Taxonomy & naming
Geophagus sveni was described in 2010 by the Brazilian ichthyologists Paulo H. F. Lucinda, Carlos A. S. Lucena and Newton C. Assis, in a paper in the journal Zootaxa (vol. 2429, pp. 29–42) that simultaneously erected two new eartheaters from the Rio Tocantins drainage: Geophagus sveni and the sympatric Geophagus neambi. The type material was collected from the middle portion of the Tocantins River in the Brazilian state of Tocantins, and the species' nomenclatural act is registered in ZooBank. The specific epithet honours Sven O. Kullander of the Swedish Museum of Natural History, the foremost modern reviser of Neotropical cichlids, whose 1986–1998 work redefined Geophagus in the strict sense and underpins the framework into which sveni was placed. The genus Geophagus was erected by Johann Jakob Heckel in 1840; sveni belongs to Geophagus sensu stricto — the assemblage of true sand-sifting eartheaters built around the type species Geophagus altifrons — and within it to the so-called surinamensis species group, the cluster of superficially similar, plain-bodied eartheaters that includes Geophagus surinamensis, Geophagus altifrons, Geophagus abalios, Geophagus brokopondo and Geophagus parnaibae. It is placed in the eartheater tribe Geophagini of the subfamily Cichlinae. Lucinda and colleagues diagnosed sveni against this crowded background largely on markings and bar count (see Morphology), distinguishing it both from the head-marked Geophagus and from the other plain-faced members of the group.
Morphology
Geophagus sveni is a medium-sized, laterally compressed eartheater with the high, rounded forehead and underslung, protrusible mouth typical of the genus. The original description records a maximum of about 6.5 in standard length; as with most geophagines, well-fed aquarium fish grow somewhat larger and heavier-bodied than wild specimens, and total length runs a few centimetres beyond the standard-length figure. Its single most useful field mark is a negative one: the head carries no markings at all. This absence of an infra-orbital (suborbital) stripe immediately separates it from Geophagus grammepareius, Geophagus taeniopareius, Geophagus harreri, Geophagus argyrostictus and Geophagus gottwaldi, all of which wear a complete cheek stripe, and the absence of a dark preopercular spot separates it from Geophagus dicrozoster, Geophagus winemilleri, Geophagus brachybranchus and Geophagus proximus. Among the remaining plain-faced eartheaters it is identified by its flank pattern: five faint, parallel vertical bars (versus none in Geophagus megasema, Geophagus camopiensis and Geophagus altifrons, four in Geophagus surinamensis, and six in Geophagus abalios and Geophagus brokopondo) and a prominent, rounded mid-lateral blotch five to six scales wide that reaches the scale row just above the upper lateral line. From the very similar Geophagus parnaibae of the neighbouring Parnaíba basin it differs in having solid (rather than medially bisected) flank bars and a caudal fin patterned with alternating vertical light and dark bars rather than horizontal ones. In life the horizontal body stripes are reddish anteriorly and orange behind, and three iridescent blue spots run along the preopercle — features that, together with only five faint bars, separate it from the sympatric Geophagus neambi, which shows seven or eight conspicuous bifurcated bars and lacks the blue preopercular spots. Sexual dimorphism is slight and follows the geophagine norm: mature males tend to grow larger, deeper-bodied and to develop slightly more extended fins, but the sexes are not reliably told apart on colour alone and venting or breeding behaviour is the surest guide.
Habitat
The species is endemic to Brazil and, as originally known, to the middle reaches of the Rio Tocantins, with hobbyist and aquarium-trade material also attributed to the adjoining Rio Araguaia — the two great rivers of the Tocantins–Araguaia basin, the largest drainage lying wholly within Brazilian territory. FishBase places the documented range between roughly 9°S and 11°S, a demersal fish of warm, generally clear lowland waters. Within these rivers it occupies main channels as well as marginal floodplains and marshes, foraging over the sand and fine sediment of the bottom. In 2018 the species turned up far outside its native basin: specimens taken in 2015 from the Yacyretá reservoir on the Paraná River in Argentina were identified, by both morphology and cytochrome-oxidase-I sequence, as Geophagus sveni — the first record of Geophagus sensu stricto in Argentina, and almost certainly the result of an introduction rather than a natural range extension. The water of the middle Tocantins is warm, soft to moderately mineralised and near-neutral clearwater; in keeping with that, the species is maintained in soft to moderately hard water around pH 5.5–7.0 and temperatures of roughly 79–86 °F. As a bottom-oriented river fish it lives in the shallow, sunlit band over sand and is not a deep-water species; precise depth soundings for sveni itself have not been published, so the figures given here are the typical river-margin band for the surinamensis-group eartheaters rather than a measurement specific to this species.
Feeding
Geophagus sveni is a classic sand-sifting eartheater. Like its congeners it feeds by taking mouthfuls of substrate, sorting the edible fraction within the buccal cavity and gill rakers, and expelling the cleaned sand through the mouth and gill openings — the winnowing behaviour that gives the genus its name ("earth-eater"). Dietary records for the species classify it as a detritivore that also takes small crustaceans and aquatic insects, and FishBase estimates a low trophic level of about 2.3, consistent with a benthic micro-invertivore and detritus feeder rather than a predator. In its native rivers it works the soft-sediment and sand zones of channel margins and floodplains, processing the thin organic film, micro-crustaceans, insect larvae and small invertebrates bound up in the bottom. As a constant, gentle excavator of the substrate it plays the eartheater's usual role in the benthic community, turning over and aerating surface sediment as it forages.
Mating
Direct courtship observations specific to Geophagus sveni are scarce in the literature, but its reproductive biology can be read with confidence from the surinamensis species group to which it belongs, in which spawning behaviour is well documented and consistent. These eartheaters form monogamous pairs that establish a shared territory on the open bottom, typically over sand or a cleaned flat surface such as a stone or submerged wood. Pair formation and the defence of the spawning site are a joint effort, and outside the immediate spawning area the fish are gregarious and only moderately territorial — they are not the relentless boundary-holders that many Central American or rift-lake cichlids are. Courtship runs to the geophagine pattern of lateral displays, quivering and substrate-cleaning by both partners before eggs are laid. The pair bond and biparental territory carry through into the brood-care phase described below.
Breeding
Geophagus sveni has not been the subject of a published wild spawning study, but as a member of the surinamensis group it is a larvophilous ("delayed") biparental mouthbrooder — the reproductive mode confirmed for its near relative Geophagus surinamensis and shared across most of the group. In this strategy the pair first spawns in the open: adhesive eggs are deposited on a cleaned substrate — sand, a flat stone or wood — and guarded and fanned there by both parents through incubation. Once the eggs hatch, one or both parents take the larvae (wrigglers) into the mouth, chewing them free of the egg membranes, and brood them orally until they are free-swimming, periodically releasing and re-collecting the developing fry and admitting them back to the buccal cavity at any sign of danger. This intermediate mode — substrate spawning followed by mouthbrooding of the larvae — sits evolutionarily between ancestral biparental substrate brooding and the fully "immediate" (ovophilous) mouthbrooding seen in a few other Geophagus, and yields a smaller number of larger, better-protected young than open substrate spawning alone. Specific clutch counts for sveni have not been published; surinamensis-group eartheaters lay on the order of one to a few hundred eggs per spawning. Parental care is biparental and ends once the fry are foraging independently.
In the aquarium
Geophagus sveni is far less often seen in the aquarium trade than the staple eartheaters such as Geophagus surinamensis (sensu lato), Geophagus proximus or the "red-head Tapajós"; at the time of its description it had not been encountered in the trade at all, and it remains an enthusiast's fish reaching keepers mainly through specialist channels and as Araguaia/Tocantins collections. Where it is kept, its husbandry follows the well-understood blueprint for medium-sized surinamensis-group eartheaters, and on that basis it is not difficult given space and clean water. A single fish or a pair needs a long tank of at least about 59 in (around 90–120 US gal); because these fish do best as a loose group that can sort out a soft social hierarchy, a larger footprint housing five or more is better than a single pair, which can become one-sided. The substrate is non-negotiable: a deep bed of fine, smooth sand is essential, both because the fish sift it constantly for food and because coarse gravel frustrates that behaviour and can abrade the gills. Décor of smooth water-worn rock, driftwood and a few sturdy or potted plants suits them, with open sand left clear for foraging and a flat stone available as a potential spawning site.
Water should be warm and clean: roughly 79–86 °F, pH about 5.5–7.0, and soft to moderately hard. Eartheaters are sensitive to accumulated nitrogenous waste and to a lesser extent prone to head-and-lateral-line erosion when water quality slips, so generous filtration and large, regular water changes are the core of good keeping; gentle to moderate flow is appropriate for a river fish. Feeding is straightforward — they accept quality sinking pellets and a varied menu of frozen and live foods (bloodworm, daphnia, brine shrimp, chopped invertebrates) — but a sand bed to sift is part of natural feeding and digestion and should not be omitted. Temperament is peaceable for a cichlid of this size: sveni is not aggressive toward dissimilar tankmates and combines well with other medium South Americans of the Tocantins–Araguaia community — peaceful larger characins, other calm geophagines, and similarly sized non-pugnacious cichlids — provided companions are not small enough to be swallowed or so boisterous that they outcompete the eartheaters at feeding time. The usual cautions are intraspecific: rival males and competing pairs can squabble, and spawning fish will hold a territory, so the answer is space and numbers rather than a confrontational single pair. Captive breeding has not been widely reported for this particular species but should follow the group's larvophilous mouthbrooding pattern, triggered as for its relatives by soft, warm, clean water and conditioning on rich foods.
Conservation
The IUCN Red List assesses Geophagus sveni as Least Concern, in an assessment carried out on 7 November 2018 by Brazil's Instituto Chico Mendes de Conservação da Biodiversidade (ICMBio) and published in 2022. The population trend is judged stable. The rationale notes that the species is frequent and abundant through its range in the middle Tocantins, with no significant threats identified at the species level. It carries no CITES or CMS listing. That said, the Tocantins–Araguaia basin as a whole is among the most heavily impounded river systems in Brazil — the Tucuruí dam on the lower Tocantins is one of the largest hydroelectric projects in the Amazon region, and a cascade of further dams, together with agricultural expansion and deforestation across the cerrado headwaters, has fragmented and altered flow regimes throughout the basin. None of these pressures is documented as a present danger to this widespread, generalist eartheater, but they are the backdrop against which its long-term status in a fast-developing basin should be read. The recent establishment of the species in the Paraná basin of Argentina is a separate concern of a different kind — an example of how readily traded or transported eartheaters can colonise reservoirs well outside their native range.