Taxonomy & naming
Sven O. Kullander described Aequidens mauesanus in 1997, in a short paper devoted entirely to the new species in Ichthyological Exploration of Freshwaters (volume 7, pages 377-383). The holotype is held at the Museu de Zoologia da Universidade de São Paulo (MZUSP 7306), with paratype lots split between that institution and the Swedish Museum of Natural History in Stockholm (NRM). Eschmeyer's Catalog of Fishes records the type locality as an igarapé (forest stream) of the Rio Maraú within the Rio Maués drainage, in the município do Maués, Amazonas State, Brazil. The genus Aequidens was erected by Eigenmann and Bray in 1894 — originally as a subgenus of Astronotus — and sits in the subfamily Cichlinae, the South American cichlids. As a 'true' Aequidens in Kullander's modern, restricted sense of the genus, Aequidens mauesanus belongs to the medium-bodied acaras that are biparental substrate spawners, distinct from the eartheaters and the dwarf cichlids. It has no junior synonyms. Kullander treated it again in his 2003 checklist of the freshwater fishes of South and Central America, which — with the original description — is essentially the whole of the published literature on the fish. The IUCN assessment notes morphological differences between specimens from the Maués and the Arapiuns systems and flags that further comparative study may be warranted, a hint that the name as currently applied may not be the last word.
Morphology
This is a small cichlid: the largest measured specimen reached about 5.5 in in standard length, the figure Kullander (2003) gave and FishBase and the IUCN repeat (the IUCN expressing it as total length). That places it among the more modest members of its genus. In build it is a typical acara — a moderately deep, laterally compressed, oval-bodied cichlid with the group's continuous spiny-and-soft dorsal fin and a rounded-to-lanceolate caudal. The details that separate it from its congeners are technical, drawn from the original description's counts and proportions rather than from striking field marks. Like most Aequidens it carries a dark midlateral blotch or spot and faint vertical barring that the fish can intensify or suppress with mood. No image of a living wild specimen is widely published, and FishBase carries no photograph for the species, so any description of its precise live coloration should be treated with caution. Sexual dimorphism has not been documented for this species in particular. In Aequidens generally, males tend to grow somewhat larger and develop more produced (extended) dorsal- and anal-fin tips while ripe females are rounder-bodied — but whether, and how strongly, that holds in Aequidens mauesanus is unrecorded.
Habitat
The species is endemic to Brazil, confined to two neighbouring drainages on the south side of the lower Amazon: the Rio Maués (Maués-Açu) system in Amazonas State and the Rio Arapiuns in Pará. This is the stretch of the Amazon's right bank roughly opposite the confluence of the Rio Negro, and the IUCN characterises the fish as probably restricted to the region's blackwater streams. FishBase summarises it as 'found mainly in black-water rivers,' and the Red List adds that it occurs in black- or clearwater streams (igarapés) with low pH — the soft, acidic, tannin-stained, ion-poor water typical of the Amazon's blackwater tributaries. No in-situ temperature, conductivity, or pH measurements specific to Aequidens mauesanus have been published, so any chemistry figures are general to the blackwater biotope rather than measured for this fish: lower-Amazon blackwater igarapés are characteristically warm (roughly 75–84 °F), strongly acidic (pH often around 4.5-6.5), and very soft and ion-poor. As a lowland Amazonian fish it lives in the warm range typical of the basin's small forest streams, and FishBase records no depth range for it. The known range is small and the species is described as naturally infrequent and not abundant; whether it is genuinely rare or simply under-collected in a remote, lightly surveyed region is unclear.
Feeding
There is no published study of the diet of Aequidens mauesanus, and FishBase lists no food items for it; its trophic level there (about 3.2) is a model estimate based on body size and the diets of close relatives, not a measured value. By inference from the genus, it is almost certainly a generalist micro-predator and omnivore of the leaf-litter and soft-sediment zones of small streams. Aequidens species typically forage over the bottom, taking aquatic insect larvae, small crustaceans, and other benthic invertebrates along with some plant and detrital material, and they will pick at and winnow mouthfuls of substrate in the loosely 'eartheating' manner common to many Cichlinae — though true Aequidens are not the specialised sand-sifters that the geophagine eartheaters are. In a blackwater forest stream such a fish would function as a small mid-level predator on the invertebrates of the leaf-pack community. All of this is reasonable extrapolation rather than observation; what Aequidens mauesanus actually eats in the Maués and Arapiuns remains undocumented.
Mating
No courtship or pairing behaviour has been described for Aequidens mauesanus, in the wild or in captivity — the species is essentially absent from the aquarium hobby, and its reproduction has not been observed. What can be said is drawn from the genus. Aequidens in the strict sense are biparental substrate spawners that form monogamous pairs for the duration of a breeding cycle. A pair establishes and jointly defends a territory around its chosen spawning surface, becoming markedly more aggressive toward conspecifics and other fish while breeding. Courtship in these acaras runs through the familiar cichlid repertoire of lateral displays, fin-spreading, jaw-locking, and substrate-cleaning by both partners ahead of spawning. Whether Aequidens mauesanus follows this pattern in detail is unrecorded; it is the expected mode for a true Aequidens, but read it as an inference about the genus rather than a description of this species.
Breeding
Breeding has not been documented in Aequidens mauesanus, in the wild or in aquaria. As a member of the genus it is expected to be a biparental open-substrate spawner: such acaras lay an adhesive clutch of eggs on a cleaned firm surface — a flat stone, a root, or a leaf — which both parents then guard and fan, moving the wrigglers to pits and shepherding the free-swimming fry as a defended school, in the way characteristic of substrate-brooding Neotropical cichlids. Clutch sizes in comparable Aequidens are moderate: FishBase reports about 1,000 eggs per spawning for the genus's type species, Aequidens tetramerus, laid on stone or wood with both parents tending the young, and biparental care of eggs and fry is the norm. None of these particulars — clutch size, egg and larval development times, the duration of parental care — has been measured for Aequidens mauesanus itself, and no spawning of the species appears to have been reported. The figures and behaviours above are therefore genus-level expectations, not records for this fish.
In the aquarium
Aequidens mauesanus is essentially absent from the hobby: no commercial import is on record, and the few references in aquarium literature are passing notes in discussions of the genus rather than first-hand husbandry accounts. What follows is honest inference from the fish's biology and from the well-documented care of closely related small acaras such as Aequidens tetramerus and Aequidens diadema — a likely-good framework, not verified practice.
Tank: A single pair could be kept in a 25–30 US gal aquarium (roughly a 90 × 17.5 in footprint). Because the species is expected to defend a breeding territory vigorously, a longer tank — 47 in or more — gives other fish room to retreat; adding cichlids to a smaller tank alongside a breeding pair is asking for trouble.
Water: The fish comes from soft, acidic blackwater igarapés (pH 4.5-6.5, 75–84 °F, very low mineral content). Aim to approximate them: temperature 77–82 °F, pH 5.5-6.8, general hardness below 8 °dGH, and conductivity near zero for best colour and any chance of spawning. Soft tap water cut with peat, or RO water blended back up to target, works well; skip hard alkaline tap water. Tannin from dried catappa (Indian-almond) leaves or alder cones suits the biotope. Run efficient filtration with a moderate current — the igarapés are not fast-moving — and keep up the water changes, because this is a fish from clean, nutrient-poor water that won't tolerate a dirty tank.
Setup: A biotope layout suits it — sandy or fine-gravel substrate, leaf litter, smooth stones, and tangles of driftwood for visual breaks and potential spawning sites. Strong lighting isn't necessary and, in a tannin-stained tank, probably unwelcome. Soft-acid-tolerant live plants (Eleocharis, Cryptocoryne, floating plants) can be used but aren't essential.
Tankmates: Like other substrate-spawning acaras, pairs are expected to become assertive — even seriously aggressive — when breeding, chasing other fish from the territory; outside breeding condition a single specimen or a non-breeding pair should be a mid-range community fish, assertive but not habitually violent. The most sensible companions are blackwater species of similar size and water needs: small South American tetras (Hyphessobrycon, Moenkhausia), pencilfish, or peaceful catfish (Corydoras, small Hypancistrus or Peckoltia). Avoid rougher cichlids, and avoid nano fish small enough to be nipped.
Feeding: As a presumed benthic invertivore it should take most sinking foods without fuss — quality pellets and granules of appropriate size, frozen or live bloodworm, Daphnia, Artemia, and small earthworms. Vary the diet rather than leaning on one staple, and expect it to rummage through substrate and leaf litter for hidden invertebrates.
Common mistakes: The pitfalls are hard alkaline water, a dirty tank, and underestimating a breeding pair's territoriality. Not recommended for beginners, mainly because of the water-quality demands of a strict blackwater fish and the absence of any established hobby experience to draw on. For a keeper comfortable with soft-water South Americans it is not an especially difficult cichlid — the expected biology is that of a straightforward, moderately-sized, substrate-spawning acara — but go in with eyes open about the lack of published husbandry records and be prepared to work empirically.
Conservation
The IUCN Red List assesses Aequidens mauesanus as Least Concern (assessed 7 November 2018, published 2022; assessed by the Instituto Chico Mendes de Conservação da Biodiversidade, ICMBio, and reviewed by P. Reis). The assessment treats the species as endemic to Brazil in the Maués and Arapiuns drainages, considers it naturally infrequent and not abundant, and identifies no significant threats that would currently put its population at risk — hence the Least Concern listing despite the small known range and a population trend recorded as unknown. The species carries no CITES listing and has no known use in trade. It is not recorded from any protected area, and the assessors flag both a need for further taxonomic and life-history research and the observation that Maués and Arapiuns specimens differ somewhat, which could bear on how the species is delimited. The broader backdrop is the accelerating pressure on Amazonian freshwaters — deforestation, dam-building, mining, and altered flood and flow regimes — which threatens the basin's narrow-range endemics generally; none of these is documented as a present danger to this particular cichlid, but a fish known from so few localities has little margin if its streams are degraded.