Taxonomy & naming
Petrotilapia sp. "Black Flank" is not a formally described species. It has no scientific name of its own, no describing author, no year of publication, and no entry in Eschmeyer's Catalog of Fishes, because no taxonomist has published a description establishing it. It is an informal taxon recognised in the field and aquarium literature, principally through Ad Konings's documentation of the northern Malawian shore. The placeholder name follows the standard convention for undescribed cichlids: the genus Petrotilapia, the abbreviation "sp." (species), and a descriptive tag, here "Black Flank," for the dark male colouration. Petrotilapia is one of the most taxonomically unsettled genera in Lake Malawi — only a handful of species have ever been formally described (among them P. tridentiger, P. nigra, and P. genalutea), while the genus is understood to comprise dozens of geographically distinct, largely undescribed local populations distinguished chiefly by subtle differences in male colour and pattern. This form should be regarded strictly as one such Konings-recognised undescribed population, and no authority or date should be attached to it.
Morphology
Petrotilapia species share a robust, moderately elongate body and a distinctively wide, blunt head carrying rows of comb-like, closely packed unicuspid teeth adapted for scraping algae from rock — a diagnostic feature of the genus visible on close inspection of the jaws. Genus-typical adult size runs roughly 4–6 in. Consistent with its trade name, dominant males in this population are understood to show dark, near-black flank colouration, a common pattern across many Petrotilapia forms in which otherwise blue or grey-based males develop strong melanism when in full breeding condition. Females and subordinate males are typically much plainer — mottled brown or grey — the standard camouflage pattern for female mbuna that must forage on open rock without drawing predator attention. Beyond the flank colouration named in the trade name, no further population-specific detail has been documented.
Habitat
This form is recorded from Katale Island and Mphanga Rocks, both near Chilumba on the Malawian shore in the northern part of the lake — a stretch of coast well known for extensive rocky habitat and a rich mbuna fauna. Petrotilapia is a strictly rock-associated genus, grazing algal films ("Aufwuchs") from rock surfaces in the shallow-to-moderate depths typical of mbuna habitat, rarely straying onto open sand. The water column here follows the lake-wide profile: warm, hard, and strongly alkaline, broadly mid-20s Celsius with pH on the alkaline side of neutral. As an undescribed, locality-based form its depth range and full distribution beyond the Katale–Mphanga stretch of coast are not established.
Feeding
Petrotilapia is a genus of specialised algae grazers, using its comb-like unicuspid teeth to scrape algal films and the associated small invertebrates (collectively "Aufwuchs") from rock surfaces in well-lit, shallow-to-moderate water. No feeding study exists for this population specifically, but this genus-typical herbivore/detritivore mode is the reasonable characterisation absent contrary data — a diet built around algae scraped directly from rock, rather than sand-sifted or open-water food.
Mating
Like other Lake Malawi mbuna, Petrotilapia sp. "Black Flank" is presumed polygynous, with dominant, dark-flanked males holding and defending small territories on the rock face, from which they court passing females while driving off rival males and other territory intruders. There is no pair bond. Females move between male territories, choose largely on the basis of colour and territory quality, and spawn briefly on the rock before departing with the fertilised clutch in the mouth. No behavioural study specific to this undescribed population exists; the account follows the general pattern documented for Petrotilapia and for rock-dwelling mbuna more broadly.
Breeding
Petrotilapia sp. "Black Flank" is a maternal mouthbrooder, as are all Lake Malawi mbuna. After spawning on the rock face, the female carries the fertilised eggs in her mouth for roughly three to four weeks, feeding little during this period, before releasing free-swimming fry with no further care from either parent. Clutch size has not been documented for this form specifically; by analogy with similarly sized Petrotilapia species, a modest clutch of a few dozen eggs is a reasonable expectation rather than a measured fact.
In the aquarium
Petrotilapia sp. "Black Flank" is an uncommon aquarium fish, traded mainly through specialist Malawi importers working the Chilumba area. Where it is kept, husbandry follows the well-established mbuna template.
Dense, stable rockwork built directly on the tank floor — not on top of substrate, which can collapse it — is essential, since this is a strictly rock-associated grazer that needs abundant surface area and shelter. A footprint of at least 47 in suits an adult group. Water chemistry should mirror Lake Malawi's profile: temperature in the mid-20s Celsius, pH roughly 7.7–8.6, and moderately hard to hard water, with prolonged soft or acidic conditions avoided. Diet should emphasise vegetable matter — a good spirulina-based flake or pellet as the staple, with algae wafers and blanched vegetables as supplements — reflecting its herbivore/grazer biology; meaty, high-protein foods should be limited, as Petrotilapia and other herbivorous mbuna are prone to digestive problems on protein-heavy diets. As with other rock-dwelling mbuna, males are territorial and often aggressive, so a single male with multiple females and adequate rockwork to break sightlines works better than a pair in a small tank.
Conservation
Petrotilapia sp. "Black Flank" has not been evaluated by the IUCN, and as an undescribed form without a formal scientific name it has no Red List assessment of any kind. That silence is a statement about taxonomy, not about safety. As a rock-restricted population near Chilumba, it is exposed to the pressures typical of popular mbuna collection sites: ornamental fish collection, and the broader basin-wide stresses of sedimentation and fishing pressure affecting Lake Malawi's rocky shore communities. No conservation status should be fabricated for it. The honest statement is that it is unassessed and that its true range and status remain to be established by formal study.