Taxonomy & naming
Petrotilapia sp. "Nigra Tumbi" has no describing author, no publication date, and no entry under a validated name in Eschmeyer's Catalog of Fishes. Its name follows a recurring pattern in the field literature: "nigra," Latin for black, is used informally across several unrelated Petrotilapia populations around the lake to flag a predominantly black or near-black male breeding colour, with a locality qualifier — here "Tumbi," for Tumbi Point on the Mozambican shore — distinguishing this particular black-morph population from the many others recorded elsewhere, including the formally described species Petrotilapia nigra Marsh, 1983, itself named for the same colour and split from P. tridentiger in Marsh's 1983 revision. The genus Petrotilapia is notorious for producing multiple distinct male colour morphs, sometimes several at a single reef, which has left species boundaries within the genus unusually difficult to establish and left the great majority of named forms undescribed. Whether this Tumbi Point black morph is conspecific with, or merely convergent on, the described P. nigra has not been resolved by any published study.
Morphology
As implied by its trade name, breeding males of Petrotilapia sp. "Nigra Tumbi" are described as predominantly black or near-black, a striking departure from the barred or blue-toned patterns typical of most other Petrotilapia forms. Detailed morphometrics specific to this population have not been published. The body should follow the genus-typical robust build — a thick-set head and strong, closely packed teeth adapted for scraping tough algal turf from wave-washed rock, larger and heavier than most other mbuna. Genus-typical size for Petrotilapia runs to roughly 4.5–6 in total length, with males somewhat larger than females. As with other "nigra" Petrotilapia forms recorded elsewhere in the lake, the black colouration is confined to breeding males; females and non-breeding males are expected to be far plainer and paler.
Habitat
The form is recorded from Tumbi Point, on the Mozambican shore of Lake Malawi. Petrotilapia are specialists of wave-exposed, high-energy rocky shoreline, where strong surf keeps algal turf short, and this form is expected to occupy the same shallow-to-moderate rocky habitat typical of the genus. The Mozambican shore of the lake is comparatively less intensively surveyed than the well-studied Malawian coast, and the precise extent of this form's range beyond Tumbi Point is not established. Water in this part of the lake follows the general Malawi profile: warm, hard, and alkaline.
Feeding
Petrotilapia are the lake's rock-scraping algae specialists, using broad, closely packed teeth to tear and scrape tough, wave-hardened algal turf from exposed rock, a niche their robust jaws are particularly well suited to compared with the more delicately toothed grazing mbuna. Petrotilapia sp. "Nigra Tumbi" is presumed to feed in this genus-typical manner, defending a feeding territory on exposed rock, though no feeding study specific to this population has been published.
Mating
Mature males are expected to hold and defend a feeding-and-breeding territory on exposed rock, displaying their black breeding colour to attract passing females — a pattern consistent with other "nigra" Petrotilapia forms recorded elsewhere in the lake. Petrotilapia territorial defence is generally more intense than in many other mbuna genera, given the value of well-exposed, algae-rich rock. Given the genus's tendency to produce multiple coexisting male colour morphs at a single site, other Petrotilapia colour forms may share these Mozambican reefs, though this has not been documented specifically.
Breeding
As in all Lake Malawi haplochromines, reproduction is by maternal mouthbrooding: the female collects the fertilised eggs into her mouth after spawning and carries them, without normal feeding, for several weeks until the fry are ready for independent release, with the male taking no further role. Clutch sizes in Petrotilapia tend toward the larger end for mbuna given the genus's larger adult body size, though exact figures for this Tumbi Point population have not been documented.
In the aquarium
Petrotilapia sp. "Nigra Tumbi" would be kept as a robust rock-grazing mbuna: substantial rockwork offering exposed grazing surfaces and retreats, a sand substrate, and good water movement suit its natural high-energy habitat. Water chemistry should match Lake Malawi's rocky zone — hard and strongly alkaline, pH roughly 7.7 to 8.6, temperature 75–82 °F. As with other Petrotilapia, expect pronounced aggression, generally more intense than typical mbuna, so a generously sized tank, careful stocking, and one male to several females (or a very large, densely decorated mixed-mbuna system) help manage conflict. Diet should be strongly vegetable-based — spirulina flake or pellet, algae wafers welcome — with fatty, protein-heavy foods minimised given the group's susceptibility to digestive bloat.
Conservation
Petrotilapia sp. "Nigra Tumbi" has not been evaluated by the IUCN and, as an undescribed form, has no Red List status. Its documented range is limited to Tumbi Point on the Mozambican shore, a part of the lake that is comparatively less surveyed than the Malawian side, meaning both its true extent and any pressures on it remain poorly known. The general caution applied to narrow-range Malawi mbuna forms — vulnerability to concentrated collection or localised habitat change — applies here in the absence of any formal assessment.