Taxonomy & naming
Geophagus dicrozoster was described in 2004 by the Venezuelan-trained ichthyologists Hernán López-Fernández and Donald C. Taphorn in the journal Zootaxa (issue 439, pages 1–27), in a single paper that simultaneously erected three new Venezuelan species — Geophagus abalios, Geophagus dicrozoster and Geophagus winemilleri — and brought order to a genus that had long been a taxonomic muddle in the region. Before that revision the fish circulated in the aquarium trade and in collection records under provisional labels such as Geophagus sp. 'Colombia'. The holotype, an adult of 7.5 in standard length, was collected by D.A. and J. Arrington on 13 April 1999 from Laguna Larga, a floodplain lagoon of the Río Cinaruco in Apure State, Venezuela, and is held at the Museo de Ciencias Naturales de la UNELLEZ en Guanare (MCNG 40996); paratypes range up to 8 in SL and include material from the Río Caroní and the Río Cinaruco. The species belongs to the Geophagus surinamensis species group, a cluster of around twenty closely similar, notoriously hard-to-identify eartheaters that share a single mid-lateral spot and span the Guiana Shield, the Orinoco and the Amazon. Within that group López-Fernández and Taphorn separated dicrozoster from the species lacking head markings (Geophagus abalios, Geophagus altifrons, Geophagus megasema and others) by its dark preopercular mark, and from the other marked species by its possession of seven parallel vertical bars on the flank — four in Geophagus winemilleri, none in Geophagus proximus and Geophagus brachybranchus. The genus Geophagus itself was raised by Heckel in 1840 and has been repeatedly redefined; dicrozoster sits in Geophagus sensu stricto, the true sand-sifting eartheaters, as distinct from the various lineages once lumped under the name.
Morphology
Geophagus dicrozoster is a moderately elongate, laterally compressed eartheater with the steep forehead and protrusible, downturned mouth typical of the genus. The type series spans from juveniles of a few centimetres to adults of 8 in standard length (roughly 8 in SL, near 10 in in total length), and the species is sometimes reported in the aquarium literature at sizes around 10–12 in; it is a large eartheater by hobby standards. The original describers found the sexes to be isomorphic — there is no reliable external difference in shape, fin length or pattern between males and females — so sexing is generally done only by venting or by behaviour at spawning, with males tending to reach somewhat greater bulk. The diagnostic markings are a vertical blackish bar in the corner of the preopercle (the cheek) and a row of seven dark-gray vertical bars descending the flank. A well-defined black mid-lateral spot sits on the third bar, pierced by the upper lateral line. The fourth and fifth bars are the signature: bar four descends and angles back to meet bar five at the upper lateral line, the two together tracing a 'Y'-shaped figure — the forked band that the species name records. In life the background is a yellowish olive-green, the head silvery with an iridescent blue stripe along the upper lip running back to the preopercular mark, and the unpaired fins are reddish, the anal and caudal carrying iridescent blue stripes and spots; breeding fish intensify in colour. As in other surinamensis-group Geophagus the pelvic fins draw out into trailing filaments in larger adults, and the bar pattern, vivid in preserved specimens, is often muted or broken into rows of spots on a living, settled fish.
Habitat
Geophagus dicrozoster ranges across the Orinoco basin of Venezuela and Colombia and reaches into the upper Amazon, the IUCN listing it from Brazil (Amazonas), Colombia and Venezuela. The original description records it as common in the black waters of the Caura and Caroní drainages of the Guiana Shield, and present in essentially all the major tributaries of the middle and upper Orinoco — among them the Cataniapo, Ventuari, Atabapo, Ocamo and Mavaca — as well as the Casiquiare, the natural channel that links the Orinoco to the Río Negro, and into the headwaters of the Negro itself. In the open llanos savannas the species is more restricted: it occurs in the moderately blackwatered Río Cinaruco (the type locality) but the describers found no specimens in white water or in the clear-water llanos drainages such as the Aguaro-Guariquito. It is, in short, a fish of clear and especially black waters rather than the sediment-laden whitewater rivers. Within those drainages it inhabits large, medium and small rivers, seasonal floodplain lagoons and morichales — the moriche-palm swamps characteristic of the region — typically over sand or soft bottoms in the shallow, well-lit margins where an eartheater can sift the substrate. Published in-situ chemistry specific to dicrozoster is sparse, but its blackwater Orinoco haunts are characteristically warm, soft and acidic to near-neutral. No precise depth range has been published; like its congeners it is a shallow-water, shoreline-and-floodplain fish working the upper metre or two of the water column over the bottom rather than a deep-river species.
Feeding
Geophagus dicrozoster is a benthic sand-sifter, the feeding mode that gives the genus its name (Geophagus, 'earth-eater'). It takes mouthfuls of sand and fine sediment, sorts edible particles in the buccal cavity and over the gill rakers, and expels the cleaned substrate through the mouth and gill openings — the patient, head-down winnowing seen throughout the genus. FishBase places it low on the food web, with an estimated trophic level near 2.3. In practice it is an opportunistic omnivore with a leaning toward small animal prey: stomach studies of similar surinamensis-group eartheaters turn up benthic invertebrates, insect larvae, small crustaceans, micro-organisms and detritus extracted from the bottom, along with incidental plant and algal material. In the river community it functions as a small-invertebrate and detritus forager of the sandy, leaf-strewn shallows, ecologically equivalent to the other Geophagus and Satanoperca that share its range. It is not a piscivore, though a large adult will opportunistically take very small fishes.
Mating
Detailed field accounts of pair formation in Geophagus dicrozoster specifically are scarce, but its reproductive behaviour follows the well-documented pattern of the surinamensis group. These are not harem or polygamous spawners; they form monogamous pairs that jointly hold and defend a spawning site and then cooperate in caring for the brood. Because the sexes are isomorphic, pairs in the wild and in captivity generally sort themselves out from a group as the fish mature, two individuals gradually pairing off and becoming intolerant of others around the chosen site. Courtship involves the usual cichlid repertoire of lateral displays, quivering and substrate cleaning, with both partners intensifying in colour — in dicrozoster the reds of the fins and the iridescent blue head markings brighten as breeding condition comes on. Territoriality is centred on the spawning patch and tends to be strongest during the brief substrate-spawning phase, before the eggs hatch and the pair switches to mouthbrooding the larvae.
Breeding
Geophagus dicrozoster is a larvophilous biparental mouthbrooder — sometimes called a 'primitive' or 'delayed' mouthbrooder — and is among the surinamensis-group species that have been bred in the aquarium. In this strategy the pair first spawns in the open: eggs are laid and fertilized on a cleaned patch of substrate, often a flat stone or firm area of sand, in the manner of an ordinary substrate-spawner. The parents tend and fan the eggs for the first day or two, and then, as the embryos hatch into wrigglers, the adults take the larvae up into their mouths and brood them orally, alternating the load between the two fish and continuing to shelter the free-swimming fry in the buccal cavity at any sign of danger for some time after they can swim. This two-stage care — substrate spawn followed by oral brooding of the young — is exactly what was reported for the closely related Geophagus parnaibae bred by Stawikowski and Werner, and aquarists who have spawned dicrozoster describe the same biparental mouthbrooding behaviour, with both parents protecting comparatively large broods of fry. Precise wild clutch counts for dicrozoster have not been published, but surinamensis-group mouthbrooders produce moderate clutches (typically on the order of one to a few hundred eggs) consistent with the substantial fry schools keepers report. The species is described as not especially difficult to breed once a compatible pair has formed and is given clean, warm, soft water and a sandy bottom.
In the aquarium
Geophagus dicrozoster has a real, if modest, presence in the hobby, where it has long been traded — sometimes under the old provisional name Geophagus sp. 'Colombia' — and prized as one of the more colourful Orinoco eartheaters. It is a large, active fish and needs a correspondingly large tank: a footprint of at least 59 in (around 120 US gal) is a sensible minimum for an adult group, and more is better given the species' eventual size and its habit of sorting out pairs by squabbling. The single non-negotiable feature is a deep bed of fine, smooth sand: this is a fish that must be able to take mouthfuls of substrate and winnow it, and coarse gravel both frustrates that behaviour and risks damaging the gills. Décor of driftwood and rounded rocks suits it; rooted plants should be firmly anchored or potted, since the fish will rework the sand constantly. Water should mirror its warm, soft, clear-to-blackwater home — roughly 79–86 °F, pH on the soft-acidic to neutral side (about 6.0–7.5), and low to moderate hardness — and, crucially, it should be clean. Like most eartheaters, dicrozoster is sensitive to accumulated dissolved waste and is prone to head-and-lateral-line erosion if maintenance lapses, so generous filtration and large, regular water changes matter more than chasing exact parameters. Feeding is straightforward: good-quality sinking pellets as a staple, supplemented with frozen and live foods (bloodworm, brine shrimp, small crustaceans) and the occasional vegetable matter; food must reach the bottom, as these fish forage there rather than at the surface. Temperament is peaceful for its size outside of breeding, but it is an opportunist — small tankmates may be eaten, and fin-nippers will quickly ruin the long pelvic filaments. The best companions are similarly sized, non-aggressive South Americans (other eartheaters, larger characins, peaceful medium cichlids). The common keeping mistakes are an undersized tank, gravel instead of sand, and neglected water quality.
Conservation
The IUCN Red List assesses Geophagus dicrozoster as Least Concern (assessed 28 October 2020 by C. Lasso, reviewed by D.C. Taphorn, and published in 2023). The assessment rests on the species' wide distribution across the Orinoco and upper Amazon basins and its presence in numerous protected areas — among them El Tuparro, Sierra de La Macarena, Tinigua, Nukak and Puinawai in Colombia and Canaima, Yapacana, the Orinoco Delta, Duida-Marahuaca, Aguaro-Guariquito and Santos Luzardo in Venezuela. It carries no CITES listing, and population size and trend are formally unknown. The assessors nonetheless flagged real, ongoing pressures rather than a clean bill of health: the fish is taken in subsistence fisheries (including by spearfishing and diving), is collected for the ornamental trade in Colombia and Venezuela, and — most seriously — overlaps heavily with Venezuela's Arco Minero del Orinoco (Orinoco Mining Arc), where legal and illegal mining for gold, bauxite, coltan, diamonds, iron and copper drives deforestation, riverbed alteration, sedimentation, and contamination by mercury, fuel and sewage across the Caura, Caroní, Ventuari, Casiquiare and upper Orinoco. The fragile morichal palm swamps it uses are additionally threatened by desertification, salinization and increasing turbidity. Because the species reproduces through parental care in small subpopulations, the assessors note that localized over-extraction at particular blackwater and morichal sites can have an outsized effect even where the species as a whole remains widespread.