Laubuka tenella, Kullander, Sven O., Rahman, Md. Mizanur, Noren, Michael & Mollah, Abdur Rob, 2018

Kullander, Sven O., Rahman, Md. Mizanur, Noren, Michael & Mollah, Abdur Rob, 2018, Laubukatenella, a new species of cyprinid fish from southeastern Bangladesh and southwestern Myanmar (Teleostei, Cyprinidae, Danioninae), ZooKeys 742, pp. 105-126 : 109-115

Laubuka tenella sp. n. Figs 1, 2, 3

Holotype.

(Fig. 1A). DU 9004, 42.1 mm SL. Bangladesh: Chittagong Division: Cox’s Bazar District: Naf River drainage, Domdomia stream, 10 km north of Teknaf town, 70 km south of Cox’s Bazar; 20°55'24"N, 92°15'47"E; M.M. Rahman et al., 9 May 2015.

Paratypes.

DU 9005, 6, 30.8-42.2 mm SL; DU 9006/ NRM 67845, 1, 47.4 mm SL; DU 9007/ NRM 67861, 1, 35.8 mm SL; NRM 67862, 1, 38.4 mm SL; NRM 69227, 1, 47.4 mm SL; NRM 68062, 7, 33.9-45.7 mm SL; same data as holotype. NRM 67380, 1, 46.8 mm SL; DU 9008/ NRM 67381, 1, not measured; Bangladesh, Chittagong Division: Bakkhali River drainage, Majerchora stream, 10 km south of Cox’s Bazar; 21°23'45"N, 92°0'16"E; M.M. Rahman et al., 8 May 2015. BMNH 2018.1.31.3-5, 3, 37.9 mm SL; NRM 40813, 15, 33.9-42.5 mm SL; NRM 69798, 10, 29.6-37.0 mm SL; Myanmar: Rakhine State; Thandwe River drainage: Nan Chaung, a stream at 3 km on road from Thandwe (market) to Ngapali; 18°27'8"N, 94°20'55"E; S.O. Kullander & R. Britz, 20 Mar 1998.

Diagnosis.

Distinguished from all other species of Laubuka except L. insularis , L. lankensis , L. ruhuna , and L. varuna by the colour pattern, including a dark stripe along the middle of the posterior third of the side or slightly shorter, anteriorly replaced by 6-11 short vertical bars (vs. presence of a dark stripe along the side but absence of bars in L. fasciata , L. parafasciata , and L. trevori ; plain sides or presence of a very narrow posterior stripe in L. khujairokensis , L. latens , and L. laubuca ; indistinct vertical bars anteriorly on the side, followed by a dark stripe ending in a triangular spot on the caudal-fin base in L. siamensis ; a few dark bars present anteriorly on the side but lateral band absent in L. caeruleostigmata ). Distinguished from L. fasciata , L. latens and L. trevori also by more dorsal-fin rays (ii. 8½ vs. ii. 7½) and from L. siamensis by the absence of a dark spot on the caudal-fin base. Distinguished from L. insularis by fewer scales in the lateral line (29-32 vs. 34-36), shorter pelvic fin (not reaching to vent, vs. reaching to bases of anal-fin rays 3-8); from L. lankensis by fewer scales in the lateral line (29-32 vs. 34-37); from L. ruhuna and L. varuna by the presence of an entire, narrow lateral band on the posterior third of the body, vs. a series of blotches along the side which may form a broad band extending anteriorly to about the middle of the side.

Description.

Elongate, strongly compressed laterally. Predorsal contour slightly ascending, levelling out close to dorsal fin-base, minor indentation at commencement of squamation. Dorsal-fin origin marking 2/3 of standard length, immediately posterior to vertical from anal-fin origin. Dorsal-fin base contour slanting, continuous with dorsal contour of caudal peduncle; caudal peduncle only slightly tapering caudad.

Snout shorter than orbital diameter, triangular in lateral aspect, rounded in dorsal aspect. Mouth terminal, lower jaw at about 50° angle, tip anterior to upper jaw, not quite reaching level of upper margin of orbit. Eyes large, lateral, in middle of head length, well visible in ventral aspect of head, in dorsal aspect only slightly. Anterior nostril tubular, opening anterolaterad. Supraorbital ending anteriorly in sharp point. Long shallow frontal and rostral neuromast grooves present. Barbels absent. Tubercles absent. Lower jaw with wide band of minute papillae, tentatively identified as neuromasts. Ventral outline more arched than dorsal; slanting about straight to under pectoral-fin base, posteriorly about straight horizontal to anal-fin insertion; anal-fin base contour straight ascending. Chest flat close to isthmus; from pectoral-fin base caudad to pelvic-fin base strongly compressed, posteriorly strongly compressed and with sharp keel formed by margins of opposed left and right side abdominal scales.

All scales cycloid, thin, transparent. Lateral line anteriorly descending for about five scales, posteriorly paralleling ventral outline, ending on lower half of caudal peduncle, continued by 1-2 scales basally on caudal fin. Single row of scales along base of anal fin. Elongate axillary pelvic-fin scale present. Lateral line scales 29 (1), 30 (2), 31 (7), 32 (3) in Cox’s Bazar specimens; 29 (1), 30 (5), 31 (7) in Rakhine specimens. Predorsal scales 16 (1), 17 (6), 18 (4) in Cox’s Bazar specimens; 16 (2), 17 (7), 18 (1) in Rakhine specimens. Circumpeduncular scales 12 (27). Scales between dorsal fin origin and lateral line ½6 (28); between lateral line and anal-fin origin 3 (28), of which distal scale part of abdominal keel.

Dorsal-fin origin at about posterior third of body, slightly posterior to origin of anal fin; with straight distal margin, rays gradually shorter from second unbranched ray to last ray. Dorsal-fin rays ii. 8½ (29). Anal fin with longer base than dorsal fin; short rounded anterior lobe, posterior rays gradually shorter. Anal-fin rays iii. 16½ (3), iii. 17½ (3), iii. 18½ (8) in Cox’ Bazar specimens; iii. 18½ (3), iii. 19½ (10, iii. 20½ (2) in Rakhine specimens. Pectoral-fin long, falcate, unbranched ray longest or unbranched and first branched ray equally long, not reaching to vent; two large scales covering bases of branched fin rays and adjacent chest. Caudal fin forked to about middle of fin. Pectoral-fin rays i.10 (1), i.11 (10), i.11 (3) in Cox’s Bazar specimens; i.10 (4), i.11 (9), i.12 (2) in Rakhine specimens. Pelvic fin inserted slightly anterior to middle of side; short, unbranched ray longest with short prolongation, not reaching to vent. Pelvic-fin rays i.5 (1), i.6 (13) in Cox’s Bazar specimens; i.6 (15) in Rakhine specimens.

Vertebrae: predorsal 16 (2), 17 (5), precaudal+caudal 15+18 (1), 15+19 (5), 16+19 (1), within caudal peduncle 5 (4), 6 (3) in Cox’s Bazar specimens; predorsal 16 (1), 17 (5), precaudal+caudal 15+18 (1), 15+19 (4), 16+19 (1), within caudal peduncle 5 (4), 6 (2) in Rakhine specimens.

Ground colour in formalin-fixed specimens (Figs 1 A–B) pale yellowish white with diffuse grey or black markings except for round black cleithral spot size of pupil. Dorsum sparsely pigmented; brown stripe on dorsal midline from occiput to end of caudal peduncle. Thin black or brown stripe along middle of caudal peduncle or slightly longer, anteriorly replaced by 6-11 grey or brown short vertical bars, not reaching ventrally onto abdomen, less distinct in some larger specimens. A few black dots along middle of abdomen. Fins hyaline. Ethanol-fixed specimens (Fig. 1C), with silvery opercle and sides; vertical bars indistinct, dorsum grey, abdominal sides pale yellow; cleithral spot and lateral band black. Live specimens observed only in the type locality, with silvery reflections dorsally, abdomen white, sides blue, along middle a wide iridescent green cleithral spot flanked by golden (Fig. 2).

Etymology.

The specific name is a Latin adjective in diminutive form, tenellus, here in the meaning of delicate, referring to the small size and the soft, delicate consistency of fresh specimens.

Comparative morphometry.

Laubuka tenella is slightly more slender than L. laubuca of similar size (Tables 1-2, Fig. 3), but size differences and potential sexual dimorphism between the measurement series prevent a conclusive comparison. The sample of Laubuka laubuca (N=7) is too small to establish significant linear regression parameters.

Phylogenetic characterization and relationships.

The Bayesian phylogenetic analysis recovered Laubuka tenella as the sister species of L. laubuca (Fig. 4). COI sequences of L. tenella differed from the most similar sequences, in L. laubuca , by 9 % uncorrected p-distance. The within-species variation in L. tenella amounted to 0.9 %, between the Majerchora and Domdomia samples. Maximum pairwise p-distance in L. laubuca was 2.5% when the sequence KT353103 was included, and 0.9 % when excluded. Species delimitation methods confirmed reciprocal distinctness of L. laubuca and L. tenella : P ID(Liberal), the mean probability of making a correct identification of an unknown specimen of the focal species was reciprocally 0.97. At 6*10-4, Rosenberg’s P(AB) failed the null hypothesis that the combined clade ( L. laubuca + L. tenella ) represents a single species.

Geographical distribution and habitat.

Laubuka tenella is known only from small streams in the vicinity of Cox’s Bazar and Teknaf in Bangladesh, and Thandwe in Myanmar (Fig. 5).

Collections were made in the dry season when the streams had very little water. The type locality (Fig. 6) was in the lower course of the Domdomia stream, close to the mouth of the Naf River, flowing out of low forest into pasture associated with a village. At the time the stream was very shallow, not more than 10 m wide, with slightly turbid water and a bottom substrate of clay mixed with stones. The steep banks suggested that the water level in the monsoon season would reach a few meters higher. The associated fish fauna included Aplocheilus panchax (Hamilton) ( Aplocheilidae ), Megalops cyprinoides (Broussonet) ( Megalopidae ), Acentrogobius caninus (Valenciennes) ( Gobiidae ), Eleotris melanosoma Bleeker ( Eleotrididae ), Dermogenys burmanica Mukerji (Zenarchopterdae), and Oryzias cf. dancena (Hamilton) ( Adrianichthyidae ), reflecting proximity to the Naf River estuary. Other associated species in the Domdomia stream were identified as Glossogobius giuris (Hamilton) ( Gobiidae ), Channa gachua (Hamilton), C. punctata (Bloch) ( Channidae ), Danio sp., Devario anomalus Conway, Mayden & Tang, Devario coxi Kullander, Rahman, Norén & Mollah, Esomus danrica (Hamilton), Pethia ticto (Hamilton), Puntius chola (Hamilton), and Rasbora rasbora (Hamilton ( Cyprinidae ).

The Majerchora stream, in a hilly landscape with low forest near the sea, was very small, less than 1 m wide, and not more than about 30 cm deep. The water was only slightly turbid, flowing slowly over a bottom of sand and clay. Very few fish specimens were collected at this site; associated species were identified as Danio sp., Devario coxi , and Pethia ticto ( Cyprinidae ).

The locality in Myanmar was a stagnant pool in a desiccated small river, with leaf litter and sand on the bottom. The associated species were identified as Anguilla sp. ( Anguillidae ), Aplocheilus panchax ( Aplocheilidae ), Channa sp. ( Channidae ), Danio aesculapii Kullander & Fang, Pethia sp., Puntius chola , Rasbora cf. daniconius (Hamilton), Rasbora rasbora ( Cyprinidae ), and Lepidocephalichthys berdmorei (Blyth) ( Cobitidae ).