Taxonomy & naming
Dicrossus foirni was described by Uwe Römer, Ingo J. Hahn and Pablo M. Vergara in 2010, in the journal Vertebrate Zoology, together with its Rio Tapajós congener Dicrossus warzeli. The two were the last members of the genus to be formally named: Dicrossus had been erected by Steindachner in the nineteenth century, and by the time of this paper it already contained Dicrossus maculatus, Dicrossus filamentosus and (since 2008) Dicrossus gladicauda. Both new species are diagnosed against all other Neotropical cichlids by exhibiting three longitudinal rows of dark lateral spots; in foirni those spots take the form of clustered individual dots, whereas warzeli shows three rows of horizontal dash-like marks, and the two also differ in the relationship between body depth and length. The authors regarded foirni and warzeli as probably more closely related to each other than to the three older species, a hypothesis they noted would need genetic testing. The holotype, an adult female catalogued as MZUSP 106589 in the Museu de Zoologia da Universidade de São Paulo, was taken with a large paratype series in the Rio Preto near Lago Urubu, about 8 mi north-northwest of that river's junction with the Rio Negro, in the Brazilian state of Amazonas. Paratypes are split among the Senckenberg Natural History Collections in Dresden (MTD F), the California Academy of Sciences (CAS) and MZUSP. Before its description the fish had circulated among aquarists under several informal labels — Dicrossus sp. "Peru" (a misnomer, since it is a Rio Negro fish), "Rotflossen" (red-fin), "Rio Negro", "Doppelfleck" and "Doppelpunkt" (double-spot) — names that survive only as trade history. Within the family the genus sits among the small "crenicaratine" cichlids of the subfamily Geophaginae, close to Crenicara.
Morphology
This is a small, elongate, laterally compressed dwarf cichlid with a body depth of roughly 30 percent of standard length in adults, deepening to about 34–37 percent in ripe, dominant fish. The head is elongate and bears a small, narrow, terminal, pointed mouth. The genus name ("two tassels") refers to the doubled, fringe-like spinous lobes of the gill cover characteristic of these cichlids, and like its relatives foirni has a finely serrated preoperculum. Counts run to fifteen or sixteen dorsal spines, six or seven dorsal soft rays, three anal spines and six or seven anal soft rays. The defining mark is the flank pattern: three rows of round to vertically ovoid, alternating dark-grey to blackish spots, here formed by clusters of smaller dots that distinguish foirni from the dash-marked warzeli. Adult males also carry a black blotch on the dorsum and the basal part of the dorsal fin at about the seventh and eighth hard rays. The species shows pronounced sexual size and colour dimorphism. Males are much larger — reported in the description up to about 3 in SL, with FishBase listing roughly 3 in — and develop a pointed, rhombic to spade-like caudal fin, elongated pointed ventral fins, a blue submarginal band on the dorsal, and reddish, milky-blue and violet tones in the unpaired fins. Females stay small (to roughly 2 in SL, about 2 in), keep a rounded caudal fin, and during reproduction flush their ventral fins intense red-orange with a blue leading edge — a brood-care signal repeated across the genus.
Habitat
Dicrossus foirni is endemic to the middle and upper Rio Negro system in Amazonas, Brazil, where its known collecting sites lie exclusively in left-bank (northern) tributaries upstream of the Negro's confluence with the Rio Branco. It is, above all, a blackwater fish. Juveniles have been found exclusively in very soft, highly acidic clear and black water — water with no detectable hardness and a pH regularly measured below 5, in most cases below 4 — among submerged leaf litter and root tangles in the dim margins of forest streams and pools. Intriguingly, the species proved less rigidly stenotopic than first assumed: sub-adults have also turned up in whitewater brooks, so that across its life history the fish has been recorded in clear, black and white water, sometimes in mixed shoals with its congener Dicrossus filamentosus, several Apistogramma (Apistogramma gibbiceps, Apistogramma paucisquamis, Apistogramma pertensis) and the cardinal tetra Paracheirodon axelrodi. These are shallow, structurally complex habitats — sandy or leaf-strewn bottoms in water typically only knee- to waist-deep — and warm in the manner of lowland Amazonian blackwaters. No formal depth survey exists; the fish occupies the shallow litter zone rather than open or deep water.
Feeding
Like other checkerboard cichlids, Dicrossus foirni is a micro-predator of the leaf litter. Field observers describe it working the bottom by picking up and turning over submerged leaves, then taking the small invertebrates — insect larvae, micro-crustaceans and other meiofauna — sheltering beneath them, a foraging style well matched to its small, narrow, pointed mouth. FishBase places it at a trophic level of about 3.2, consistent with a carnivorous bottom-feeder rather than a sediment-sifter. In the nutrient-poor blackwaters it inhabits, this leaf-turning habit lets it exploit the rich community of small animals concealed in the drowned-forest litter, the same "basement level" of decaying leaves and twigs that supports many of the dwarf cichlids and tetras it lives alongside.
Mating
Detailed in-situ reproductive observations of Dicrossus foirni specifically are scarce, but the genus follows a consistent pattern that the species shares. Dicrossus are not pair-bonding cichlids in the strict sense; they breed best in a harem, with one male loosely holding a territory across which several females each defend a smaller spawning area. The female, not the male, chooses and prepares the spawning site and leads courtship and brood care. Courtship is a display of fin-spreading and body-curving, with the male flaring his enlarged, colourful fins while the female solicits with tail-waving and assumes her vivid red-orange brood-care livery. Females can be assertive, and spawning bouts may leave fish looking briefly battered; in foirni and warzeli the male's role narrows largely to fertilisation and broad territorial patrol once eggs are laid. Sexual size and colour dimorphism is marked, which makes the larger, brighter, longer-finned males easy to tell from the smaller females once mature.
Breeding
Dicrossus are open substrate spawners with female-led, often effectively maternal, brood care — not cave or mouthbrooding cichlids. Specific spawning data for Dicrossus foirni in the wild have not been published, but its close relatives are well documented and provide the genus-typical picture. The female selects and cleans a site — a broad leaf, a flat surface, or a piece of décor — and lays a modest clutch, on the order of dozens to perhaps a couple of hundred small eggs in congeners, which she alone tends while the male patrols. A striking flexibility recurs in the genus: when the eggs are laid on a stable, attached surface the female guards a fixed nest, but when she chooses a loose, detached leaf she may carry that leaf of eggs around with her, becoming a "mobile platform" brooder. Eggs in congeners hatch in roughly two to three days; the female then gathers the wrigglers into tended piles, shepherds the free-swimming fry, and signals to them with her brightly coloured ventral fins. Successful rearing demands very soft, acidic water — congeners hatch best at pH well under 5 — and a sex ratio skewed toward females, since a lone female is often badgered into re-spawning before she can raise a brood. None of this has been confirmed for foirni itself, which is rarely bred, but as a blackwater Dicrossus it is expected to follow the same template.
In the aquarium
Dicrossus foirni is an advanced, infrequently traded fish, and almost everything written about keeping it is extrapolated from its better-known congeners Dicrossus filamentosus and Dicrossus maculatus plus the species' own field chemistry. It is not a beginner's cichlid. The single hardest requirement is water: this is a fish of extreme blackwater, recorded in nature at no detectable hardness and pH often below 4. To keep and especially to breed it, soft, ion-poor, acidic water is non-negotiable — reverse-osmosis water re-mineralised lightly and acidified with peat, alder cones and leaf litter, aiming for very low conductivity and a pH in the 4.5–6.0 range, with temperatures around 81–86 °F in the warm lowland-Amazon manner. Hard, alkaline tap water will not do.
A single male with two or more females in a well-furnished tank of at least 20 US gal (a 20-gallon "long" or larger) suits the harem social structure better than an isolated pair. The aquascape should recreate the leaf-litter biotope: a soft sand bottom, generous drifts of dried oak, beech or catappa leaves, driftwood and roots for sightline breaks, shade-tolerant plants such as Anubias or Java fern, dim lighting, and gentle, well-oxygenated flow. Tannin-stained "tea" water is ideal and helps hold pH down. Tankmates should be small, peaceful blackwater associates — pencilfish, small tetras such as cardinals, and the like — kept partly as dither fish to draw out the females' protective behaviour; avoid boisterous or large fish entirely. Feed small live and frozen foods (bloodworm, daphnia, brine shrimp, micro-crustaceans), which these litter-pickers take far more readily than dry flake. The commonest ways keepers lose this fish are nitrogenous-waste accumulation in under-maintained tanks, water that is too hard or too alkaline, and trying to keep it in a bare, brightly lit setup that denies it the litter and cover it needs.
Conservation
The IUCN Red List assesses Dicrossus foirni as Data Deficient (assessed 7 November 2018, by the Instituto Chico Mendes de Conservação da Biodiversidade, ICMBio). There is simply not enough information on its distribution, population size and trends to assign a threat category — unsurprising for a fish described only in 2010 and known from a handful of remote blackwater tributaries. It carries no CITES or CMS listing. Two broad pressures frame its outlook. First, it is a narrow-range Rio Negro endemic restricted to soft, hyper-acidic blackwaters, a habitat type especially sensitive to disturbance; the original authors explicitly noted that hydroelectric and infrastructure development planned across the Amazonian wilderness threatens the functional ecosystems on which both the fish and the region's indigenous peoples depend. Second, the species enters the ornamental trade in small numbers, though there is no evidence that collection presently threatens it. Its very name honours FOIRN, the federation of indigenous organisations of the Rio Negro whose stewardship of these tribal lands made the species' discovery possible — a reminder that its conservation is bound up with the integrity of the blackwater basin and the people who manage it.