Devario kysonensis ( Nguyen, Nguyen & Mua, 2010 )

Kottelat, Maurice, 2020, Redescription of Devario kysonensis and first record from Laos (Teleostei: Cyprinidae), Raffles Bulletin of Zoology 68, pp. 556-561 : 556-559

Devario kysonensis ( Nguyen, Nguyen & Mua, 2010)

( Figs. 1 , 2 )

Danio kysonensis Nguyen, Nguyen & Mua, 2010: 62 , fig. 1 (type locality: Vietnam: Nghe An Province: Ky Son District: Nam Can commune [19°10′20″N 104°15′48″E]: Nam Khien spring, Song Lam drainage).

Material examined. CMK 27892, 13 formalin-fixed, 2 ethanol-fixed, 25.9–67.5 mm SL; ZRC 61298, 2 , 41.3–45.9 mm SL; Laos: Bolikhamsai Province: Nam Kading watershed: Houay Taen, small tributary of Nam Tuk , at ford about 3 km southeast of Ban Vangphieng on road to Viengthong ; Nam Tuk is a tributary of Nam Chouan , which it enters at Ban Vanghieng ; 18°47′34.4″N 104°29′44.5″E; 472 m asl; M. Kottelat et al., 25 March 2018 . — CMK 27822, 1, 52.6 mm; Laos: Bolikhamsai Province: Nam Kading watershed: Houay Kapong, at mouth, a small stream entering Nam Ma about 800 m upstream of confluence of Nam Ma and Nam Sang , about 3 km northeast of Ban Vangphieng ; 18°49′15.6″N 104°30′30.9″E; 439 m asl; M. Kottelat et al., 23 March 2018 .

Diagnosis. Devario kysonensis is distinguished from all other species of the genus by its unique colour pattern in adults, made of 2 or 3 rows of black irregularly shaped spots on flank. Other characters useful for identification but not unique to the species are: lateral line complete, on 31–33 + 2–3 scales, 9½–11½ branched dorsal-fin rays and 11½–12½ branched anal-fin rays, infraorbital process present, and 2 pairs of barbels, one rostral and one maxillary.

Description. See Figs. 1 , 2 for overall appearance, and Table 1 for morphometric data of 10 specimens. Body moderately deep, compressed. Dorsal profile straight to slightly convex on head, with a marked concavity at nape, and convex behind, body depth increasing until dorsal-fin origin, then decreasing regularly to posterior extremity of dorsal-fin base, then about straight horizontal. Ventral profile convex from chin to posterior extremity of anal-fin base, then about straight horizontal. Caudal peduncle 1.3–1.6 times longer than deep.

Infraorbital process present ( Fig. 3 ). Danionine notch present. Mouth terminal to slightly superior. Knob at dentary symphysis weakly developed, fitting in depression in upper jaw. Maxilla reaching to below anterior half of orbit. Lower jaw with only few and small conical tubercles, in a short line in 1 or 2 rows along outer edge of dentary and a few along the danionine notch. No visible tubercles on pectoral fin. Rostral barbel short, almost reaching base of maxillary barbel; maxillary barbel one third length of rostral barbel, reaching below middle of orbit.

Dorsal fin with 2 (9) or 3 (1) unbranched and 9½ (2), 10½ (7), or 11½ (1) branched rays; distal edge straight. Pectoral fin with 11 (1) or 12 (9) rays; posterior edge straight; not reaching pelvic-fin base; a small axillary lobe present. Pelvic fin with 7 (9) or 8 (1) rays; posterior edge straight; axillary scale present. Anal fin with 3 unbranched and 11½ (2) or 12½ (8) branched rays; distal edge slightly concave; origin below base of branched dorsal-fin rays 3–5; not reaching anal-fin origin. Caudal fin forked, with 10+9 principal rays, 9+8 branched (except for one specimen with damaged and regrown fin with 8+8 branched rays).

Lateral line complete, along 31+3 (1), 32+2 (2), 32+3 (3), 33+2 (3), or 33+3 (1) scales (3–5 unperforated scales in steep anterior part). 14 (3) or 15 (7) predorsal scales. ½6 (3) or ½7 (7) scale rows between dorsal-fin origin and lateral line; 2½ (1) or 3½ scale rows between lateral line and midventral line (counted about 2–3 scales in front of pelvic-fin base); 1½ scales between lateral line and pelvicfin origin; ½3/1/1½ (9) or ½4/1/1½ (1) transverse rows of scales on caudal peduncle.

Sexual dimorphism: none observed.

Colouration. In formalin, about 1 month after fixation. Body olive brown, darker brown on back, pale yellowish on belly. In largest specimens, 2 or 3 longitudinal rows of irregular black spots (smaller than eye) from gill opening to caudal peduncle (corresponding to stripes P, P+1 and P- 1 in other Devario species ). In smallest specimens (about 25–30 mm SL; Fig. 1a ), a faint grey midlateral stripe (P) from about above pelvic-fin until base of caudal fin, continued anteriorly by 2 or 3 blotches immediately in front of stripe. Stripe margined above and below by a yellowish stripe (I+1, I-1); space between blotches and stripes same yellowish. A greyish stripe (P-1) from lower edge of blotches, parallel to midlateral stripe and ending on caudal peduncle. A faint greyish stripe (P+1) above upper yellow stripe.

In specimens 40–50 mm SL ( Fig. 1b–d ), midlateral stripe P becoming dark grey to black. Blotches becoming black and vertically elongated, reaching stripe P-1. Additional black blotches appearing overimposed on stripe P, more distinct anteriorly. A row of blackish spots appearing on stripe P+1, which disappears. Small spots on stripe P-1, connected or not with those on stripe P. With increasing size ( Figs. 1e–h , 2 ), stripe P paler or indistinct except on caudal peduncle; stripe P-1 is fainter or missing. Spots on stripe P are smaller, more numerous, more contrasted, and extend until caudal-fin base. Spots on stripe P-1 are more numerous, and extend until above end of anal-fin base. Spots on stripe P+1 are more numerous, irregularly shaped, and extend until about midlength of caudal peduncle. With increasing size, spots are less regularly shaped and less regularly set; additional spots appear between the main rows.

A small vertically elongated black spot behind upper extremity of gill opening, visible at all sizes, although less contrasted in largest individuals.

Dorsal fin: a black band made of pigments on membranes in median position; distal part hyaline, proximal part greyish. Anal fin: a black band made of pigments on membranes in median position; distal part yellowish, proximal part pale grey. Caudal pale yellowish; proximal half of median rays black, continuing stripe P. Pectoral and pelvic fins yellowish.

In fresh specimens, about 10 hours after fixation (heavy rain did not allow to examine material at day light earlier; Fig. 2 ): body pale brown with greenish to orangish hue, blotches black with greenish hue, back brown, belly silvery. Dorsal fin: median stripe dark brown, distal part whitish to pale yellowish, proximal part pale brown. Anal fin: median band pale brown, distal part yellow to orange, proximal part greyish to orange. Caudal fin pale orange. Pectoral and pelvic fins orange.

Geographical distribution. Devario kysonensis was originally described from the Song Lam drainage in Nam Can commune ( Ky Son district , Nghe An Province, Vietnam; Nguyen et al., 2010). No additional records have appeared in available Vietnamese literature as far as could be ascertained. Nam Can is located at 19°10′20″N 104°15′48″E, along the border with Laos , which corresponds to the water divide between the Song Lam and Mekong drainages. The Song Lam drains eastwards and enters the Gulf of Tonkin.

In Laos, the species has been observed only in the Nam Chouan. The Nam Chouan originates along the divide between the Mekong drainage and the Song Lam. It is formed by the confluence of two small streams, Nam Xang and Nam San, at 18°49′20″N 104°30′10″E. After about 40 km, at 18°51′39″N 104°14′42″E the Nam Chouan meets the Nam Sang and becomes the Nam Mouan, which after about 100 km enters the Nam Kading at 18°23′00″N 104°18′41″E. Headwaters of the Nam Sang originate on the Mekong–Song Lam divide, where it makes the limits of Nam Can commune. The species is expected to be more widespread in headwaters on both sides of the water divide .

Field observations. In the Nam Chouan, D. kysonensis was obtained in small streams, 1–3 m wide, 20–100 cm deep, with riffles, along shores in sheltered areas. The water was cool and clear. Other species present in syntopy included Neolissochilus blanci , Scaphiodonichthys acanthopterus , Balitora cf. annamitica , Schistura corruscans , S. cf. crassa, S. dorsizona , S. ephelis , S. obeini , S. sombooni , Oreoglanis delacouri , and Channa limbata .

In March 2018, temperatures in the upper Nam Chouan watershed were very low, air temperature about 10–15° at night and it had reportedly been lower in December–January. Water was cold (estimated 10–15°C) and fish were at low density. Most collected specimens of all species were relatively large. It is noteworthy that, of most species, there were very few or no juveniles, especially of the smallsized species that usually would have spawned around December–February (end of rainy season or beginning of dry season). This suggests a cold spell in the preceding months (at spawning time or early in life-cycle), which villagers indicated; however, there are no available meteorological data to confirm or refute this hypothesis. For most smallsized species (e.g., Schistura spp. ), the presence of adults (estimated 2 years old) in numbers equivalent to those usually observed in the previous days in other areas (e.g., Nam Ngiep watershed) under similar sampling conditions but warmer water, suggests that the cold temperature could be responsible for the low number of fry and juveniles. Massive overfishing, for example with electricity or ichthyocides, seems excluded because this would have resulted in the absence of large individuals and the numerical dominance of juveniles of the year (pers. obs.).

The upper Nam Chouan is included in the Nam Chouan– Nam Xang Biodiversity Offset Site for the Nam Ngiep 1 hydropower scheme. Human activity is presently limited in this area, there is no known mining and there is a very limited number of ‘roads’ and the resulting accumulation of sediments in the river bed. Because of its small known range and two documented populations, with the present knowledge the species would probably have to be rated as Data Deficient or Vulnerable using IUCN Red List criteria (IUCN, 2001).

Figures

Fig. 1. Devario kysonensis; Laos: Nam Kading: Nam Chouan: a, CMK 27892, 31.3 mm SL; b, CMK 27892, 43.3 mm SL, right side, reversed; c, CMK 27892, 43.5 mm SL; d, CMK 27892, 45.3 mm SL, right side, reversed; e, CMK 27892, 53.1 mm SL, right side, reversed; f, CMK 27822, 52.6 mm SL; g, CMK 27892, 60.6 mm SL, right side, reversed; h, CMK 27892, 59.2 mm SL.

Fig. 2. Devario kysonensis, CMK 27892; Laos: Nam Kading: Nam Chouan; shortly after fixation; a, 53.1 mm SL; b, 60.6 mm SL; c, 65.1 mm SL. All right side, reversed.

Fig. 3. Devario kysonensis, CMK 27892, 59.2 mm SL; head, showing infraorbital process (arrow).