Taxonomy & naming
Geophagus pyrineusi was described in 2022 by Gabriel C. Deprá, Willian M. Ohara and Hugmar Pains Silva in the journal Zootaxa (vol. 5162, no. 1, pp. 37–53), in a paper devoted entirely to this one fish. The holotype, an adult of about 3.5 in standard length catalogued as MZUSP 119715 in the Museu de Zoologia da Universidade de São Paulo, was collected in the rio Cristalino, a tributary of the rio Teles Pires, near Alta Floresta in Mato Grosso State, Brazil (roughly 9°36′S, 55°56′W, about 797 ft elevation). Paratypes are distributed across several Brazilian collections, including the INPA in Manaus and the MNRJ in Rio de Janeiro. Eschmeyer's Catalog of Fishes lists the name as valid. The genus Geophagus, erected by Johann Jakob Heckel in 1840, is the core of the Neotropical eartheater radiation (subfamily Cichlinae, tribe Geophagini); modern usage restricts it to the so-called Geophagus sensu stricto, of which the surinamensis group — the lineage that includes pyrineusi — is the dominant Amazonian assemblage. Within that group the authors found pyrineusi most similar to Geophagus argyrostictus, Geophagus gottwaldi and a handful of other species, but separated from all congeners by its colour pattern. One point of nomenclatural housekeeping is worth flagging for hobbyists: this fish is NOT the popular aquarium 'Tapajós red head' or 'orange head' eartheater, which was given the name Geophagus pyrocephalus in a separate 2022 paper. The two were described almost simultaneously, both from the Tapajós drainage, and the similar epithets are easy to confuse — but they are distinct species, and pyrineusi is the more cryptic, rarely seen of the pair.
Morphology
Geophagus pyrineusi is a medium-sized eartheater. The type series tops out at about 4.5 in standard length (roughly 4.5 in SL, not counting the tail fin), so even large adults are modest by the standards of giants like Geophagus proximus or Geophagus altifrons. The body is the typical Geophagus shape — moderately deep, laterally compressed, with a steep forehead and the protrusible, downturned mouth of a substrate-sifter. Its diagnostic features are all about pigment. Most surinamensis-group eartheaters carry a busy caudal fin marked with spots, vermiculations, bands or stripes; pyrineusi, uniquely among its congeners, has a caudal fin essentially free of any discrete pattern, and its dorsal and caudal fins lack the interradial scales many relatives bear. On the flank it shows the genus-typical round midlateral blotch covering four to nine scales, but its signature is that the two anteriormost vertical bars are darkened to nearly the same intensity as that blotch and as the bold, complete bar that runs down through the eye (the infraorbital bar) — while the remaining bars stay much fainter. The fish is also polymorphic and changes with age: the description figures specimens from 1 in to over 4.5 in SL, and notes that the presence and shape of the dark markings can differ between the left and right sides of the same individual. Sexual dimorphism in this species has not been documented in detail; as in most Geophagus, males likely grow somewhat larger and develop longer fin extensions with maturity, but the original description does not establish reliable external sex differences.
Habitat
The species is known only from the lower rio Teles Pires and the rio Cristalino, in the rio Tapajós basin of Mato Grosso, Brazil — a tight endemic range on the northern slope of the Brazilian Shield, within FEOW's Tapajós–Juruena freshwater ecoregion. The Teles Pires is one of the clearwater, rapid-strewn headwater rivers of the Tapajós system, draining Cerrado savannah country before plunging through a series of falls and cataracts. Deprá and colleagues recorded pyrineusi from both clearwater (rio Teles Pires) and blackwater (rio Cristalino) stretches, always in moderately flowing water near rapids, over bottoms of small rocks mixed with sand. This places it among the more current-tolerant, semi-rheophilic members of its group rather than the still-water, lake-dwelling eartheaters. Despite broad and intensive collecting across the wider Teles Pires drainage, the fish has turned up only in this lower-river band, suggesting a genuinely restricted distribution rather than under-sampling. In-situ water chemistry was not published with the description, but these are warm, soft, low-conductivity shield rivers: clearwater and blackwater Tapajós tributaries typically run acidic to near-neutral (pH roughly 5.5–7.0) and warm (about 79–86 °F), the general envelope for shallow upper-river habitats where a benthic sifter forages over rock-and-sand substrate.
Feeding
As its genus name promises — Geophagus means 'earth-eater' — this is a sand-sifting benthic invertivore. Eartheaters feed by taking mouthfuls of substrate, sorting edible particles from grit inside the buccal cavity with specialised gill-raker and pharyngeal structures, and expelling the cleaned sand back out through the mouth and gill openings. Working over the rock-and-sand bottoms of its rapids-adjacent habitat, Geophagus pyrineusi would extract small benthic invertebrates — insect larvae such as chironomids, micro-crustaceans, worms — along with detritus and the organic film bound up in the sediment. The gut contents of this particular species have not been formally reported, but the trophic ecology of the surinamensis group is well established and consistent across its members: they are low-trophic-level omnivores that glean the soft bottom rather than chase prey. In the field, current-loving eartheaters of this kind are often seen foraging alongside the larger, more boisterous Retroculus, which flip rocks and stir up sediment and so expose food items that the smaller geophagines opportunistically snap up.
Mating
No courtship or pair-formation behaviour has been described for Geophagus pyrineusi specifically — it is a recently named, rarely collected fish that, as of writing, has essentially no documented life history in the wild or in captivity. What can be said is drawn from its close relatives. Geophagus of the surinamensis group are typically gregarious, forming loose foraging shoals outside the breeding season, then breaking into pairs that establish and defend a spawning territory over open substrate. Pairs generally form from within a group, and the bond is reinforced through mutual substrate-cleaning and lateral displays at a chosen site. Aggression in the group runs the gamut: the still-water species tend to be relatively peaceable, while the rheophilic, current-dwelling members — the ecological company pyrineusi keeps — are often the more territorial and quarrelsome. Until someone observes this species breeding, its mating system should be regarded as inferred rather than known.
Breeding
The reproductive mode of Geophagus pyrineusi has not been documented; it had not been bred in captivity at the time of its description and no wild spawning has been reported. The honest position is that we do not yet know which of the group's strategies it uses. The surinamensis group is unusual in deploying all three of the common cichlid breeding tactics: a few species are open substrate-brooders that lay eggs on a rock or the sand and guard a large free-swimming brood without ever taking eggs into the mouth (e.g. Geophagus argyrostictus, Geophagus taeniopareius); others are ovophilic (immediate) mouthbrooders that pick the eggs up shortly after laying (e.g. Geophagus altifrons, Geophagus proximus); and the largest contingent are larvophilic (delayed) mouthbrooders that let the eggs hatch on the substrate before scooping up the wrigglers (e.g. Geophagus surinamensis, Geophagus dicrozoster, Geophagus megasema). Several of pyrineusi's nearest relatives are mouthbrooders, so some form of mouthbrooding is a reasonable expectation, but this is genus-typical inference, not an observation of this species. Clutch sizes in the group are moderate — typically on the order of one to a few hundred eggs — and care is generally biparental or female-led, with parents shepherding fry for some weeks after release. None of these figures has been confirmed for pyrineusi.
In the aquarium
Geophagus pyrineusi is, for practical purposes, not an aquarium fish — it is a 2022-described endemic of a remote stretch of the Teles Pires, and the eartheater commonly sold under Tapajós trade names ('red head Tapajós', 'orange head') is a different species, Geophagus pyrocephalus. There is therefore no established body of hobbyist experience with pyrineusi itself, and a keeper is far more likely to encounter its look-alike relatives than this fish. That caveat aside, anything kept as pyrineusi should be managed as a semi-rheophilic surinamensis-group eartheater, and that guidance is well worked out. These are sand-substrate cichlids above all: a deep bed of fine, smooth sand is non-negotiable, because the fish must be able to take mouthfuls of substrate and sift them to behave and feed normally — gravel both frustrates that behaviour and can injure the gills. Because this is a current-loving fish from rapids-adjacent water, it wants more flow and oxygen than the placid lowland eartheaters: brisk circulation from a powerhead or an oversized canister return, and large, frequent water changes to keep nitrate low, since rheophilic Geophagus are comparatively sensitive to degraded water. A footprint of at least 47 in (about 100+ US gallons) suits a small group; eartheaters show their best behaviour and colour kept in groups of six or more rather than singly or in pairs, which lets social aggression diffuse across the shoal. Aim for warm, soft, slightly acidic to neutral water (about 79–86 °F, pH roughly 6.0–7.0) over a sand-and-smooth-rock layout with driftwood, mirroring the clear- and blackwater shield rivers it comes from. Décor can be sparse, but plants will be uprooted rather than eaten, so anchor or pot them. Feeding is undemanding — a varied diet of frozen bloodworms, small crustaceans, quality sinking pellets and the occasional vegetable component keeps them in condition — but resist overfeeding, which causes bloat in this group; a weekly fast day is sensible. Tankmates should be peaceful, similarly-sized South Americans that won't outcompete a deliberate bottom-sifter at feeding time: medium characins, peaceful pike cichlids such as dwarf Crenicichla or Teleocichla, plecos, and calm cichlids like Heros, Mesonauta or Satanoperca. Avoid greedy, fast-eating fish (large serrasalmids) and anything aggressive enough to bully a slow feeder.
Conservation
Geophagus pyrineusi has not been assessed by the IUCN Red List; its conservation status is formally Not Evaluated, and the Cichlid Room Companion confirms it carries no Red List category. That is unsurprising for a species described only in 2022 and known from a single short reach of one river system. The more pressing concern is that its entire documented range — the lower rio Teles Pires and rio Cristalino — sits in one of the most heavily dammed and rapidly developing parts of the Amazon. The Teles Pires drainage has seen a cascade of large hydroelectric projects (including the Teles Pires and São Manoel dams) built across exactly the kind of rapid-and-fall habitat this fish occupies, fragmenting the river, drowning rapids beneath reservoirs and altering flow, sediment and temperature regimes. The wider Tapajós basin faces additional pressure from deforestation, agricultural expansion across its Cerrado headwaters, and gold mining with its attendant mercury and sedimentation. For a narrow-range, current-dependent endemic, these are precisely the threats that matter most. None has been demonstrated to be reducing pyrineusi's numbers — no one has studied them — but its restricted distribution within a stressed, dam-laden basin is a flag that a future assessment would need to weigh carefully.