Taxonomy & naming
The species was described by the French ichthyologist Jacques Pellegrin in 1909, in the Bulletin de la Société Zoologique de France, under the name Astatotilapia nigrescens. The single type specimen (holotype MNHN 1909-0508, in the Muséum national d'Histoire naturelle in Paris, catalogued by Blanc in 1962) came from Kavirondo Bay — the Kavirondo (Winam) Gulf in the northeastern corner of Lake Victoria, in present-day Kenya. The name was later combined into the catch-all genus Haplochromis, where it has remained valid; van Oijen and colleagues retained it as Haplochromis nigrescens in their 1991 CLOFFA treatment, and Eschmeyer's Catalog of Fishes lists it under that combination today. The generic placement is a matter of convention rather than settled phylogeny: Lake Victoria's flock of more than 500 endemic haplochromines is a very recent, explosively diversified radiation, and the assignment of its members among Haplochromis, Astatotilapia, Pundamilia, Neochromis, Lithochromis and other genera has been repeatedly revised. We keep the long-standing Haplochromis heading here, as the most widely recognized name, while noting that nigrescens was erected in Astatotilapia and that its exact position within the radiation has never been resolved. The species has no established common name.
Morphology
Haplochromis nigrescens is a small cichlid, reaching about 4.5 in in total length, with the laterally compressed, deep-bodied build typical of the Lake Victoria haplochromines. Its defining feature is its colour, and indeed its name: the specific epithet nigrescens is Latin for 'becoming black' or 'darkening'. Pellegrin's material, and the etymology recorded by The ETYFish Project, describe a chocolate-coloured body set off by blackish fins — the dark, melanic appearance that gives the species its name. As in the great majority of the lake's haplochromines, sexual dichromatism is expected to be pronounced: throughout the flock it is the breeding males that develop the bold, often dark or boldly patterned nuptial colours used in courtship, while females and non-breeding fish are a plainer silvery-brown. Detailed morphometric and meristic data for this poorly studied species are sparse, and because so many superficially similar dark haplochromines coexist in the lake, reliable field identification rests on a combination of head and tooth characters that have never been fully published for nigrescens.
Habitat
The species is endemic to Lake Victoria, the vast, shallow East African lake shared by Tanzania, Uganda and Kenya. FishBase places it between roughly 1°N and 3°S, and the type locality — Kavirondo (Winam) Gulf — is a broad, comparatively shallow, turbid inshore embayment on the Kenyan side of the lake. Lake Victoria's water is warm and moderately alkaline, generally in the range of about 73–81 °F with a pH around 7.2 to 9.0, and the lake is famously shallow for its size (mean depth roughly 131 ft, maximum around 262 ft). Like most members of the flock, nigrescens is a fish of the productive, well-lit inshore and sub-littoral zone rather than the deep open water; no measured depth range has been published for the species specifically, so the figures given here reflect the inshore haplochromine band rather than a direct record. Its precise biotope — whether it favoured rock, sand or open water — was never documented in detail before the lake was transformed.
Feeding
Haplochromis nigrescens is a piscivore — a fish-eater. FishBase records its diet simply as 'feeds on fish' and estimates a high trophic level of about 4.2, consistent with a predator near the top of the haplochromine food web. Lake Victoria's species flock is the textbook example of trophic radiation: from a single ancestor it diversified into a full spectrum of feeding guilds — algae-scrapers, detritivores, zooplanktivores, insectivores, mollusc-crushers, scale-eaters, paedophages (which prey on the broods of mouthbrooding females) and piscivores — a framework set out in detail by Witte and Witte's classic monograph on the lake's haplochromine trophic groups. The piscivores, the guild to which nigrescens belongs, were among the most diverse, with many species specialized on hunting other, smaller haplochromines. That specialization became a liability: as fish-eating predators, the piscivorous haplochromines both competed with, and were preyed upon by, the introduced Nile perch, and they were among the hardest-hit groups when the lake's ecology collapsed.
Mating
No courtship has been described for Haplochromis nigrescens itself, but its reproductive biology can be read from the well-studied behaviour of the Lake Victoria flock to which it belongs. These cichlids are lek-like maternal mouthbrooders in which mate choice is driven overwhelmingly by male nuptial colour: ripe males develop bright or strongly contrasting breeding dress (in this dark species, the chocolate-and-black livery that names it) and display to females, which choose among them visually. Seehausen and van Alphen's work showed that this colour-based female choice is the engine of reproductive isolation across the flock — closely related species are kept distinct chiefly by differences in male colour rather than by gross morphology. That dependence on vision has a sting in its tail: where human activity has clouded the water, females can no longer discriminate between males of different species, mate choice breaks down, and previously distinct species hybridize and merge.
Breeding
Haplochromis nigrescens is a maternal mouthbrooder, as are all of Lake Victoria's haplochromine cichlids. FishBase records the mode simply as 'mouthbrooder', and no clutch count or developmental data have been published for this particular species. In the genus-typical pattern, the female takes the fertilized eggs into her mouth — often nipping at egg-shaped markings on the male's anal fin, which prompt him to release sperm over the brood she already holds — and then carries and incubates the eggs and newly hatched fry in her buccal cavity for a few weeks, taking no food during the period. The young are released as well-developed free-swimming fry and may shelter back in the mother's mouth when threatened for a short time afterward. Clutches in small Lake Victoria haplochromines of this size are typically modest, on the order of a few dozen eggs (commonly cited figures for comparable species run to roughly 20–60), reflecting the trade-off inherent in mouthbrooding — fewer, larger, better-protected young rather than the many small eggs of a substrate spawner. The specific clutch size, spawning season and fry development of nigrescens remain undocumented.
In the aquarium
Haplochromis nigrescens is not a fish of the aquarium hobby. It is a poorly known, possibly vanished Lake Victoria endemic with no commercial collection, no aquarium-trade presence, and no body of captive husbandry to report — unlike a handful of colourful Victoria haplochromines (such as members of the Pundamilia and 'Astatotilapia' groups) that are maintained in conservation-minded breeding programs and occasionally reach specialist keepers. Anyone wishing to keep Lake Victoria haplochromines in general should treat them as warm-water, hard, alkaline-water fish (around 75–81 °F, pH well above 7) kept in spacious tanks with the males' aggression managed by crowding and broken sight-lines, and should be aware that maintaining species identity in captivity requires keeping each species separate, because these fish hybridize readily. For nigrescens specifically, however, the honest position is that it is not kept, and no reliable account of it in captivity exists. The most valuable thing aquarists do for fish like this is the deliberate, pedigreed conservation breeding of the Victoria haplochromines that are still available, not the pursuit of this one.
Conservation
The IUCN Red List assesses Haplochromis nigrescens as Data Deficient, in an assessment by Witte and de Zeeuw dated 31 March 2010; the population trend is listed as unknown. The assessors note that the threats are not fully understood but are likely to include hybridization driven by decreased water transparency, which disrupts the colour-based mate recognition that keeps the lake's species distinct, and that the fish is taken in fisheries though not specifically targeted. Tellingly, Eschmeyer's Catalog of Fishes flags the species' distribution as 'extinct' — a reflection of how comprehensively this group was reduced. The wider story is one of the most studied ecological disasters of the modern era: from the 1980s the introduced Nile perch (Lates niloticus) erupted across Lake Victoria, and as documented by the Leiden Haplochromis Ecology Survey Team and by Witte and colleagues, the haplochromine flock — once perhaps 500 or more species and the bulk of the lake's fish biomass — was decimated, with the piscivorous and deep-water guilds among the worst affected. Eutrophication and falling water clarity from catchment deforestation and nutrient loading compounded the collapse by undermining the visual mate choice that maintained species boundaries. Some haplochromines have rebounded as Nile perch were fished down, but for many narrowly known species like nigrescens it is genuinely unclear whether populations survive, which is precisely why the species sits in the Data Deficient category rather than a definite one.