Diagnosis: Haplochromis planus differs from all other species of Haplochromis, except H. retrodens from Lake Victoria, by a shallow, broad and rectangular lower jaw with ascending arms steeply sloping upwards, enlarged and distinctly visible intrabuccally; oral-tooth bands U-shaped and, in large specimens of more than 75 mm standard length, set posteriorly with large and stout inner and outer teeth, especially in upper jaw; all oral mucosa in lower jaw at same height and oral-breathing valves strongly reduced to narrow, rectangular skin folds (Ref. 126074). It differs from H. retrodens by larger eyes, 35.7-41.6% of head length vs. 23.8-29.6%, correlated shorter snout, 24.6-28.8% of head length vs. 29.0-36.8%, and more slender interorbital width, 19.1-22.1% of head length vs. 24.2-33.8%, and dominant males with scarlet to red vs. dark grey-green flanks (Ref. 126074). It differs from H. concillans and H. erutus by a shallow vs. deep lower jaw, a narrower interorbital region, 19.1-22.1% of head length vs. 22.5-26.4% and 23.4-27.2% respectively, and dominant males with scarlet to red vs. blue-silver and silver-beige flanks, respectively; it differs further from H. concillans by a longer premaxillary pedicel, 21.9-24.4% of head length vs. 16.1-21.6%, and from H. erutus by a longer upper jaw, 27.6-32.1% of head length vs. 20.9-26.0% (Ref. 126074).

Description: Small species with a relatively slender body; head straight to slightly convex, snout relatively acute and sloping at 40°-55° (Ref. 126074). Eye large, interorbital distance small and jaws broad, isognathous and with a gape inclination of 15°-25°; upper jaw enlarged posteriorly and maxilla extends to just past anterior margin of pupil; lower jaw very shallow anteriorly and laterally, with a square outline and ventral margin of dentigerous arm horizontal in rostral view, hereby jaw somewhat resembles a shovel; ascending arm of lower jaw steeply sloping upwards and enlarged; in lower jaw, intrabuccal oral mucosa at same height as dentigerous area of jaw and oral breathing valve reduced to a narrow and rectangular skin fold (Ref. 126074). Lips and oral mucosa large; neurocranium generalised with a low and convex occipital crest (Ref. 126074). Chest scales small; size transition to flank scales gradual; 19-23 scales on upper lateral line, 10-12 scales on lower lateral line; 6-8 scales between pectoral and pelvic fins; 2-4/8-10 cheek scales (Ref. 126074). Outer teeth small, with slender and slightly recurved necks and broad and strongly recurved crowns; in small specimens smaller than 65 mm standard length: crowns bicuspid, major cusps short and isocline while bearing a small flange to obliquely truncated, minor cusps stout, large and horizontally inclined; in large specimens of more than 70 mm standard length: crowns unicuspid to bicuspid and major cusps isocline, broad, relatively short and bluntly pointed, minor cusps small, if present; dental arcades broad and with square outlines; outer teeth closely set with an inter-tooth distance of 0.5-1 outer tooth width; in anterior part of lower jaw, outer teeth implanted procumbently in anterior margin of bone; inner teeth small, tricuspid and set in 2-4 rows without a distinct gap between inner and outer rows; dental arcades long in upper jaw with 1-4 posteriormost outer teeth large, stout, unicuspid and buccally orientated; mostly short in lower jaw, reaching to before ascending arm, although, in large specimens of more than 70 mm standard length 1-3 stout outer teeth may occur halfway on ascending arm; tooth bands in both jaws narrow posteriorly, until only outer row remains over posterior quarters; in large specimens of more than 70 mm standard length, outer and one inner row remains posteriorly and posteriormost part of upper jaw set with inner teeth equal in size and form to the outer teeth (Ref. 126074). Caudal fin emarginate; dorsal and anal fins reach level between caudal fin base and two scales anterior to this point; pectoral fins reach level between genital opening and anal fin; pelvic fins between first spine and first branched ray of anal fin in males, between genital opening and anal fin in females (Ref. 126074). Gill rakers on the outer row of the first gill arch small, unifid and stout; posteriormost ceratobranchial rakers can be bifid (Ref. 126074). Lower pharyngeal bone relatively fine, short, broad, shallow posteriorly and with a slightly deeper keel; all pharyngeal teeth slender, with concave major cusp gaps, small but distinct minor cusps and often cusp protuberances; teeth set in 25-32 rows, teeth in median two rows at most slightly stouter; teeth in posteriormost row with blunt major cusps, straight major cusp gaps and distinct but small minor cusps (Ref. 126074).

Colouration: Life colouration of dominant males: flanks and dorsal part of head scarlet to red, chest white, snout bright blue; cheeks and opercula orange-red; dorsal part of head and anterior half of flanks scarlet to red; branchiostegal rays black; flanks of dominant males sometimes with 4-5 blueish vertical stripes; pelvic fins black; dorsal and caudal fins orange-red and lappets of dorsal fin black anteriorly and red posteriorly; pectoral and anal fins hyaline, anal fin with 2-3 large yellow egg spots, each spanning two fin rays; lacrimal, preorbital and supraorbital stripes and a vague nape band present (Ref. 126074). Life colouration of females and juveniles: body whitish, dorsally with a greenish sheen, snout dusky; pelvic, anal and ventral part of caudal fin yellow; dorsal part of caudal fin dusky; anal fin with 2-3 small spots resembling egg spots; dorsal fin dusky and pectoral fins hyaline (Ref. 126074). Colouration in alcohol: body dorsally brown-yellow, ventrally brown-white and branchiostegal membrane black; lacrimal stripes and a nape band present in dominant males, while faint in females; nostril, interorbital and vertical preopercular stripes faint; flanks of dominant males with 4-5 faint vertical stripes; all fins hyaline, except dorsal and caudal fins dusky and dominant males with pelvic fins black and anal fin with dusky posterior margin and 2-3 large egg spots; anterior part of dorsal fin with black lappets (Ref. 126074).