Taxonomy & naming
Geophagus rufomarginatus was described by José L. O. Mattos and Wilson J. E. M. Costa in 2018, in the open-access journal Zoosystematics and Evolution (volume 94), in a paper titled 'Three new species of the Geophagus brasiliensis species group from the northeast Brazil (Cichlidae, Geophagini)' — the other two being Geophagus multiocellus from the rio de Contas and Geophagus santosi from the rio Mariana. The holotype (UFRJ 9994, about 4 in SL) was collected in 2016 from a small stream crossing road BA-001 in the rio Buranhém basin, in the municipality of Porto Seguro, Bahia. The species sits in the 'Geophagus' brasiliensis species group — a distinct, still-unnamed lineage of pearl cichlids endemic to the Atlantic-draining rivers of eastern Brazil, conventionally written with the genus in quotation marks because it does not belong to Geophagus sensu stricto as restricted by Kullander (1986). The 2018 description was integrative, combining morphology with mitochondrial DNA: rufomarginatus was recovered as an exclusive genetic lineage of the Buranhém basin, grouping in the widespread clade alongside 'G.' brasiliensis and 'G.' iporangensis, and diagnosed by both unique colour characters and unique nucleotide transformations. The work is part of a broader effort, including Argolo and colleagues' (2020) systematic study of the brasiliensis complex, to resolve a tangle of look-alike coastal pearl cichlids into discrete, basin-endemic species.
Morphology
Geophagus rufomarginatus is a small pearl cichlid, the type series running to about 3.5–4 in standard length. It has the robust, deep-bodied, large-scaled look typical of the brasiliensis group — heavier-headed and less elongate than the Amazonian sand-sifters — with a body scattered in the iridescent pearly spots that give the group its common name. The diagnostic feature, and the source of the name, is the colour of the dorsal fin: the membranous lappets between the dorsal-fin spines are edged in red in life (versus grey or dark brown in related species). Beyond that, it is distinguished from its relatives by a combination of colour-pattern characters, including longitudinal series of small light-blue spots on the flanks and specific markings on the head and fins, as set out in the original diagnosis, together with exclusive DNA signatures. As is usual in the group, sexual dimorphism is limited; the description does not report strong external sex differences, and as in related pearl cichlids mature males tend to grow somewhat larger and develop more produced fin extensions, with breeding fish intensifying in colour.
Habitat
The species is endemic to the rio Buranhém basin, a small coastal river of southern Bahia that reaches the Atlantic at the historic town of Porto Seguro. It is known from the middle and lower sections of the drainage, at low altitudes — roughly 213 ft above sea level or less, with the type locality near Porto Seguro sitting at only about 33 ft. Collections span the basin from Porto Seguro upstream to Eunápolis, in small streams and the main Buranhém channel crossed by the coastal highways. This is a fish of the warm, low-elevation Atlantic Forest coastal plain of eastern Brazil — a region of short, independent rivers, each isolated from its neighbours by the sea and by intervening uplands, which is exactly the geography that has driven the brasiliensis group to split into so many small-range endemics. Detailed published water-chemistry figures for the wild habitat are limited, but the setting implies warm, generally soft to moderately mineralised coastal-river water; in captivity, brasiliensis-group pearl cichlids are notably adaptable across a wide temperature and hardness range.
Feeding
Like the rest of the 'Geophagus' brasiliensis group, rufomarginatus is an omnivorous, substrate-oriented feeder — a pearl cichlid that grazes and sifts the bottom for small invertebrates, plant matter, detritus and the micro-organisms in soft sediment. The brasiliensis-group eartheaters are less specialised, more generalist omnivores than the obligate sand-winnowers of Amazonian Geophagus sensu stricto, and 'G.' brasiliensis itself is well known as a hardy, adaptable opportunist that will take a very broad diet. No species-specific gut-content study has been published for rufomarginatus, but its ecology can be read confidently from its close relatives: a bottom-foraging omnivore of coastal streams, working sand and leaf litter for whatever edible matter it yields, and rising into the water column to take drifting or introduced food. A soft substrate it can browse remains important to its natural behaviour.
Mating
The 'Geophagus' brasiliensis group are pair-forming, biparental substrate spawners rather than mouthbrooders, and rufomarginatus is expected to follow that well-documented pattern, though its reproduction has not been studied directly in the wild or described in detail in captivity. In the group generally, a male and female form a bond, jointly select and clean a hard spawning surface, and defend a territory around it; courtship involves the usual cichlid repertoire of lateral displays, fin-spreading and substrate-cleaning. Pearl cichlids of this lineage are seasonal, opportunistic breeders that can be quite territorial around a nest. Sexing is difficult outside breeding condition; in related species males average larger with longer fin tips, and ripe females round out and show a drooped ovipositor. As with much of the group, pairs are best obtained by raising a group of young fish together and letting a bond form.
Breeding
Reproduction in Geophagus rufomarginatus specifically has not been documented, so the account here is drawn from the well-known biology of the 'Geophagus' brasiliensis group, of which it is a member. These pearl cichlids are biparental, open or semi-open substrate spawners: the adhesive eggs are laid in rows on a cleaned flat rock, root or pit and fertilised by the attending male, then tended by both parents — the female concentrating on fanning and direct egg care, the male on defending the surrounding territory. Unlike the Amazonian Geophagus sensu stricto, the brasiliensis group does not mouthbrood; the eggs are guarded externally, the wrigglers are moved to pits, and once free-swimming the fry are herded and protected as a school by both parents, often for an extended period. The best-studied relative, 'G.' brasiliensis, is a prolific and reliable breeder producing large broods of several hundred fry and showing strong, sustained parental care — the template against which rufomarginatus is reasonably read until species-specific data appear.
In the aquarium
Geophagus rufomarginatus is, at the time of writing, essentially a wild, scientifically described species rather than an established aquarium fish: it is not (or only very rarely) in the ornamental trade, and there is no body of hobbyist breeding reports for it specifically. What can be said comes from the broader 'Geophagus' brasiliensis group, which is one of the most forgiving of all cichlids in captivity and a good guide to keeping this species should it become available.
Pearl cichlids of this lineage are robust, adaptable and tolerant of a wide range of temperature (roughly 68–86 °F) and hardness, from soft to fairly hard water, around neutral pH — a reflection of the variable coastal rivers they inhabit. Being a smaller member of the group, rufomarginatus would need less room than the large 'G.' brasiliensis, but it is still a substantial, digging, territorial cichlid best given a generous tank with a soft sand substrate it can sift, plus driftwood, rounded stones and a few flat surfaces to serve as territory markers and spawning sites. Strong filtration and regular water changes suit any bottom-stirring eartheater. Feeding is straightforward given the group's omnivory: a varied diet of sinking prepared foods, frozen and live invertebrates, and some vegetable matter. Temperament in the group ranges from peaceful to fairly aggressive, sharpening markedly around spawning, so tankmates should be robust and similarly sized, and a breeding pair benefits from space to hold a territory. Anyone keeping it should treat it as a coastal-Brazilian endemic and prioritise stable, clean water and a sandy bottom.
Conservation
Geophagus rufomarginatus has not been formally assessed on the IUCN Red List, which is typical for a coastal-endemic cichlid described as recently as 2018. Its conservation profile is, however, a concern worth flagging on first principles: it is known only from a single short coastal river, the rio Buranhém, in the densely settled and heavily deforested Atlantic Forest belt of southern Bahia around Porto Seguro and Eunápolis. Narrow-range endemics of small eastern-Brazilian drainages are inherently vulnerable — a single basin's worth of pollution, urban and tourist development, water abstraction, deforestation and sedimentation can affect the entire global range of such a species at once. None of this is a documented, quantified threat to rufomarginatus specifically, because the species is too new and too little studied for that, but the combination of a tiny distribution and an intensively developed coastal setting is the standard recipe for elevated extinction risk among Atlantic-slope freshwater fishes. It carries no CITES listing, and there is no evidence of meaningful collection pressure. A formal assessment, and basic ecological and population study, would be valuable.