Diagnosis: The dark submarginal band in the spinous part of the dorsal fin of both male and female distinguishes Petrotilapia mumboensis from P. tridentiger, P. xanthos, P. flaviventris, and P. palingnathos, which lack such a band (Ref. 87181). Males of P. mumboensis are light blue with 8-10 dark blue vertical bars, light blue cheek, and a light blue to gray gular region, which distinguishes it from P. genalutea, P. nigra, P. chrysos, P. microgalana, and P. pyroscelos; Petrotilapia genalutea are dull gray-blue with 5-7 black vertical bars with orange-yellow cheek and a black throat; males of P. nigra and P. chrysos are predominantly blue-black with 7-10 gray/brown bars, have a dark blue cheek, and a black throat; males of P. microgalana are bright blue with 5-7 faint black vertical bars, and have a light blue cheek and a yellow throat while those of P. pyroscelos are blue with bronze highlights and have a red pelvic fin (Ref. 87181). Females of P. mumboensis are gray-brown to light brown with a conspicuous black submarginal band in the dorsal fin and are distinguished from those of P. genalutea, P. nigra, P. chrysos, and P. microgalana by the lack of horizontal elements in the pigmentation pattern on the flank and from females of P. pyroscelos, which are brown with faint blue and yellow highlights and often have dorso-lateral series of faint spots (Ref. 87181).

Description: Body depth at origin of dorsal fin 36.3-42.5% of standard length; dorsal head profile concave between snout tip and interorbital; large males with small hump at interorbital (Ref. 87181). Horizontal and vertical eye diameters 23.4-30.2% of head length and 24.7-31.5% of head length, respectively; preorbital depth 20.0-28.5% of head length; snout 32.3-42.8% of head length; isognathous jaws and thickened lips; teeth on lower jaw in 9-21 rows with teeth in outer and inner rows tricuspid (Ref. 87181). Dorsal fin with XVI-XVIII spines and 8-10 rays; anal fin with III spines and 7-9 rays; first dorsal-spine less than half length of ninth spine; caudal fin subtruncate to emarginate; depressed pelvic fin not reaching anal fin in most females and reaching to second anal-fin spine in males; pectoral fin round and paddle-shaped (Ref. 87181). Flank scales large, ctenoid; cheek with 3-5 rows of scales; lateral line with 29-31 pored scales (Ref. 87181).

Colouration: Live males with dark blue/gray head and two light blue interorbital bars, opercle and pre-opercle light blue and black opercular spot; throat light blue/gray; flank blue/green with 8-10 dark blue bars; breast and belly white with black/gray markings; caudal peduncle dark blue/gray; dorsal fin with broad black submarginal band; proximal 1/4 of dorsal fin light blue, lappets light blue with orange tips; caudal fin with gray rays, orange tips, and light blue membranes; anal fin blue/gray with black spinous portion, and broad black marginal bar over rayed portion; 2-6 dark orange ocelli on anal fin; pelvic fin light blue with white leading edge; first two rays of fin and membrane black, rest of membranes clear; pectoral fin with gray rays and clear membranes (Ref. 87181). Live females with brown head and gray throat; flank brown with light brown to white belly, dark gray breast, and 8-10 faint dark vertical bars; anal fin black with 1-4 small orange ocelli; pelvic, pectoral, caudal, and dorsal fins as in males (Ref. 87181). Preserved males with dark brown head, brown opercle, and brown throat; flank dark brown to gray with 8-10 faint dark vertical bars, white to gray belly, and dark brown breast; dorsal fin light brown to dark, with broad submarginal black band; dorsal fin membranes light brown; caudal fin brown to gray with light brown membranes; anal fin gray with black marginal band over spines and first ray; spines black with dark brown rays and light brown membranes; pelvic fin light brown with black spine; light brown membranes and black marginal band over spines and first ray; pectoral fin light brown with clear membranes (Ref. 87181). Preserved females with dark brown head, brown opercle, and yellow to light brown throat; flank brown with 8-10 vertical bars, yellow to light brown breast, and light brown belly; dorsal fin light brown with broad submarginal band and light brown membranes; caudal fin brown with light brown membranes; anal fin light brown to gray, light brown membranes, and black spines with submarginal band; pelvic fin gray to white; black spine with first three rays dark and last two rays white to gray; pectoral fin dark gray with clear membranes (Ref. 87181).