Taxonomy & naming
The species was described in 1998 by Ole Seehausen and Niels Bouton, in a monographic paper co-authored with Erwin Lippitsch and Hidde Zwennes that named three new genera and fifteen new species of Lake Victoria rock cichlids — the 'mbipi', a Tanzanian (Kerewe) word for the rock-dwelling haplochromines (Seehausen, Lippitsch, Bouton & Zwennes, 1998, Ichthyological Exploration of Freshwaters 9(2):129–228). The original combination was Neochromis omnicaeruleus, placed in the newly erected genus Neochromis for blunt-snouted epilithic algae-scrapers; the holotype (RMNH 33160) and a large paratype series are held in the Naturalis collection at Leiden (RMNH) and at the Tanzania Fisheries Research Institute (TAFIRI). The type locality is Makobe Island, in the Speke Gulf of Lake Victoria, Tanzania — the population that has since become the focus of decades of speciation research. Like almost the entire Lake Victoria haplochromine flock, the species sits in a thicket of unresolved generic names: it is variously cited as Neochromis omnicaeruleus or Mbipia/Pundamilia-adjacent in the evolutionary literature, while FishBase and several checklists park it in the catch-all genus Haplochromis pending a stable revision of this explosively young species flock. We keep the broad Haplochromis heading here, the convention this atlas uses across the Victoria mbipi, while noting that the generic placement is genuinely provisional. The epithet 'omnicaeruleus' — 'all-blue' — was coined for the entirely blue coloration of the most common male morph.
Morphology
This is a small, fusiform cichlid reaching about 4.5 in standard length (the largest measured type specimen is 4.5 in SL), with the blunt snout and steeply set, scraping dentition typical of the epilithic algae-grazing mbipi. Fin counts run to 15–17 dorsal spines and 9–11 soft rays, 3 anal spines and 8–10 anal soft rays, over 30 vertebrae. What sets the species apart is not its build but its colour, and colour here is bound up with sex in a way that is rare among fishes. Three distinct colour morphs occur, and they are distributed very unequally between the sexes. The 'plain' (P) morph — the putatively ancestral form — has metallic blue (sometimes yellow) males and plain brown females; the all-blue male is the namesake. Two blotched morphs occur predominantly, often almost exclusively, in females: orange-blotched (OB) females carry irregular orange and black blotches on a pale ground, while white-blotched (WB, also called black-and-white-blotched) females show black blotches on a brassy-to-white background when heterozygous and become nearly all-black when homozygous. Sexual dimorphism is therefore pronounced and unusually structured: males are larger and bear the bright nuptial blue, while the blotched patterns are essentially a female phenomenon. This polychromatism is the species' diagnostic and scientifically defining feature.
Habitat
Haplochromis omnicaeruleus is endemic to Lake Victoria and, as far as is known, restricted to the clear-water rocky shores of the south-eastern part of the lake: islands in the open lake off the western shores of Ukerewe Island, the Sengerema region, and the western half of the Speke Gulf (Seehausen & Bouton, 1998; CLOFFA). It is one of the obligate rock-dwellers, never venturing far from the boulder habitat, and occupies the littoral zone at depths of roughly 3–20 ft. It is found across the structural range of that habitat — from steeply sloping shores of large boulders to gently sloping shores of small rocks — and its diet shifts with slope (see Feeding). Lake Victoria is a warm, well-buffered, alkaline lake: surface waters typically run pH on the order of 7.2 to 9.0 and temperatures around 73–81 °F in the shallow rocky zones these fish inhabit. Water clarity matters to this species in a way that goes beyond habitat preference — the colour-based mate choice that maintains its polymorphism depends on clear water, and the lake-wide eutrophication and turbidity of recent decades is implicated in the collapse of exactly this kind of visual signalling elsewhere in the flock.
Feeding
It is an epilithic algae grazer with a low trophic level (FishBase estimates ~2.0), scraping the algal film that coats the rocks. The diet tracks the habitat: on gentle slopes the fish feeds mainly on filamentous green (Chlorophyta) and blue-green (Cyanophyta) algae, whereas on steep large-boulder shores it takes more Cyanophyta and diatoms (Seehausen & Bouton, 1998). The blunt snout and short, robust scraping jaws are the tools of this guild; like other mbipi grazers it works the biofilm methodically rather than chasing prey, and it is usually encountered solitary rather than in foraging shoals. Within the rocky community it occupies the same broad grazer niche as several co-occurring Neochromis and related species, the kind of fine ecological packing that characterises the Victoria rock flock.
Mating
The species is polygynous: males hold territories throughout the depth range down to 20 ft, with the highest territorial densities at 7–13 ft, and court females from these sites with their nuptial blue. Mate choice is colour-based, and this is where the species earns its place in the evolutionary-biology textbooks. The colour morphs are associated with assortative mating and morph-biased aggression: females and males show own-morph mating preferences, and females are more aggressive toward their own morph. Behavioural-dominance work on the Makobe population found a consistent hierarchy among female morphs — WB females dominate both OB and plain females, with evidence that colour and dominance are genetically linked. Seehausen, van Alphen and Lande (1999) argued that this combination of colour polymorphism, assortative mating and morph-specific aggression represents an incipient stage of sympatric speciation by sexual selection: disruptive mate choice pulling one population toward becoming several. The same sex-linked colour system, with very similar dynamics, occurs in the closely related Neochromis greenwoodi.
Breeding
Like essentially all Lake Victoria haplochromines, Haplochromis omnicaeruleus is a maternal mouthbrooder with no paternal care: after the female lays and the eggs are fertilised (the male displaying the egg-spot lure on his anal fin), she takes the clutch into her mouth and broods the eggs and then the fry there until they are free-swimming. Spawning occurs throughout the year. Brooding females defend small territories around a cave or crevice between rock boulders at various depths, distinct from the open courtship territories the males hold. Published clutch counts specific to this species are scarce; mbipi haplochromines of this size typically produce only small clutches — on the order of a few dozen comparatively large eggs (roughly 20–60 in congeners) — the classic trade-off of mouthbrooders, which invest in few, well-provisioned young carried to a relatively advanced stage. The polymorphism adds a genetic twist to reproduction: because the blotched patterns are sex-linked and tied to sex-ratio distortion, the morph composition of a brood depends on the parents' genotypes, which is part of why the system has been so useful for studying how a sexually-selected polymorphism can persist and even drive divergence.
In the aquarium
This is a scientist's fish far more than a hobbyist's. It is rarely kept and is not in general commercial trade; the aquarium specimens that exist are largely the descendants of research collections (the Makobe Island OB/WB lines maintained for behavioural and genetic work), occasionally circulated among specialist keepers of Lake Victoria haplochromines. None of that makes it difficult to house in principle — its needs are those of any small mbipi grazer — but it is genuinely hard to obtain, and its real interest lies in the colour genetics, which only express properly in groups.
For anyone who does keep it, the husbandry mirrors the rocky Victoria shoreline. A footprint of at least 47 in (around 200+ litres) suits a colony, aquascaped with a stack of rounded boulders and rockwork to create the caves and territories males and brooding females both demand, over a sand base. The water should be hard, well-buffered and alkaline — pH roughly 7.5–8.5, temperature about 75–79 °F — and kept clean and well-oxygenated; algae grazers are messy feeders and Victoria cichlids are sensitive to deteriorating water and to 'bloat' from an over-rich, protein-heavy diet. Feed a spirulina- and vegetable-based menu (the natural diet is algal film), not a meaty one. Stock as one male to several females, with enough rock to let subordinate fish and the dominant WB females stake out separate space; the morph-biased, own-morph aggression that makes the species scientifically interesting also means same-morph females will quarrel. Keep it as a single-species colony — hybridisation with other Victoria haplochromines is rampant in captivity and destroys exactly the colour lines worth maintaining. Difficulty is moderate; the challenges are sourcing genuine, unhybridised stock and resisting the temptation to mix it.
Conservation
The IUCN Red List assesses Haplochromis omnicaeruleus as Least Concern (assessed 1 April 2010 by Witte, F., de Zeeuw, M.P. & Brooks, E.). The assessment notes that the species is restricted to rocky littoral shores of south-eastern Lake Victoria but is probably more widely distributed than the few documented sites, and that as an obligate rock-dweller it was partly buffered against the single greatest catastrophe to befall the lake's fishes — the explosive rise of the introduced Nile perch (Lates niloticus), which from the 1980s drove an estimated several hundred Victoria haplochromine species to extinction or near-extinction, mostly in open and inshore non-rocky habitats where the perch hunts most effectively. Rocky shores gave many mbipi a refuge. The persisting threat for this lineage is more insidious: lake-wide eutrophication and turbidity. Because the species' identity and its very polymorphism are maintained by colour-based mate choice in clear water, loss of water clarity can collapse assortative mating and erode reproductive boundaries between morphs and between related species — a documented mechanism of 'speciation in reverse' in turbid parts of Lake Victoria. So while its formal status is reassuring, the long-term fate of this particular fish is tied closely to the optical state of the lake.