Nothobranchius elucens, Nagy, 2021

Nagy, Béla, 2021, Nothobranchius elucens, a new species of seasonal killifish from the upper Nile drainage in Uganda (Cyprinodontiformes: Nothobranchiidae), Zootaxa 4915 (1), pp. 133-147 : 139-143

Nothobranchius elucens , new species

( Figs. 2–4 )

Nothobranchius species Madi Opei: Nagy 2017a: 169; Nagy 2017b: 48; Nagy 2018b: 77; Nagy & Watters 2018: 177; Nagy et al. 2020: 194; van der Merwe et al. 2020: 11.

Holotype. MRAC 2020.007 .P.0001, male, 31.9 mm SL; Uganda: upper Nile drainage: Achwa drainage: ephemeral swamp in the Aringa system, about 2.6 km south of Madi Opei township, 3.575556 N, 33.079167 E; B. Nagy, K. Østergaard & G. Petneházy, 6 June 2017, preserved in the field.

Paratypes. MRAC 2020.007 .P.0002-7, 6, 3 males, 33.6–35.6 mm SL & 3 females, 23.5–24.8 mm SL; collected with the holotype, preserved in the field .— MRAC 2020.007 .P.0008, male, 35.7 mm SL; collected with the holotype, preserved after 2 months in captivity .

Diagnosis. Nothobranchius elucens is distinguished from all other members of the genus by the following characters in males: body colouration golden-grey with brown scale margins creating irregular vertical stripes on trunk; anal fin yellow with brown spots proximally, with slender brown median band, followed by a slender light blue subdistal band and a slender black distal band; caudal fin brown proximally and medially, followed by a slender light blue subdistal band and a slender black distal band; dorsal fin golden with irregular brown blotches and stripes, with narrow light blue subdistal band and narrow black distal band. Furthermore, it differs from N. taiti by the morphometric characters of having a smaller head length of 29.5–33.1 % SL (vs. 35.0–38.6 %); smaller prepectoral length of 31.2–33.9 % SL (vs. 34.2–38.6 %); greater head depth of 81–87 % HL (vs. 71–80 %); greater interorbital width of 43–49 % HL (vs. 31–40 %); and greater caudal peduncle length of 145–152 % of its depth (vs. 134–143 %).

Description. General body features illustrated by Figures 2–4 . Morphometric and meristic characters of holotype and paratypes summarised in Table 1. Relatively small-sized Nothobranchius species, maximum observed length of specimens examined 35.7 mm SL. General body shape robust, laterally compressed and deep. Greatest body depth at vertical passing through pelvic-fin origin: 28.4–33.8 % SL. Greatest body width at pectoral-fin base with body progressively narrowing towards caudal-fin base. Dorsal profile convex from tip of snout to base of last dorsal-fin ray; postdorsal profile straight to slightly concave on caudal peduncle. Ventral profile convex from lower jaw to base of last anal-fin ray; postanal profile straight to slightly concave on caudal peduncle from insertion of posteriormost anal-fin ray to caudal-fin base. Caudal peduncle shallow, length 1.45–1.52 times of its depth. Anus just anterior to anal-fin origin.

Head moderately long (29.5–33.1 % SL), laterally compressed, deeper than wide (head width 66–72 % of its depth). Snout slightly pointed, smaller than eye diameter. Mouth supraterminal, slightly oblique in profile. Jaws subequal, lower jaw longer than upper, posterior end of rictus at same level or slightly ventral to centre of eye. Premaxilla and dentary with many irregularly distributed conical, slightly curved teeth at outer row of lower and upper jaws. Orbit large (28–32 % HL), mainly in anterior half of head (snout to eye end length 50–54 % HL), at median portion of head side. Branchiostegal membrane projecting posteriorly from opercle.

Dorsal-fin origin anterior to anal-fin origin, both fins originating posterior to mid-length of body. Distal border of dorsal and anal fins rounded, with small contact organs in form of papillae on fin rays and distal margin with short filamentous rays. Posterior extremity of dorsal fin reaching caudal-fin base. Dorsal fin, 16–18; anal fin, 17–19. Pectoral fin subtriangular; its insertion slightly posterior to margin of opercular opening; base slightly oblique, upper fin rays placed slightly anteriorly to lower fin rays, tip reaching or slightly surpassing base of pelvic fin. Pectoral fin, 18–20. Pelvic fin subabdominal, its origin at about mid-length of body, short, bases medially separated, tip reaching urogenital papilla. Caudal fin subtruncate, with 17–19 branched rays, plus three or four procurrent rays at dorsal and ventral origins.

Scales cycloid, body and head entirely scaled, except for ventral surface of head. No scales on dorsal and anal fins. Scales in mid-longitudinal series, 28–30 plus two or three small scales on caudal-fin base. Transverse rows of scales in front of dorsal-fin origin, 11; scale rows around caudal peduncle 12.

Frontal scalation pattern variable. Anterior nostril at anterior tip of snout, tubular opening lateral to upper lip. Posterior nostril in front of orbit, with oblique oval aperture. Frontal neuromasts arranged in two rows of shallow grooves, one neuromast in each groove. Cephalic sensory system at supraorbital level fragmented into two discontinuous portions, in shallow groove with one or two exposed neuromasts at anterior level, continued at the posterior supraorbital level with two exposed neuromasts over the orbit, whereas at supratemporal level in slightly curved shallow groove, with four exposed neuromasts. Preorbital canal in shallow groove with three exposed neuromasts; postorbital canal in deep groove with one long exposed neuromast; infraorbital level with about ten small buttons or neuromasts at ventral and posterior margin of orbit. Preopercular system with exposed neuromasts in deep groove on preopercle portion, whereas in shallow groove with about twelve small buttons on ventral portion. Mandibular canal in shallow groove with about ten to twelve small buttons or neuromasts. One neuromast on each scale along trunk mid-longitudinal series.

Females smaller than males, maximum observed size 24.8 mm SL. Body less laterally compressed and more slender than in males (head depth 68–72 % HL vs. 81–87 %; head width 77–79 % of its depth vs. 66–72 %). Branchiostegal membrane not projecting posteriorly from opercle. Dorsal and caudal fins rounded. Anal fin subtriangular, tip rounded, central rays longer and more rigid.Anal fin positioned more posteriorly than in male (preanal length 60.9–62.8 % SL vs. 56.5–60.5 %, respectively). Base length anal fin smaller than in male (anal-fin base length 16.7–20.2 % SL vs. 22.2–25.2 %). No papillae or epidermal tissue present on dorsal and anal fins. Pelvic fin short, tip reaching anus. Caudal peduncle less deep than in male (caudal peduncle length 1.79–1.85 times in its depth vs. 1.45–1.52, respectively).

Colouration. Live male: illustrated in Figure 3 . Scales on trunk and head golden-grey with brown scale margins creating irregular vertical stripes on trunk. Scales with iridescent light blue on side of body. Scales on abdomen light golden to silver. Snout and throat grey, dorsal portion of head light brown. Posterior scale margins on post-orbital portion of opercle creating three brown oblique bars. Exposed branchiostegal membrane grey. Iris golden, with dark grey spots on upper and lower parts, creating dark vertical bar through centre of eye. Dorsal fin golden with brown spots and brown irregular stripes proximally and medially, and with narrow light blue subdistal band and narrow black distal band. Anal fin golden with brown spots proximally, followed by slender brown median band, then broad light blue subdistal band, and broad black distal band. Caudal fin brown proximally and medially, with slender light blue subdistal band and slender black distal band. Pelvic fin hyaline to brown with broad light blue distal margin. Pectoral fin hyaline with light blue posterior distal margin.

Live female: illustrated in Figure 4 , scales on trunk and head pale grey-brown, darker on dorsum and lighter to silver on venter. Scales with dark grey-brown margin, creating reticulation on trunk and irregular vertical stripes on dorsal and posteroventral portions of flank. Light blue iridescence on trunk and opercular region. Iris golden. All fins hyaline.

Distribution. Nothobranchius elucens is currently known from a temporary swamp formed in the floodplain of the seasonal Aringa River system ( Fig. 5 ). The Aringa flows into the Pager, which is a tributary of the Achwa within the upper Nile drainage in northern Uganda.

Ecology. The northern highlands of Uganda experience a mean annual rainfall of around 1500 mm concentrated with two peaks in May and August ( Hughes & Hughes 1992). Wetlands are at their lowest levels in January, when seasonal streams and most temporary habitats are dry. At the type locality, Nothobranchius elucens was the only fish species found. The accompanying fauna consisted of tadpoles of unidentified species .

The type locality is a large temporary pool formed on the floodplain of the seasonal Aringa River ( Fig. 6 ). The habitat belongs to category 1.1, as defined by Watters (2014), representing an isolated pool in a shallow depression on a floodplain, formed primarily as a result of natural processes. At the end of the first peak of the rainy season, the habitat was a large and shallow pool more than 50 m long and about 30 m wide, and about 1 m at the deepest point. At the type locality, all collecting took place in the same general habitat. Aquatic vegetation comprised Nymphea sp., whereas the edge of the habitat was partially overgrown with grass. The water was turbid, the bottom of the pool was not visible. The water temperature around noon was 33.6° C, the pH was 7.8 and it contained 107 ppm of total dissolved solids.

Stomach content of an adult wild-caught specimen showed that N. elucens is a micropredator, feeding on small aquatic crustaceans, worms, insect larvae and other zooplankton.

Conservation status. Nothobranchius elucens is recommended to be listed as Vulnerable. The species is currently known only from the type locality. It might exist at some other sites within the drainage system of the Aringa River , but its distribution will remain restricted and any potential additional populations are expected to be fragmented. The extent of occurrence is expected to be less than 5.000 km ². The only known location is close to human population and, during the collection of the types, it was observed that the water of the habitat was used to wash the surface of truck platforms, with the fouled water being allowed to drain back into the habitat. Increasing pressure on land and water resources result in habitat degradation and represents an important threat to the species. Using IUCN version 3.1 (2012), the species meets the criteria D2 for Vulnerable .

No conservation measures are in place for this species. It is not known from any protected areas. There is a need for improved habitat protection at the type locality, currently the only site where the species is known to occur. Additional collecting efforts targeting suitable habitats should be conducted in the region in order to better understand the geographic distribution of this species and to elaborate on the appropriate conservation measures.

Biology. Aquarium maintenance of selected specimens was undertaken for observation of breeding behaviour and biology. Nothobranchius elucens has an annual mode of reproduction, typical of the genus. Eggs are deposited in the habitat substrate, where they survive the dry season and hatch with the onset of the wet season. Under captive conditions, peat moss was used successfully as an artificial spawning substrate. An embryonic development period of three to five months was observed at a temperature of about 22–24 °C.

Etymology. The specific name elucens is a Latin participial adjective meaning ‘shining out’ and is given with reference to the distinctive light blue subdistal band in the anal and caudal fins, contrasting and shining out of the overall dark colouration of body and fins of the males.

Figures

FIGURE 2. Nothobranchius elucens, MRAC 2020.007.P.0001, holotype, male, 31.9 mm SL; Uganda: upper Nile drainage: ephemeral swamp in the Aringa system, about 2.6 km south of Madi Opei town.

FIGURE 3. Nothobranchius elucens, MRAC 2020.007.P.0008, paratype, male, 35.7 mm SL, live; Uganda: upper Nile drainage: ephemeral swamp in the Aringa system, about 2.6 km south of Madi Opei town. Photographed after one month in captivity.

FIGURE 4. Nothobranchius elucens, female, about 25 mm SL, live; Uganda: upper Nile drainage: ephemeral swamp in the Aringa system, about 2.6 km south of Madi Opei town. Photographed after two weeks in captivity.

FIGURE 5. Map of Uganda, showing the type locality of Nothobranchius elucens (blue triangle) and N. taiti (green squares). Map prepared by Brian Watters.

FIGURE 6. Type locality of Nothobranchius elucens; Uganda: upper Nile drainage: ephemeral swamp in the Aringa system. Photographed on 6 June 2017.