Diagnosis: Species with a piscivorous morphology; body shallow, body depth 29.0-32.3% of standard length; snout very acute in dorsal and lateral views; outer oral teeth few and large, number of upper outer teeth 28-46; vertebrae many, 30-32; dominant males yellow-green with a red anterior part of flank (Ref. 126312). Amongst piscivorous species from the Lake Edward system, Haplochromis mentatus differs from H. latifrons by the combination of a longer dorsal fin base, dorsal-fin base 50.3-54.2% of standard length vs. 47.2-50.1%; a weakly vs. strongly prominent premaxillary pedicel; a steeper sloping lower jaw side, 30-45° vs. 25-30°; and absence vs. presence of a well-defined mid-lateral band (Ref. 126312). It differs from H. rex and H. aquila by the combination of a longer caudal peduncle, caudal peduncle length 15.7-17.5% of standard length vs. 13.5-16.2%; a gentler sloping snout, 30-35° vs. 35-50°; and dominant males yellow-green with a red anterior part of flank vs. cream-coloured with an orange operculum, or light grey with a black head, respectively; further from H. rex by a lower number of infraorbital cheek scales, 3-4 vs. 5-6, rarely 4 or 7; further from H. aquila by smaller eyes, eye diameter 25.4-29.9% of head length vs. 30.0-31.5% (Ref. 126312). It differs from H. simba by the combination of a broader interorbital area, interorbital width 51.3-61.0% of head width vs. 45.5-50.4%; a larger number of outer upper jaw teeth, 28-46 vs. 22-31; absent to weakly prominent vs. strongly prominent premaxillary pedicel and mentum; and dominant males yellow-green with a red anterior part of flank vs. yellow with an orange anterior part of flank (Ref. 126312). It differs from H. glaucus by the combination of a longer caudal peduncle, caudal peduncle length 15.7-17.5% of standard length vs. 13.4-16.1%; a narrower lower pharyngeal bone, lower pharyngeal width 83.6-85.7% of lower pharyngeal length vs. 93.3-95.1%; a slightly shorter pre-pectoral distance, 33.1-38.2% of standard length vs. 36.4-39.4%; and dominant males yellow-green with a red anterior part of flank vs. uniformly blue (Ref. 126312). It differs from H. kimondo and H. quasimodo by the combination of a narrower head, head width 39.4-42.3% of head length vs. 42.0-48.1%; and dominant males yellow-green with a red anterior part of flank vs. grey dorsally and yellow or blue-black ventrally; further from H. kimondo by a very acute vs. blunt snout, and a gentler sloping snout, 30-35° vs. 40-50°; further from H. quasimodo by a shallower body, body depth 27.2-30.1% of standard length vs. 33.5-41.7% (Ref. 126312). It differs from H. falcatus by the combination of a shorter predorsal distance, 33.3-36.4% of standard length vs. 36.9-41.1%; a shorter head, head length 33.4-37.0% of standard length vs. 36.6-39.6%; straight to weakly recurved vs. strongly recurved outer oral teeth, a steeper lower jaw side, 30-45° vs. 15-25°; absence vs. presence of well-defined mid-lateral and dorsal-lateral bands (Ref. 126312). It differs from H. curvidens and H. pardus by the combination of a deeper lacrimal, 18.1-20.9% of head length vs. 16.0-18.3%; and smaller eyes, eye diameter 25.4-29.9% of headclength vs. 29.4-34.1%; further from H. pardus by dominant males yellow-green with a red anterior part of flank vs. speckled to uniformly black (Ref. 126312). It differs from H. squamipinnis by the combination of large vs. small outer oral teeth and a smaller number of outer upper jaw teeth, 28-46 vs. 46-71; a shallower body, body depth 29.0-32.3% of standard length vs. 32.4-39.3%; absence vs. presence of minute scales on proximal part of dorsal fin; and dominant males yellow-green with a red anterior part of flank vs. uniformly slate blue (Ref. 126312).

Description: Body shallow and oval; caudal peduncle long (Ref. 126312). Head very narrow and with a straight dorsal outline and a slightly convex nape; cheek average in depth; lacrimal deep; eye small; interorbital area narrow in comparison to generalised Haplochromis elegans, but relatively broad for a piscivorous species (Ref. 126312). Snout long, very acute in dorsal and lateral views, and slopes very gentle at 30-35°; premaxillary pedicel long and weakly prominent; jaws long, very narrow, acute in dorsal view, relatively slim, and isognathous; gape large and slopes gently at 20-30°; posterior tip of maxilla reaches a vertical just past anterior margin of orbit; lower jaw slim and with a straight to weakly concave ventral outline in lateral view, mental prominence absent or weakly developed, and lower jaw side steep with an inclination of 30-45° to horizontal in anterior view; upper jaw expanded anteriorly and weakly ventrally; lips and oral mucosa large (Ref. 126312). Neurocranium shallow, ethmo-vomerine block decurved, preorbital region very shallow, 18-22% of neurocranium length, orbital region shallow, 28-31% of neurocranium length, and supraoccipital crest shallow and wedge-shaped (Ref. 126312). Outer oral teeth few, unicuspid, and large; necks stout, conical, and straight; crowns slightly recurved and acutely pointed; dental arcades acute; outer teeth widely and irregularly set with neck-distances of 1/2-2 neck-widths; in upper jaw, 1-3 posteriormost teeth slightly enlarged; inner teeth small, strongly recurved, unicuspid, and acutely pointed; tooth bands very slander crescent-shaped with 1-2 rows of inner teeth, and narrow posteriorly until only outer row remains past 2/3 length of tooth band in upper jaw, past 1/2 length of tooth band in lower jaw; inner teeth closely and regularly set on 1-2 outer neck-widths from outer row; implantation recumbent; size uniform (Ref. 126312). Lower pharyngeal bone long, narrow, slim, and shallow over whole length; pharyngeal teeth relatively large and slender; major cusps acutely pointed; cusp gaps concave; minor cusps and cusp protuberances mostly absent; teeth in two median longitudinal rows equal in size and form to lateral teeth, 11 in each row; posterior transverse row with 14-21 teeth, implanted recumbently with a lateral inclination; major cusps nearly straight, bluntly pointed, and laterally compressed; minor cusps mostly absent (Ref. 126312). Chest scales small; transition to larger flank scales gradual; in some specimens, basal parts of membranes between anal-fin spines covered by few, 1-3, minute, ellipsoid scales, nearly invisible to naked eye; remaining part of anal and dorsal fins scale-less; minute scales on proximal half of caudal fin; 32-35 scales on longitudinal line, 19-23 upper lateral line scales, 8-14 lower lateral line scales, 5-7 upper transverse line scales, 10-13 lower transverse line scales, 16-19 scales around caudal peduncle, 5-8 scales between pectoral and pelvic fins, 3-4 infraorbital cheek scales, 8-12 postorbital cheek scales (Ref. 126312). Caudal fin emarginate; dorsal and anal fins reach to between vertical through caudal-fin base and two scales anterior to this vertical; pectoral and pelvic fins reach to between genital opening and first anal-fin spine; pelvic fin reaches to third anal-fin spine in males; first branched pelvic-fin ray slightly elongated in all specimens (Ref. 126312). Ceratobranchial gill rakers in outer row of first gill arch short, stout, and simple; posteriormost rakers mostly anvil-shaped or weakly bifid; epibranchial rakers slender and simple (Ref. 126312).

Colouration: Colouratio in life of dominant males: body, cheek, and lower jaw yellow-green with blue sheen; flank, dorsal part of head, and operculum bright red; belly and chest speckled black; snout and lips dusky; branchiostegal membrane black; eye with (dark) grey outer ring and yellow inner ring; flank with very faint mid-lateral, dorsal-lateral, and 5-7 vertical stripes; nostril stripe faint; dorsal and anal fins dusky and with black lappets; anal fin with a crimson distal part and 2 small orange egg spots with dusky rings; caudal fin dusky with a crimson distal part; pectoral fin dusky yellow and pelvic fin black (Ref. 126312). Colouration in life of females and juveniles: dorsal parts of body and operculum green-yellow; gradual transition to white ventral parts of body and operculum, cheek, and lower jaw; flank with a blue sheen; lacrimals, snout, and lips dusky green; lacrimals with a blue sheen; eye with (dark) grey outer ring and yellow inner ring; nostril stripe faint; dorsal fin dusky and with black lappets; anal fin white-yellow and with 1-2 small spots resembling egg-spots; caudal fin dusky; pectoral fin yellowish; pelvic fin white-yellow (Ref. 126312). Preserved colouration: in dominant males, body uniformly brown, pectoral fin dusky, pelvic fin black, and anal fin dusky and with 1-2 small egg-spots; in females, dorsal part of body yellowish, gradual transition to white ventral part of body, cheek light yellow, pectoral fin hyaline, pelvic fin yellowish, and anal fin with a white base and a dusky distal part; in all specimens, snout dusky and nostril, interorbital, and lacrimal stripes faint, dorsal fin dusky and with black lappets, caudal fin dusky (Ref. 126312).