Taxonomy & naming
Petrotilapia sp. "Nigra Tanzania" 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 "Tanzania," for the northern Tanzanian shore — distinguishing this particular black-morph population from the many others recorded elsewhere. The genus Petrotilapia (Regan, 1922), the lake's dedicated rock-scraping mbuna specialists, 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 Tanzanian black morph represents a distinct species, or one colour morph within a wider Petrotilapia population sharing these reefs with other morphs, has not been resolved by any published study.
Morphology
As implied by its trade name, breeding males of Petrotilapia sp. "Nigra Tanzania" 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 morphometric data 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 three points along the northern Tanzanian shore of Lake Malawi — Lundu, Magunga, and Ngwasi. 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, generally within the top 33–49 ft. The Tanzanian shore of northern Lake Malawi is comparatively less intensively surveyed than the well-studied Malawian coast further south, and the precise extent of this form's range beyond the three recorded points 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 set in strong jaws to tear and scrape tough, wave-hardened algal turf from exposed rock — a niche their robust dentition is particularly well suited to compared with the more delicately toothed grazing mbuna such as Maylandia, which tend to favour softer, more sheltered algal growth. That dietary specialisation lets Petrotilapia exploit the highest-energy, most wave-battered rock faces on the reef, where algal turf grows dense but is difficult for other grazers to harvest efficiently. Petrotilapia sp. "Nigra Tanzania" is presumed to feed in this genus-typical manner, defending a feeding territory on exposed rock against both conspecifics and other grazing mbuna, though no feeding study specific to this Tanzanian 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 Tanzanian 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 developed enough for independent release. The male takes no further role once spawning is complete and provides no protection to the female or the developing brood. Brooding females generally withdraw from exposed feeding rock into more sheltered parts of the reef for the duration, reducing both feeding opportunity and the risk of confrontation with territorial males while unable to feed normally. Clutch sizes in Petrotilapia tend toward the larger end for mbuna, reflecting the genus's larger adult body size compared with most other rock-dwelling cichlids, though exact figures for this Tanzanian population have not been documented.
In the aquarium
Petrotilapia sp. "Nigra Tanzania" 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 Tanzania" has not been evaluated by the IUCN and, as an undescribed form, has no Red List status. Its documented range is limited to a stretch of the northern Tanzanian shore that is comparatively less surveyed than the Malawian side of the lake, 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. Several formally described Petrotilapia species elsewhere in the lake have themselves been assessed by the IUCN, a reminder that the genus is not automatically low-concern simply because many of its colour morphs appear locally abundant where they occur.