Taxonomy & naming
The French naturalist François de Laporte, Comte de Castelnau, named this fish in 1855 from material taken in the lakes of the Rio Araguaia during his expedition through central South America. He placed it, with evident uncertainty, in the genus Chromys — writing it as 'Chromys?? acora' — and figured a specimen on plate 9 of the second volume of his Animaux nouveaux ou rares. No type material is known to survive; the name is anchored by the original description, figure and type locality alone. The species drifted in obscurity for well over a century, lumped with the broader 'festivus' flag-cichlid complex, until Sven Kullander and Anna Silfvergrip redescribed it in their 1991 review of Mesonauta (Revue suisse de Zoologie 98: 407–448), the monograph that first sorted the genus into modern species. They fixed acora as a valid species diagnosed in part by an unusual count of two to four (most often three) scales on the preopercle, and Kullander reaffirmed it in the 2003 Checklist of the Freshwater Fishes of South and Central America. Kullander had, in fact, already begun using the combination Mesonauta acora informally in a 1986 checklist, repeating it with Nijssen in 1989, well before the 1991 paper gave the species its first formal modern diagnosis. That same 1991 review flagged an undescribed Mesonauta sympatric with acora in the Tocantins — material too limited to describe at the time — a reminder that the genus's diversity in that basin may still not be fully catalogued. Kullander and Silfvergrip also proposed, in the same paper, that Mesonauta belongs to a broader natural group together with Pterophyllum, Symphysodon, Uaru and a restricted Heros lineage (H. severus and H. appendiculatus) among the cichlasomine cichlids. The genus Mesonauta was erected by Albert Günther in 1862 and sits in the Neotropical subfamily Cichlinae (the heroine cichlids); flag cichlids are not eartheaters but deep-bodied, substrate-brooding relatives of Heros, Pterophyllum and the festive-cichlid lineage. Modern DNA-barcode work by Rianne de Oliveira and colleagues (2025) confirmed Mesonauta acora as a distinct lineage centred on the Tocantins–Araguaia and Xingu basins, even as the same study collapsed some other nominal Mesonauta into wider-ranging species — a reminder that the genus remains taxonomically restless.
Morphology
Mesonauta acora is a small, strongly compressed cichlid: the largest measured specimen on record is only about 3 in in standard length, making it one of the smaller flag cichlids, though aquarium fish with the tail included look larger and deeper than that figure suggests. The body is the characteristic Mesonauta lozenge — tall, thin, and disc-like — carried on long, filamentous pelvic fins that the fish trails beneath itself like a pair of feelers. Ground colour is a silvery to brassy tan, and the defining mark of the genus is a dark oblique stripe that runs from the snout up through the eye and back to the rear of the dorsal fin, the 'flag' of the common name. What sets acora apart in life is how variable that dark pattern is: importers have remarked that within a single shipment the stress-and-mood blotching differs so much from fish to fish that they could almost be mistaken for several species, where in most flag cichlids the dark pattern is reliably species-specific. Across the genus, species are chiefly told apart by variation on an ancestral pattern of about eight vertical bars crossing the oblique flag stripe — which bars are retained, doubled or lost differs species to species — but acora's markings are unusually unstable even within that framework. The firmest diagnostic character is not visible to the naked eye on a swimming fish — the low count of two to four (usually three) scales on the preopercle — so identification in the trade leans on locality and the variable markings. Sexual dimorphism is slight and the sexes are notoriously hard to tell apart; as in related flag cichlids, ripe females round out in the belly while males may grow marginally larger with somewhat extended fin tips, but there is no bold, reliable external difference.
Habitat
The species is endemic to Brazil and confined to the eastern Amazon periphery — the Tocantins and Araguaia rivers (the Araguaia being a major affluent of the Tocantins) and, according to recent molecular and distributional work, the neighbouring lower Xingu. The type series came from lakes along the Araguaia, and the fish is a creature of quiet, marginal still water rather than open river. A well-documented collection south of São José do Xingu, described by Werner (1990), gives a clear picture of its biotope: a narrow blackwater creek that widened at the collecting site to perhaps a metre deep, floored with sand and mud and choked in places with dense aquatic vegetation; the fish were taken from calm shore zones among reed and grass. The water there was warm and extremely soft and acidic — about 72 °F, pH 5.5, and a conductivity of just 5 microsiemens, near the bottom of the range any cichlid tolerates. These figures describe blackwater chemistry: tannin-stained, mineral-poor, sunlit shallows along the floodplain margin. The Tocantins–Araguaia is largely a clearwater system, so acora occupies the soft, vegetated backwaters and creek mouths within it rather than the main channel.
Feeding
No dedicated dietary study of Mesonauta acora has been published, but its place in the food web can be read from its relatives and from its estimated trophic level of about 3.3 — that of a small omnivore rather than a specialist predator or a true herbivore. Flag cichlids are generalist mid-water and surface feeders that pick at small invertebrates, insects and their larvae, crustaceans, and soft plant and algal material among the vegetation they live in. The long pelvic feelers and the habit of hanging quietly among reed stems suit a fish that gleans rather than digs; unlike the eartheaters of the same basins it does not sift sand for a living. In its vegetated creek margins it would take drifting and clinging invertebrates, fallen terrestrial insects, and bites of the soft growth on submerged plants, functioning as one of the small, unobtrusive omnivores of the floodplain shallows.
Mating
Reproductive behaviour has not been described from the wild for Mesonauta acora specifically, and the species is so seldom kept that detailed captive accounts are scarce; what is known is inferred from the consistent behaviour of the genus. Flag cichlids form monogamous pairs that, once settled, are said to bond for life, and they are notoriously difficult to pair deliberately because the sexes look so alike — the usual route is to raise a group and let a pair form on its own. A forming pair selects and cleans a spawning surface together and becomes territorial around it, but outside of breeding these are mild, retiring fish that shoal loosely and rarely trouble tankmates they cannot swallow. The pre-spawning courtship is the quiet substrate-cichlid routine of mutual surface-cleaning, lateral displays and side-by-side patrolling rather than the digging or jaw-locking dramatics of more aggressive cichlids.
Breeding
Mesonauta acora is a biparental, open-substrate spawner. FishBase records that both parents care for the eggs and larvae and that the larvae are deposited among plants near the water surface — the genus-typical pattern in which a pair cleans a chosen site, the female lays a sheet of adhesive eggs and the male fertilises them, and after hatching the wrigglers are moved to fresh plant surfaces high in the water column where the parents fan and guard them. Spawning on the underside of a broad submerged or floating leaf, or on a cleaned flat stone or root, is the rule among flag cichlids, and the brood is defended jointly until the fry are free-swimming and foraging. No published clutch count exists for acora itself; congeneric flag cichlids lay on the order of several hundred small eggs per spawning, and that is the best available guide to its fecundity. Flag cichlids in general have a reputation as difficult to breed in captivity — slow to pair, fussy about water, and prone to eating early clutches before settling into reliable parenthood — and given how rarely acora reaches aquarists, deliberate spawnings of this particular species are barely documented.
In the aquarium
Mesonauta acora is, by the testimony of the importers who handle flag cichlids, the rarest of the described Mesonauta in the hobby — it reaches the trade only occasionally, usually as wild fish labelled by their Tocantins or Araguaia origin, and was for a long time not imported under a confident identification at all. As a result there is no deep body of keeping experience specific to it, and the husbandry advice below is drawn from the well-established care of the genus, scaled to acora's documented biotope and small adult size.
This is a peaceful, somewhat shy, plant-oriented cichlid that should be kept as a group in a generously planted tank. Because the species stays small — under about 3 in — a single pair does not need a vast aquarium, but flag cichlids are happiest in a small shoal and do best given room to spread out; a tank of roughly 55 US gal or more suits a group, with taller dimensions favoured to match their deep-bodied, mid-water habits. Provide dense planting, driftwood and root tangles, and the subdued, tannin-tinted light of a blackwater setup; tall plants and floating cover are important both for security and as potential spawning surfaces, since these fish hang in the upper water and spook in bright, bare tanks.
Water should mirror the soft, warm, acidic creeks the species comes from. Aim for a temperature around 79–84 °F and a pH on the acidic side of neutral — the wild biotope was measured at pH 5.5 and near-zero hardness, so soft, slightly acidic water (pH roughly 5.5–7.0) suits it far better than hard, alkaline conditions. Tannins from leaf litter, peat or driftwood are beneficial. Like most blackwater fish it dislikes dissolved-waste accumulation, so steady filtration and regular partial water changes matter, but keep flow gentle: these are fish of calm, vegetated margins, not current.
Feeding is straightforward for an omnivore. A varied diet of small live and frozen foods — bloodworm, daphnia, brine shrimp, small insects — alongside a quality flake or micro-pellet and occasional vegetable matter keeps colour and condition. Tankmates should be calm and not large enough to bully or eat them: pencilfish, small tetras and other peaceful characins, corydoras and similarly mild South Americans make natural companions. Avoid boisterous or fin-nipping species, which stress these retiring fish and can shred the long pelvic filaments. The main caution is simply scarcity and the difficulty of telling males from females — anyone hoping to breed acora should buy the largest group they can find and let a pair sort itself out.
Conservation
The IUCN Red List assesses Mesonauta acora as Least Concern (assessed 7 November 2018 by the Instituto Chico Mendes de Conservação da Biodiversidade, ICMBio, and published in 2022), with an unknown population trend and an occurrence recorded only from Brazil. It carries no CITES or CMS listing and is of no commercial fishery interest. The species is collected in modest numbers for the ornamental trade, but its rarity in shipments reflects how seldom it is targeted rather than any documented decline. The real conservation backdrop is the Tocantins–Araguaia basin itself, which is among the most heavily dammed and developed of the major eastern-Amazon drainages: large hydroelectric impoundments, agricultural expansion across the Cerrado headwaters, and the loss of marginal floodplain vegetation all alter the quiet, soft-water creek and lake habitats this fish depends on. None of these pressures has been tied to a measured population change in acora, but as a small endemic restricted to one developing river system it is more exposed than the wide-ranging flag cichlids of the central Amazon, and its status is best read against the ongoing transformation of its home basin.