Taxonomy & naming
The species was described by the French ichthyologist Jacques Pellegrin in 1904 as Tilapia guiarti, in a preliminary diagnosis of new fishes from Lake Victoria collected by the explorer Charles Alluaud (Bulletin de la Société Zoologique de France, vol. 29). The type locality is Kavirondo Bay (the Winam Gulf) on the Kenyan side of the lake, and the unique holotype — MNHN 1904-0150 — is held at the Muséum national d'Histoire naturelle in Paris; Pellegrin figured the fish the following year. One later name, Haplochromis nigroventralis Lohberger, 1929, also from Lake Victoria, falls into synonymy, established by van Oijen and colleagues in their 1991 CLOFFA treatment of Haplochromis. The Catalog of Fishes lists the species as valid Haplochromis guiarti (Pellegrin 1904). Like most members of the Lake Victoria flock it has been shuffled between genera: the piscivorous haplochromines were split by Greenwood into a series of small genera, and guiarti has at times been placed in Prognathochromis (and is allied to the Harpagochromis–Prognathochromis assemblage of fish-eaters). The ETYFish Project and the Catalog of Fishes both treat these as synonyms of the inclusive genus Haplochromis, the usage retained here; the generic boundaries within Victoria's haplochromines remain genuinely unsettled.
Morphology
Haplochromis guiarti was one of the larger members of the Lake Victoria flock, reaching about 7 in in standard length (FishBase, after the CLOFFA checklist) — a substantial fish by the standards of a radiation in which most species are well under 4.5 in. The body conformed to the piscivorous-haplochromine pattern: comparatively elongate and streamlined, with a fairly large, oblique mouth and slender, unicuspid teeth set for seizing prey rather than scraping or crushing, the hallmark dentition of the fish-hunting clade revised by van Oijen (1991). Detailed in-life coloration is poorly documented, as is true for many Victoria haplochromines described from preserved museum material; as in the group generally, breeding males would have been the more colourful sex, and females and non-breeding fish drabber and more cryptic. Several closely similar piscivores were later separated from guiarti — Greenwood named Haplochromis paraguiarti in 1967 precisely for its superficial resemblance to this species — a measure of how subtle the morphological distinctions within the flock can be.
Habitat
Haplochromis guiarti was endemic to Lake Victoria, the largest tropical lake in the world (about 68,497 mi²), shared by Tanzania, Uganda and Kenya and lying within the Lake Victoria Basin freshwater ecoregion. It was widely distributed across the lake rather than confined to a single gulf, and FishBase records it from a depth range down to about 20 ft — that is, the inshore and shallow sublittoral waters rather than the deep offshore. Victoria is a shallow, warm equatorial lake; its surface waters typically run roughly 73–81 °F and its well-buffered water is alkaline, with pH commonly in the range of about 7.2 to 9.0 and conductivity higher than the soft Amazonian or western-Rift waters. Historically the lake was clear enough for the haplochromines' visual mate-recognition to function across these inshore habitats — a condition that has since broken down, with direct consequences for this species (see Conservation).
Feeding
Haplochromis guiarti was a piscivore — a predator on other fishes, principally the smaller members of the haplochromine flock itself. FishBase records the diet simply as 'feeds on fish' and places it at a high trophic level (around 4.2), consistent with a top or near-top position in the inshore food web. In the pre-perch lake, piscivorous haplochromines formed a large and diverse guild, partitioning prey by size, depth and hunting style; van Oijen's 1991 revision treated guiarti within this assemblage of fish-hunters distinguished by their elongate bodies and grasping, unicuspid dentition. The slender jaws and teeth describe the mechanics: prey was seized and swallowed whole rather than processed, the standard strategy of the lake's open-water predators.
Mating
No courtship or social behaviour specific to Haplochromis guiarti has been documented — like the great majority of Victoria haplochromines, it was described and catalogued largely from preserved specimens, and its reproductive biology is inferred from the well-studied pattern of the flock as a whole. In these cichlids reproduction is polygynous and built on visual signalling: breeding males develop bright, species-specific nuptial colours and display to females in the inshore shallows, and females choose mates partly on those colour cues. This visual mate-choice system is central to the flock's explosive speciation — and, as it turned out, to its collapse, because it depends on clear water. Territoriality and the details of male display in guiarti specifically are not on record.
Breeding
Haplochromis guiarti is, like all members of the genus, a maternal mouthbrooder: the female takes the fertilized eggs into her mouth and broods the clutch and the newly hatched young there until the fry are free-swimming, providing no biparental substrate care. Specific fecundity for this species has not been published; Lake Victoria haplochromines are characteristically low-fecundity, small-clutch brooders, typically producing on the order of a few tens of eggs per spawning (commonly cited at roughly 20–80, varying with female size and species), a clutch traded off against the protection and large size of mouthbrooded young. No clutch count, egg size or brooding-duration figure specific to guiarti appears in the literature, and none should be inferred beyond the genus-typical band noted here.
In the aquarium
Haplochromis guiarti is essentially never kept in the aquarium hobby. It is not bred or traded as an ornamental fish, it has no established common name, and as a Critically Endangered (Possibly Extinct) species it is not available through normal channels; the handful of modern records are field collections by researchers and cichlid specialists rather than aquarium stock (a 2019 Ugandan collection is documented on the Cichlid Room Companion). Were it kept, its needs would follow those of other large Lake Victoria haplochromines: a spacious tank for an active, predatory fish; hard, alkaline water (pH roughly 7.5–8.5) at about 75–81 °F, matching the lake; and, given its diet, only tankmates too large to be eaten. As a piscivore it would treat smaller fish as food. This profile is included for completeness — in practice the species is a conservation and museum subject, not a hobby fish, and may no longer exist to be kept at all.
Conservation
The IUCN Red List assesses Haplochromis guiarti as Critically Endangered (Possibly Extinct) under criterion C2a(ii), assessed on 15 August 2010 by F. Witte and M.P. de Zeeuw, with a decreasing population trend. The assessment notes that the species was known from Lake Victoria in the past but has not been confirmed since the 1980s despite targeted surveys. Its decline is part of the broader catastrophe that overtook Victoria's endemic cichlid flock from the 1980s onward, when the predatory Nile perch (Lates niloticus), introduced earlier in the century, surged to dominance and an estimated 200-plus haplochromine species disappeared or crashed. For guiarti the Red List flags two specific pressures: hybridization driven by falling water transparency — eutrophication, erosion and sedimentation have clouded the lake, degrading the visual mate-recognition cues that keep closely related species reproductively distinct — and incidental capture as bait for the long-line fishery. As a once-common, large-bodied piscivore the species was doubly exposed: directly, as both predator on and competitor with Nile perch, and indirectly, through the loss of the clear-water conditions its breeding system required. It carries no CITES listing.