Taxonomy & naming
Haplochromis azureus was described in 1998 by Ole Seehausen and Erwin Lippitsch, in the landmark monograph 'Mbipi, the rock-dwelling cichlids of Lake Victoria' (Seehausen, Lippitsch, Bouton & Zwennes, Ichthyological Exploration of Freshwaters 9(2): 129–228), which erected three new genera and fifteen new species at once. The authors placed it in the new genus Pundamilia, and the name is still most often written Pundamilia azurea — the combination used by Eschmeyer's Catalog of Fishes and the Cichlid Room Companion. We follow FishBase in keeping the lacustrine and riverine haplochromines of the Lake Victoria catchment under the older umbrella genus Haplochromis Hilgendorf, 1888 (after van Oijen et al., 1996) until a complete revision of this enormous, recently radiated flock settles the generic boundaries; the heading therefore reads Haplochromis azureus, but the fish is the same one the literature calls Pundamilia azurea. The holotype (RMNH 33313, Naturalis, Leiden) and a long series of paratypes were collected at Ruti Island in the Speke Gulf at a depth of about six metres. Within Pundamilia, azurea is the sister of the blue Pundamilia pundamilia and forms, with the red Pundamilia nyererei and P. 'red head', a tight cluster of 'blue versus red' species pairs that has become one of the best-studied model systems in the whole of speciation research.
Morphology
This is a small cichlid: FishBase gives a maximum of about 4 in standard length, so mature fish are roughly hand-length including the tail. The body is fusiform and laterally compressed in the generalised haplochromine mould, with 14–17 dorsal spines, 8–10 dorsal soft rays, three anal spines and 8–9 anal soft rays over 29–31 vertebrae. As in almost all Lake Victoria haplochromines the sexes look entirely different, and it is the colour of the breeding male that defines the species. Males in nuptial dress carry the 'blue' nuptial pattern that runs through several Victorian lineages — a metallic blue to blue-grey flank, often with a paler greenish or silvery belly and dark vertical bars — together with the egg-spots (ocelli) on the anal fin that are the haplochromine signature. Females and non-breeding males are a drab, cryptic silvery-grey to brownish, almost impossible to tell from the females of co-occurring species in the field. The blue male phenotype is what separates azurea from its red-dorsum neighbour nyererei; the two are so similar in body shape and ecology that they were long treated as colour morphs of one fish before being recognised as distinct, reproductively isolated species.
Habitat
Haplochromis azureus is a narrow-range endemic, known only from island groups in the middle of the Speke Gulf, an arm of Lake Victoria in Tanzania — the type series came from Ruti Island. It is strictly a rock-dweller: the original description and FishBase both note that it occurs only over rocky substrates, favouring moderately to steeply sloping shores built of large boulders. This is classic mbipi habitat — the wave-washed, light-filled boulder zone of the lake's islands and headlands — and the fish does not stray onto the open sand or mud that separates one rocky island from the next, which is a large part of why such species are so finely subdivided across the lake. Within the rocky zone azurea tends to occupy the shallower, better-lit water, where its blue colour is most visible, while its red relative nyererei shades into slightly deeper, redder-lit water nearby; this depth-and-light partitioning is one of the threads in the sensory-drive story of how the pair stays apart. Lake Victoria is warm, well-buffered equatorial water: surface temperatures generally run about 73–81 °F and pH is alkaline, broadly in the 7.2–9.0 range, with the shallow rocky margins the species inhabits sitting at the warmer end of that band. Reported depths for the genus on these reefs are modest, on the order of one to a few metres down to the ~20 ft of the type locality.
Feeding
FishBase records Haplochromis azureus simply as a plankton feeder, and field studies of the Pundamilia complex place these blue-and-red reef fish among the zooplanktivorous and insectivorous haplochromines of the rocky zone rather than the algae-scrapers or snail-crushers. In practice that means a diet of small drifting and bottom-living invertebrates — zooplankton in the water column over the boulders, plus insect larvae and other small prey gleaned from the rock surfaces and the thin algal film. Its estimated trophic level of about 3.4 fits a small mid-level invertebrate predator. A telling behavioural detail from the original work: females and sexually inactive males gather into roving foraging shoals, and these shoals are often mixed with those of Pundamilia nyererei. The two species feed side by side in the same way over the same rocks and are kept apart chiefly by who the ripe females choose to breed with, not by how they make a living — a separation of reproductive niche from feeding niche that is central to why so many of these fish can coexist on one reef.
Mating
Like the rest of the Lake Victoria flock, Haplochromis azureus is a polygynous maternal mouthbrooder with no pair bond. Outside of breeding the fish travel in those mixed foraging shoals; when a male comes into condition he develops his bright blue nuptial colour, sets up and defends a small display territory among the boulders, and courts passing females with the lateral displays and quivering runs typical of haplochromines, flashing his anal-fin egg-spots. Females are the choosers, and here colour does the work that ecology does not: ripe female azurea preferentially mate with blue males, while the females of the sympatric red nyererei prefer red males. This direct, colour-based mate choice is what keeps the two species reproductively isolated where they shoal together, and the Pundamilia pair has become a textbook case of speciation by 'sensory drive' — the way the light environment of the water (bluer in the shallows, redder with depth and turbidity) tunes both female vision and male colour, splitting one ancestral population into a blue, shallow form and a red, deeper one.
Breeding
No dedicated breeding study of Haplochromis azureus has been published, but its reproduction follows the well-documented Lake Victoria haplochromine template, and FishBase records it explicitly as a polygynous female mouthbrooder with maternal care. After the male fertilises the clutch on his territory, the female takes the eggs into her mouth — the anal-fin egg-spots are thought to help coordinate this — and broods them there. She incubates the developing eggs and larvae in her buccal cavity for roughly two to three weeks with no help from the male, releasing fully-formed free-swimming fry and continuing to shelter them in her mouth when danger threatens for a short period afterward. Clutch size has not been counted for this species specifically; small rock-dwelling Victorian haplochromines of this body size are typically modest brooders carrying on the order of a few dozen eggs (commonly ~15–40), so any figure should be read as genus-typical rather than measured. The combination of small clutches and prolonged oral care is the standard trade-off of the flock: few young, each given a strong head start.
In the aquarium
Haplochromis azureus is rarely kept. It has none of the trade presence of its red sibling Pundamilia nyererei, which is a fixture of the Lake Victoria hobby; azurea turns up mainly through specialist keepers and Victorian-cichlid enthusiasts, and is not a fish a general aquarist will find offered for sale. Anyone keeping it should treat it as a typical small, rock-dwelling Victorian mbipi. That means hard, alkaline water (pH comfortably on the basic side of neutral, roughly 7.5–8.5) at about 75–81 °F, a tank aquascaped with piles of rock to create territories and broken sightlines, and an open swimming lane over an easily-cleaned substrate. Because the males are aggressive territory-holders and the species is polygynous, the standard approach is to keep one male with several females, or a group large enough to spread the aggression, in a tank of at least a few feet in length; a lone pair usually ends with the female harassed. Diet is straightforward — small frozen and dry foods (cyclops, daphnia, brine shrimp, a quality cichlid flake or micro-pellet) suit a planktivore of this size. The single most important caution is conservation rather than husbandry: this is a Vulnerable narrow endemic that hybridises freely with its relatives, so it must be kept as a clean, single-species line and never crossed with nyererei or other blue Victorians. Keepers who maintain pure stock are, in effect, holding an insurance population for a fish under real pressure in the wild.
Conservation
The IUCN Red List assesses Haplochromis azureus as Vulnerable under criterion D2 (assessed 1 April 2010 by Witte, de Zeeuw and Brooks; published 2016), with a population trend listed as stable. The D2 criterion reflects exactly the situation here: a species with a very small range — confined to island groups in the middle of the Speke Gulf — that could be tipped toward extinction by a single localised event. As a rock-restricted endemic it is somewhat buffered from the catastrophe that overtook Lake Victoria's open-water haplochromines, hundreds of which vanished after the introduced Nile perch (Lates niloticus) exploded across the lake in the late twentieth century; the rocky reefs gave the mbipi places to hide that the soft-bottom species lacked. But the rock-dwellers face their own, subtler threat. Lake Victoria has become badly eutrophic and turbid as its catchment has been deforested and fertilised, and Seehausen and colleagues showed that murky water collapses the colour-based mate choice these fish depend on: when females can no longer tell a blue male from a red one, the reproductive barriers between species break down and they hybridise back together, eroding the very diversity that turbidity does not directly kill. The threats the assessment lists — fishing, invasive species, and urban, industrial and agricultural pollution — all feed into that water-clarity problem, which for a colour-signalling reef fish like azureus is as dangerous as any predator.