Nothobranchius luekei Seegers, 1984

Watters, Brian R. & Nagy, Béla, 2025, A review and redefinition of the subgenus Aphyobranchius Wildekamp, 1977, in the seasonal killifish genus Nothobranchius Peters, 1868 (Teleostei: Nothobranchiidae), from ephemeral wetlands of eastern Tanzania, Zootaxa 5570 (3), pp. 401-446 : 411-414

Nothobranchius luekei Seegers, 1984

( Figs. 4A–C )

Material examined. 5 males, 21.2–22.9 mm SL, 1 female, 21.3 mm SL; Tanzania: Mbezi drainage, Mkuranga; 07°11.615’ S, 39°10.289’ E; B. Watters, B. Cooper, O. Schmidt; 16 June 2000; field code: TAN 00-23 ; – 2 males, 25.4–26.2 mm SL, 1 female, 24.6 mm SL; Tanzania: Mbezi drainage, Mkuranga; 07°11.617’ S, 39°10.299’ E; B. Nagy; 13 January 2009; field code: TZN 09-9 .

Diagnosis. Nothobranchius luekei belongs to the N. janpapi species group within the subgenus Aphyobranchius by presenting the following combination of characters in the male: light blue body colouration with narrow red scale margins and yellow-brown to yellow-green dorsum stripe; rearward positioned dorsal fin with predorsal length of 68–73.0 % SL and predorsal length 117–122 as % of preanal length; dorsal fin relatively small with low number of rays, 11–13; cephalic sensory system at supraorbital level in an undivided, slightly curved, almost flat, shallow groove with three exposed neuromasts and pronounced lobes ( Fig. 3b ). Both sexes with central 2–3 rays of the caudal fin slightly elongated forming a posterior lobe (vs. central lobe absent). Nothobranchius luekei is distinguished from the other members of the N. janpapi species group by male colouration of anal fin with slender yellow subdistal band and narrow red margin (vs. slender orange-red distal band with narrow light blue margin in N. janpapi ); caudal fin base and proximal zone grey to light red with slender red subdistal band and narrow black distal band (vs. base and proximal zone olive, and with light grey to translucent subdistal band and narrow dark grey distal band in N. janpapi , and plain red to orange-red with narrow black distal margin in N. geminus ); pelvic fin light blue with yellow subdistal band and narrow red distal band (vs. plain orange-red with narrow light blue distal band in N. janpapi , and yellow to red with narrow black distal band in N. geminus ); as well as morphometric characters in males of body depth at pelvic-fin origin 21.5–23.6 % SL (vs. 25.3–30.2 in N. geminus and 27.4–31.0 in N. janpapi ); head depth 55–63% HL (vs. 66–72 in N. geminus and 74–81 in N. janpapi ); head width 87–97 as % of its depth (vs. 73–83 in N. geminus and 74–81 in N. janpapi ); and caudal peduncle length 148–156 as % of its depth (vs. 121–130 in N. geminus and 133–140 in N. janpapi ). Further, it differs from N. janpapi by predorsal length 68.0–73.0 % SL (vs. 63.3–67.9 % SL); head width 52–56 % HL (vs. 58–65 %); and suborbital depth 8–12 % HL (vs. 16–20).

Description. General body features illustrated by Figures 4A–C . Morphometric and meristic characters of examined material summarized in Table 4 .

Male: Small Nothobranchius species, maximum size of examined specimens 26.2 mm SL. General body shape laterally compressed. Greatest body depth at vertical in front of pelvic-fin origin (21.5–23.6 % SL). Greatest body width at pectoral-fin base with body progressively narrowing towards caudal-fin base. Dorsal profile straight from tip of snout to dorsal-fin origin, slightly concave on caudal peduncle. Ventral profile convex from lower jaw to base of last anal-fin ray, straight to slightly concave on caudal peduncle. Caudal peduncle deep, length 148–156 % of its depth. Anus situated directly in front of anal-fin origin.

Head moderately long (27.1–31.1 % SL), laterally compressed, slightly deeper than wide (head width 87–97 % of its depth). Snout distinctly 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 teeth on inner rows and slightly curved teeth on outer row. Orbit large, eye diameter 32–37 % HL, mainly in anterior half of head (snout to eye end length 52–58 % HL). Branchiostegal membrane projecting posteriorly from opercle.

Dorsal-fin origin markedly posterior to anal-fin origin (predorsal length 117–122 as % of preanal length), both fins originating posterior to mid-length of body (predorsal length 68.0–73.0 % SL; preanal length 55.7–60.5 % SL). Dorsal fin relatively small with low number of rays, 11–13; extremity rounded. Anal fin trapezoidal in shape, with 17–19 rays; lower margin quite strongly concave in profile, resulting in mildly lobate rear part of fin. Dorsal and anal fins with short fin-ray extensions. Pectoral fin subtriangular, insertion slightly posterior to margin of opercular opening, base slightly oblique, upper fin rays positioned slightly anterior to lower fin rays, with tip reaching or slightly overlapping base of pelvic fin. Pelvic fin subabdominal, origin at about mid-length of body, short, base medially separated, with tip reaching urogenital papilla. Caudal fin rounded, with 13–16 branched rays, plus 3–4 dorsal and ventral procurrent rays; central 2–3 rays of fin relatively slightly elongated resulting in mild central lobe.

Scales cycloid, body and head entirely scaled, except for ventral surface of head. No scales on base of dorsal and anal fins. Scales in mid-longitudinal series 26–28 plus 2–3 small scales on caudal-fin base. Transverse rows of scales in front of dorsal-fin origin 10; scale rows around caudal peduncle 12.

Cephalic squamation pattern G-type. Frontal neuromast in single shallow groove behind upper lip. Cephalic sensory system at supraorbital level in an undivided, slightly curved zigzag pattern, almost flat, shallow groove with three exposed neuromasts and pronounced lobes ( Fig. 3b ), whereas at supratemporal level with three exposed neuromasts in slightly curved arrangement on flat surface, with pronounced lobes. Preorbital canal in deep groove with two exposed neuromasts; infraorbital level with series of about 20 small buttons at ventral and posterior margin of orbit; postorbital canal in deep groove with one exposed neuromast. Preopercular system with six exposed neuromasts in deep groove on preopercle portion, whereas in shallow groove with six small neuromasts on ventral portion. Mandibular canal in shallow groove with about 20 small buttons. One neuromast on each scale along trunk mid-longitudinal series.

Female: Smaller than male, maximum recorded size 24.6 mm SL. Branchiostegal membrane not projecting posteriorly from opercle. Dorsal fin rounded with low number of fin rays, 11–12. Caudal fin basically rounded but with central 2–3 rays elongated resulting in pointed posterior margin. Anal fin subtriangular, tip rounded, central rays longer and more rigid; posterior lower margin may be very slightly concave, mimicking in very mild way, that of male. Anal-fin base length smaller than in male (19.7–20.3 % SL vs. 21.9–26.3). Caudal peduncle shallower than in males (caudal peduncle length 161–167 % of its depth vs. 148–156).

Colouration. Live male ( Figs. 4A–B ): Scales on the trunk and head are light iridescent blue to blue-green with red margins, forming a regular reticulation pattern. Scales on the abdomen are faint blue to cream with less pronounced margins. Scales on the dorsum anterior to the dorsal fin have yellow-green centres forming a narrow stripe. Snout, and frontal and dorsal portions of head, are yellow-green (Mbezi population) or grey (Luhule/Luhute population); the throat is yellow-green. Operculum has blue to blue-green, and lesser red, markings with intervening yellow areas. Exposed part of the branchiostegal membrane is yellow with white edging. Iris is blue to blue-green. Dorsal fin background light blue-green in the basal and proximal zones grading to yellow through the medial zone and into the distal zone; background overlaid by irregular red spots and patches that become coarser in the medial and distal zones (Luhule/Luhute) that may also take the form of streaks parallel to fin rays (Mbezi), merging in the distal zone as an irregular marginal band. Anal fin background comprises light blue basal and proximal zones grading to blue-green through the medial zone to yellow in the subdistal zone. In the Luhule/Luhute population the anal fin background is overlaid by narrow irregular red bands over the proximal, sub-medial, medial and distal zones, the latter forming a distinct marginal band; in the Mbezi population the background is overlaid by prominent red streaks arranged parallel to fin rays that end at the yellow subdistal zone and there is a red marginal band in the distal zone. Basal, proximal and medial zones of the caudal fin are grey (Luhule/Luhute) or light red (Mbezi), with red spots and streaks, the latter aligned parallel to the fin rays; in the Luhule/Luhute population the distal zone comprises a broad red band followed by a black marginal band; in the Mbezi population the distal zone grades from a relatively dark red outwards to a lighter red, with no black margin. Pelvic fins mimic, in part, the pattern of anal fin with light blue overlaid by red markings, followed by an irregular red band or red spots, a yellow band and red edging. Pectoral fins are dominantly hyaline or pale yellow with prominent blue margins.

Live female ( Fig. 4C ): Trunk and head are light grey, darker on the dorsum and lighter ventrally. Scales have narrow darker grey margins on the flanks, becoming broader on the back; scale centres on the trunk above the abdomen are iridescent pale blue; the operculum is iridescent blue-green. All fins are hyaline. The iris is golden.

Distribution. Nothobranchius luekei is endemic to seasonal freshwater habitats in coastal Tanzania and is relatively sparsely distributed over a restricted range. Confirmed sites occur in association with the Mbezi and Luhule/Luhute river systems in the near coastal region south of Dar es Salaam. It appears to be confined to the so-called Mbezi Triangle (sensu Watters et al. 2007; Watters 2009), a region that is separated to varying degrees from the Ruvu and Rufiji systems by the Pugu and Mtoti hills, respectively ( Fig. 1 ).

Conservation status. Nothobranchius luekei has been assessed as Endangered B1ab(iii)+2ab(iii) ( Nagy & Watters 2019b). The known extent of occurrence (EOO) is 238 km ², with a maximum estimate of around 1,000 km ², whereas the area of occupancy (AOO) is 24 km ², with a maximum estimate of around 100 km ². Within its restricted area of distribution, the occurrence of the species is fragmented and currently it is known from only five threat-based locations. As observed directly by the authors during the course of repeated visits to localities over a number of years, there is increasing pressure on land resources due to the expansion of agriculture in both principal areas where this species occurs. Such expansion of agricultural activity is resulting in habitat degradation and modification, rendering them unsuitable to support the semi-annual life cycle of the species and by altering conditions of the habitat niche preferred by this species. This category is assigned based on using the upper estimates of the EOO, AOO, and number of locations as best estimates. As presently known, this species is restricted to the Mbezi Triangle (sensu Watters et al. 2007; Watters 2009) in coastal Tanzania, and its distribution will remain restricted by geographic confinement and configuration of the drainage systems.

Figures

FIGURE 1. Distribution of species belonging to the subgenus Aphyobranchius, all of which occur in the east-central and southern coastal lowlands of Tanzania: N. janpapi (yellow-filled circle), N. luekei (red-filled triangle), N. geminus (orange-filled diamond), N. lourensi (green-filled square), N. fuscotaeniatus (blue-filled inverted triangle), N. cf. janpapi (dark magenta-filled circle). The inset map shows the area of distribution in a somewhat broader geographic context, to include the distribution of N. willerti (red-filled star) in the lower Tana River area of northeastern coastal Kenya, a species that herein is considered to belong to the subgenus Adiniops, rather than Aphyobranchius to which it was ascribed in the original description by Wildekamp (1992). Note that individual symbols on the map may represent more than one locality or population where the sites are close to one another and could not reasonably be resolved at the scale of the map.

FIGURE 3. Cephalic structure: a, Nothobranchius janpapi; b, N. luekei; c, N. geminus; d, N. lourensi and N. fuscotaeniatus. Key to label abbreviations: an, anterior nostril; pn, posterior nostril; f, frontal neuromast; so, supraorbital canal; st, supratemporal canal.

FIGURE 4. Wild-caught specimens of Nothobranchius luekei and example of typical habitat. A. N. luekei Mbezi River TAN 00-23, male, Mbezi River drainage, in immediate vicinity of the type locality; B. N. luekei Luhule River TAN 02-8, male, Luhule/Luhute River drainage; C. N. luekei Mbezi River TZN 09-9, female, Mbezi River drainage (photograph by B. Nagy); D. Location Luhule River TAN 02-8, a typical heavily vegetated shallow habitat (photograph June 2, 2002), hosting N. luekei, N. lucius, N. rubripinnis and N. ruudwildekampi. Photographs by B.R. Watters except where otherwise noted.