Taxonomy & naming
Haplochromis obesus was described by the Belgian-British ichthyologist George Albert Boulenger in 1906, in the original combination Pelmatochromis obesus, from material taken in Lake Victoria. The type locality is Bunjako, on the Ugandan shore of the lake; the lectotype (BMNH 1906.5.30.311, designated by Eschmeyer's Catalog if not previously fixed) and a paralectotype from Entebbe are held by the Natural History Museum, London. Boulenger erected the name in the course of his vast survey of African freshwater fishes, of which the Lake Victoria haplochromines formed one of the most bewildering parts: well over three hundred — by some counts more than five hundred — endemic species ultimately came to be recognized from this one lake, the bulk of them placed, then as now, in the catch-all genus Haplochromis. Greenwood reviewed the species in his 1959 revision of the Victoria flock and the combination Haplochromis obesus has been standard since. The genus is a maternal-mouthbrooding lineage of the subfamily Pseudocrenilabrinae, and the Victoria assemblage is the textbook example of an explosive adaptive radiation from a recent common ancestor. Because of its dentition and feeding mode, Haplochromis obesus has at times been placed in Lipochromis, the genus Greenwood split off for the lake's specialized paedophagous (egg- and embryo-eating) haplochromines; most recent treatments, including the ETYFish Project and the standard checklists, fold Lipochromis back into Haplochromis. The atlas keeps the conventional Haplochromis heading while noting that, on diet and jaw build, the fish sits with the paedophage guild — and is the species to which the small-toothed Haplochromis microdon is most often compared.
Morphology
Haplochromis obesus is a robust, deep-bodied cichlid — the very feature its name records. FishBase gives a maximum of 6.5 in standard length (from the CLOFFA checklist), which places it among the larger members of the flock. The ETYFish Project glosses the epithet as "fat or stout, presumably referring to (the) much deeper and rounded body compared to the allied Haplochromis microdon," so the diagnostic impression is one of a heavy, full-bodied haplochromine rather than the slender profile of the lake's open-water zooplanktivores. No full in-life colour description is available in the accessible literature, and FishBase holds no photograph of the species — a measure of how poorly the living fish is documented. One specific point was recorded, though: Fryer & Iles, in their landmark survey of the African Great Lakes cichlids, cited Haplochromis obesus as one of only a handful of Lake Victoria examples of male nuptial-colour polymorphism — multiple distinct breeding colour morphs occurring among males of a single species, the same phenomenon better known from Lake Malawi mbuna such as Pseudotropheus zebra, Ps. tropheops and Petrotilapia tridentiger. The individual colour morphs themselves were not described in the source available here, so the fact establishes that this species was polymorphic without pinning down what the morphs looked like. As in haplochromines generally, mature males would have carried the brighter nuptial colours and the egg-spot ocelli on the anal fin used in spawning, while females and non-breeding fish were the duller, more cryptic sex. The mouth and jaws are those of a paedophage — distensible and adapted to engulfing the broods of other cichlids rather than to scraping, biting or crushing.
Habitat
Haplochromis obesus occurs in Lake Victoria, the great shallow equatorial lake shared by Uganda, Tanzania and Kenya, and in Lake Kwania, part of the Kyoga lake-and-swamp system that lies just north of and about 98 ft lower than Victoria. The two waters belong to the same Lake Victoria Basin freshwater ecoregion, the Kyoga complex draining into the Victoria Nile. The IUCN assessment lists the species from Kenya and Uganda. It is a fish of the littoral and sublittoral zones, recorded to depths of about 49 ft. FishBase notes that most specimens were caught over a hard substrate — sand, shingle and rock — and only a few over mud, so it is associated with firm-bottomed inshore habitat rather than open soft mud. Lake Victoria is warm, well-lit and mildly alkaline: surface temperatures generally sit around 73–82 °F and pH runs from near-neutral to strongly alkaline (about 7.2 to 9.0+). Crucially, the Victoria of Haplochromis obesus's heyday was a clear-water lake with a diverse, structured cichlid community; the eutrophic, turbid, oxygen-poor lake of recent decades is a very different and far less hospitable habitat for a visually hunting littoral predator that depends on finding and approaching brooding cichlids.
Feeding
Haplochromis obesus is a paedophage — a predator on the eggs and larvae of other fish. FishBase records that it "feeds on fish embryos or larvae" and assigns it a high trophic level of about 4.0, consistent with a predator rather than a grazer. Witte and van Oijen's classification of the Lake Victoria haplochromines into trophic groups treats Haplochromis obesus among the lake's specialized feeders, and a published study by Goldschmidt and others specifically examined the feeding and fecundity of Haplochromis obesus alongside Haplochromis quiarti. The Victoria and Edward paedophages feed in two main ways: by ambushing and seizing free-swimming fry, and by "snout-engulfing" — approaching a mouthbrooding female head-on, taking her snout into a widely distensible mouth, and drawing out the clutch of eggs or larvae she is carrying. The deep, rounded body and capacious mouth of Haplochromis obesus fit this brood-snatching habit. In the lake's food web it functioned as a higher-order predator that turned the reproductive investment of other cichlids into its own meals — a guild that depended entirely on a large, breeding host community of mouthbrooders to exploit.
Mating
No courtship or social behaviour has been described for Haplochromis obesus specifically. Like the rest of the Lake Victoria flock it belongs to a maternal-mouthbrooding lineage, in which the pre-spawning behaviour follows a well-known pattern: males develop nuptial colour, establish and defend a small spawning station, and display to passing females, while the egg-spot ocelli on the male's anal fin play a role in fertilization. Mate choice in the Victoria haplochromines is strongly visual and colour-based, which is one reason the radiation produced so many closely related species separated chiefly by male coloration — and also why the lake's later turbidity, by blurring those colour signals, is thought to have eroded the reproductive isolation between species. The particulars of a display and a defended spawning territory were never recorded for Haplochromis obesus specifically, but the species was at least flagged as unusual in this arena: Fryer & Iles listed it among the few Lake Victoria cichlids known to show multiple distinct male nuptial-colour morphs, a form of variation more commonly associated with mate choice and incipient speciation in Lake Malawi's mbuna.
Breeding
Haplochromis obesus is a maternal mouthbrooder; FishBase records "mouthbrooding by females." Within the Lake Victoria flock that means the female takes the fertilized eggs into her mouth after spawning and broods the developing clutch through hatching and the early free-swimming stage, releasing the fry once they are large enough to fend for themselves and offering them refuge in her mouth when threatened — there is no male parental care. Clutch sizes in Victoria haplochromines are modest by cichlid standards: congeners commonly carry broods in the tens (roughly 20–80 eggs) rather than the hundreds laid by open substrate-spawning cichlids, the trade-off of a mouthbrooder that invests in fewer, well-protected young. A larger-bodied species such as Haplochromis obesus would sit toward the upper end of that range, but a precise published fecundity figure for it is not available in the accessible literature. There is a pointed irony in its biology: the very maternal mouthbrooding that protects most of the flock's young is exactly what the paedophage guild exploits, and a brooding female with a mouthful of eggs is the snout-engulfer's target — so within this lake the strategy that shelters one species' brood feeds another's.
In the aquarium
The true Haplochromis obesus — the deep-bodied paedophage described here — is essentially never kept. It has no established aquarium strain, and given its Critically Endangered status and the collapse of its native community, it is rarely if ever available to be collected. The name has, however, circulated in the hobby attached to a fish that almost certainly is not this species. Per Ad Konings (Enjoying Cichlids), an aquarium line known as 'Haplochromis' sp. "citrus" — a slender, lemon-yellow fish up to about 4.5 in, marked with a thin black dorsolateral band and broad, irregular black blotches forming an interrupted midlateral band, introduced to Sweden by the aquarist Selbrink — was identified in the trade as H. obesus. That identification does not fit the true species' known deep, stout body plan and is now considered a misidentification: 'citrus' instead belongs to a small, largely undescribed group of slender, pelagic, phytoplankton-feeding Lake Victoria haplochromines that has itself declined dramatically, with several members possibly extinct — whether 'citrus' still survives in captivity is an open question. Anyone seeking to keep Lake Victoria cichlids responsibly should look to the species actually available through reputable breeders and conservation programmes (Pundamilia, Mbipia and other 'Victoria haps'), understanding that a fish sold as H. obesus is not the true species. In general terms, were the true H. obesus kept it would call for warm (75–81 °F), hard, alkaline water (pH around 7.5–8.5) over a firm sand-shingle-and-rock littoral layout — the standard regime for Victoria haplochromines — but this is hypothetical: there is no genuine husbandry experience to draw on, and its egg- and fry-eating habits would make it a destructive tankmate to any breeding cichlids kept with it, raiding their broods rather than coexisting.
Conservation
The IUCN Red List assesses Haplochromis obesus as Critically Endangered under criterion C2a(ii), assessed on 31 March 2010 by Witte, de Zeeuw and Brooks and published in 2010, with a decreasing population trend. The assessment names predation by the introduced Nile perch (Lates niloticus) as the main threat — a pressure it regards as potentially reversible — together with possible hybridization driven by decreased water transparency, and incidental capture in the lake fishery (the species is not specifically targeted). Its plight is part of the best-known mass decline of any vertebrate radiation: from the 1980s the Lake Victoria haplochromine flock collapsed, with an estimated 200 or more species lost or driven to the brink. The Nile-perch explosion consumed the small native cichlids wholesale; this was compounded by severe eutrophication and the resulting turbidity and bottom-water deoxygenation, which both stressed the fish directly and broke down the colour-based mate recognition that kept species distinct. Specialized predators such as the paedophages were doubly exposed — hit by the perch like every other haplochromine, and additionally undercut by the wholesale loss of the brooding host species their feeding mode depended on. Whether Haplochromis obesus persists in viable numbers, in Victoria or in the Kyoga lakes, would require targeted survey effort to establish.