Taxonomy & naming
George Albert Boulenger described this fish in 1906 as Tilapia nigricans, from two specimens (syntypes, BMNH 1906.5.30.469) collected at Entebbe on the Ugandan shore of Lake Victoria — the type locality and, for many years, the only place from which it was recorded with certainty. As the haplochromine flock of the great lakes was sorted out over the twentieth century the species was moved into the catch-all genus Haplochromis, the name still used as the heading here and across much of the hobby and older literature. Peter Humphry Greenwood, in his 1980 generic revision of the Lake Victoria haplochromines, treated nigricans as the sole Victorian representative of the genus Neochromis, a lineage of strictly rock-restricted (lithophilic) algae scrapers; he had been surprised to find such specialists in the lake at all, having long believed they were a Lake Malawi phenomenon. For decades nigricans stood as the group's only formally described member even as field workers kept turning up more of them — Ole Seehausen's own surveys around the Mwanza and Speke Gulfs, building on collections by Bo Selbrink in Uganda in 1985 and the long-term Haplochromis Ecology Survey Team founded in 1990, eventually brought the number of recognised epilithic-scraper species to at least twenty. When Seehausen, Erwin Lippitsch, Nicolas Bouton and Han Zwennes published their 1998 monograph 'Mbipi, the rock-dwelling cichlids of Lake Victoria' — describing three new genera and fifteen new species — they formally placed the fish as Neochromis nigricans, and Eschmeyer's Catalog of Fishes today lists Neochromis nigricans (Boulenger, 1906) as the valid combination. 'Mbipi' is the local Kerewe name for these rock cichlids. The genus has since accumulated several relatives (the 'giant scraper', 'blue scraper', velvet-black and omnicaeruleus forms), but nigricans remains its anchor and its diagnostic type.
Morphology
Haplochromis nigricans is a small cichlid — FishBase gives a maximum of about 3.5 in standard length — with a fusiform, moderately deep body (around 38% of standard length deep), a moderately to strongly decurved dorsal head profile — the steep, downward-sloping forehead typical of this whole scraper group, not the convex profile of a more generalized haplochromine — a short snout and a terminal mouth. The jaws carry broad bands of closely set, subequally bicuspid teeth in the outer row and five or more inner rows (four to five in one known form); weakly bicuspid or unicuspid teeth turn up only in very old, long-captive fish. Behind that dentition sits a long, tightly coiled intestine that can run up to four times the standard length, the plumbing a herbivore needs to process a diet of algae. Fin counts run to 15–16 dorsal spines with 9–10 soft rays, three anal spines and nine anal soft rays. As the Latin name ('blackish') promises, the resting body colour is dark, and like other 'Vertical bar Mbipi' it carries a series of simple vertical bars on the flanks that may be crossed by a faint mid-lateral band, a pattern clearest in juveniles and females. The nuptial dress itself, though, is not the blackish colour the name might suggest: diagnostic accounts of the group describe breeding males as turning grey-blue to light blue with five or more vertical stripes running from the pectoral fins back to the end of the dorsal fin, and note that all known nigricans populations share a red caudal fin. Exceptions occur locally — at some Mwanza Gulf islands, yellowish and very dark males have been recorded living alongside normally coloured fish, and it is unresolved whether black-and-yellow-white piebald individuals collected on a Speke Gulf island belong to this species at all. Seehausen and colleagues separated nigricans from the other Neochromis on hard anatomy — it has enlarged cephalic (head) pores and unusually large, deeply embedded scales over the chest (a trait that persists even in tank-bred 'Mwanza Gulf' stock) that only grade down to small ones near the pelvic-fin bases, and relative to N. simotes it is deeper-bodied with a shorter, narrower snout and larger eyes.
Habitat
The species is endemic to Lake Victoria and the Victoria Nile. It is one of the Mbipi — the cichlids of the lake's rocky shores, which make up only about a tenth of a shoreline otherwise dominated by sand and swamp. Like most Mbipi it is a habitat specialist: it has occasionally been taken over sand, but hardly ever more than fifty metres from rock, and it lives on and among the boulders themselves, grazing the sunlit upper surfaces where algal growth is richest. It is a shallow-water fish. Seehausen's diving observations put the highest densities of Neochromis at roughly 3–10 ft depth, with the genus as a whole ranging down to about 16–20 ft; nigricans specifically was never observed or caught beyond about 16 ft. FishBase records the water it lives in as hard and alkaline — pH around 8 and hardness of roughly 9–19 dH — at tropical temperatures near 75–79 °F, consistent with Lake Victoria's well-buffered, mineral-rich open water. Because stretches of sand and deep water are impassable barriers to such a rock-bound, shallow specialist, populations on separate rocky islands are effectively isolated from one another, a fragmentation that helped drive the flock's rapid speciation.
Feeding
Haplochromis nigricans is an epilithic algae scraper — it feeds by rasping the 'Aufwuchs', the carpet of unicellular and filamentous green and blue-green algae that coats sunlit rock surfaces. In the lake it has been directly observed grazing algae from rock, and its anatomy is built for the job: a short snout, a terminal mouth, and broad bands of contiguously set, subequally bicuspid teeth that work the substrate like a comb. FishBase places it near the bottom of the food web, at a trophic level of about 2.2. Within the Neochromis guild a row of similar scrapers partition the same rocky slope by depth, their population peaks usually separated by no more than a metre of water, so that several specialists graze in parallel without directly displacing one another. As one of the lake's grazers, nigricans belongs to the functional group whose collapse was felt across the whole ecosystem: when the algae-eating cichlids were lost, algal blooms went unchecked and helped drive the deoxygenation of the deeper water.
Mating
Like the rest of the Lake Victoria flock, Haplochromis nigricans is a polygynous maternal mouthbrooder with no pair bond, and its social system is organised around territorial males. Sexually active males defend small territories on the rocky slope — among the Mbipi these territories are distributed across much the same depth band the fish forages in, and densities can be very high, with several species breeding in sympatry at a given depth and males squabbling along closely spaced boundaries. A territorial male darkens into nuptial colour and displays to passing females, which move over the habitat in loose shoals; a ripe female visits to spawn and then leaves. Mate choice in these rock cichlids is strongly visual and colour-based: females discriminate between anatomically near-identical forms largely by male coloration, a behaviour Seehausen's group showed in the laboratory and which underpins the way so many similar-looking species coexist. That same colour-based mate recognition is the species' Achilles' heel — when the water turns turbid, the visual cues blur and the reproductive barriers between forms break down.
Breeding
Reproduction follows the maternal-mouthbrooding pattern universal to the Lake Victoria haplochromines: the female takes the fertilised eggs into her mouth — typically nipping at the male's anal-fin egg-spots as part of the fertilisation sequence — and broods the clutch buccally with no help from the male, who returns to defending his territory and courting other females. Specific clutch counts for nigricans do not appear to have been published; Victorian rock haplochromines of this size are small-clutch brooders, generally carrying on the order of a few dozen eggs, the trade-off of a mouthbrooder that invests heavily in each large, well-provisioned egg. After releasing the free-swimming fry for the first time, brooding females of most Mbipi — unlike many Malawi mbuna — continue to guard the young for a week or more, defending a very small territory around a crevice or hideout into which the fry can bolt. FishBase rates the species as highly resilient, with a minimum population-doubling time under fifteen months, which reflects this protected, fast brood cycle.
In the aquarium
Haplochromis nigricans is one of a handful of Lake Victoria rock cichlids that have been in the hobby for some years, but it remains a specialist's fish and is only rarely kept; Seehausen noted in 1996 how little had been published on keeping Victorian rock cichlids compared with the vast literature on Malawi and Tanganyika. What is known follows general mbipi husbandry rather than species-specific experience. These are aggressive, territorial fish for their size, so the tank should be furnished like their natural biotope — a rockwork of piled stones forming caves and broken sight-lines, over sand — in a footprint large enough to spread out the aggression of nuptial males; a single male typically dominates, so keeping one male with several females, or a larger group in a bigger tank, distributes the harassment. Water should match the lake: hard and distinctly alkaline (pH roughly 7.5–8.5, the species tolerates up to about 9), with temperatures around 75–81 °F. Diet must be vegetable-based — a spirulina or algae-based prepared food, with only sparing protein — because, like Malawi mbuna, these dedicated algae grazers are prone to digestive disease ('Malawi bloat') on too rich a diet. The most important conservation-minded point of husbandry is to keep any Victorian haplochromine as a clean, single-species line and never to hybridise it with the many superficially similar forms in the trade; with so much of this flock gone or going from the wild, aquarium populations have real value only if their identity is kept pure.
Conservation
The IUCN Red List assesses Haplochromis nigricans as Data Deficient (assessed 1 April 2010 by F. Witte and M.P. de Zeeuw, published 2017), with an unknown population trend — the data simply are not adequate to judge its status, a common verdict for the Lake Victoria flock given the taxonomic confusion and the difficulty of surveying so many similar rock-bound forms. The assessment names two threats. The first is hybridisation: as Lake Victoria's water has lost its transparency to eutrophication and sediment, the colour-based visual cues by which females recognise mates fail, and previously distinct species interbreed and collapse into hybrid swarms — Seehausen's work made this loss of biodiversity through 'speciation reversal' a famous case. The second is incidental capture as bait for the long-line fishery. Behind both sits the wider catastrophe of Lake Victoria: the introduced Nile perch (Lates niloticus), whose population exploded from the 1980s, preyed the endemic haplochromine flock down catastrophically — hundreds of species are estimated to have declined or vanished — while nutrient loading and algal blooms deoxygenated the deeper water. As a shallow, rock-restricted grazer, nigricans was somewhat buffered (Nile perch hunt the deeper rocky shelf hardest), but it remains exposed to the turbidity-driven breakdown of mate recognition that the Red List flags as its principal threat.