Taxonomy & naming
The Swedish ichthyologist Sven O. Kullander described Aequidens plagiozonatus in 1984, in the fifth part of his series on the cichlids of the La Plata basin (Zoologica Scripta, vol. 13), working from 49 specimens taken in the northern Paraguay River system. The holotype (MZUSP 28232) came from internal lakes of the Piquiri-Itiquira system at Itiquira, Mato Grosso State, Brazil, with paratypes lodged in São Paulo (MZUSP), Philadelphia (ANSP 53925), Chicago (FMNH 70476), and Stockholm (NRM 18009). The genus Aequidens, erected by Eigenmann and Bray in 1894, sits in the subfamily Cichlinae and the well-supported tribe Cichlasomatini — the Neotropical 'acaras,' a lineage distinct from the true eartheaters (Geophagini) despite a broadly similar build. About seventeen valid species make up the genus, most described from the Amazon-Orinoco-Guianas region; Aequidens plagiozonatus is the lone Aequidens of the La Plata drainage, and Kullander's original description flagged it as the only species of the genus then known from that basin. In the rio Guaporé it shares water with Aequidens viridis (Heckel, 1840). The name has stood as valid since 1984 with no recognized synonyms (Eschmeyer's Catalog of Fishes; Kullander in Reis et al. 2003). Records from drainages outside the natural La Plata/Guaporé range, including the upper rio Araguaia, are read as recent dispersals through headwater stream capture rather than part of the original distribution.
Morphology
This is a small cichlid. FishBase gives a maximum of about 4 in standard length (snout to tail base), citing Kullander's 2003 checklist — modest even by the standards of the genus. The build is the typical acara shape: laterally compressed and moderately deep, with a rounded snout and a large eye, dressed in an understated brassy to greyish-olive. The diagnostic mark, and the source of the name, is the series of dark flank bars that slant obliquely forward rather than running straight up and down; Kullander singled it out to separate the fish from its bar-flanked relatives. A dark blotch usually sits on the cheek, paired with a midlateral spot or an interrupted lateral band, and in well-conditioned fish the unpaired fins carry faint spotting or banding. Sexual dimorphism is slight and poorly described. As in most substrate-spawning acaras, males tend to grow somewhat larger and develop more produced, pointed dorsal and anal fin tips, while ripe females are rounder-bellied — but no detailed morphometric account exists for the species. The Brazilian name acará-pixuna, roughly 'dark acara' in Tupí-derived usage, reflects that generally muted, dusky look.
Habitat
Aequidens plagiozonatus is a freshwater, benthopelagic fish of tropical lowland South America. Its native range spans the upper Paraguay River drainage of the La Plata (Paraná-Paraguay) basin and the upper Guaporé drainage of the Amazon basin, across Brazil — notably Mato Grosso and Mato Grosso do Sul — and Bolivia. The IUCN range statement lists Bolivia and Brazil, with Brazilian occurrences in Mato Grosso, Mato Grosso do Sul, Goiás, Paraná, Rondônia, and São Paulo, and an estimated extent of occurrence of about 1.15 million square kilometres. It is a characteristic fish of the seasonally flooded Pantanal and similar lowland country, recorded from streams, rivers, and lakes (Casatti et al. 2010) and from floodplain lagoons. Surveys tend to find it in the shallower, more open water: a Northern Pantanal study of temporary ponds recorded it alongside Hoplias malabaricus and Synbranchus marmoratus (Tondato et al. 2013), and it has turned up in floodplain lagoons of the Cuiabá River and in urban tributaries of the upper Paraguay basin. Species-specific water chemistry is sparse. One Paraguay-basin collection reports very soft, near-neutral water (pH about 7.3 at a conductivity of only ~8 µS/cm), but no preferred temperature or pH range is established — Brazil's Instituto Hórus risk assessment found no evidence for the species' temperature, pH, or depth tolerances, and FishBase carries only 'freshwater, benthopelagic, tropical.' The figures usually quoted (roughly 72–82 °F, pH about 6–7.5) are therefore the soft, warm, near-neutral conditions general to lowland floodplain habitat rather than measured values for the fish. FishBase publishes no depth range, but this is a fish of shallow lakes, ponds, and stream margins: in the Tondato et al. (2013) survey it occupied the shallower parts of temporary ponds no more than about 7 ft deep, so a working band of roughly 0–7 ft fits, with the fish favouring the shallow end.
Feeding
FishBase estimates a trophic level of about 3.2, which places Aequidens plagiozonatus as a small carnivore/omnivore rather than a strict predator or a pure detritivore. Reports describe broad omnivory: Brazil's Instituto Hórus calls it omnivorous, taking plant matter, algae, microcrustaceans, and other fishes, while its staple is small invertebrates — small crustaceans and aquatic insect larvae picked from the bottom and from among submerged plants. That fits the foraging style of small acaras, which probe the substrate and leaf litter rather than chasing open-water prey. A study of cichlid assemblages in floodplain lagoons of the Cuiabá River (Pantanal) recorded the species as present but among the least abundant cichlids there. In its shallow, vegetated waters it holds the lower-middle tier of the floodplain food web, a small benthic invertivore-omnivore; dedicated gut-content data for the species remain sparse in the accessible literature.
Mating
No field study of courtship and pair formation specific to Aequidens plagiozonatus was located, so its mating behaviour is inferred from its place in the tribe Cichlasomatini and from well-documented relatives. Acaras of this group are typically monogamous, forming a pair bond and jointly establishing and defending a spawning territory in the run-up to breeding. In Aequidens paraguayensis, a close La Plata-basin relative, both partners cooperate from courtship through brood care — the biparental pattern Aequidens plagiozonatus is expected to share. Away from breeding it is a peaceful fish, and pre-spawning aggression amounts to territorial sparring around the chosen site rather than the heavy combat of larger cichlids. The species reaches the aquarium trade with a reputation as undemanding and good-natured, but published accounts of its reproductive behaviour in captivity are thin.
Breeding
Among the sources consulted, Aequidens plagiozonatus has never been the subject of a published wild or captive breeding study, so its reproductive biology is read from genus- and tribe-level pattern. Aequidens are biparental substrate (open) spawners: a pair cleans a firm surface — a flat stone, or a submerged leaf, root, or log — and the female lays an adhesive clutch on it, after which both parents guard and fan the eggs, then escort and defend the free-swimming fry. The congener Aequidens paraguayensis has been documented spawning on a leaf lifted from the litter and giving biparental care, with the parents able to move the brood; that is the most likely template here. FishBase's model-based estimate of fecundity under about 1,000 eggs matches the modest clutch expected of a 10-cm substrate-spawning acara, but no measured clutch count, egg size, or developmental timetable exists for the species — those particulars are best treated as undocumented rather than assumed.
In the aquarium
Aequidens plagiozonatus is an undemanding, genuinely peaceful cichlid — a good fit for intermediate keepers who want a small, well-behaved acara without the husbandry puzzles of more specialised Neotropical cichlids. Published aquarium accounts are sparse; it draws less trade attention than relatives such as Aequidens pulcher or Aequidens rivulatus, so the guidance here is assembled from its natural history and the better-documented care of close relatives.
Tank: A single pair does well in 25 US gal (roughly an 80 × 14 in footprint); a colony or a busy community wants 40 US gal or more. This is a shallow-water benthic fish, so lateral swimming room and a long footprint matter more than depth. Furnish it like floodplain shallows — fine to medium sand it can sift without scraping its snout, driftwood and root tangles for cover, and clumps of hardy plants (Vallisneria, Echinodorus, floating cover all work) — but leave open sand rather than a wall-to-wall thicket.
Water: The natural habitat sets the target — 72–82 °F, pH 6.0–7.5, soft to moderately hard. One in-situ Paraguay-basin record gives pH ≈ 7.3 at very low conductivity (~8 µS/cm), in keeping with the dilute Pantanal floodplain. Hard, alkaline tap water (above roughly 12 °dH or pH 8) sits outside its documented habitat and should be softened or blended with RO for long-term keeping and spawning. Good filtration and regular water changes matter; like most cichlids it handles accumulated nitrate poorly. Strong current is unnecessary — it comes from still to slow water.
Tankmates: One of the easier acaras for a community. Aggression outside spawning is minimal, and even breeding pairs rarely threaten fish that keep clear of the nest. Good companions include medium-sized characins (Hyphessobrycon spp., Myleus, silver dollars), peaceful barbs, Corydoras on the bottom, and calm upper-level tetras. Avoid very small fish (below ~1 in) that could look like food, and large or boisterous cichlids — this is not a fighter. A Pantanal or Guaporé biotope also works: the species co-occurs in nature with Hoplias malabaricus, Synbranchus marmoratus, and various small characins, though any predator must be chosen carefully given its modest size.
Feeding: An unselective benthic omnivore that takes most standard foods. Base the diet on quality sinking pellets or granules, then add frozen or live bloodworm, Daphnia, and brine shrimp, plus the occasional vegetable (blanched spinach, spirulina wafers). It picks at the substrate constantly, so dropping food onto the sand rather than at the surface plays to that habit and makes sure it gets its share.
Breeding: Given the right water, a compatible pair usually spawns without special prompting, cleaning a flat surface — a smooth stone, broad leaf, or piece of slate — for the female's adhesive clutch. Both parents then guard and fan the eggs and escort the wriggling larvae, which are free-swimming within roughly a week (temperature-dependent) and take newly hatched brine shrimp and fine powdered food from the start. Soft, slightly acidic water (pH 6.2–6.8, conductivity under 100 µS/cm) and a nudge to the warm end of the range (79–82 °F) tend to trigger spawning. Parental aggression toward tankmates climbs once fry appear, so either remove the other fish or give the pair a dedicated tank.
Common mistakes: Difficulty is easy to intermediate. The usual error is hard, high-pH tap water left unadjusted, which suppresses spawning and wears the fish down over months. Understocking is the second: a dominant fish, usually the male, then harasses its partner, so give plenty of visual breaks and let a pair form on its own rather than forcing two fish together. The third is overfeeding and the nitrate that follows — small frequent water changes beat large infrequent ones.
Conservation
The IUCN Red List assesses Aequidens plagiozonatus as Least Concern (assessed 29 October 2020 by R.G. Frederico, reviewed by T.J. Lyons, published 2023). The reasoning is straightforward: despite limited biological information, the species has a very wide distribution — an extent of occurrence of roughly 1.15 million square kilometres across the Amazon and Paraná basins in Bolivia and Brazil — and appears common, with no known threats currently affecting it. The population trend is listed as unknown, and it carries no CITES listing (Not Evaluated). It is used as an ornamental fish and harvested for the aquarium trade at the national level within Brazil, regulated under Brazilian ornamental-fishery rules (IBAMA 2012), but this is not flagged as a population-level concern. As a small, adaptable generalist of widespread floodplain and stream habitats, it is far less exposed than the many narrow-range endemics of the same basins. The broader pressures on its waters — degradation of Pantanal and Cerrado wetlands through agriculture, altered flood regimes, pollution — shape its long-term outlook, but none is documented as a present danger to the species.