatus (Adrianichthyidae) , Indostomus crocodilus (Indostomidae) , Betta imbellis , B. pi, Parosphromenus paludicola , Trichopodus trichopterus , Trichopsis vittata (Osphronemidae) , Channa limbata , and C. lucius (Channidae) .

Syntopic species collected from Kuala Brang in Terengganu include the following: Osteochilus vittatus (Cyprinidae) , Rasbora bankanensis , R. dusonensis , Trigonopoma gracile (Danionidae) , Acanthopsoides sp., Lepidocephalichthys furcatus , Pangio cuneovirgata , P. piperata , P. semicincta , P. muraeniformis (Cobitidae) , Homalopteroides nebulosus (Balitoridae) , Nemacheilus selangoricus (Nemacheilidae) ,

RAFFLES BULLETIN OF ZOOLOGY 2020

Barbucca diabolica (Barbuccidae) , Hemibagrus capitulum , Nanobagrus fuscus , Pseudomystus stenomus (Bagridae) , Silurichthys hasseltii (Siluridae) , Parakysis verrucosus (Akysidae) , Clarias leiacanthus (Clariidae) , Neostethus smithi (Phallostethidae) , Bihunichthys sp. ( Chaudhuriidae ), Hemirhamphodon pogonognathus (Zenarchopteridae) , Xenentodon canciloides (Belonidae) , Doryichthys martensii (Syngnathidae) , Brachygobius xanthomelas (Gobiidae) , Pristolepis grooti (Pristolepididae) , Luciocephalus pulcher , and Parosphomenus paludicola (Osphronemidae) .

Syntopic species collected from Dungun swamp in Terengganu include the following: Barbodes cf. binotatus , Desmopuntius hexazona , Osteochilus vittatus , O. waandersii (Cyprinidae) , Boraras maculatus , Rasbora einthovenii , R. cephalotaenia , R. paucisqualis , Trigonopoma gracile , T. pauciperforatum (Danionidae) , Lepidocephalichthys furcatus , Pangio semicincta (Cobitidae) , Homalopteroides nebulosus (Balitoridae) , Nemacheilus selangoricus (Nemacheilidae) , Neostethus smithi (Phallostethidae) , Hemirhamphodon pogonognathus (Zenarchopteridae) , Nandus nebulosus (Nandidae) , Betta stigmosa , Luciocephalus pulcher , and Parosphromenus paludicola (Osphronemidae) .

Etymology. From the Latin ‘ truncus ’, meaning cut off, in allusion to the caudal apex of the axine not reaching the base of the caudal-fin. Used as a noun in apposition.

Remarks. Trigonostigma truncata , new species, can be further differentiated from T. heteromorpha in the following characters: adpressed pectoral fin not reaching pelvic-fin origin (vs. reaching); adpressed pelvic fin not reaching analfin origin (vs. surpassing); more lateral scale count (26–30 [mode 27], vs. 25–28 [mode 26]); perforated lateral line scales leading horizontally away from head (vs. curving gently downwards away from head); relatively longer head (head length 28.5–31.6% SL [mean 30.2], vs. 27.2–31.7% [mean 28.9]); relatively slimmer caudal region (body depth 22.5–29.3% SL [mean 25.8], vs. 24.4–30.2% [mean 27.9]; caudal peduncle depth 11.0–14.5% SL [mean 12.9], vs. 12.6–15.8% [mean 13.8]); more slender head (head depth 62.1–76.9% HL [mean 69.6], vs. 68.1–80.6% [mean 74.7]; head width 39.4–51.9% HL [mean 46.8], vs. 42.9–53.5% [mean 49.1]); relatively smaller eye (orbital diameter 33.8–40.4% HL [mean 36.8], vs. 35.6–44.8% [mean 40.2]).

Trigonostigma truncata , new species, can be differentiated from the other congeners ( T. espei , T. hengeli , and T. somphongsi ) by having a larger adult size (up to 33.7 mm SL, vs. less than 30 mm); large black axine on body with dorsal and ventral borders not exceeding one scale spacing to edge of body (vs. 1½–2½ scale width).

In the naming of the new genus Trigonostigma by Kottelat & Witte (1999: 54, fig. 10), they presented a figure of a preserved specimen of T. heteromorpha which is from the series ZRC 42076. This is actually part of the type series for the new species, T. truncata . The two taxa are so similar that they were misidentified.

Collins’ et al. (2012: 6, fig. 4) material of T. heteromorpha consist of seven specimens from two genetic populations, with COI divergence of more than 3%. They further commented on the presence of orange pigments on the anal fin (supplementary table 1), which is congruent with the present new species; however all of their material is from the ornamental fish trade, which is without precise locality data, and thus of limited use for taxonomical studies.

Figures

Fig. 2. Trigonostigma truncata, new species, holotype, ZRC 61240, 24.2 mm SL; Thailand: Narathiwat.

Fig. 3. Trigonostigma truncata, new species, live specimen of ca. 30 mm SL (trade material, not preserved).

Fig. 4. A, Trigonostigma truncata, new species, ZRC 54736, 33.7 mm SL, trade material; B, T. heteromorpha, ZRC 61239, 28.4 mm SL, trade material; C, T. espei, ZRC uncat, 28.2 mm SL, trade material (obese individual); D, T. hengeli, ZRC uncat, 23.3 mm SL, Sumatra: Jambi; E, T. somphongsi, not preserved, ca. 15 mm SL (right-side reversed; photograph by N. Panitvong).

Fig. 5. Trigonostigma truncata, new species, ZRC 54736, 33.7 mm SL (left); and Trigonostigma heteromorpha, ZRC 61239, 28.4 mm SL (right); detail of body, showing axine.

Fig. 6. Map of Southeast Asia showing distribution of Trigonostigma: T. heteromorpha (solid circle), T. somphongsi (hollow circle), T. espei (diamond), T. hengeli (square), and T. truncata, new species (pentagon). Each symbol may represent more than one location.