Taxonomy & naming
Mazarunia mazarunii is a slow-burn discovery. The two specimens on which its description rests were collected in the upper Mazaruni by the Swiss naturalist Patrick de Rham in 1976. Sven O. Kullander of the Swedish Museum of Natural History recognized by 1982 that the pair belonged to an undescribed genus but held off formally naming it — reportedly because two small specimens felt too thin a sample on which to build a description — until he finally did so in 1990, in the journal Ichthyological Exploration of Freshwaters (volume 1, pages 3–14), describing both a new genus and a new species at once; the original combination has stood unchanged since. The holotype (catalogued MHNG 1553.96 at the Muséum d'histoire naturelle, Geneva) and the single paratype (ZMA 115036) both came from the upper Mazaruni River near the village of Kamarang in western Guyana.
Kullander placed the genus in the eartheater assemblage of South American cichlids, the tribe Geophagini within subfamily Cichlinae. His original 1990 paper allied Mazarunia with Crenicara and Dicrossus — a trio he grouped informally as the 'Crenicarini' — rather than with the genus it is now considered most closely related to. That placement did not hold: Hernán López-Fernández, Donald C. Taphorn and Elford Liverpool's 2012 phylogenetic diagnosis and expanded description of the genus instead recovered Mazarunia as sister to Guianacara, another Guiana-Shield genus described only a year before Mazarunia. Their work nearly tripled what was known: they redescribed Mazarunia mazarunii from a much larger series and erected two additional species, Mazarunia charadrica and Mazarunia pala, both likewise confined to the upper Mazaruni. The genus is diagnosed by an unusual suite of skeletal traits — most strikingly the complete loss of the sixth infraorbital bone, a feature shared by no other cichlid — along with details of the palatine, hyomandibula and gill arches. Among its own congeners, Mazarunia mazarunii is set apart by the presence of two foramina (rather than one) on the ascending process of the premaxilla, by ctenoid scales on the subopercle and chest, and by being the only Mazarunia with a discernible lateral band; in large adults a dark bar behind the head gives the impression of a collar. An early misspelling, 'mazaruni,' appears in some literature but carries no nomenclatural weight.
Morphology
Mazarunia mazarunii is a true dwarf cichlid. FishBase records a maximum of 2 in in standard length, and even the largest examples in the genus barely exceed 3 in, so this is a fish that comfortably fits the popular notion of an apistogramma-sized cichlid rather than a robust eartheater. The body is moderately deep and laterally compressed, and the fins are notably short and low — Kullander singled out the 'short, low vertical fins' as a generic hallmark. The dorsal fin carries about 17 spines followed by 7 to 8 soft rays, the anal fin three spines and roughly 8 soft rays, and there are 26 vertebrae. Coloration is restrained and patterned rather than gaudy: the diagnostic feature is a series of about eight dark vertical bars running down the flank, the foremost of which passes through the eye, while the fourth bar from the front bears a large dark blotch at mid-body. The unpaired fins are essentially immaculate, lacking the bright spangling seen in many other Geophagini. In larger adults the bar immediately behind the head darkens into a collar-like mark, and the mid-line blotches link into a faint lengthwise band — the only species in the genus to show one. A further diagnostic detail separates females of this species from their superficially similar relatives in Crenicara and Dicrossus: the ventral fins of a female M. mazarunii are dark, often black, rather than the red seen in those two genera. Sexual dimorphism in this genus follows the typical dwarf-cichlid pattern, with males growing somewhat larger and developing more produced fins, and ripening females becoming deeper-bodied, but the differences are subtle and the published descriptions emphasise meristics and osteology over external sexual characters.
Habitat
Everything about this fish is bound to one place: the upper Mazaruni River drainage, a headwater tributary of the Essequibo River basin in western Guyana, and the species is found nowhere else. The type locality is the blackwater Kamarang River near the town of Kamarang. What makes this range so remarkable is its isolation. The upper Mazaruni sits on a highland plateau of the Pakaraima Mountains, ringed by rapids and falls and hydrogeographically and climatically cut off from the rest of the Essequibo for millions of years. Ichthyologists who have surveyed it — López-Fernández, Taphorn, the University of Guyana biologist Calvin Bernard and colleagues — describe it as a 'lost world' with perhaps the highest proportion of endemic freshwater fishes of any river fauna in the Neotropics; by their estimates somewhere between two-thirds and the great majority of its fish species live in no other river on the planet, a rate rivalling or exceeding Lake Titicaca. Mazarunia mazarunii is one of those endemics. It is a benthopelagic fish of soft, warm, tannin-stained water. FishBase gives a pH range of about 5.8 to 7.2, hardness up to roughly 8 dH, and a temperature range spanning 68–86 °F — the soft, acidic, warm profile expected of a shaded Guiana-Shield blackwater stream. Field data place it among submerged structure, where it shelters and breeds in the cavities and root tangles of the streambed. The Peaima Falls, further downstream on the Mazaruni, are thought to act as a further barrier limiting the species' dispersal, which may help explain why it has never been recorded outside the immediate Kamarang area despite decades of searching by subsequent collectors.
Feeding
Direct dietary studies of Mazarunia mazarunii are lacking — it has been collected and described far more than it has been observed feeding — but its anatomy and relationships make it a benthic invertivore. FishBase models place it at a trophic level near 3.1, the value typical of a small carnivore that works the bottom for aquatic insect larvae, micro-crustaceans and other small invertebrates rather than rasping algae or sifting large mouthfuls of sand. Unlike the larger, deep-bodied eartheaters such as Geophagus and Satanoperca, which take and winnow sediment, this is a small-mouthed dwarf that picks prey from the substrate, leaf litter and submerged wood of its blackwater stream. In a tea-coloured, nutrient-poor highland river the available menu is dominated by the larvae of midges, mayflies and other stream insects, the staple of most small Guiana-Shield fishes, and it can be assumed to forage opportunistically among the root masses and crevices where it also breeds.
Mating
As a substrate-spawning dwarf cichlid, Mazarunia mazarunii almost certainly forms pair bonds around a defended site rather than holding a harem or schooling to spawn, but detailed courtship has been documented from only a single captive pair: a male and female collected in the upper Mazaruni in 1993 by the Dutch killifish specialist Frans Vermeulen and kept by Willem Heijns, whose account and photographs appear in the Cichlids Yearbook. That pair showed a striking, mood-linked colour language. At rest both sexes carry the barred pattern of the formal description, but an excited male loses his markings almost entirely, retaining only his yellowish ground colour and a faint reticulated pattern, while an excited female loses most of her bars too, leaving a simplified cross-shaped pattern made of the horizontal stripe and the central vertical bar, her ground colour shifting from yellow toward grey-blue. Courtship intensifies the change further: a courting female drops the horizontal stripe altogether and darkens the central bar, producing a pattern that, in Heijns's description, recalls the unrelated Central American cichlid Neetroplus nematopus; a courting male shows a similar pattern shift but keeps his yellowish ground colour, which keeps the sexes distinguishable even at the height of display. Pair formation and the division of labour between the sexes otherwise follow the general Geophagini template of biparental cichlids, but because the species is so rarely kept, much of its behaviour beyond this one documented pair remains undescribed.
Breeding
Mazarunia mazarunii is a biparental substrate spawner, and the one consistently reported detail of its reproduction is an evocative one: both parents tend the clutch, and the larvae are deposited in small holes among submerged roots. This is open- or cave-style substrate spawning rather than mouthbrooding — eggs are laid on a sheltered surface, guarded and fanned by the parents, and the hatched larvae are then moved into the protective cavities and root-hole nurseries where the pair continues to care for them. A precise clutch count for the species has not been published; as a fish around 2 in long it lays a small clutch in keeping with its size, and genus-typical figures for these dwarf substrate spawners run on the order of a few dozen to perhaps a hundred or so eggs per spawn rather than the hundreds or thousands of a large eartheater. Parental care is biparental and extended through the free-swimming fry stage, with the young guarded among the same root structure in which they were hatched. Actual spawning has not, however, been confirmed in the aquarium: the one detailed captive account — Willem Heijns's pair, collected by Frans Vermeulen in 1993 — got no further than courtship and colour display before the fish stopped progressing, and no aquarist has published a documented spawning, egg count, incubation time, or fry-rearing record for the species. What is described above as its reproductive biology therefore rests on field inference and generic Geophagini biology rather than on an observed spawn — a reflection of how little this remote endemic has actually been studied at close range.
In the aquarium
Mazarunia mazarunii is essentially a collector's and specialist's fish rather than a hobby staple. It has reached European aquarists only rarely: the best-documented case is a pair collected in the upper Mazaruni in 1993 by the Dutch killifish specialist Frans Vermeulen, most of whose catch died in transit, leaving just one male and one female that survived on in Willem Heijns's aquarium and were photographed there for the Cichlids Yearbook — the pair courted repeatedly but was never observed to spawn. It has never entered the trade in the way that Apistogramma or the popular eartheaters have. Anyone who does obtain it should treat it as a small, soft-water blackwater dwarf. A modest tank suits it: because adults top out near 2 in, a footprint in the region of 23.5–31.5 in (roughly 20–30 US gal) is ample for a pair, and a species or quiet biotope setup is far wiser than a busy community.
Water should mirror its highland stream: warm (the low-to-mid 20s °C, within the documented 68–86 °F envelope), soft, and acidic to neutral (pH around 5.5–7.0, toward the lower end for breeding). Tannin-stained 'blackwater' conditions created with leaf litter, peat or botanicals are both natural and beneficial, and a gentle, well-filtered flow over a fine sand or leaf-litter substrate replicates the streambed it inhabits. The single most important furnishing is cover: a tangle of driftwood, roots and leaf litter offering the small cavities and root-holes the fish uses for shelter and for depositing fry. Subdued lighting and a dark substrate suit its restrained coloration and help it feel secure.
Feeding is straightforward in principle — small live and frozen foods (bloodworm, daphnia, micro-crustaceans, baby brine shrimp) are ideal for a small benthic invertivore, supplemented with quality micro-pellets — but the real obstacle is simply availability. Tankmates, if any, should be small, peaceful soft-water companions; this is not a fish for boisterous community tanks. Because it is so rarely kept, husbandry guidance is necessarily extrapolated from its biotope and from its close relatives rather than drawn from a deep well of aquarium experience, and prospective keepers should approach it as a conservation-minded specialist project rather than a beginner's cichlid.
Conservation
The IUCN Red List assesses Mazarunia mazarunii as Near Threatened (NT), under criterion B1b(iii), in an assessment dated 12 March 2023 and published in 2023 by O.M. Lasso-Alcalá, E. Quintero-T., I. Mikolji and E.O. Martínez; the population trend is recorded as unknown. The listing reflects the fish's tiny, single-drainage range rather than any evidence of decline — a species confined to the upper Mazaruni is, by definition, exposed to whatever befalls that one river. The threats the assessors name are exactly those that hang over the wider 'lost world' fauna: mining, deforestation, contamination, and heavy sediment loads stripped into the waters of the Guiana Shield. Gold and diamond mining is the dominant concern; in the upper Mazaruni it has already degraded aquatic habitats, increasing siltation and pollution, and researchers including López-Fernández and Taphorn have warned that the basin's exceptionally high endemism makes it disproportionately vulnerable to biodiversity loss for its small area. They have repeatedly urged formal protection — a reserve or national park, regulated mining, controls on deforestation and flow alteration, and prevention of non-native fish introductions — to safeguard a fauna in which the great majority of species, this little cichlid among them, exist nowhere else on Earth.