Taxonomy & naming
Uwe Römer and Frank Warzel described Apistogramma arua in 1998 in the journal aqua (Journal of Ichthyology and Aquatic Biology, vol. 3, no. 2, pp. 45–54), under the title 'Apistogramma arua sp. n. … a new species of dwarf cichlid from the Rio Arapiuns system, Pará State, Brazil.' Their type series was small — four specimens — collected in a little igarapé (forest creek) near the village of Aruã, in the lower course of the Rio Aruã. Römer later repeated the description in German, with more photographs, in the first volume of his Cichlid Atlas (Mergus, 2000/2001). The naming act is registered in ZooBank, and both the Catalog of Fishes and FishBase carry Apistogramma arua as a valid species with no synonyms; the IUCN's Brazilian assessors likewise record no taxonomic problems and no synonymy.
Apistogramma is the largest genus of Neotropical cichlids — well over a hundred described species, and many more undescribed — and sits in the tribe Geophagini (subfamily Geophaginae, Cichlinae). The standard working framework within the genus is Mike Wise's species-group classification, maintained on apistogramma.com. Wise does not place Apistogramma arua in any well-defined species-group; instead he treats it as one of a small set of 'relic ancestral' members of the trifasciata lineage, alongside Apistogramma salpinction, an undescribed Apistogramma cf. salpinction from the Rio Itapeaçu, and Apistogramma sp. 'Doppelfleck'. These fish live on the far eastern edge of the lineage's range, in the Amazon basin between the Rio Madeira and the Rio Tapajós, and Apistogramma arua is singled out as the best-studied of them — broadly resembling the sort of ancestral fish the whole trifasciata lineage may have sprung from. It also mixes group traits: a net-like reticulate flank pattern reminiscent of the brevis group over black markings roughly like those of the trifasciata group. The honest summary is a species that is taxonomically stable but phylogenetically lonely, a peripheral relic whose exact place in Apistogramma is still unresolved.
One caution on the type material: the original description sits behind a paywall we could not open, so the holotype's museum register number is left out here rather than guessed at. The type locality, and a second, later lot in the Museu Paraense Emílio Goeldi (MPEG), are confirmed by the IUCN assessment.
Morphology
Apistogramma arua is small even by Apistogramma standards. FishBase, following the original description, gives a maximum of about 1.5 in standard length for males and roughly 1.5 in SL for females; Mike Wise classes it among 'medium to moderately large' apistos at around 2.5–3 in total length, a figure that counts the long-finned tail and reflects fully grown aquarium males. The body is the usual fusiform, slightly compressed dwarf-cichlid shape. Meristics reported by FishBase are 15 dorsal spines with 6–7 soft rays and 3 anal spines with 6–7 soft rays, all consistent with the genus.
Its most distinctive feature is the scale pattern: the rear edges of the flank scales are darkened, so the side of the fish carries a fine, net-like mesh — the trait Römer and Warzel leaned on in diagnosis, and the one Wise compares to the brevis group. Over that mesh runs the standard Apistogramma set of dark marks — a lateral band and a series of vertical bars, roughly trifasciata-like, fading toward a marking at the tail base. Like most of the genus, the fish is sexually dimorphic: males grow larger, with more extended, pointed dorsal and caudal fins and stronger colour, while females stay smaller and plainer outside breeding. Beyond that, live coloration is poorly documented in the reachable literature, and the few hobby photographs that exist — such as one of Oliver Lucanus's aquarium male — are the main visual reference.
Habitat
Apistogramma arua is endemic to Brazil and, as far as the formal record goes, to a single small drainage. It is restricted to the sub-basin of the Rio Aruã, a tributary within the Rio Arapiuns system on the lower Amazon in Pará State — the Arapiuns being a major clearwater affluent that joins the Rio Tapajós near Santarém. The type locality is a small right-bank igarapé of the Rio Aruã, roughly 2 mi upstream of the village of Aruã and, crucially, above the Aruã waterfall; the species has not been found below the falls, which appear to bound its range. The IUCN's Brazilian assessors put the extent of occurrence at just 3,171 mi² for the whole Aruã sub-basin above the cachoeira, and stress that the fish is formally known from only two records: the type series and a single further lot in the MPEG collection. A handful of other records are informal, most likely from illegal ornamental collecting.
Ecologically it is a fish of terra firme (upland) forest streams near várzea (floodplain) areas. FishBase, citing the original description, reports that it keeps close to the substrate, above the leaf litter, in calm, shallow water — the classic micro-habitat of small Apistogramma, where drifted leaves and submerged wood make the dark crevices these cichlids use for cover and spawning. The IUCN classes its habitat as tropical/subtropical swamp forest plus permanent rivers, streams and creeks. No published in-situ water chemistry from the type locality was available to us, but the setting — a small Amazon-tributary forest creek over leaf litter — implies the soft, acidic, low-conductivity water typical of such biotopes, and aquarists keep the species in soft, acidic conditions (commonly cited hobby ranges are roughly 72–84 °F and pH around 5–7). Those temperature and pH figures are aquarium-husbandry guidance rather than measured field data, and should be read that way.
Feeding
No dedicated diet study exists for Apistogramma arua, so its feeding ecology has to be read off the genus and its micro-habitat. Apistogramma are micro-predators — small benthic carnivores and omnivores that pick tiny invertebrates (insect larvae, micro-crustaceans, worms and other meiofauna) from the substrate, leaf litter and biofilm, often taking mouthfuls of fine sand or detritus and sorting the edible parts out. FishBase puts Apistogramma arua at an estimated trophic level near 3.3, in keeping with that invertebrate diet, though the value is modelled from relatives rather than measured. The fish's reported habit of hugging the bottom over leaf litter in calm water is exactly where such prey gathers. In the aquarium it takes the usual dwarf-cichlid fare of small live and frozen foods — Artemia, micro-worms, daphnia, cyclops. Its likely community role is that of a small leaf-litter invertivore, but that is reasoned inference, not documented fact.
Mating
Courtship in Apistogramma arua has essentially never been described in the reachable literature, so what follows is the well-established genus pattern, offered as the working expectation rather than confirmed for this species. Most Apistogramma are polygynous or facultatively harem-forming cave spawners: a dominant male holds and patrols a large territory that overlaps the smaller territories of one or more females, each guarding her own spawning cavity. Pair bonds are brief and built around the brood, not a lasting partnership. Females in spawning condition typically turn vivid yellow with intensified black markings, actively soliciting the male and defending their chosen cave, while males display with spread fins and lateral posturing and jostle rival males for territory and access. Whether Apistogramma arua is strictly harem-polygynous or pairs more monogamously in the wild has not, as far as we know, been recorded — and its small, isolated populations in narrow forest creeks would be worth studying on exactly that question.
Breeding
Apistogramma arua is a cave-spawning, female-tended substrate brooder — the one part of its life history with a direct, if brief, statement in the literature. FishBase, citing reproductive data for the species, reports that the eggs are laid on the ceiling of a cave and tended by the female, who cares for both eggs and larvae. That fits the general Apistogramma mode: the female picks and cleans a hidden cavity — a crevice under wood, a hollow in the leaf litter, or in the aquarium a cave or inverted vessel — attaches a fairly small clutch of adhesive eggs to its ceiling, and does essentially all of the direct brood care, fanning and guarding the eggs and then leading a tight school of free-swimming fry, while the male defends the wider territory. Figures specific to Apistogramma arua — clutch size, egg and larval timing, how long the fry are guarded — were not in the sources we could reach; for the genus, clutches commonly run from a few dozen to a couple of hundred eggs depending on female size, hatch within a few days, and yield free-swimming fry about a week after spawning. Soft, warm, acidic water is generally held to matter for egg development across the genus, and brood sex ratio is temperature- and pH-sensitive in some species — but neither point has been shown specifically for Apistogramma arua. It stays a thinly documented fish in captivity, kept and occasionally bred by specialists rather than widely traded.
In the aquarium
Apistogramma arua is one of the rarest apistos in the hobby, surfacing now and then through specialist importers and almost always as wild-caught fish. Finding it at all is the first hurdle, and it is no fish for a beginner.
Tank: A single pair, or a male with one female, is comfortable in a tank of about 23.5–31.5 in (roughly 10–15 US gal) — enough for the female to hold territory away from the male, and for the male to patrol without cornering her. Favour long, low footprints over tall tanks; this is a bottom fish whose world is the lowest 4–6 in of the water column. Use fine, pale sand it can sift for food, and — most important of all — a deep bed of dried Indian almond (Catappa) or oak leaves, replaced gradually as they break down: the litter mildly acidifies the water, grows the biofilm micro-fauna the fish graze, and gives the female constant cover. Add crevice-formers as spawning caves (smooth driftwood, coconut halves, short clay tubes) and edge planting (Java fern, Bucephalandra, mosses, small Anubias), leaving the centre open to view.
Water: Matching the clearwater forest creek of the Rio Aruã is the single biggest factor in keeping the fish well. Aim soft and acidic — pH around 5.5–6.5, conductivity below about 100 µS/cm (total hardness under 5° dH). FishBase's documented envelope of pH 5.0–7.0 at 72–84 °F is the outer tolerance, not the target; treat the upper ends as limits. Wild imports out of very soft blackwater struggle if moved abruptly into harder or more alkaline water, so acclimate them slowly over several days. RO water lightly remineralised, or an RO-and-rainwater blend, is the practical route unless your tap water is already very soft. Temperatures around 79–82 °F seem to favour spawning, and flow should be gentle — a sponge filter or a diffused low-flow outlet is plenty.
Feeding: The fish takes small live and frozen foods readily, and these should form the bulk of the diet — freshly hatched Artemia nauplii, micro-worms (Panagrellus), cyclops, daphnia, frozen bloodworm — with live or frozen Artemia and small chironomid larvae especially useful for conditioning breeders. A fine dried food can supplement but should not be the staple.
Tankmates: A species-only tank needs none. If you want dither fish to draw the cichlids out, choose calm soft-water, upper-water swimmers — small Hyphessobrycon or Moenkhausia tetras, or Nannostomus pencilfish. Avoid anything pushy enough to strip food from the bottom, and avoid other cichlids and large bottom catfish entirely. Never keep a second Apistogramma species alongside it; intra-genus rivalry, especially between males, tends to end with a dead fish.
Breeding: Possible, if achieved by few. The trigger is very soft, slightly acidic water, a temperature at or a little above mid-range, and a suitable cave. The female chooses and cleans a crevice or coconut-shell cave, draws the male in briefly to spawn, then becomes sole guardian and should be left to it; the male can be moved out if he stresses her, though in a roomy, well-decorated tank he usually just keeps to his own end. Fry become free-swimming at roughly 7–10 days and take infusoria and newly hatched Artemia nauplii; raise them in the same very soft, acidic water, since hardness swings are badly tolerated at that stage.
Common mistakes: The species is rated difficult mainly for its water-chemistry demands and the scarcity of tank-bred stock. The usual errors with sensitive eastern-Amazon apistos are water kept too hard or alkaline (a pH consistently above 6.8–7.0 brings poor colour, no spawning and chronic low-grade stress), too little cover so the male harasses the female relentlessly, and overfeeding dry food in a small tank without the biological capacity to handle the waste. Source legally imported fish patiently, commit to genuinely soft water, and the rest follows.
Conservation
The IUCN Red List assesses Apistogramma arua as Data Deficient (DD). The global assessment was made by Brazil's Instituto Chico Mendes de Conservação da Biodiversidade (ICMBio) for the national Red Book of threatened fauna, assessed on 7 November 2018, published in 2022, and reviewed by P. Reis. The Data Deficient rating reflects how little is known: the species is endemic to Brazil and formally recorded from just two collections, with a calculated extent of occurrence of only 3,171 mi² confined to the Rio Aruã sub-basin above the Aruã waterfall in Pará. Its population is described as uncommon, with an unknown trend and no quantitative data.
The assessors judged the region itself to be relatively well preserved, with no evident habitat degradation threatening the fish; part of its range lies within the Maró Indigenous Land and within environmental protection areas. The one flagged pressure is the aquarium trade: Apistogramma arua is occasionally taken illegally for ornamental export in small, collector-directed quantities that cannot be measured, and given its tiny range the assessors urge that such collecting be curbed. The species is not listed under CITES and is not covered by Brazil's interministerial ornamental-fish harvest regulation, so the recommended actions are precautionary — legal protection and enforcement, research into the impact of ornamental collection, and monitoring of harvest. In short, this is a naturally rare, narrowly endemic dwarf cichlid whose status is unresolved less because it is known to be declining than because it has barely been studied at all.