Taxonomy & naming
Sven O. Kullander described Aequidens epae in 1995 in the journal Ichthyological Exploration of Freshwaters, in a paper that erected three southern-Amazonian acaras at once: Aequidens gerciliae, Aequidens epae, and Aequidens michaeli (Kullander 1995: 158, fig. 10). The holotype — MZUSP 21979 at the Museu de Zoologia da Universidade de São Paulo — came from an igarapé, or forest creek, at Barreirinha in the Rio Tapajós drainage of Pará, Brazil. The name has stood unchallenged since. Eschmeyer's Catalog of Fishes lists it as valid with no synonyms, under the ZooBank registration 56F9303A-EAE0-4623-B5F6-1E6258856C3D. Eigenmann & Bray raised the genus Aequidens in 1894, originally as a subgenus of Astronotus, for acaras whose teeth are all of even size — the opposite of the enlarged front teeth of Astronotus. It sits in the subfamily Cichlinae and the tribe Cichlasomatini, the medium-sized Neotropical acaras. The genus has been pruned repeatedly over the decades, its species dispersed to Andinoacara, Bujurquina, Cleithracara, Krobia, Laetacara, and others; what remains is a tighter core of true Aequidens around the type species Aequidens tetramerus, and Aequidens epae belongs to that core.
Morphology
Aequidens epae is a small cichlid. The largest specimen in Kullander's original series measured 4.5 in standard length, so total length including the tail runs somewhat greater. The build is the familiar acara plan: a moderately deep, laterally compressed, oval body with a rounded snout and the genus's even dentition. Detailed colour notes are scarce in the public literature, but like its congeners it is broadly a brownish to greenish fish carrying the dark markings typical of the genus — the lateral and caudal-peduncle blotches acaras commonly show, plus iridescent flecking on the flanks and head. On sex differences the original description is unusually helpful for so obscure a fish: Kullander (1995:162) found no consistent external differences between the sexes, making Aequidens epae effectively monomorphic. His largest male measured about 4.5 in SL and his largest female about 4 in SL, so males may average a little larger — but there is no reliable colour or finnage cue for sexing, unlike the modest but visible dimorphism of many substrate-spawning Cichlasomatini.
Habitat
The species is endemic to Brazil and, as currently understood, restricted to the lower Rio Tapajós drainage in Pará, around Jacareacanga and São Luis (Kullander 1995, 2003). The IUCN notes possible but unconfirmed records farther upstream in the Tapajós basin and in Mato Grosso, plus questionable ones from the Floresta Nacional de Caxiuanã; all await confirmation of identity, so the firmly established range stays small. The Tapajós is one of the Amazon's classic clearwater rivers — low in suspended sediment, often transparent and turquoise, its water generally soft and running from acidic to near-neutral (reported pH for the system spans roughly the mid-4s to near-neutral). FishBase records the fish from clearwater streams over muddy, sandy, or rocky bottoms, and the IUCN assessment adds that it turns up in the main rivers and in tributary creeks, in still (lentic) water and in small tributaries alike. FishBase gives a preferred temperature of about 75–86 °F (75–86 °F), consistent with a warm lowland-to-shield clearwater fish. No in-situ chemistry measured at Aequidens epae collection sites has been published in the sources consulted.
Feeding
No published diet study or gut-content analysis exists for Aequidens epae in the sources consulted, so its trophic ecology has to be read from the genus. FishBase places it at an estimated trophic level near 3.2 — that of a small, invertebrate-leaning omnivore — inferred from its size and the diets of close relatives. True acaras of the Aequidens tetramerus group are generalist micro-predators: they forage over the bottom and through leaf litter, taking aquatic insect larvae, small crustaceans, and other benthic invertebrates along with incidental plant and detrital material, picking and sifting at the substrate as they go. As a small fish of clearwater creeks over muddy, sandy, and rocky bottoms, Aequidens epae almost certainly fills the same benthic-invertivore and omnivore role in its community — inference, though, rather than a documented observation.
Mating
The reproductive and social behaviour of Aequidens epae has not been described from the wild, and the species is essentially absent from the aquarium-breeding literature, so no species-specific account can honestly be given; what follows is the genus pattern. True Aequidens are monogamous, biparental cichlids. A male and female form a pair and jointly hold and defend a territory, with mutual courtship displays — lateral flaring, jaw-locking, and substrate cleaning — leading up to the spawn, and pairs generally grow more territorial and aggressive toward their own kind and other fish as spawning nears. Whether Aequidens epae departs at all from this widespread acara routine is simply unknown.
Breeding
Breeding has not been documented for Aequidens epae, in the wild or in captivity, in any source consulted, so this too rests on the genus. True Aequidens are open, biparental substrate spawners. The pair cleans a flat surface — a stone, a submerged root, or a leaf — and the female lays a sheet of adhesive eggs that both parents then guard and fan, the male usually patrolling the wider territory while the female tends the clutch most closely. After hatching, the wrigglers are typically moved to one or more pre-dug pits; once free-swimming, the fry are shepherded and defended by both parents for a while before independence. Clutch sizes in small Aequidens run from roughly a hundred to a few hundred eggs. For Aequidens epae specifically, none of these particulars — clutch size, spawning trigger, fry development — has been recorded, so they stand here only as the expected genus pattern.
In the aquarium
Aequidens epae is, in practice, not kept in the hobby: there is no captive-breeding record, no importer list, and no established husbandry tradition for it. What follows is an informed projection from the species' documented biology and from the well-studied care of its closest analogues, the true acaras of the Aequidens tetramerus group — not a first-hand report.
Tank: at a confirmed maximum of 4.5 in standard length (perhaps 5–5.5 in total length for a large male), this is a small cichlid. A pair could reasonably live in a 90–120-litre (24–32 US gallon) footprint, though length matters more than volume — 35.5 in or longer — because territory here is measured across the bottom in two dimensions. Use fine sand or small-particle gravel, since acaras of this group are habitual bottom-sifters that take mouthfuls of substrate and sieve them for invertebrates and will dig spawning pits; add a flat stone or broad piece of driftwood as a potential spawning site. Modest planting and some leaf litter (dried catappa or oak leaves, renewed every few weeks) earn their place, acidifying and faintly tinting the water in an echo of the lower Tapajós creek floor.
Water: track the Tapajós system — soft (GH 1–6 °dH, KH ideally below 4), pH 5.5–6.8, temperature 77–84 °F. Forest creeks in the native system run as low as pH ~4.5, but the moderate soft-acid end is the safe captive target; avoid near-neutral-to-hard, alkaline water. Keep filtration gentle — sponge or slow-turnover canister — both to hold quality in a low-buffering system and because clearwater acaras are not built for strong flow, and change 20–25% of the water weekly to manage nitrate.
Diet: weight it toward meaty foods. Frozen and live bloodworms, Daphnia, and brine shrimp are all taken readily, with quality sinking micropellets as a base. Like all true acaras it is likely an efficient, opportunistic forager and no fussy eater, but keep the menu varied to mirror the invertebrate-rich creeks it comes from.
Tankmates: expect the genus temperament — generally peaceful toward dissimilar fish, but meaningfully territorial as a pair bonds and especially after spawning. In a 35.5 in tank, keep a single pair alone or with peaceful, similarly-sized dither fish that stay off the bottom; small tetras or pencilfish from soft South American waters suit it and draw the pair into the open. Adding other substrate-holding cichlids to a small tank invites chronic conflict.
Common mistakes: sexing is unusually hard, because Kullander's description reports no consistent external sex differences — males average slightly larger, but that is unreliable in a small group of unknown-age fish. The standard route for monomorphic acaras is to buy five or six juveniles, grow them up together, and let a pair form on its own; the spare fish can then be removed. Pairing comes with the mutual lateral-flaring and jaw-locking displays common to the genus.
None of this makes Aequidens epae a casual community fish, but the difficulty is mainly availability and water-quality maintenance rather than temperament or fragility. Any experienced aquarist comfortable with soft, acidic, low-KH systems — and willing to track down a fish with essentially no trade presence — would have this quiet corner of the acara world largely to themselves.
Conservation
The IUCN Red List assesses Aequidens epae as Least Concern (assessed 7 November 2018, published 2022; the assessment by the Instituto Chico Mendes de Conservação da Biodiversidade, ICMBio, reviewed by P. Reis). The fish is described as uncommon and not abundant, with an unknown population trend, and it is not used commercially. The reasoning is pragmatic rather than reassuring: although it has a small, apparently restricted range on the lower Tapajós — where two hydroelectric dams, the UHEs São Luiz and Jatobal, are planned — its presence in lentic waters and small tributaries suggests it may tolerate the impoundment effects of damming, and on that basis it was not listed as threatened. It carries no CITES listing. The backdrop is the rapid transformation of the Tapajós basin, a long-clearwater river facing a cascade of proposed dams alongside the wider Amazonian pressures of deforestation, mining, and altered flow. The assessors explicitly flagged the species' taxonomy, distribution, and basic life history as research needs — a fair measure of how little is firmly known about this cichlid.