Taxonomy & naming
Haplochromis katunzii was formally described in 2010 by Saskia ter Huurne and Frans Witte, in a paper by M.P. de Zeeuw, M. Mietes, P. Niemantsverdriet, ter Huurne and Witte titled 'Seven new species of detritivorous and phytoplanktivorous haplochromines from Lake Victoria' (Zoologische Mededelingen, Leiden, volume 84, part 9, pages 201–250). The holotype, an adult male of 2.5 in standard length, is held at Naturalis (RMNH 73255); it was collected in the Mwanza Gulf on 20 April 1979 by the Haplochromis Ecology Survey Team (HEST), the long-running Leiden University programme that has surveyed the Tanzanian shore of Lake Victoria since the late 1970s. Paratypes are distributed among Naturalis, the American Museum of Natural History, the Natural History Museum in London, and Japan's National Museum of Nature and Science.
Long before it had a Latin name, the fish circulated in the HEST literature under the provisional code 'Haplochromis 75', and from the mid-1980s onward as 'Haplochromis (Enterochromis) 75' — Enterochromis being the informal subgeneric grouping Greenwood proposed for the lake's small detritivorous and phytoplanktivorous haplochromines. The species belongs to an assemblage of small, deep-bodied, small-mouthed (micrognathic) Victoria cichlids that also contains Haplochromis cinctus, Haplochromis paropius and the simultaneously described Haplochromis antleter, all closely similar in build and most readily separated by the live breeding colours of the males. Like nearly all of the roughly 500 endemic cichlids of Lake Victoria, katunzii is placed in the catch-all genus Haplochromis; the Victoria 'superflock' is so young and so densely speciated that a fully resolved generic classification has never been agreed, and most workers retain the broad Haplochromis heading rather than split the flock into the many small genera that have been proposed.
Morphology
This is a small cichlid: FishBase records a maximum of 3 in standard length, and the type series of 47 males ran 2–3 in SL (mean about 2.5 in). The body is relatively deep — body depth averages around 39% of standard length, comfortably above the 35% threshold that helps define the species group — with a moderately curved to straight dorsal head profile. The mouth is small (micrognathic, lower-jaw length under 45% of head length), and the oral jaws carry relatively small, mainly bicuspid teeth, the dentition of a fish that grazes fine material rather than seizing prey. Diagnostic counts from the description include 28–31 vertebrae and eight to nine gill rakers on the lower limb of the first arch.
Colour is the most practical way to tell katunzii from its near-identical relatives. Sexually active males are sandy-brown with rather distinct vertical bars — darker and more sharply defined than the faint barring of the dusky Haplochromis antleter, and lacking the red dorsum that marks Haplochromis cinctus and Haplochromis paropius. Against Haplochromis antleter the species also shows a deeper cheek (18.6–24.6% of head length versus 16.1–21.1%) and a more pronounced bony prominence at the chin; against Haplochromis cinctus it has a narrower space between the eyes. Females and non-breeding fish are drab and, as the describers note, genuinely hard to distinguish from other detritivorous 'curved-head' haplochromines — earlier ecological studies simply lumped katunzii females together with Haplochromis cinctus. As in essentially all haplochromines, the species is sexually dimorphic and dichromatic: it is the male that carries the bright nuptial colour and the yellow-ringed 'egg dummies' (ocelli) on the anal fin, while females stay cryptic.
Habitat
Haplochromis katunzii is known only from Lake Victoria, and within the lake only from the Mwanza Gulf on the Tanzanian side — the stretch of water the HEST programme has sampled most intensively. It is a benthic, demersal fish of the sub-littoral zone, caught mainly over soft mud bottoms at depths of 7–49 ft. That is distinctly shallower than the closely related Haplochromis cinctus, which the early surveys took from 43–197 ft, so depth as well as colour separates the two. By night the fish were observed to rise slightly higher off the bottom than during the day. Lake Victoria is a large, shallow, equatorial lake; its open waters are warm (broadly in the mid-20s Celsius) and run from near-neutral to distinctly alkaline, with surface pH typically in the high 7s to around 8–9 — soft-water aquarium chemistry is not what this fish evolved in. The species' narrow documented range and its dependence on the gulf's muddy sub-littoral floor are central to why it has fared so badly: it had nowhere to retreat to when conditions in that exact habitat changed.
Feeding
Before the ecological upheaval of the 1980s, katunzii was a detritivore. Stomach analyses summarised in the description show it fed mainly on detritus — the organic ooze and associated micro-organisms of the mud bottom — while also taking phytoplankton, insect (chironomid) larvae and copepods. It fed mostly during the day. Its small bicuspid teeth, modest mouth and long gut (intestine roughly 1.5–3.3 times standard length) are all the equipment of a fish processing large volumes of low-value plant and organic material rather than hunting. FishBase estimates a trophic level around 3.3, reflecting that mixed detritus-and-invertebrate diet. Within the extraordinarily fine-grained trophic partitioning of the Victoria flock, katunzii occupied the detritivore guild — one of the many narrow feeding niches into which the lake's cichlids radiated, and a guild that was hit especially hard by the lake's eutrophication.
Mating
No direct observations of courtship in Haplochromis katunzii have been published; the species is known almost entirely from preserved museum material and trawl ecology rather than from watching live fish breed. Its reproductive behaviour is inferred from the universal pattern of the Lake Victoria haplochromines. In that flock the males are the colourful, display-driving sex: a ripe male takes up a small temporary territory and courts passing females, flashing his nuptial colours and presenting the egg-spot ocelli on his anal fin. Mate choice by females, keyed to male colour, is widely held to be the engine that drove the lake's explosive speciation — and katunzii's sandy-brown-with-bars male livery, the trait that distinguishes it from its red-backed relatives, is exactly the kind of species-recognition signal that theory predicts. There is no evidence of pair bonding; as in its relatives, the association between the sexes almost certainly lasts only as long as spawning itself.
Breeding
Breeding in Haplochromis katunzii has not been directly observed. The describers classify it as a female mouthbrooder, an inference drawn from the egg-dummy ocelli on the males' anal fins — the classic apparatus of haplochromine maternal mouthbrooding, in which a female lays a small clutch, takes the eggs into her mouth, and is stimulated to collect the male's milt by snapping at the egg-spots. This is the reproductive mode of essentially the entire Lake Victoria flock. No clutch count is published for katunzii specifically; congeneric Victoria haplochromines of this small size typically produce only a few dozen eggs per spawning (broods on the order of 10–50 are usual for the smaller members of the flock), trading fecundity for the safety of oral incubation. The female alone broods the eggs and larvae in her mouth and continues to shelter the free-swimming fry there for a period after they can swim; the male provides no parental care. Spawning triggers, brooding duration and fry development for this species are simply undocumented.
In the aquarium
Haplochromis katunzii is not an aquarium fish in any practical sense. It has no common name, no trade presence, and no record of being kept or bred by hobbyists; it is a small, drab, scientifically-collected detritivore known essentially only from museum jars and research trawls, and given its Critically Endangered (Possibly Extinct) status it may no longer exist to be collected at all. Nothing approaching a husbandry protocol exists for it.
What can be said is general and inferred from the lake it comes from. Were living fish ever to reach a conservation aquarium, they would need hard, alkaline water (pH roughly 7.5–8.5) at warm temperatures around 73–81 °F — the opposite of the soft, acidic setup suited to West African or South American dwarf cichlids — over a soft, fine substrate that mimics the muddy sub-littoral floor, with a diet built around finely processed vegetable and detrital matter rather than meaty foods. A handful of other Lake Victoria haplochromines (the 'zebra obliquidens', the 'flameback', a few others) are maintained as managed populations in the hobby and in public aquaria precisely because the wild flock collapsed; katunzii is not among them. For anyone interested in the group, those established species are the realistic subjects, and katunzii is better understood as a cautionary case study than as a candidate for the fish room.
Conservation
The IUCN Red List assesses Haplochromis katunzii as Critically Endangered (Possibly Extinct) under criterion C2a(ii), assessed on 31 March 2010 by F. Witte, M.P. de Zeeuw and E. Brooks, with a decreasing population trend. The species is, in other words, formally listed as one collecting expedition away from a confirmed extinction. Its decline is part of the best-documented mass collapse of a vertebrate fauna in modern times. From the 1950s the predatory Nile perch (Lates niloticus) was introduced to Lake Victoria, and from the late 1970s its population exploded; over the 1980s an estimated several hundred haplochromine species were driven to extinction or near-extinction, with detritivores and other open-water guilds among the hardest hit. The collapse was compounded by rapid eutrophication — nutrient loading from a growing human catchment lowered water clarity and deep-water oxygen, erasing the visual environment in which the colour-based species barriers of the flock had been maintained.
When Nile-perch stocks were later thinned by an intensive fishery in the 1990s, the HEST surveys recorded a partial recovery of some haplochromine trophic groups in the Mwanza Gulf — but the recovery was uneven, favoured certain guilds over others, and did not bring every species back. Haplochromis katunzii appears to be one that did not recover, hence its Possibly Extinct tag. Its fate shows how little resilience the lake's narrow-niche endemics have, and how a single introduced predator, acting together with eutrophication, can dismantle an entire adaptive radiation.