Taxonomy & naming
The Swedish ichthyologist Sven O. Kullander described Aequidens gerciliae in August 1995, in a paper that erected three new southern-Amazonian cichlids at once: Aequidens gerciliae, Aequidens epae and Aequidens michaeli (Ichthyological Exploration of Freshwaters 6(2): 149-170). The holotype (INPA 974) came from the Núcleo Aripuanã, in the Igarapé do Aeroporto above the waterfall at roughly 10°10'S, 59°25'W, in Mato Grosso, Brazil, with paratype lots held at INPA in Manaus and at the Swedish Museum of Natural History (NRM) in Stockholm. The genus Aequidens was raised by Eigenmann & Bray in 1894 — originally as a subgroup of Astronotus — and gathers the medium-sized 'acaras' of the subfamily Cichlinae. Within it, Kullander set Aequidens gerciliae in a knot of similar species — Aequidens diadema, Aequidens epae, Aequidens metae and Aequidens michaeli — that share black-margined dorsal scales and dark stripes on the cheek. The fish had been seen before it had a name: Maria Gercília Mota Soares, in her 1980 ecological study of the fishes of the Igarapé do Porto at Aripuanã, had recorded it as Aequidens tetramerus, and Kullander re-identified her material as the new species when he described it, naming it in her honour. Both the Catalog of Fishes and Kullander's own 2003 CLOFFSCA checklist treat Aequidens gerciliae as valid; collection records once attributed to the rio Curuá in Pará and to the lower Aripuanã in Amazonas were judged by Kullander to be misidentifications and are not accepted.
Morphology
This is a modest-sized, fairly slender acara. Kullander's description gives a maximum of about 5 in (5 in) standard length and a body depth of 42-50% of SL — proportions that read as 'relatively elongate' for the genus; FishBase lists the largest measured male or unsexed specimen at 5 in SL and the largest female at 4 in SL. The build is typical Aequidens: a laterally compressed, oval body, a continuous dorsal fin, and a rounded to slightly lyrate tail. The diagnosis lives in fine detail. The predorsal scales sit in three rows (triserial), and the E1 longitudinal row usually carries 25 scales — one more than its closest relatives, which typically have 24. Like Aequidens diadema, Aequidens epae, Aequidens metae and Aequidens michaeli, it bears a dark midlateral blotch on the flank and a pattern of dark 'buccal' stripes across the lachrymal and cheek, though in both Aequidens gerciliae and Aequidens epae that stripe pattern can be absent altogether. Two features set it apart from the very similar Aequidens epae: in Aequidens gerciliae the vertical extension of the midlateral spot, when present, runs straight up to the dorsal-fin base rather than slanting backward, and most of the caudal fin is uniformly dark with only the upper and outer margins flecked with light spots, instead of the whole tail being clearly spotted. Sexual dimorphism appears limited and is poorly documented; the size figures suggest males reach a somewhat larger maximum than females, as is usual in the genus, but no distinct nuptial coloration has been described.
Habitat
Aequidens gerciliae is a clear-water fish of small Amazonian streams, and its known range is narrow: the upper Aripuanã River drainage — itself a tributary of the Madeira within the greater Amazon basin — in northern Mato Grosso, near the Cachoeira de Dardanelos and at a headwater creek along the Juína-Vilhena road. The IUCN treats it as endemic to Brazil and restricted to the upper Aripuanã in Mato Grosso, with an unconfirmed (and possibly erroneous) record from the lower river in Amazonas. FishBase classifies it simply as a freshwater, benthopelagic, clear-water species. No detailed in-situ chemistry has been published for the type localities, and FishBase carries no temperature, pH or depth figures. What can be said is framed by the basin: Amazonian clearwater rivers and streams are characteristically soft, very low in conductivity and slightly acidic — Duncan & Fernandes (2010) measured clearwater pH at 6.5 ± 0.4 (range about 6.0-8.0) with conductivity around 12-25 µS/cm — while field and biotope records from the Aripuanã drainage report pH near 6.6 and water temperatures of roughly 82–84 °F. Those are basin-typical conditions rather than measurements taken on Aequidens gerciliae, but they bracket the likely envelope: warm, soft, near-neutral to slightly acidic clear water. Its congener Aequidens epae, described in the same paper, was recorded from clear-water streams over muddy, sandy or rocky bottoms — a reasonable stand-in for the biotope. The range is genuinely small, confined to a headwater region, yet the assessors note that many igarapés in the area are well preserved and hard to reach, so more populations may still turn up.
Feeding
The one substantive dietary record for this fish comes from Soares's (1980) study of the Igarapé do Porto at Aripuanã — the same material Kullander later reassigned to Aequidens gerciliae. She found the stomachs and intestines packed mostly with insects and insect larvae, marking the species out as primarily insectivorous. The IUCN assessment, drawing on Kullander, describes it the same way, with insects and insect larvae dominating the gut contents, and FishBase places it at a trophic level of about 3.1 — in line with a small invertebrate-feeder. That fits the general Aequidens pattern: opportunistic micro-predators that pick small invertebrates from the substrate, leaf litter and water column, working the bottom and margins of a forest stream.
Mating
No courtship or pair-formation behaviour has been described for Aequidens gerciliae — the species has essentially no published reproductive observations of its own. What can be said comes from its membership in Aequidens proper, a genus of biparental substrate spawners in which a male and female form a pair bond, jointly stake out and defend a territory, and clean a spawning site before laying. Courtship in these cichlids typically runs through mutual displays, lateral posturing and substrate cleaning, and the pair grows territorial and intolerant of intruders as spawning nears. Aequidens gerciliae can reasonably be expected to follow that template, but with no direct field or aquarium accounts, this is inference from close relatives rather than documented fact.
Breeding
Breeding has not been documented for Aequidens gerciliae in the wild or, in any published account, in aquaria. By the reproductive biology of the genus, it is expected to be a biparental open-substrate (open-brooder) spawner: the pair lays a single layer of adhesive eggs on a cleaned hard surface such as a flat stone, root or leaf, and both parents tend the clutch — fanning and guarding the eggs, then shepherding the free-swimming fry and moving them between pits in the substrate. Aequidens of this size typically produce a few hundred to around a thousand eggs — FishBase records the genus type species, Aequidens tetramerus, releasing about 1,000 eggs per spawning, with both parents then tending the young — and parental care generally continues for some weeks after the fry become mobile. Even so, no clutch count, egg size, incubation period or developmental timeline has been reported for this particular species. Spawning triggers in related stream cichlids are usually tied to the seasonal rains and the flush of soft, slightly warmer water and abundant invertebrate food they bring. Until someone observes or breeds Aequidens gerciliae directly, the specifics stay unknown; everything here is inference from the genus.
In the aquarium
Aequidens gerciliae is essentially unknown in the hobby. It has never been offered through the ornamental trade, and given its confinement to a handful of remote headwater creeks in northern Mato Grosso — a region collectors rarely visit — that is unlikely to change soon. If specimens ever surfaced, the most honest guide is the husbandry of its better-known genus-mates, above all the closely related saddle cichlid Aequidens tetramerus.
Tank: Seriously Fish sets a base of at least 120 × 17.5 in as the minimum for Aequidens tetramerus; because Aequidens gerciliae tops out around 5 in SL against up to 6.5 in for Aequidens tetramerus, a 100 × 15.5 in footprint might just hold a single pair with no tankmates, though more room always helps once the pair spawns and turns territorial. Use fine sand — this is a fish of small sandy- and rocky-bottomed igarapés — with tangles of driftwood and submerged roots, scattered leaf litter, and a smooth flat stone or two as potential spawning surfaces. Dense planting isn't essential or strongly indicated by the wild habitat, but clumps along the back and sides break up sightlines and take the edge off aggression. Keep the lighting subdued.
Water: reflect the clearwater upper Aripuanã — soft (GH and KH both below 6 °dH), pH 6.0–7.0, and temperature 75–84 °F, with a sensible target near the basin-typical 79–82 °F and pH around 6.5. Like other Aequidens, treat it as sensitive to deteriorating water quality: heavy, mature biological filtration, weekly water changes of at least 40–50 %, and no overstocking. Skip strong flow — the natural habitat is a sluggish headwater stream, not a torrent.
Feeding: feed it as the wild insectivore it is — high-quality sinking pellets as a staple, backed generously with frozen or live bloodworm, Daphnia, Artemia and small earthworm pieces. Variety matters more than any single food.
Tankmates: expect the standard biparental acara temperament — a compatible pair is tolerant and fairly peaceable day to day, then emphatically territorial around the spawning site, the male usually the more aggressive during breeding. Small or thin-bodied fish risk being eaten or harassed into the corners, so a species-only or near-species setup is safest; a pair in a well-structured tank with enough broken sightlines is a realistic goal.
Common mistakes: don't introduce the fish to an immature aquarium — Aequidens are reliably reported to develop head-and-lateral-line erosion and stunted growth when water quality is chronically suboptimal. Any breeding in captivity would follow the biparental open-substrate template described in the Breeding section: eggs on a cleaned flat surface, both parents fanning and guarding the clutch, then herding the free-swimming fry around the tank. There are, to date, no documented aquarium observations of Aequidens gerciliae; everything above is calibrated inference from its congeners.
Conservation
The IUCN Red List assesses Aequidens gerciliae as Least Concern (assessed 7 November 2018 by the Instituto Chico Mendes de Conservação da Biodiversidade, ICMBio, and published in 2022), with a population trend judged stable. The reasoning: although the species has a restricted, headwater distribution, it is known from more than twenty collection lots and numerous specimens across several nearby localities, shows no sign of decline, and appears resilient to human disturbance — there are recent records close to the type locality, which itself sits near urbanised, farmed ground. The assessors do flag real pressures on the upper Aripuanã basin: agriculture and livestock ranching, mining and quarrying, and planned hydroelectric dams, with the type locality subject to anthropogenic change. The species occurs in no protected area, is not in trade, carries no CITES listing, and has no conservation programme directed at it. For a narrow-range endemic, those basin-scale pressures — damming above all, plus the spread of agriculture and mining — are the factors worth watching over the long run, even though none is presently documented as causing a decline.