Taxonomy & naming
Sven O. Kullander, the Swedish ichthyologist, and Efrem J. G. Ferreira, his Brazilian co-author, named this species in 'A new Aequidens species from the Rio Trombetas, Brasil, and redescription of Aequidens pallidus (Teleostei, Cichlidae)', published in Zoologica Scripta (volume 19, pages 425–433). The paper appeared in the 1990 volume, but the formal nomenclatural act is consistently dated to 1991, so the fish is universally cited as Aequidens tubicen Kullander & Ferreira, 1991. It was described in its current genus — an original combination — and carries no synonyms. Within the Neotropical subfamily Cichlinae it belongs to the tribe Cichlasomatini. Kullander and Ferreira showed that Aequidens tubicen and the more widespread Aequidens pallidus form a closely allied species pair, united by a high vertebral count (14 + 13 = 27) and three shared colour-pattern traits: a lateral band set high on the flank, a midlateral spot placed far back, and a caudal spot preceded by a large pale blotch. Aequidens itself is a difficult genus. Molecular work on the Cichlasomatini by Musilová and colleagues (2008) found it to be paraphyletic — an unnatural grouping in which some nominal Aequidens sit nearer to genera such as Krobia than to one another — so the genus as currently drawn is provisional, and a future revision may move some of its species. The type series came from the Trombetas and Mapuera rivers a little above Cachoeira Porteira, Pará.
Morphology
This is a moderately sized acara. The largest specimen in the type series measured 4.5 in (about 4.5 in) in standard length, and FishBase gives the same figure as the maximum. The body is the typical fusiform, laterally compressed acara shape. Kullander and Ferreira diagnosed it by a triserial predorsal scale pattern, a fairly long pectoral fin (32.5–37.7% of standard length), and 25–26 scales in the E1 (upper lateral) row. Its one feature unique within the genus is an irregularly rounded blackish spot at the inner angle of the preopercle and the neighbouring cheek. The flank markings are nearly as diagnostic: a dark lateral band runs from the head to the upper edge of the caudal peduncle, broken by a large, elongate midlateral spot and splitting into two or three elongate spots toward the rear. That midlateral spot sits against — or just below — a broad dark bar climbing to the base of the dorsal fin, so spot and bar together form a strongly oblique, backward-slanting mark. As in its close relative Aequidens pallidus, a wide pale spot sits just ahead of the caudal-fin-base spot. The original account described no sexual dimorphism, and no reliable account of how the sexes differ is available for this species.
Habitat
Aequidens tubicen is endemic to Brazil and, as far as is known, to a single drainage: the Rio Trombetas and its tributary the Rio Mapuera, just upstream of the Cachoeira Porteira rapids in Pará. The Trombetas drains the southern slopes of the Guiana Shield into the lower Amazon, placing the species in the Amazonas Guiana Shield freshwater ecoregion. The IUCN assessment expects it to range more widely through the Trombetas above Cachoeira Porteira than the few collection sites suggest, though documented records stay confined to that stretch. FishBase sums up its preference as small, fast-running creeks and rapids — a current-loving habitat that sets it apart from the many acaras of still backwaters and floodplain lakes. The Trombetas is a Guiana Shield clearwater river draining ancient bedrock, which by basin convention means soft, low-ionic, near-neutral-to-acidic, warm water. Physical-chemical surveys of the Amazon's Guiana Shield left-bank tributaries report pH around 6.0–7.0 and low conductivity, with lowland temperatures typically in the mid-20s to about 86 °F. No in-situ chemistry has been published for this species' Trombetas and Mapuera sites, and FishBase carries no temperature, pH or depth range for it, so the figures here are habitat-typical for the drainage rather than measurements taken where the fish was caught.
Feeding
FishBase reports that Aequidens tubicen eats fish, insects and plant detritus at an estimated trophic level of about 3.2 — a small generalist that leans carnivorous rather than a strict detritivore or a dedicated piscivore. That diet fits its biotope. In fast creeks and rapids, drifting insects and small fishes are plentiful, and detritus gathers in the slacker pockets among the stones. No direct gut-content study of this species appears to have been published; the picture is extrapolated from the genus and from the closely related Aequidens pallidus, whose Rio Negro diet was reported by Goulding and colleagues as chiefly fish and detritus. In its rapids community, Aequidens tubicen would work as a small benthic-to-midwater forager, taking invertebrates and small prey from the current and the substrate.
Mating
No courtship, pair formation or social behaviour has been documented for Aequidens tubicen, in the wild or in captivity — it is rarely collected and has essentially never entered the aquarium hobby. What can be said comes from the genus. Aequidens are biparental cichlids that form monogamous pairs and hold a territory around a spawning site, both partners defending it. Pair bonds in the genus typically form through mutual display, often with sharp aggression toward their own kind and other fishes. Whether Aequidens tubicen follows that pattern in its rapids habitat is a reasonable inference rather than an observation, and should be read as one.
Breeding
Breeding has not been documented for Aequidens tubicen, in the wild or in aquaria. By the standard of its genus, it is expected to be a biparental, open-substrate spawner: Aequidens lay an adhesive clutch on a cleaned hard surface — a stone, root or leaf — which the female tends and fans while the male guards the surrounding territory, after which both parents shepherd and defend the free-swimming fry. Clutch size, spawning triggers, egg and larval development times, and the duration of parental care are all unrecorded for this species. Given how little it is kept, any specific breeding account should be treated as absent until a documented spawning is reported.
In the aquarium
Aequidens tubicen is, in plain terms, not an aquarium fish: it has never established itself in the hobby and is absent from the commercial trade. Every statement below is therefore extrapolated from the biology of the species and from well-documented acara relatives, not from accumulated keeper experience — read it with that caveat.
Tank: a fish reaching roughly 5–5.5 in total length (the type-series maximum of 4.5 in SL corresponds to perhaps 5 in TL) needs room for a territory. A footprint of at least 100 × 15.5 in — the conventional 'medium' cichlid minimum for an acara this size — is the floor, and more length is always better. Because the recorded habitat is shallow, fast-running creeks and rapids, aim for strong, well-oxygenated flow from a directional powerhead or sump return and avoid stagnant conditions. Filtration must be efficient; a cichlid that evolved in moving water has little tolerance for accumulated nitrate.
Water: mirror the Trombetas basin — a Guiana Shield clearwater river that is soft, low in dissolved salts and mildly acidic to neutral. Target pH 6.0–7.0, a general hardness under 10 °dH, and 75–82 °F; the lower end of the natural 75–86 °F band is safer for long-term health than sustained highs. Reproductively inactive fish will tolerate moderately hard, buffered tap water if softening is impractical, but soft, slightly acidic water closer to the natural chemistry is the honest recommendation.
Decor: follow the biotope — fine sand with smooth river pebbles and cobbles, driftwood roots wedged among the stones, and a scattering of dried leaf litter in the calmer corners. Dense planting is not called for, though robust, current-tolerant plants such as Microsorum and Anubias anchored to hard furnishings do no harm and add surface area. The fish will claim a corner or cave, so break up the sight-lines and offer a few potential spawn sites.
Tankmates: with the species essentially unrepresented in captivity, compatibility data are absent, so the general acara rule applies — house it with similarly sized, non-aggressive South American cichlids, or robust South American tetras and loricariids. Aggression between males is likely to match relatives such as Aequidens diadema and Andinoacara pulcher, where males will not tolerate one another at close quarters, so a single pair in a well-decorated tank is the safest bet; skip large, fin-nipping or very territorial tankmates.
Difficulty sits between intermediate and challenging — not because the fish is delicate, but because obtaining specimens is the real obstacle, and because a rapids-adapted cichlid stays in condition only with steady attention to flow and water quality. Anyone who does get hold of it would serve the hobby by attempting a documented spawning and reporting the result; nothing is on record.
Conservation
The IUCN Red List assesses Aequidens tubicen as Least Concern (assessed 7 November 2018 by the Instituto Chico Mendes de Conservação da Biodiversidade, ICMBio; reviewed by P. Reis; published 2022). The assessment treats the species as endemic to the Rio Trombetas basin and its tributaries above Cachoeira Porteira in Pará, and notes that although it is naturally infrequent and low in abundance, no significant threats to its population have been identified — hence the Least Concern listing. It is recorded in use as an aquarium and display animal in the ornamental trade, but not at a level flagged as a population-level concern, and it carries no CITES listing. Much of its known range lies within or beside protected areas of the Trombetas region. As with any narrow-range Amazonian endemic, the broader pressures on the basin — damming, deforestation, mining and altered flow — form the backdrop against which its long-term outlook should be read, even if none is documented as a present danger to this fish.