Taxonomy & naming
Johann Jakob Heckel, the Austrian ichthyologist, described this fish in 1840 as Acara viridis, in the second volume of the Annalen des Wiener Museums der Naturgeschichte, working from material collected in Mato Grosso State, Brazil. The Catalog of Fishes lists its syntypes in two places: Vienna's Naturhistorisches Museum (NMW 16247, 33833 and a questionable 91433, all but one dry) and the Senckenberg museum in Frankfurt (SMF 2925, transferred from Vienna in 1844). The genus Aequidens arrived later — Carl H. Eigenmann and William L. Bray erected it in 1894, first as a subgenus of Astronotus — and Sven O. Kullander's authoritative 2003 checklist of South American freshwater fishes treats the species as valid: Aequidens viridis (Heckel 1840). Two names fall into its synonymy, both described by John D. Haseman in 1911 from the Rio Guaporé on Carnegie Museum collections now held at the Field Museum: Aequidens awani (type locality São Antonio de Guaporé) and Aequidens guaporensis, which Kullander formally synonymized in 2003. The genus sits in the tribe Cichlasomatini within the Neotropical subfamily Cichlinae, and as currently drawn it is not monophyletic. Molecular work by Musilová and colleagues in 2009 showed that some nominal Aequidens belong instead in Krobia, and that the familiar 'Aequidens' pulcher-rivulatus group is a separate lineage, later named Andinoacara. The genus's true core is the 'tetramerus group' — a more restricted set of mostly substrate-brooding South American cichlids that holds the type species and, with it, Aequidens viridis.
Morphology
Aequidens viridis has the standard acara build: a deep, oval, laterally compressed body, a moderately steep forehead, and long-based dorsal and anal fins. FishBase, following Kullander (2003), gives a maximum of about 6.5 in standard length — snout to tail base — which puts it among the medium-sized species of the genus. Its trade names describe what a keeper actually sees. Golden acara and acará cascudo both point at a brassy, golden-bodied fish; the scientific name records the colour those names miss. Heckel called it viridis, 'green', for the living animal, and the ETYFish Project, drawing on his original description, glosses that as a green head and back, blue-green sides to the head near the snout, a grey-green forehead, and bright green bands along the flanks. The green belongs to the fish while it is alive and vanishes in alcohol, leaving the pallid yellowish-brass of the museum syntypes. Like most acaras it carries a dark lateral spot and dark barring or a stripe through the body, but no modern redescription of its pigment pattern or meristics has been published, and sexual dimorphism has not been formally documented. Its relatives hint that mature males run somewhat larger with more extended unpaired fins and ripe females rounder-bodied — but for Aequidens viridis, that is an inference from the genus rather than a recorded fact.
Habitat
Aequidens viridis is endemic to the Guaporé-Iténez basin — the clearwater river Brazilians call the Rio Guaporé and Bolivians the Río Iténez, which runs a long stretch of the Bolivia-Brazil border before joining the Mamoré to form the Madeira. The sources draw its range slightly differently. The Catalog of Fishes gives the Guaporé drainage and adjacent western Amazon in Bolivia and Brazil; FishBase lists the Guaporé and Amazon basins; the IUCN confines it to the Guaporé basin in Bolivia and the Brazilian states of Rondônia and Mato Grosso, with an estimated extent of occurrence near 297,000 square kilometres. The basin itself (Freshwater Ecoregion 319) is geologically and chemically distinct from its Andean-fed neighbours in the Madeira system: a clearwater river draining the Brazilian Shield and the eastern Bolivian lowlands, soft and mildly acidic at a basin pH of roughly 6.3 to 6.8. Its margins hold extensive seasonally flooded forests and savannas, oxbow and subsidence lakes, and permanent swamps and marshes, among them the vast Llanos de Moxos wetlands. FishBase records the fish on flood plains and the IUCN classes its habitat as permanent rivers, streams and creeks, but the assessment is candid that 'there is no information about this species' habitat and ecology.' What can be said is the biotope it belongs to — warm, soft, clearwater floodplain of the southwestern Amazon. Field surveys of Guaporé biotopes, which list the species under its synonym Aequidens awani among the resident cichlids, describe water that is exceptionally soft (carbonate and general hardness both below 1 °dH), mildly acidic at about pH 6.5, and seasonally variable from roughly 68–86 °F, rarely dropping below 68 °F even when winter air temperatures fall sharply. No in-situ reading has ever been taken from Aequidens viridis itself; these are basin-typical figures, not species-specific ones.
Feeding
No diet study of Aequidens viridis exists, so its feeding has to be read from its genus and its habitat. FishBase treats it as a benthopelagic fish and estimates a trophic level of about 3.2 — the figure for a small to mid-level predator rather than a herbivore or pure detritivore. Acaras are opportunistic benthic omnivores as a rule: they work over soft bottoms and leaf litter for insect larvae, small crustaceans, worms and other invertebrates, round the diet out with plant matter, seeds and detritus, and routinely take mouthfuls of substrate to sort edible morsels from sand and silt. The flooded forests, marshes and floodplain lakes of the Guaporé offer exactly the invertebrate-rich, detritus-laden bottom such a feeder exploits, and the seasonal flooding that powers Amazonian floodplain productivity would make terrestrial and aquatic invertebrates abundant in turn. The fish is taken locally as subsistence food, which fits a common floodplain generalist. Without gut-content data, the safe description is a generalist benthic invertivore-omnivore of floodplain still waters.
Mating
No account this article could verify describes the pre-spawning behaviour of Aequidens viridis, from the wild or from the aquarium. What is reliable is the reproductive style of the genus. True Aequidens are biparental substrate spawners that pair off monogamously out of a larger social group, with both partners then holding a territory around the chosen spawning site. In congeners such as Aequidens tetramerus and Aequidens metae, a pair comes together after a bout of mutual display, jaw-locking and lateral threat; the bond firms up as the two clean a surface together and drive off intruders side by side, and aggression toward their own kind and toward tankmates climbs sharply once the pair commits to a site. Aequidens viridis is presumed to follow the same course of pair formation and shared territoriality — but with no spawning of the species on record, that reads as an extrapolation from its relatives rather than an observation of the fish itself.
Breeding
Breeding has never been documented for Aequidens viridis. The genus is uniformly made up of biparental open-substrate spawners: a bonded pair cleans a flat, firm surface — a rock, a submerged log, a leaf, or in aquaria a slate or the inside of a flowerpot — and the female lays a single sheet of adhesive eggs that both parents then tend, the female fanning and mouthing the clutch while the male patrols the territory. Once the eggs hatch, the wrigglers are usually moved to one or more pits in the substrate, and the free-swimming fry are shepherded and guarded by both parents as a school, sometimes for several weeks. Clutch sizes in comparable mid-sized acaras run from the low hundreds to roughly a thousand eggs, scaling with the female's size. For Aequidens viridis, none of this is recorded — not clutch size, not egg or larval development time, not spawning triggers or the length of care — and a hobby note in the Deutsche Cichliden-Gesellschaft's DCG-Info even asked whether the species had ever been kept. So everything here is the expected genus mode, offered as the honest minimum: in the wild and in aquaria Aequidens viridis is in all likelihood a biparental substrate spawner like its close relatives, but its reproduction is effectively unstudied.
In the aquarium
Aequidens viridis is rare in the hobby and almost absent from the aquarium literature — the Deutsche Cichliden-Gesellschaft's index once asked whether it had been kept at all. The husbandry below is built from its biology and the well-known care of its 'tetramerus group' relatives; anyone keeping it is working with a blank page.
Tank: a pair approaching the published 6.5 in SL needs at least a 120 × 19.5 in footprint — about 80 US gal — for defensible home ranges and the territorial surge that comes with spawning. It uses the whole water column but, being benthopelagic and drawn to shallow, structured lake margins, spends most of its time near the bottom.
Water: match the Guaporé — very soft (GH below 1 °dH), mildly acidic at pH 6.3–6.8, and warm, a comfortable 75–82 °F within the basin's 68–86 °F span — using RO water remineralised to single-digit GH, or natural rainwater. Robust acaras tolerate harder, more alkaline tap water (above 10–12 °dH, pH over 7.0), but for a fish this understudied and this tied to soft clearwater, keeping it hard is a needless gamble. Filter efficiently but gently, with a sponge pre-filter over the inlet to suit its substrate-sifting.
Decor: fine dark sand it can mouth and sift, driftwood and root tangles, dried leaves such as Terminalia catappa to stain and acidify the water, and a few flat stones or slate that can double as spawning sites. Subdued lighting suits it and brings up the green iridescence that harsh light washes out; low-light plants such as Microsorum, Anubias and floating Salvinia or Pistia take the soft, acidic conditions well.
Tankmates: treat it as a moderately aggressive medium cichlid — fine alongside similarly sized, peaceable soft-water fish, but liable to bully smaller or timid ones and potentially lethal toward its own kind in a cramped spawning tank. Good companions in a Guaporé setup are same-drainage characins (Hyphessobrycon or Moenkhausia), armoured catfish such as Corydoras sterbai and other callichthyids, and appropriately sized loricariids; avoid small tetras that read as food and thin-bodied fish that cannot defend themselves.
Feeding is the easy part: sinking pellets, frozen bloodworm, Daphnia, artemia and earthworm as the mainstay, with the odd plant wafer or blanched vegetable and a regular live or frozen invertebrate component to condition a pair.
Common mistakes: the real difficulty is simply finding the fish. It shows up only as occasional bycatch in mixed South American shipments or through specialist Guaporé collectors, with no captive population and no trade demand to build one. Because it has so rarely been kept, the hobby's mistakes for it aren't catalogued yet — so keep it as you would any moderately large soft-water acara from a clearwater floodplain: soft, warm, roomy, well-structured, and carefully noted.
Conservation
The IUCN Red List assesses Aequidens viridis as Least Concern, evaluated on 28 October 2020 by R.G. Frederico, reviewed by T.J. Lyons and published in 2023. The reasoning is brief and rests almost entirely on geography: 'despite there being little information available on this species, it is assessed as Least Concern due to its wide range' — an extent of occurrence near 297,000 square kilometres across the Guaporé basin of Bolivia and Brazil. The assessment carries no population estimate and an unknown trend, noting only that the fish 'appears to be uncommon.' It is used nationally as an ornamental fish and locally as subsistence food, yet no threats are documented and it holds no CITES listing. The wider picture is more reassuring than for many Amazonian endemics: the Guaporé-Iténez is one of the better-preserved clearwater basins of the southern Amazon, draining the sparsely populated Brazilian Shield and the Llanos de Moxos wetlands. Even so, the southern Amazon faces mounting pressure from deforestation, cattle ranching, agricultural expansion and proposed Madeira-system hydropower, and a fish this poorly known could shift status before anyone had the baseline data to notice. For now its standing is secure on range alone — with the caveat that it rests on an almost complete absence of ecological information.