Taxonomy & naming
Astronotus mikoljii was described in 2022 by Alfredo Pérez Lozano, Oscar M. Lasso-Alcalá, Pedro Senna Bittencourt, Donald C. Taphorn, Nayibe Pérez and Izeni Pires Farias, in the open-access journal ZooKeys (volume 1113, pages 111–152; DOI 10.3897/zookeys.1113.81240). The holotype is an adult of about 9 in standard length, catalogued as MCNG 56677 in the fish collection of what was the Universidad Nacional Experimental de los Llanos Occidentales (Guanare, Venezuela), taken in June 2015 from a small stream tributary of the Arauca River in Pedro Camejo Municipality, Apure State — that floodplain stream is the type locality. Paratypes are spread across Venezuelan and Brazilian museums (MCNG, MBUCV, MHNLS, INPA, EBRG) and the Academy of Natural Sciences in Philadelphia (ANSP). The species is registered in ZooBank, the formal record of zoological nomenclatural acts. The genus Astronotus sits in its own tribe, Astronotini, within the Neotropical cichlid subfamily Cichlinae, and for a long time was thought to hold just two valid species: the wide-ranging Astronotus ocellatus and the more southerly Astronotus crassipinnis. Material from the Orinoco had been filed under Astronotus ocellatus for decades. Using an integrative approach — mitochondrial DNA, skeletal characters, and the geometry of the sagitta otolith (the largest ear-stone) — the describers showed the Orinoco population to be a reciprocally monophyletic, diagnosable lineage and named it as new. The same study flagged several further undescribed Astronotus lineages (informally 'East', 'Negro' and 'Jurua'), so the genus is plainly more diverse than the textbook two-species picture, and the boundaries between these forms are still being worked out — muddied by the species' long history of being moved around the continent as a food fish.
Morphology
Mikolji's oscar has the classic oscar build: a deep, laterally compressed, oval body, a large head, and a big oblique mouth, with long-based dorsal and anal fins that taper back toward a rounded tail. FishBase records a maximum of 9.5 in standard length from the type series; in practice, like its congeners, it commonly reaches roughly 10–12 in in total length, and aquarium specimens can be bulky, heavy-bodied fish. Counts run to 17–21 dorsal soft rays and 16–20 anal soft rays. Colour is variable and changes with age and locality: juveniles are strikingly marbled and colourful, while adults settle into a darker olive-to-brown ground broken by irregular pale blotches and reticulations, often with rusty or orange highlights and the dark ocelli on the upper body and caudal region typical of spotted oscars. Honestly, the fish is genuinely hard to tell apart from Astronotus ocellatus and Astronotus crassipinnis by eye — the describers found external morphometrics and meristics of 'little promise' for separating them, and reliable identification leans on internal features. The diagnostic combination is technical: two or three supraneural bones (versus two in its congeners), absence of a small spinous process on the upper border of the parahypural bone in the tail skeleton (present in the others), and an oval sagitta otolith with strongly crenulated margins, a projecting elongate rostrum, and a rounded posterior region — proportions captured by a higher aspect ratio and roundness index than in either Astronotus ocellatus or Astronotus crassipinnis. Sexual dimorphism is not well documented for this species specifically; across the genus the sexes look alike, with mature males tending to grow somewhat larger and breeding adults of both sexes intensifying in colour, so external sexing is unreliable outside the spawning season.
Habitat
Astronotus mikoljii is native to the middle and lower Orinoco River basin and the adjacent Gulf of Paria drainages, across Venezuela and Colombia, at altitudes up to about 820 ft above sea level. Within the Orinoco it is recorded the length of the floodplain of the main channel and through a long list of tributaries — the Atabapo, Inírida, Guaviare, Vichada, Bita, Meta, Tomo, Arauca, Apure, Caura and Morichal Largo among them — down into the Orinoco Delta, plus the San Juan and Caño La Brea systems on the Gulf of Paria side. It is a floodplain generalist by water type, turning up in white, clear and black waters alike (in the classic Sioli sense), and in both flowing and still habitats: large and small rivers, channels, lagoons and seasonally flooded forest and savanna. Young fish shelter among floating and semi-rooted vegetation — grasses and water hyacinth (Paspalum repens, Eichhornia crassipes) — while adults keep to vegetated riparian margins among grasses and sedges. Measured in-situ data are sparse, but part of the type series came from an almost-stagnant, muddy-bottomed flooded stream in the Apure llanos at 81 °F with heavy aquatic vegetation, which is a fair picture of its preferred habitat: warm, weedy, low-current floodplain water. The species has also been introduced within Venezuela beyond its native basin — into Lago de Valencia and several Caribbean-slope drainages — where it is not naturally found.
Feeding
No diet study has yet been published for Astronotus mikoljii specifically, so its trophic role is inferred from the genus, which is well studied. Oscars are large opportunistic carnivore-omnivores: ambush and bottom-foraging predators that take aquatic insects and their larvae, shrimp and other crustaceans, snails and small fishes, supplemented by detritus sucked from soft bottoms and, seasonally, by fruits and seeds dropped into flooded forest. FishBase places Astronotus mikoljii at a trophic level near 3.0, consistent with a mixed invertebrate-and-fish diet rather than strict piscivory. Field studies of Astronotus ocellatus in the Venezuelan llanos and the central Amazon similarly describe a diet dominated by insects and fishes with notable ontogenetic shifts, so the broad picture for Mikolji's oscar is that of a flexible mid-level predator of weedy floodplain habitats, hunting from cover among grasses and flooded vegetation and switching prey with the flood pulse and with its own growth.
Mating
Reproductive behaviour has not been described in the wild for Astronotus mikoljii, and the account here is drawn from the genus, whose biology is consistent and well known to aquarists. Oscars are monogamous, biparental substrate spawners that form lasting pair bonds. Pairs typically arise from a group of young fish raised together, sorting themselves out as they mature; once bonded, a pair becomes territorial and intolerant of rivals. Courtship involves jaw-locking, lateral displays, fin-flaring and cooperative cleaning of a chosen spawning surface — usually a flat rock or other hard, open substrate — and the deepened colour and contrast of breeding adults. Pair aggression can be intense and, in confinement, dangerous: an unsettled or mismatched pair will fight hard, which is the usual obstacle to breeding oscars in the home. There is no reason to expect Astronotus mikoljii to differ in any of this from its better-known relatives.
Breeding
Like other oscars, Astronotus mikoljii is expected to be a biparental, open-substrate (lithophilic) spawner rather than a cave brooder or mouthbrooder. In the genus the pair cleans a flat surface and the female lays a large adhesive clutch — commonly several hundred to over a thousand whitish eggs in a single spawning for a full-grown female — which both parents guard and fan. Wrigglers are typically moved to pits dug in the substrate, and once free-swimming the fry are shepherded and defended by both parents for an extended period; oscars are attentive, protective parents. None of this has been documented for wild Astronotus mikoljii, and clutch figures here are genus-typical values, not measurements of this species. In its first stages of life Astronotus mikoljii is associated in the wild with floating and semi-rooted vegetation in the floodplain, the nursery cover where small fish find food and shelter — but how and exactly where it spawns in nature remains undescribed and is a clear gap for future field work.
In the aquarium
Mikolji's oscar only reached the trade under its own name after 2022, so dedicated husbandry records are thin; the honest foundation is oscar-typical care adapted to the chemistry of a floodplain fish from the soft, warm, often blackwater-influenced Orinoco basin. The first and non-negotiable requirement is space. A single adult, which will commonly reach 10–12 in total length and be a heavy, actively swimming fish, needs no less than 120 US gal as a practical minimum — 130–160 US gal is more comfortable for a kept pair, and larger is better. A standard 71 in footprint gives meaningful territory. Anything smaller simply cannot buffer the ammonia and waste output of a large cichlid reliably, and Astronotus of all kinds are notorious for being heavily medicated in undersized tanks, which is needless and easily avoided. Water quality demands strong mechanical and biological filtration — external canister filters at a combined turnover of 8–10× the tank volume per hour are a reasonable starting point — because these fish eat heavily, produce large waste loads, and will suffer gill damage and susceptibility to hole-in-the-head disease (HLLE) in chronically poor water. Regular partial water changes, at least 30–40% weekly, are the best health tool available.
Water parameters should target the species' natural range: temperature 77–84 °F, pH 5.5–7.5 (a neutral to mildly acidic 6.5–7.0 is widely used and appropriate), and low to moderate hardness consistent with soft Orinoco floodplain water. The fish tolerates a fairly broad band — it occupies white, clear and black waters across its range — so rigid precision matters less than stability. Décor should reflect the floodplain environment: a sandy or fine-gravel substrate that the fish can dig in without abrading its mouth, large smooth rocks and pieces of driftwood for territory markers and hiding places, and robust or artificial plants (live plants will be uprooted by a pair that begins digging in earnest before or during spawning). Avoid sharp substrates, gravel-packed corners with no drainage, or any décor small enough to be swallowed.
Tankmate selection is straightforward in principle but demands realistic honesty about the fish's size and appetite: any fish small enough to fit in a 10–12 in cichlid's mouth will eventually disappear. Suitable companions are other large South American cichlids of comparable size and temperament — species of Cichla, Geophagus, Satanoperca, or comparably sized Acarichthys — large armoured catfish such as Pseudacanthicus or robust Hypostomus, and big plecos generally. Small tetras, rasboras, dwarf cichlids, small catfish, and any fish under roughly 4–4.5 in are not suitable. A bonded breeding pair of Astronotus mikoljii will additionally chase and injure even large tankmates during spawning preparation, so pairs are often best maintained in a dedicated species tank or with a few durable, fast diel-space-dividing dither fish large enough to escape aggression.
Breeding in captivity follows the genus pattern described above: the pair cleans and defends a flat, open surface — a large smooth rock, an inverted slate slab, or even the bare tank floor — deposits a large adhesive clutch of several hundred to over a thousand eggs, and both parents guard and fan eggs through hatching and the wriggler stage. The usual obstacle is forming a compatible pair in the first place. The safest method is to raise a group of six or more juveniles together and allow them to pair naturally; forced pairing of two adults that have not chosen each other reliably ends in injury or death of one fish. Once a genuine pair is established, breeding usually follows without intervention if water quality and temperature are maintained. Fry are mobile and closely herded by both parents from the free-swimming stage onward, and the pair will aggressively defend the brood against any intrusion; non-breeding tankmates should be removed or given ample refuge.
The most common keeper mistakes with Mikolji's oscar mirror those well known from the genus at large: housing juveniles in tanks that will be inadequate within months and then failing to upsize in time; underestimating filtration needs and accepting chronically elevated nitrates; feeding an exclusively pellet or feeder-fish diet without variety (insects, earthworms, frozen shrimp, and occasional vegetable matter better replicate natural feeding); and purchasing a single animal, keeping it alone in a featureless tank, and then attributing any listlessness or disease to the species rather than to husbandry. This is an intelligent, long-lived fish — oscars regularly live 10–15 years in well-managed tanks — and it repays appropriate care with recognisable individual behaviour and a willingness to breed. The difficulty level is moderate: not a fish for a beginner's first tank, but straightforward for a keeper with experience of large cichlid systems and a commitment to the necessary tank volume and maintenance schedule.
Conservation
Astronotus mikoljii has not yet been assessed for the IUCN Red List — unsurprising for a fish described only in 2022 — so its formal status is Not Evaluated, and it carries no CITES listing. That absence of a category is not the same as safety. The describers single out one concrete pressure: the spread through the Orinoco basin of the invasive cichlid Caquetaia kraussii, transferred outside its native range within Venezuela. They report a floodplain lagoon in the Portuguesa River drainage, between Guanare and Guanarito, where Astronotus mikoljii was collected repeatedly across a decade of field trips (1978–1988) and is now wholly absent, replaced by Caquetaia kraussii — a stark, if localized, sign of competitive displacement. Beyond that specific threat, the species is a heavily used food fish across the llanos: type material was bought from fishermen and market stalls, and the long tally of indigenous and Spanish names testifies to how thoroughly it is fished and eaten. As an adaptable, wide-ranging floodplain generalist it is in far less immediate danger than a narrow-range endemic, but the Orinoco floodplains it depends on face the same basin-wide stresses as the rest of tropical South America's lowland rivers — dams, deforestation, mining, altered flood pulses and the continued movement of non-native fishes. With no population assessment in hand, the honest summary is that Astronotus mikoljii looks common and widespread today, but its status genuinely has not been measured, and at least one introduced competitor is already reshaping parts of its range.