Taxonomy & naming
Geophagus diamantinensis was described in 2015 by the Brazilian ichthyologists José Leonardo de Oliveira Mattos, Wilson J. E. M. Costa and Alexandre Clistenes de Alcântara Santos, in the journal Ichthyological Exploration of Freshwaters. The species name refers to the Chapada Diamantina, the highland region of central Bahia — literally the 'Diamond Highlands,' named for the 19th-century diamond rush — to which the fish is endemic. Eschmeyer's Catalog of Fishes, FishBase and the Catalogue of Life list it as valid as Geophagus diamantinensis Mattos, Costa & Santos 2015. Taxonomically it belongs not to Geophagus in the strict sense but to the 'Geophagus' brasiliensis species group, the assemblage anchored on the widespread pearl cichlid 'Geophagus' brasiliensis. This group is morphologically and genetically distinct from the Amazonian-Guianan true eartheaters, and is frequently written as 'Geophagus' in quotation marks because it is widely expected to be moved to its own genus (the resurrected name Acaronia or another) when the genus is formally revised; for now the combination Geophagus diamantinensis stands. A subsequent molecular study (Mattos & Costa 2018) placed diamantinensis in a basal clade of the brasiliensis group restricted to the Rio Paraguaçu basin, together with its only basin co-occurring relative, the resurrected Geophagus obscurus.
Morphology
This is a small eartheater. FishBase gives a maximum of about 3 in in standard length, and the type series ran to only around 2.5–3 in SL, making diamantinensis one of the more diminutive members of its group. It has the typical brasiliensis-group form — a moderately deep, laterally compressed body, a fairly steep forehead and the small, low, protrusible mouth of a substrate-sifter — and the genus-typical dark mid-lateral blotch. The species was diagnosed from its relatives on a combination of internal and external characters, notably a deep urohyal bone and details of its colour pattern: the brasiliensis-group eartheaters are characteristically patterned with rows of pale iridescent blue-green spots, dots and short streaks scattered over the flanks, cheeks, operculum and unpaired fins, set against a darker olive-to-grey ground, and diamantinensis carries its own particular arrangement of this spangling. As is usual in the group, sexual dimorphism is modest: males tend to grow somewhat larger and develop marginally extended fin tips, but the sexes are not strongly differentiated.
Habitat
Geophagus diamantinensis is known only from the upper section of the Rio Paraguaçu basin, within the Chapada Diamantina of Bahia state, north-eastern Brazil — the highland headwaters around towns such as Andaraí, in and near the Chapada Diamantina National Park. The Paraguaçu rises in these quartzite uplands and runs some 311 mi east to the Atlantic at Todos os Santos Bay; the upper basin that diamantinensis occupies is a landscape of clear, often tea-stained rivers and streams flowing over sand, gravel and rock through a seasonally dry, semiarid interior. The Chapada Diamantina is recognised as the area of greatest freshwater endemism within the north-eastern Mata Atlântica region, and diamantinensis is one of a suite of small fishes confined to these isolated upland drainages. It is a benthopelagic freshwater fish of warm tropical waters; no specific published temperature or chemistry envelope exists for the species, but the upper-Paraguaçu regime is warm, soft-to-moderate, and ranges from clear to lightly stained over a hard, mixed substrate.
Feeding
Like all members of its genus and group, Geophagus diamantinensis is a substrate-sifter. It takes mouthfuls of sand and fine sediment from the river bed, sorts edible invertebrates, detritus and organic particles from the inorganic grit over its gill rakers, and expels the cleaned substrate through mouth and gills. The brasiliensis-group eartheaters are documented as highly abundant sand-sifting fishes whose diet is dominated by benthic invertebrates — insect larvae, small crustaceans and worms — together with plant matter, seeds and detritus, placing them low on the food web as invertivore-detritivores. There is no diet study specific to diamantinensis, but its mouth and gill-raker morphology and its membership in this well-characterised group leave little doubt that it works the sand and gravel of the upper Paraguaçu in the same winnowing manner.
Mating
The 'Geophagus' brasiliensis species group differs reproductively from the true Amazonian eartheaters: its members, including the well-studied pearl cichlid, are biparental substrate spawners and guarders rather than mouthbrooders. The reproductive biology of diamantinensis specifically has not been observed and is inferred from this group pattern. A male and female form a pair, select and clean a spawning surface — a flat stone or firm patch of substrate — and defend a territory around it together. Courtship is the usual cichlid repertoire of lateral displays, fin-spreading and joint substrate-cleaning. Unlike the mouthbrooding steindachneri-group and the delayed-mouthbrooding surinamensis-group eartheaters, the brasiliensis group practises straightforward open or semi-open substrate spawning with shared, attentive biparental care of eggs and fry — the strategy that has helped make the pearl cichlid one of the most adaptable and successful cichlids of eastern South America.
Breeding
No captive or wild breeding account exists specifically for Geophagus diamantinensis; the pattern below is that of the brasiliensis group, exemplified by its widespread relative the pearl cichlid. The pair lays a clutch of adhesive eggs on a cleaned hard surface and tends them biparentally — the female concentrating on fanning and direct egg-care, the male on perimeter defence. After hatching, the wrigglers are typically moved to one or more pre-dug pits in the substrate, and once free-swimming the fry are herded and guarded as a school by both parents, who continue to defend them vigorously for some time. Brasiliensis-group cichlids are noted for producing sizeable broods and for tenacious, sometimes aggressive, parental defence — a contrast to the small broods of the mouthbrooding eartheater lineages. Given diamantinensis's small adult size its broods are presumably modest, but no clutch counts have been published. The group's robust, conventional substrate-spawning biology is part of why the pearl cichlid has thrived and become invasive outside its native range; how well that translates to a narrow upland endemic like diamantinensis is unknown.
In the aquarium
Geophagus diamantinensis is effectively unknown in the aquarium hobby. It has only been described in the last decade, comes from a remote and protected upland region, and is not exported; the brasiliensis-group eartheater that hobbyists actually keep is the hardy, widely available pearl cichlid, 'Geophagus' brasiliensis. There is therefore no body of aquarist experience with this species, and any husbandry guidance is necessarily extrapolated from its relatives.
That extrapolation is reasonably safe, because the brasiliensis group is undemanding by eartheater standards. A keeper would provide a spacious tank with a deep bed of fine, soft sand — these are substrate-sifters, and gravel both frustrates feeding and risks gill abrasion — over which the fish can forage and dig. The natural upper-Paraguaçu habitat suggests clear-to-tannic, warm water (around 75–82 °F) of soft to moderate hardness over sand, gravel and rock, with scattered stones, driftwood and some current; the brasiliensis group is tolerant of a fairly wide range of conditions, so stability and cleanliness matter more than precise targets. Strong filtration and regular water changes suit a fish that constantly stirs the bed. Feeding is straightforward: sinking pellets and frozen or live invertebrates taken off the bottom match the natural invertivore-detritivore diet. The chief caution, again from the group, is temperament around breeding: brasiliensis-group cichlids can become markedly territorial and pugnacious when spawning, so a small, narrow endemic like diamantinensis would still want space and the option to hold a defensible territory away from tankmates.
Conservation
Geophagus diamantinensis has not yet been formally evaluated for the IUCN Red List (Not Evaluated). This is, however, exactly the kind of fish that warrants attention: a small species with a very restricted known range, confined to the upper Rio Paraguaçu in the Chapada Diamantina, in a semiarid interior region whose freshwaters are under pressure from water abstraction, agriculture, and the effects of a drying, seasonally water-stressed climate. The Chapada Diamantina is both a centre of freshwater endemism and the site of historic and ongoing mining and tourism development. Part of the upper basin lies within the Chapada Diamantina National Park, which affords some protection to the headwater habitats, but narrow-range upland endemics of this sort are inherently vulnerable to localised disturbance — drought, damming of small rivers, sedimentation and the introduction of non-native fishes. A formal conservation assessment of diamantinensis and the other Paraguaçu-endemic fishes would be valuable. The species has no CITES listing and no commercial or ornamental significance.