Taxonomy & naming
Haplochromis dichrourus was described by the British ichthyologist Charles Tate Regan in 1922, in his landmark monograph 'The cichlid fishes of Lake Victoria' (Proceedings of the Zoological Society of London). The name was published in the original combination Haplochromis dichrourus and has remained valid in that genus ever since. The type series is a single specimen — the holotype is therefore unique — registered at the Natural History Museum, London as BMNH 1906.5.30.265 and collected by Edward Degen in Uganda. Regan gave the type locality as Buganga, on the Ugandan (north-western) shore of Lake Victoria. The species was retained in subsequent treatments of the flock, including P.H. Greenwood's revisionary work on Lake Victoria Haplochromis (1967) and the haplochromine account of van Oijen and colleagues in the Check-list of the Freshwater Fishes of Africa (CLOFFA, 1991). Like almost all of the Lake Victoria haplochromines, dichrourus belongs to the explosively diverse endemic species flock that some workers split into many smaller genera (Pseudocrenilabrinae); the conservative, widely used practice — followed here — is to keep these fishes under the umbrella genus Haplochromis. No junior synonyms are listed, and the species has not been the subject of any modern redescription.
Morphology
Haplochromis dichrourus is a medium-sized haplochromine, reaching a maximum recorded length of about 7.5 in SL (standard length) — large for a member of the flock and at the upper end of the lake's piscivore size range. As is typical of the predatory Lake Victoria haplochromines, it has an elongate, fusiform body, a fairly large mouth and a pointed snout suited to seizing other fishes. Regan's 1922 account distinguished it within the genus on meristic and proportional characters rather than colour, and because the species is known from few specimens and has not been seen alive in decades, no detailed account of live coloration survives. The scientific name records the one feature Regan singled out: dichrourus means 'two-coloured tail', from the Greek for a tail bearing two colours, indicating a contrastingly patterned caudal fin. Sexual dimorphism is presumed to follow the haplochromine norm — territorial, breeding males developing brighter nuptial colour and (in many congeners) egg-spots on the anal fin, females and non-breeding fish remaining drabber — but this has not been documented for dichrourus specifically.
Habitat
The species is endemic to Lake Victoria, the great shallow equatorial lake shared by Uganda, Tanzania and Kenya; the type locality lies on the Ugandan shore, and the best-studied populations were in the Mwanza Gulf at the Tanzanian end. Exact details of its distribution within the lake are not known. It occupied the littoral and sub-littoral zones as well as more open offshore water, and was taken over sand, shingle and mud bottoms in both sheltered and exposed situations, including beaches. FishBase gives a depth range of roughly 10–89 ft, spanning the well-lit shallows down into the deeper sub-littoral. Lake Victoria is warm, with surface waters generally in the mid-to-upper 20s Celsius, and its water is hard and alkaline — pH typically around 8 to 9 — so dichrourus lived in conditions very different from the soft, acidic blackwaters of many South American cichlids. A central feature of its environment, and of its eventual fate, was the lake's water clarity: the haplochromines are visual fish, and the progressive loss of transparency from eutrophication and sedimentation altered the habitat profoundly during the twentieth century.
Feeding
Haplochromis dichrourus is classified as a piscivore in the strict sense — a fish-eating specialist. Examination of the gut contents of collected specimens found fish remains, predominantly other haplochromines and cichlids, together with fragments of larval insects. FishBase places its trophic level near 4.0, consistent with a predator that feeds high in the food web. Within Lake Victoria's species flock, dichrourus was one of a large guild of haplochromine piscivores that preyed on the lake's enormous standing crop of smaller, mostly detritus- and plankton-feeding haplochromines — the predators and their prey together forming the most species-rich vertebrate radiation known. This dependence on other native cichlids for food made the piscivores especially vulnerable when the prey haplochromines collapsed.
Mating
No courtship or pair-formation behaviour has been described for Haplochromis dichrourus itself. Its reproductive biology is inferred from the well-studied pattern shared across the Lake Victoria haplochromine flock: these are polygynous, lek-style breeders in which sexually active males are brightly coloured and defend small courting territories, while females visit, assess and choose among them on the basis of colour and display. Mate choice in the flock is strongly visual, and this is more than a behavioural footnote — the spectacular diversity of the radiation is thought to have been driven and maintained partly by female preference for male nuptial colour. That same dependence on clear water for mate recognition became a conservation problem: as the lake grew turbid, the visual cues separating species blurred, promoting hybridisation between previously distinct forms.
Breeding
Like the Lake Victoria haplochromines generally, Haplochromis dichrourus is a maternal (female) mouthbrooder: the female takes the fertilised eggs into her mouth and broods the developing young there, releasing free-swimming fry after they have absorbed the yolk. No clutch count, brooding duration or fry-development data have been published for this species specifically. Lake Victoria haplochromines are small-clutch mouthbrooders — congeners of comparable size typically carry on the order of a few dozen relatively large eggs, trading fecundity for the protection that oral incubation affords — so dichrourus is expected to fall in that range, but the figure for it is genuinely undocumented. Parental care is by the female alone and ends once the brood is released and independent; there is no biparental or substrate-guarding phase. Because the species has not been collected since the 1980s, no captive spawning has ever been recorded.
In the aquarium
Haplochromis dichrourus has no aquarium history. It is a rarely-collected Lake Victoria endemic that has not been seen since 1986 and is feared possibly extinct, so it is not — and effectively cannot be — kept by hobbyists; no living stock exists in the trade or in conservation populations. Any husbandry guidance can only be drawn by analogy from related Lake Victoria haplochromines that are maintained by specialists and species-survival programmes. Those fishes do well in hard, alkaline water (pH roughly 7.5 to 8.5) at about 24 to 28 Celsius, in tanks with open sand, rockwork retreats and robust filtration to cope with their appetite and waste. A piscivore of this size would have needed a large tank, a meaty diet and tankmates too big to be swallowed; the explosive male-to-male aggression typical of breeding haplochromines makes a surplus-of-females, single-male harem the usual approach for its relatives. For dichrourus, however, all of this is hypothetical — the honest statement is that the species is gone from aquaria and very probably from the wild.
Conservation
The IUCN Red List assesses Haplochromis dichrourus as Critically Endangered (Possibly Extinct) under criterion C2a(ii), assessed on 31 March 2010 by F. Witte, M.P. de Zeeuw and E. Brooks, with a decreasing population trend. The assessment notes that the species has not been recorded since 1986 despite repeated surveys within its known range; if it survives at all it must be extremely rare, and it may now be extinct. Its decline is part of the best-known mass-extinction event in modern vertebrates: the introduction of the Nile perch (Lates niloticus) to Lake Victoria, whose population exploded in the 1980s and drove a collapse of over 90% in haplochromine density in the northern Mwanza Gulf, eliminating or nearly eliminating an estimated hundreds of endemic cichlid species. As a piscivore dependent on other haplochromines for food, dichrourus was doubly exposed. The assessment identifies predation by Nile perch (a potentially reversible pressure, as perch numbers fell again after the 1990s) as the principal threat, compounded by hybridisation driven by decreased water transparency interfering with the visual mate-recognition cues these fish rely on (Mrosso et al. 2003), by eutrophication and sedimentation from agricultural, urban and industrial runoff, and by general fishing pressure even though the species is not specifically targeted. Many Lake Victoria haplochromines have shown partial recovery as Nile perch declined; dichrourus has not reappeared.