Diagnosis: Nothobranchius sonjae is distinguished from all other species of the genus by the following characters in males: body colouration light blue to blue-green with red-brown, brown or grey scale margins and a golden-brown to grey stripe on the dorsal scale row between nape and dorsal-fin origin; anal fin light blue-grey to yellow-grey with irregular red to red-brown spots proximally, followed by a narrow brown submedial band, a slender yellow medial band and a broad black distal band; caudal fin yellow-brown, grey-brown, blue-grey or grey, with red-brown, brown, dark grey or red spots; some populations with radiating red stripes, the density of which varies from moderate to highly dense such that in the latter cases the basal, proximal and medial zones may appear dominantly a deep red; distal zone comprises a slender to broad semi-translucent dark grey to black distal band; pelvic fin light blue-grey to yellow-grey, with broad yellow medial band and narrow black and blue distal margin (Ref. 122068). Furthermore, it differs from the other members of the Nothobranchius taeniopygus group in central and western Tanzania by a head length of 25.8-27.0% of standard length vs. 27.3-34.6%; and caudal peduncle length 153-159% of its depth vs. 119-149% and 160-169%, respectively (Ref. 122068).

Description: General body shape robust, laterally compressed and moderately deep; greatest body depth at vertical in front of pelvic-fin origin, 30.5-33.5% of standard length; greatest body width at pectoral-fin base with body progressively narrowing towards caudal-fin base (Ref. 122068). Dorsal profile convex from tip of snout to base of last dorsal-fin ray, 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 relatively shallow, length 1.53-1.59 times its depth; anus situated directly in front of anal-fin origin (Ref. 122068). Head relatively short, 25.8-27.0% of standard length, laterally compressed, deeper than wide, head width 64-69% of its depth; snout slightly pointed, smaller than eye diameter; mouth supraterminal, sligthly 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, 33-37% of head length, entirely in anterior half of head, snout to eye end length 47-49% of head length, in dorsal portion of head side; branchiostegal membrane projecting posteriorly from opercle (Ref. 122068). Dorsal-fin origin anterior to anal-fin origin, both fins originating posterior to mid-length of body; extremity 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 15-18 rays; anal fin 18-19 rays; pectoral fin subtriangular, 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 overlapping base of pelvic fin; pelvic fin subabdominal, origin at about mid-length of body, short, bases medially separated, tip reaching urogenital papilla; caudal fin subtruncate, with 17-19 branched rays, plus 2 or 3 unbranched smaller rays at dorsal and ventral origins (Ref. 122068). 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 11-12; scale rows around caudal peduncle 12-14 (Ref. 122068). Cephalic squamation pattern variable; frontal neuromasts separate in two rows osensoryf shallow grooves behind the upper lip, with one neuromast in each groove; cephalic sensory system at supraorbital level in continuous shallow groove, with three to four exposed neuromasts; supratemporal level with four exposed neuromasts in undivided, slightly curved shallow groove; preorbital canal in shallow grooves with two exposed neuromasts; infraorbital level with series of more than 20 small buttons at ventral and posterior margin of orbit; postorbital canal in deep groove with one exposed neuromast; preopercular system with exposed neuromasts in deep groove on preopercle portion, whereas with exposed neuromasts and small buttons on ventral portion; mandibular canal in shallow groove with four small neuromasts; one neuromast on each scale along trunk mid-longitudinal series (Ref. 122068).

Colouration: Live male: scales on trunk and head light iridescent blue to blue-green with broad dark red-brown, brown or, less commonly, dark grey margins, forming a regular reticulation pattern; scales along the dorsum between the nape and dorsal-fin origin may be darker with a golden-brown or grey hue and more prominent scale margins; scales on abdomen faint blue to white with less pronounced margins; snout, frontal and dorsal portions of head blue to blue-grey; throat pale blue to pale grey; exposed part of branchiostegal membrane very pale brown to cream; iris golden; background colour of dorsal fin is light blue-grey, yellow-grey or pale brown with irregular brown to red-brown dots and stripes, usually vermicular in basal zone, becoming finer distally and elongated parallel to the rays; small black markings may be present on membrane between first 2-3 anterior fin rays; some populations show a very narrow black distal margin to the dorsal fin; anal fin with narrow light blue-grey to yellow-grey basal zone followed by a broader yellow to yellow-brown proximal zone overlaid by irregular deep red to red-brown spots, the density of which varies from sparse to highly dense, depending on the population; in cases where the density spots in this zone is high it appears as an almost solid red band; proximal zone followed by a narrow deep red to brown submedial band comprising a row of merged or partially merged spots; medial zone comprises a slender yellow band that may overlaid with irregular deep red or red-brown spots which, if present, tend to be concentrated in the central parts of this zone; in rare cases the medial band may be pale yellow to almost white; the distal zone of the anal fin is broad and black; caudal fin, in detail, variable on both inter- as well as intra-population levels; basal to medial zones yellow-brown, grey-brown, blue-grey or grey, with red-brown, brown, dark grey or red spots; some populations with radiating red stripes; density of spots and stripes varies from moderate to highly dense such that in the latter cases the basal, proximal and medial zones may appear dominantly deep red; the distal zone comprises a slender to broad semi-translucent dark-grey to black band, with transitional inner boundary; pelvic fins roughly mimic pattern of anal fin with a narrow blue-grey to yellow-grey basal zone with some red markings proximally, a broad yellow medial zone and black, with blue in some specimens, distal zone; pectoral fins dominantly hyaline with light blue margins (Ref. 122068). Live female: trunk and head light grey-brown, darker on dorsum and lighter ventrally; scales with a very narrow grey margin; scale centres iridescent silver to very pale blue; all fins hyaline; iris golden (Ref. 122068).