Taxonomy & naming
Cichla monoculus was described by Spix and Agassiz in 1831 (originally spelled Cychla monoculus), and Eschmeyer's Catalog of Fishes lists it today as a valid species, Cichla monoculus Spix & Agassiz 1831. FishBase, following the same nomenclatural trail, credits the name to Agassiz alone — a minor authorship discrepancy that traces to how the genus's 19th-century plates and text were attributed. The genus name comes from the Greek kichle, an old word for wrasse that early naturalists reused for cichlids; the epithet monoculus, "one-eyed," points to the single dark, light-ringed ocellus the genus carries at the base of the tail.
The modern understanding of the species rests on two works. Kullander redescribed it from Peruvian material in 1986, and Kullander and Ferreira's 2006 revision of the genus (Ichthyological Exploration of Freshwaters 17(4):289-398) recognized fifteen Cichla species, nine of them newly described, and placed monoculus within a tight cluster of look-alike "yellow peacock" species alongside Cichla kelberi, Cichla pleiozona, Cichla nigromaculata and Cichla ocellaris. That tidiness is contested. Molecular work by Willis and colleagues (2007, 2012) argued that the yellow peacocks may represent a single, variable species — Cichla ocellaris in the broad sense — with morphotypes blurred by occasional gene flow, and documented introgressive hybridization both with Cichla temensis in the Amazon and, where it has been moved around, with Cichla kelberi. So the name is stable on paper but its boundaries remain a live argument between morphology-first taxonomy and the genetic data. The species' type locality lies in the Brazilian Amazon; the original 19th-century material is the basis of the name, but a single modern register number should not be invented for it here.
Morphology
This is a big, elongate, deep-bodied cichlid with a sloping forehead, a large upward-tilted mouth, and the genus's hallmark eyespot — a black, pale-ringed ocellus on the caudal fin base. Ground color runs from yellow and gold to a greenish cast, with a paler belly, and the back and dorsal fin are darker. Three dark vertical bars cross the flank, and large fish develop a pronounced occipital bar across the nape. Diagnostically, Kullander and Ferreira separate it from its relatives by what it lacks as much as what it has: no black or ocellated markings on the side of the head, but irregular dark blotches scattered on the anterior abdomen. It differs from Cichla pleiozona in scale counts (roughly 68-87 scales in a lateral row versus 84-93) and from Cichla kelberi in the absence of the light spots that kelberi shows on the anal, pelvic and lower caudal fins — though a molecular study of introduced fish in the Paraíba do Sul caught at least one genetically confirmed monoculus that carried those very spots, a hint of the hybridization muddying the group.
Reported maximum size varies with how it is measured: FishBase gives 27.6 in (27.5 in) standard length and as much as 31.5 in (31.5 in) total length, with a top weight near 20 lb (9 kg). Males grow larger than females and, in breeding condition, swell a fatty hump on the forehead. Most aquarium fish, and most fish in a heavily worked floodplain fishery, are considerably smaller than the published maxima.
Habitat
Cichla monoculus is a fish of the lowland Amazon. The Catalog of Fishes places it in the Amazon and Oyapock river basins; FishBase describes it along the main channel of the Solimões-Amazonas and the lower courses of tributaries through Peru, Colombia and Brazil, reaching the Araguari and lower Oyapock north of the Amazon mouth, and notes it is probably far more widespread in the lowland basin than the records show. It naturally occupies the floodplain lakes (várzea and the blackwater systems) of the Solimões, Negro, Trombetas and the wider Amazon drainage.
It is a warm-water, benthopelagic fish: FishBase records 77-82°F (77–82 °F) and a pH from about 7 upward, but the species spans the Amazon's three water types — nutrient-rich whitewater rivers like the Solimões, tea-stained blackwater like the Negro, and clearwater — and that tolerance is part of why it travels so well. Specimens are often taken from flood-prone shallows where it forages along shorelines and among structure, but it ranges more broadly when the waters rise and connect lakes to channels. American Cichlid Association collectors who netted Cichla monoculus on a central-Amazon várzea trip described fish that came up "very flabby," a reminder that condition swings hard with the flood pulse in these seasonally drowned habitats (Buntbarsche Bulletin 140). That same adaptability has made it a successful invader: it has been introduced to reservoirs across southeastern and southern Brazil (the Paraná and Paraíba do Sul basins, confirmed genetically by Santos and colleagues in 2016) and outside South America to Florida and Hawaii, where it can press on native fish assemblages.
Feeding
In the wild this is an apex predator — FishBase places its trophic level near 3.9 — and the diet shifts sharply with age. Juveniles take shrimp and other invertebrates, while adults become almost exclusively piscivorous, foraging along shorelines where small fish concentrate and ambushing or chasing them down with that large, terminal mouth. Studies of central Amazon floodplain fish treat Cichla monoculus as one of the top predators of the system, and classic survey work on Amazon piscivores (Goulding's on the upper Madeira) found peacock bass consuming essentially only fish. Within the floodplain community it sits at the top alongside other large predators, and its abundance, fast growth and high-quality flesh make it both an ecological keystone and an economic one.
Growth is rapid for a cichlid: a von Bertalanffy analysis of a Solimões floodplain population at Manacapuru estimated an asymptotic length around 21 in (21 in) with a growth coefficient near 0.38 per year. That combination — a high-on-the-food-web predator that grows fast but does not necessarily turn over quickly — is exactly the profile that makes a stock easy to overfish, a point that returns in the conservation picture.
Mating
Like the rest of its genus, Cichla monoculus is a substrate-spawning, biparental cichlid rather than a mouthbrooder, and reproduction begins with the formation and defense of a pair. Males mature at about a year, females at about two, and during the breeding period dominant males turn territorial and develop a fatty post-occipital hump; reproductive ecology work by Chellappa and colleagues (2003) showed that hump is a lipid store laid down for breeding and depleted in spent fish — an honest, energetically expensive signal of a male in spawning condition. A pair clears and defends a nest site on the substrate before the eggs are laid.
Reproduction is not strongly tied to a single season. The Chellappa study, on a population in semi-arid northeast Brazil, found an extended spawning season with a peak coinciding with cooler water, and described the species as a multiple (batch) spawner; FishBase notes it may spawn three to four times a year. That phenotypic flexibility — a long, repeatable breeding window rather than a brief annual pulse — is credited with helping the fish thrive in harsh and variable environments and with making it such an effective colonizer once introduced.
Breeding
After spawning, the pair lays a batch of eggs on the cleared substrate and both parents guard the eggs and the resulting school of fry — the intensive biparental care typical of Cichla. Batch fecundity for the northeast-Brazil population was estimated at around 3,100 eggs (Chellappa et al. 2003), and with three or four spawnings a year a single pair can produce a great many young.
The genus's signature caudal eyespot may itself be a piece of breeding biology. Zaret (1977) hypothesized that the pronounced ocellus — a dark spot ringed with white on the caudal peduncle — serves as a species-recognition signal that inhibits cannibalism of young by adult tucunaré: the increased predation risk from being more conspicuous is, on this argument, more than offset by the number of fry spared from being eaten by their own brood-guarding parents once they leave the school (discussed in Buntbarsche Bulletin 140). In aquaria, spawning is reported only occasionally, but keepers describe broods that can run into the hundreds of fry when a pair does settle and produce.
In the aquarium
Be honest about the size. A fish that approaches 28-32 in (27.5–31.5 in) is not a community cichlid; Seriously Fish puts the realistic adult footprint at roughly 10 x 4 x 3 ft (l x w x h) and frankly recommends it mainly for public aquaria or keepers who can build for it. The recurring story in monster-fish forums is the cute 2 in (2 in) juvenile bought without that plan. Experienced peacock-bass keepers on MonsterFishKeepers generally raise youngsters in a modest 75-125 gallon grow-out, keep groups of three or more (a lone or paired Cichla tends to be skittish and reclusive while a small shoal is bolder and less prone to bullying), and emphasize pellet-training early, because a piscivore raised on live feeders is a disease and water-quality problem waiting to happen.
Water is the easy part: it is hardy and adaptable, doing well around 77-82°F (77–82 °F), pH from roughly 6.5 to 7.5, and soft-to-moderate hardness. Filtration and swimming room are the hard parts — these are large, messy, fast-growing fish that need heavy external or sump filtration, open water, and only sparse, robust décor. Tankmates should be large enough not to be swallowed (arowana, large catfish, stingrays, other big cichlids are the usual choices) and the tank big enough to spread aggression; anything that fits in the mouth will eventually be eaten. The honest summary: rewarding, genuinely interactive, and not a beginner fish.
Conservation
The IUCN Red List assesses Cichla monoculus as Least Concern (assessed 5 November 2020 by R.G. Frederico; assessment published 2023), with the population trend listed as unknown. It is a wide-ranging, abundant, adaptable Amazon fish with no basin-wide threat to the species, and it is harmless to humans and not listed by CITES — the typical profile of a Neotropical cichlid that faces collection and habitat change rather than collapse. The genus and its place in the South American fauna are documented in the standard regional literature (Kullander's 2003 Cichlidae chapter in the Check List of the Freshwater Fishes of South and Central America, and the long line of floodplain-ecology work descending from Lowe-McConnell), which frames these fishes as resilient generalists of the várzea.
That "Least Concern" label is a species-level statement, and it should not be read as "no pressure." Cichla monoculus is one of the three peacock basses most prized by Amazon anglers and is taken simultaneously by subsistence, commercial and sport fisheries, and at the local scale that adds up: a yield-per-recruit study of the Solimões floodplain stock at Lago Grande de Manacapuru (Campos and Freitas, 2014) found fishing effort already above the level for maximum sustainable yield, with fish being caught at roughly half the size they should be, and recommended raising the minimum capture size. The broader Amazon basin is also under strain from deforestation of the floodplain forests that feed its fisheries, damming and reservoir creation that rearrange whole fish communities, and warming. So the accurate framing is the careful one: the species itself is secure across its enormous range, while particular floodplain stocks are demonstrably overfished and the habitat that sustains them is changing — a strong fish in a basin under pressure, not a threatened one.