Taxonomy & naming
Guianacara cuyunii was described in 2006 by Hernán López-Fernández, Donald C. Taphorn and Sven O. Kullander in the journal Copeia, in a paper that named two new Guianacara from the Guiana Shield of eastern Venezuela (the other being Guianacara stergiosi). The holotype, MCNG 52841, was collected from a creek of the Venamo River upstream from the Apanao rapids in Bolívar state, Venezuela, with many paratype lots held at the AMNH, MCNG and NRM. The species had circulated in the hobby for years before then under provisional labels, and a Venezuelan Guianacara had already been flagged as undescribed in the 1989 monograph that founded the genus. The genus Guianacara itself was erected by Kullander and Nijssen in 1989 (in 'The Cichlids of Surinam') to house geayi and several relatives that did not fit comfortably in Aequidens — a name meaning, simply, 'acará from the Guianas.' Guianacara cuyunii is placed in the nominotypical subgenus Guianacara, diagnosed by two supraneurals and by lappets on the dorsal-fin spines (as opposed to the subgenus Oelemaria, with one supraneural and a midlateral spot rather than a bar). Within the eartheater subfamily Geophaginae, Guianacara and Acarichthys are sister genera forming the tribe Acarichthyini, a placement proposed on morphological grounds by Kullander (1998) and broadly supported by molecular work (Farias et al. 1998; López-Fernández et al. 2005). The describers cautioned that Essequibo populations in Guyana might represent yet another undescribed species distinct from cuyunii — and indeed a further Guyanese-Brazilian species, Guianacara dacrya, was later named (Arbour & López-Fernández 2011) from the Branco and Essequibo.
Morphology
Guianacara cuyunii is a small, moderately deep-bodied cichlid; FishBase records a maximum of 3 in standard length, and across the genus males grow noticeably larger and blockier-headed than females, which are typically two-thirds or less the male's size. The base coloration is the muted yellowish-to-greenish beige shared by all bandit acaras, overlaid in good condition with rows of iridescent gold-green scales and lightly spotted unpaired fins. The diagnostic markings are melanic and subtle but stable. The most reliable is the midlateral bar — the dark vertical 'saddle' on the flank — which in this species is unusually thin, only about three scales at its widest and tapering to two below the lateral line, where in Guianacara stergiosi, Guianacara geayi and Guianacara sphenozona it is four scales wide and never thins below three. Adults normally retain a complete, uniformly dark bar rather than developing the round midlateral spot seen in several congeners, although the bar may reduce to a spot in very large fish. A second key character is that Guianacara cuyunii keeps the black coloration in the membranes (lappets) of the first three dorsal-fin spines throughout life — pigment that is lost in adult Guianacara sphenozona and Guianacara geayi. It is the only Guianacara with a branchiostegal membrane of plain background body color rather than dusky, and its pectoral fin does not reach the base of the anal fin. Over all of this runs the genus's signature bandit mask: dark supraorbital and infraorbital bars sweeping from the nape through the eye to the lower edge of the gill cover.
Habitat
The species is known from the upper Cuyuní River drainage in the Guiana Shield of eastern Venezuela, with the type material taken in the Venamo, a Cuyuní tributary in Bolívar state. The Cuyuní is a western affluent of the Essequibo, so although these are shield rivers of Venezuela the fish belongs to the Essequibo basin rather than to the Orinoco; the Catalog of Fishes and the original authors both place it in the upper Cuyuní drainage of the Essequibo system in Venezuela and Guyana, and whether its range extends into the lower Cuyuní and Essequibo remains uncertain. The genus as a whole favours clear, soft, slightly acidic water over a bottom of large rocks broken by patches of sand, often where the current runs relatively fast — the rapids-and-riffle habitat implied by the type locality 'upstream from the Apanao rapids.' These are warm tropical lowland-to-piedmont streams; congeners are reported from water around pH 6.0–7.5 and temperatures of roughly 75–82 °F, the band appropriate for a clearwater Guiana-shield cichlid. No depth range has been published for the species, but as a fish of shallow rocky creeks and riffles it occupies the shallow band typical of such habitats — generally the upper metre or two of the water column over the rock and sand substrate.
Feeding
No gut-content study of Guianacara cuyunii has been published, and its diet must be inferred from its anatomy, its genus and FishBase's modelled trophic level of about 3.3 — squarely that of a small generalist invertivore-omnivore. Guianacara are bottom-oriented foragers that work the interface of rock and sand, taking aquatic insect larvae, small crustaceans and other benthic invertebrates along with incidental plant and detrital material; they are not the dedicated, deep sand-sifting eartheaters that Geophagus or Satanoperca are, but they do graze and pick over the substrate of their rocky riffles. In its community Guianacara cuyunii functions as one of the small benthic predators of the leaf-litter, gravel and rock zones of Guiana-shield headwater streams.
Mating
Reproductive behaviour has not been observed in the wild for this particular species, but the genus is consistent and well documented in aquaria. Guianacara form pairs and are biparental, and there is pronounced sexual size dimorphism — the male substantially larger than the female. Pair formation centres on a cavity or rock cleft that the female prepares and to which she courts the male; once a bond is established the pair jointly defend a territory around the chosen site. Outside spawning the fish are only moderately aggressive and coexist with other medium-sized cichlids, but a breeding pair becomes territorial and protective of its nest site. The size difference between the sexes is itself part of the breeding strategy, allowing the smaller female to retreat into a cavity entrance too narrow for a harassing male.
Breeding
Guianacara cuyunii is a biparental cave (substrate) spawner; wild spawnings are undocumented, but the genus is bred in the hobby by a well-known method. Eggs are laid on a vertical or overhung hard surface inside a cavity — in nature a cleft or hollow among the rocks, in the aquarium most reliably an inverted clay flowerpot whose base has been chipped or knocked out to leave an entrance the female can pass through but a large male cannot. The female removes sand from the chamber, courts the male, and lays her eggs on its interior walls; the male fertilizes them, either by entering or by releasing milt through the opening above. Both parents then guard and tend the brood, the female mostly inside with the eggs and wrigglers and the male patrolling the territory, with the pair leading and shepherding the free-swimming fry once they emerge. A precise clutch count for cuyunii has not been published; congeneric cave-spawning Guianacara typically produce broods on the order of a hundred to a few hundred eggs. Where pairs prove fully compatible the entrance can be enlarged to admit both adults.
In the aquarium
Guianacara cuyunii is an uncommon aquarium fish — the bandit acaras most often seen in the trade are Guianacara sphenozona and the red-cheeked Venezuelan form near Guianacara stergiosi — but where it or its close relatives are kept the husbandry is the same and is not demanding. A small, peaceable-by-cichlid-standards fish that matures under 3 in, it can be housed as a pair in a tank from roughly 35.5–47 in (about 55–80 US gal), or in a larger community of medium-sized, non-aggressive South Americans. The décor should echo the wild biotope: a substrate of sand with patches of smooth rounded rock and cobble, arranged into caves, clefts and overhangs, plus driftwood and subdued lighting. For any attempt at breeding, at least one inverted terracotta flowerpot with an entrance chipped into its base is the single most useful furnishing — the proven surrogate for the rock cavity the female would choose in the wild.
Water should be kept clear, soft and slightly acidic to neutral, mirroring the Guiana-shield streams the genus inhabits: roughly pH 6.0–7.5, temperature about 75–82 °F, and low-to-moderate hardness. The fish appreciate clean, well-oxygenated water with some movement — these are riffle and fast-current animals in nature — so good filtration and a brisk turnover suit them, paired with regular water changes. They are unfussy feeders that take quality dried foods readily and colour up best on a varied diet of frozen and live invertebrates such as bloodworm, brine shrimp and small crustaceans.
Temperament is manageable. Outside breeding Guianacara cuyunii holds its own in a 'rowdy' medium cichlid community without being a bully; the main caution is the territoriality of a spawning pair, which will defend their cavity vigorously, and the marked size mismatch between an oversized male and a smaller female, which in a cramped tank can lead to the male harassing or injuring his mate. Adequate space, sightline-breaking rockwork and a cave entrance the female can use as a refuge are the standard remedies. As with all soft-water shield fishes, mature, well-maintained water and stable, warm conditions matter more than chasing precise chemistry.
Conservation
The IUCN Red List assesses Guianacara cuyunii as Vulnerable (VU, criterion B1ab(iii,v)), assessed on 12 March 2023 and published in 2023 by O.M. Lasso-Alcalá, E. Quintero-T., I. Mikolji and E.O. Martínez, with the population trend recorded as decreasing. The listing reflects the species' narrow range in the upper Cuyuní drainage and the intense pressures on that part of the Guiana Shield: the region is a focus of gold and other mineral mining, which brings deforestation, sedimentation and mercury contamination of the rivers, compounded by logging, dam and water-management projects, industrial effluents, and the harvesting of aquatic resources. A small-range endemic of clear, rocky headwater streams is especially exposed to the siltation and water-quality degradation that mining causes, which is the central concern behind the assessment. The species carries no CITES or CMS listing, and although it has appeared in the ornamental trade it is not a major commercial fishery. Its long-term prospects are tied directly to the fate of the Cuyuní-Venamo headwaters.