Taxonomy & naming
The Kiungululu tilapia was described by the German ichthyologist Ernst Ahl in 1924 as Tilapia chungruruensis, from the crater lake whose name it bears. Like the other small East African tilapiines, it was long carried in the old genus 'Tilapia'; the modern placement in Oreochromis — the maternal mouthbrooders — follows the revisionary work of Trewavas and the subsequent treatment by Trewavas & Teugels (1991), and is confirmed in Eschmeyer's Catalog of Fishes, which lists the species as valid as Oreochromis chungruruensis. Recent molecular and morphological work on the East African crater-lake tilapias (Ford and colleagues, 2019) re-examined this and related single-lake endemics and upheld its distinctness. The epithet chungruruensis is geographic, after Lake Chungruru; the lake itself also appears in the literature as Lake Kiungululu, the source of the species' English name. The fish is closely comparable to other small Rungwe-region and East African crater-lake Oreochromis, part of a scatter of narrow-range endemics that have diversified in isolated volcanic lakes.
Morphology
This is a small tilapia, reaching only about 6 in in standard length (around 7.5 in total length) — among the smaller members of the genus, and noticeably so. Non-breeding fish are plain: females and non-territorial males are grey-bodied and crossed by four to five vertical bars, the kind of subdued barred pattern common in juvenile and quiescent tilapias. Breeding transforms the territorial male, which turns black overall with a thin white or orange margin to the dorsal fin — a high-contrast nuptial dress displayed over his spawning territory in the shallows. Larger individuals in this food-limited, oligotrophic lake have been described as somewhat emaciated, a hint that the closed crater lake is a hard place to grow large. In meristics and general build it is a conventional small Oreochromis, with the fine grazing dentition and mouthbrooding adaptations of the genus.
Habitat
Oreochromis chungruruensis is endemic to Lake Chungruru (Kiungululu), a small, isolated, oligotrophic crater lake in the Rungwe Volcanic Province of the Rungwe District, in the Southern Highlands of Tanzania between roughly 9° and 10° S, near the Lake Masoko/Mbaka cluster of maar lakes between Mount Rungwe and the northern tip of Lake Malawi. Crucially, the lake has no outflow: it is a closed, endorheic crater basin, which makes its single fish population wholly dependent on the lake's water level and water quality. It is reported probably to occur also in nearby Lake Masoko. These are clear, nutrient-poor (oligotrophic) volcanic lakes, and the species occupies the shallows, where breeding males hold territories. The combination of a single small water body, no outlet, and an oligotrophic, food-limited environment defines both the species' biology and its acute vulnerability.
Feeding
Specific diet studies are lacking for the Kiungululu tilapia, but its small size, fine tilapiine dentition and oligotrophic crater-lake home point to the standard Oreochromis strategy: microphagous grazing and filtering of algae, diatoms, fine plant material and detritus, with the fish working low on the food web as a primary consumer. In a nutrient-poor crater lake the standing crop of food is limited, which fits the reports of emaciated larger individuals and the species' generally small adult size — there is simply not much primary production to convert into fish flesh. As with its congeners, there is no indication of piscivory; this is a broad-spectrum grazer-detritivore eking out a living in a low-productivity, closed system.
Mating
Oreochromis chungruruensis is a maternal mouthbrooder, and its mating follows the genus's lek-and-pit system. Territorial males develop their black nuptial coloration with the pale-margined dorsal fin, defend shallow-water territories and court passing females; spawning takes place over the male's cleared spot, after which the female immediately takes the fertilised eggs into her mouth. Field observations note that males defend territories in shallow areas — the visible expression of this breeding system. In a small, closed lake the breeding population is necessarily limited, and the entire reproductive future of the species turns on the few square kilometres of shallow water where these males display. There is no evidence of any departure from the standard Oreochromis pattern of male nest-tending and female oral incubation.
Breeding
Breeding is by maternal mouthbrooding: the female incubates the fertilised eggs and the resulting larvae in her buccal cavity, aerating and protecting them, then shelters the free-swimming fry in her mouth when threatened until they are independent. Clutch sizes are not published for this species; as a small Oreochromis its fecundity will be correspondingly modest — at most a couple of hundred eggs — with the usual mouthbrooder trade-off of few, well-protected young rather than many unguarded ones. For a single-lake endemic this low absolute reproductive output is part of the conservation problem: there is little buffer against a bad year. No captive-breeding programme of any scale is documented, and what is known of its reproduction is inferred from its territorial behaviour and from the well-studied biology of related Oreochromis rather than from detailed study of the species itself.
In the aquarium
The Kiungululu tilapia is not an aquarium fish in any practical sense — it is a Critically Endangered single-lake endemic of conservation interest, not a trade or hobby species, and it should not be collected from the wild. There is no established husbandry tradition and a hobbyist will not encounter it. Were it ever to be maintained, it would be in a conservation or research setting as an ex-situ safeguard, and the requirements would follow its wild ecology: clear, well-oxygenated, fairly neutral-to-hard fresh water at tropical temperatures, ample shallow open substrate for males to hold territories, and an algae- and detritus-based diet matched to its microphagous grazing. The far more important point than husbandry is that this species' survival depends on its one lake, and the constructive role for the aquarium and conservation community is supporting protection of Lake Chungruru and any properly managed ex-situ refuge population — not keeping it as a curiosity.
Conservation
The IUCN Red List assesses Oreochromis chungruruensis as Critically Endangered (assessed 22 May 2018). The reasoning is the classic single-lake endemic case: the species is confined to one small, outlet-less crater lake, so any deterioration of that lake threatens the entire global population at once. The specific threats flagged are siltation and water-level fluctuation driven by drought — a closed crater lake has no inflow buffer — together with the impact of stocking the lake with non-native tilapiines such as Coptodon (Tilapia) rendalli and other Tilapia, which compete with and can hybridise the native fish, and pressure from fishing. With no second population to fall back on and a fish already small and food-limited, the margin for error is essentially nil. Conservation of this species is, in practice, conservation of Lake Chungruru itself: protecting its water level and quality and preventing further introductions. It carries no CITES listing, and its survival is a question of whether one small Tanzanian crater lake can be kept intact.