Taxonomy & naming
Haplochromis plagiostoma was described by the British ichthyologist Charles Tate Regan in 1922, in his foundational paper 'The Cichlid Fishes of Lake Victoria' (Proceedings of the Zoological Society of London). The type locality is Bunjako, on the Ugandan shore of the lake. The name belongs to the vast, taxonomically turbulent genus Haplochromis (Hilgendorf, 1888), the catch-all that has held the great majority of Lake Victoria's haplochromine cichlids. In his sweeping 1979–1980 revision of the group, Peter Humphry Greenwood broke Haplochromis sensu lato into a series of smaller genera and subgenera based on trophic and morphological specialisation, and assigned the larger Victorian piscivores — plagiostoma among them — to Prognathochromis. That generic scheme has been heavily criticised (notably by van Oijen and colleagues at Naturalis, who argued it does not reflect monophyletic groups) and was never universally adopted; molecular work has since shown the Lake Victoria flock to be so young and so genetically intermixed that clean generic boundaries are difficult to draw. Most current databases, including FishBase and the Catalog of Fishes, therefore retain the conservative combination Haplochromis plagiostoma, and that is the heading used here. A closely allied species, Haplochromis paraplagiostoma Greenwood & Gee, 1969, was named expressly for its superficial resemblance to plagiostoma.
Morphology
Haplochromis plagiostoma is a medium-sized haplochromine, reaching about 6 in in standard length (FishBase, after the CLOFFA checklist). The body is the moderately elongate, laterally compressed form typical of the larger Victorian predators rather than the deep-bodied profile of the lake's rock-grazers. Its defining feature, and the source of its name, is the mouth: Regan described it as 'very oblique', set at a steep upward slant, an arrangement well suited to seizing prey from above and ahead. As in essentially all Lake Victoria haplochromines, the sexes differ in colour: mature males develop the bright nuptial colours used in courtship and species recognition, while females and immatures are a more cryptic silvery-brown. Detailed in-life colour descriptions for this particular species are sparse in the accessible literature, so the precise male pattern is not well documented here; the general dichromatic pattern, however, is a near-universal trait of the flock.
Habitat
The species is endemic to Lake Victoria — the great shallow lake shared by Tanzania, Uganda and Kenya — and is not known from anywhere else; the IUCN assessment lists its confirmed range as Ugandan waters. FishBase records it from sheltered, relatively shallow areas with either soft (mud) or hard (sand or shingle) bottoms, at depths down to about 30 ft, placing it firmly in the inshore, sub-littoral zone rather than the deep open water. Lake Victoria is a warm, well-buffered tropical lake: surface waters typically run about 73–81 °F and pH is on the alkaline side, broadly 7.2–9.0, with the lake's hard, mineral-rich chemistry. No in-situ chemistry measured specifically at this species' collecting sites is available in the consulted sources, so the figures given are lake-wide values rather than measurements taken where plagiostoma was netted.
Feeding
Haplochromis plagiostoma is a piscivore. The few specimens whose stomachs have been examined were found to feed predominantly on other Haplochromis and, less often, on small open-water cyprinids — the FishBase note records the prey as a small cyprinid (cited there as 'Engraulis argenteus', most plausibly the silver cyprinid Rastrineobola argentea / 'dagaa', the lake's dominant small pelagic fish). In the trophic classification of the Lake Victoria flock developed by Witte, van Oijen and co-workers, which sorts the several hundred haplochromines into fifteen feeding guilds, plagiostoma falls among the piscivores. FishBase's modelled trophic level of about 4.4 is consistent with a fish that eats other fish. The oblique, upturned mouth fits this role, helping it take prey moving in the water column above it.
Mating
Like the entire Lake Victoria haplochromine radiation, Haplochromis plagiostoma is a maternal mouthbrooder, and its reproductive biology can be described with confidence at the level of the flock even though species-specific courtship observations are not available in the consulted literature. In these cichlids the males are the colourful, displaying sex: a ripe male establishes a small territory and courts passing females with body-quivering displays and his nuptial colour pattern, which doubles as the species-recognition signal that keeps the lake's many similar species reproductively separate. Spawning is brief and is not tied to a fixed nest, because the female removes the eggs from the substrate almost immediately. This colour-mediated, lek-like mate choice is widely regarded as a key engine of the flock's explosive speciation, and there is no reason to think plagiostoma departs from the pattern.
Breeding
Haplochromis plagiostoma is a maternal mouthbrooder (FishBase). Following the universal haplochromine pattern, the female takes the fertilised eggs into her mouth and broods them there through hatching and the early free-swimming stage, releasing the fry only once they are well developed and able to feed; she gives no parental care beyond her own mouth, and the male contributes none. Species-specific figures for clutch size, egg number, brooding duration and fry size in plagiostoma have not been published in the sources consulted; for comparison, Lake Victoria haplochromines as a group are relatively small-clutch brooders, typically carrying on the order of a few dozen eggs (commonly cited in the range of roughly 20–80) — a genus-typical figure, not a measurement of this species. Breeding is presumed to occur inshore within the species' shallow sub-littoral habitat.
In the aquarium
Haplochromis plagiostoma is essentially never seen in the aquarium hobby. It has no established presence in the ornamental trade, no common name, and no body of captive husbandry or breeding reports — unlike the handful of charismatic Lake Victoria haplochromines (the so-called 'Victorian Haps' such as Haplochromis nyererei or the 'Christmas fulu') that hobbyists keep in conservation-minded programmes. Anyone attempting it would be working entirely by analogy with those better-known relatives: a warm (about 75–81 °F), hard, alkaline (pH ~7.5–8.5) tank mirroring the lake; a predator-sized footprint, since this is a fish-eating species reaching nearly 6 in; sand or fine gravel with open swimming room; and the strongly polygynous, male-aggressive social structure of the group, which generally means keeping one male with several females and providing line-of-sight breaks. As a piscivore it would regard tank-mates small enough to swallow as food. None of this is documented for plagiostoma specifically — it is inference from the flock, offered honestly as such, and the species should be regarded as a wild Lake Victoria endemic rather than an aquarium fish.
Conservation
The IUCN Red List assesses Haplochromis plagiostoma as Data Deficient (assessed 31 March 2010 by Witte, F. & de Zeeuw, M.P.; published 2010), reflecting how little is firmly known about the distribution, abundance and population trend of this narrow-range endemic. It carries no CITES or CMS listing. The 'Data Deficient' label should not be read as reassurance: it sits against the backdrop of one of the most documented faunal upheavals in modern freshwater ecology. From the 1980s onward the explosive growth of the introduced Nile perch (Lates niloticus), compounded by eutrophication, falling water clarity and heavy fishing, drove a catastrophic decline of Lake Victoria's haplochromine flock, with many species — particularly larger predators and zooplanktivores — collapsing and an unknown number lost entirely. Some haplochromine groups have since partially recovered, especially as Nile perch itself was heavily fished, but recovery has been uneven across trophic guilds and the long-term status of individual species such as plagiostoma remains genuinely uncertain.