Nothobranchius geminus Wildekamp, Watters & Sainthouse 2002
Creators: name: Watters, Brian R.
name: Nagy, Béla
Publisher: name: Zenodo
Year: 2025
Subjects: Biodiversity
Taxonomy
Animalia
Chordata
Cyprinodontiformes
Nothobranchiidae
Nothobranchius
Nothobranchius geminus

description: Nothobranchius geminus Wildekamp, Watters & Sainthouse, 2002
(Figs. 5A–B)
Material examined. RMCA 97-83 -P-1, holotype, male, 32.3 mm SL; – RMCA 97-83 -P-2-17, paratypes, 9 males 28.3–32.3 mm SL, 7 females, 25.8–30.2 mm SL; Tanzania: Kilombero drainage, Ifakara; 08°10.042’ S, 36°41.611’ E; B. Watters, R. Wildekamp, I. Sainthouse; 7 June, 1995; field code: TAN 95-4.
Additional material examined. 1 male, 28.1 mm SL; Tanzania: Kilombero drainage, Narubungo; 08°08.882’ S, 36°24.911’ E; B. Watters, B. Cooper, O. Schmidt; 9 June 2002; field code: TAN 02-24. – 1 male, 30.1 mm SL; Tanzania: Kilombero drainage, Iragua; 08°33.175’ S, 36°30.245’ E; B. Watters, B. Cooper, O. Schmidt; 12 June 2000; field code: TAN 00-16.
Diagnosis. Nothobranchius geminus 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 61.2–70.8 % SL and predorsal length 112–125 in % 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 3c). Both sexes with central 2–3 rays of the caudal fin elongated forming a posterior lobe (vs. central lobe absent). Nothobranchius geminus is distinguished from the other members of the N. janpapi species group by male colouration of anal fin with slender pale yellow subdistal band and narrow red margin (vs. slender orange-red distal band with narrow light blue margin in N. janpapi); caudal fin plain red to orange-red with narrow black distal margin (vs. base and proximal zone olive, and with light grey to translucent subdistal band and narrow dark grey distal band in N. janpapi, and base and proximal zone grey to light red with red subdistal band and narrow black distal band in N. luekei); pelvic fin yellow to red with narrow black margin (vs. plain orange-red with narrow light blue distal band in N. janpapi, and light blue with narrow red distal margin in N. luekei); by morphometric characters of head depth 66–72 % HL (vs. 74–81 in N. janpapi, and 55–63 in N. luekei) and caudal peduncle length 121–130 as % of its depth (vs. 133–140 in N. janpapi, and 148–156 in N. luekei); and, it differs from N. janpapi by dorsal-fin base length 15.6–18.6 % SL (vs. 18.8–25.1). Further, the female differs in the colouration element of small, sparsely distributed red-brown spots across mid- to upper parts of flanks and head (vs. absent in N. janpapi and N. luekei)
Description. General body features illustrated by Figures 5A–B. Morphometric and meristic characters of examined material are summarized in Table 5.
Male: Small Nothobranchius species, maximum recorded size 32.3 mm SL. General body shape laterally compressed. Greatest body depth at vertical in front of pelvic-fin origin (25.3–30.2 % SL). Greatest body width at pectoral-fin base with body progressively narrowing towards caudal-fin base. Dorsal profile slightly concave from tip of snout to nape and straight to slightly convex from nape to dorsal-fin origin, straight to 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 moderately deep, length 121–130 % of its depth. Anus situated directly in front of anal-fin origin.
Head moderately long (27.9–32.4 % SL), laterally compressed, deeper than wide (head width 73–83 % 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 28–34 % HL, mainly in anterior half of head (snout to eye end length 49–58 % HL). Branchiostegal membrane projecting posteriorly from opercle.
Dorsal-fin origin markedly posterior to anal-fin origin (predorsal length 112–125 as % of preanal length), both fins originating posterior to mid-length of body (predorsal length 61.2–70.8 % SL; preanal length 52.5–59.8 % SL). Dorsal fin relatively small with low number of rays, 11–13; extremity rounded. Anal fin trapezoidal in shape, with 17–21 rays; lower margin moderately concave in profile. 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 11; 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. 3c), whereas at supratemporal level with three exposed neuromasts in undivided, slightly curved shallow groove. Preorbital canal in shallow 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 about ten small buttons on ventral portion. Mandibular canal in shallow groove with about six small neuromasts. One neuromast on each scale along trunk mid-longitudinal series.
Female: Smaller than male, maximum recorded size 30.2 mm SL. Branchiostegal membrane not projecting posteriorly from opercle. Dorsal fin rounded with low number of fin rays, 11–13. Caudal fin basically rounded but with central 2–3 rays slightly elongated resulting in pointed posterior margin. Anal fin subtriangular, tip rounded, central rays longer and more rigid.Anal-fin base length smaller than in males (17.9–21.3 % SL vs. 25.6–28.3). Head depth smaller than in males (58–65 % HL vs. 66–72). Caudal peduncle shallower than in males (caudal peduncle length 157–164 % of its depth vs. 121–130).
Colouration. Live male (Fig. 5A): Scales on the trunk and head light iridescent blue to blue-green with prominent 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-brown centres forming a narrow zone. Snout, frontal and dorsal portions of the head and throat are yellow. Operculum yellow with sparse blue to blue-green and red markings. Exposed part of the branchiostegal membrane is yellow with white edging. Iris golden yellow to golden blue-green, reflective pale blue when viewed from above. Dorsal fin light blue-green in the basal and proximal zones grading to yellow-green through the medial zone and into the distal zone, overlaid by irregular red spots and streaks in crude parallelism with fin rays in the proximal and submedial zones but arranged somewhat concentrically conforming to the fin margin in the medial and posterior zones; the distal zone comprises a solid red band that may be followed by a very narrow black margin. Short extensions to the dorsal and anal fins are black where the similarly coloured margin is present but are otherwise red. Anal fin background colour is light blue in the basal zone and light blue or yellow in the proximal zone grading to yellow through the medial and subdistal zones; the proximal and medial zones are overlaid by red spots that commonly merge to form stripes aligned parallel to the fin rays. The density of these red markings is variable at both the inter-and intrapopulation levels to the extent that they may be present in the proximal zone but grade outwards to absent in the medial zone. There is always a narrow red margin to the anal fin and short fin-ray extensions may be red or black as discussed above. The caudal fin is red to orange-red with a narrow dark grey to black margin most prominent anteriorly; fin rays may be a darker red than the intervening membrane. Pelvic fins are yellow to light blue with a variable density of red spots, and a red marginal zone that may also show incomplete black edging. Pectoral fins are dominantly hyaline or pale yellow with narrow and irregular blue margins.
Live female (Fig. 5B): The trunk and head are overall light grey with light blue to blue-green iridescence on the scales which have pale grey edgings; small red-brown spots are sparsely distributed across the mid- to upper parts of the flanks and head, being more common over the upper body. The operculum is iridescent very pale blue-green. All fins are hyaline. The iris is golden.
Distribution. Nothobranchius geminus is endemic to seasonal freshwater habitats in central-eastern Tanzania and has been found in the seasonal marginal and peripheral zones of the Kibasira Swamp in the Kilombero River valley. The latter system represents a fairly remote component of the Rufiji River system, connected to it through the Ulanga River, the downstream continuation of the Kilombero River. Known sites are associated with the northeastern part of the Swamp, south-southwest and west-southwest of the town of Ifakara (Watters 2016b). However, the Kibasira Swamp is a vast entity and undoubtedly the distribution of this species is significantly greater than has presently been reported and is expected to also extend throughout the less accessible southwestern part of the Swamp that is yet to be fully investigated (Fig. 1).
Conservation status. Nothobranchius geminus is listed as Vulnerable B1ab(iii) (Nagy & Watters 2019c). The known extent of occurrence (EOO) is 978 km ², with a maximum estimate of around 6,000 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. However, it is expected to occur at up to 10 threat-based locations within the drainage of the Kilombero River. This species is expected to also inhabit the southwestern parts of the Kibasira Swamp that are yet to be fully investigated; all presently known sites are in the northeastern parts of the swamp. In combination with its annual life cycle in seasonal habitats, its dependence on the clay-rich vertisol-type substratum for embryonic development makes it more subject to threats, as is the case with all other members of the subgenus. This category is assigned based on the upper estimates of the EOO and number of threat-based locations as best estimates. Notably, drainage modifications associated with road construction has resulted in the destruction of the type locality for this species.
description: Published as part of 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, pp. 401-446 in Zootaxa 5570 (3) on pages 415-418, DOI: 10.11646/zootaxa.5570.3.1, http://zenodo.org/record/14735232