Taxonomy & naming
Melanorivulus illuminatus was first described by Wilson J.E.M. Costa in 2007, published in Ichthyological Exploration of Freshwaters (volume 18, pages 193–198). The original authority listed in Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & Fong, CAS) is Costa 2007; the authority string (Costa & Ramos, 2010) reflects a subsequent nomenclatural act placing the species within the genus Melanorivulus, which was erected by Costa as part of a broad re-classification of South American rivuline killifishes away from the catch-all genus Rivulus. The valid name under the Catalog of Fishes is Melanorivulus illuminatus, and the holotype is deposited at the Universidade Federal do Rio de Janeiro (UFRJ 6463), with paratypes at UFRJ 6464.
Melanorivulus is a substantial genus of South American rivulines endemic primarily to the cerrado and adjacent biomes of Brazil and neighbouring countries. Within the genus, M. illuminatus is distinguished by a combination of characters: only six pelvic-fin rays (versus seven in most congeners), 32–33 caudal-fin rays, 33–35 lateral-line scales arranged in eight transverse series, and a dorsal-fin ray count of 9–11 paired with 12–14 anal-fin rays. These meristic details, recorded by the Practical Fishkeeping report on the original description and corroborated by the It Rains Fishes husbandry database, place it firmly apart from its closest relatives and provide a reliable field-identification signature for specimens held in captivity.
The species epithet illuminatus is Latin for 'illuminated' or 'brilliantly lit', coined for the striking bright-blue iridescence on the flanks of adult males. The genus name Melanorivulus combines the Greek melas (black) with the Latin rivulus (small stream), a reference both to the dark patterning that many species in the group display and to the stream and pool habitats the genus occupies across the Brazilian interior.
Morphology
Melanorivulus illuminatus is a miniature killifish; FishBase records a maximum of 1 in SL for males, with females slightly smaller (FishBase also lists 1.5 in SL, possibly a total-length figure or reflecting a larger female in the type series). The It Rains Fishes database gives 1 in as the practical adult size. In overall body plan the species follows the typical rivuline template: a slender, slightly laterally compressed body, rounded fins, an upturned mouth well-positioned for surface feeding, and relatively large eyes.
Sexual dichromatism is pronounced. Adult males display the brilliant blue-silver iridescence on the flanks that inspired the name, overlaid with longitudinal rows of red or orange-red dots. Head pigmentation is reduced — a feature the Practical Fishkeeping description notes as diagnostic: males of M. illuminatus show less black pigmentation on the head than many congeners, making the bright flank pattern more vivid by contrast. Females are cryptic by comparison, lacking the blue iridescence and the red-dot rows; instead, they carry a pale whitish-yellow horizontal mark above the caudal-peduncle spot, a marking that the same source records as characteristic for the sex.
Habitat
The type locality is a swamp adjacent to the córrego da Queixada (a minor watercourse in the río dos Bois drainage, a right-bank tributary of the upper río Paraná) situated approximately 14 mi north of Montividiu on the Goiás state road GO-174. At the time of collection on 16 January 2007, the site consisted of shallow, interconnected floodplain pools with a maximum depth of roughly 12 in and a recorded pH of 5.85–5.9 — soft, mildly acidic water consistent with cerrado blackwater environments influenced by organic decomposition but not extreme. The elevation at the type locality is 805 metres above sea level.
The broader setting is Brazil's cerrado — a vast tropical savanna that has been drastically reduced by agricultural conversion, primarily to soy and sugarcane cultivation and cattle ranching. The shallow wetlands and seasonal pools that characterise cerrado headwater systems are particularly vulnerable because they are small, isolated, and lie in lowland areas easily drained or modified. Melanorivulus illuminatus is known only from this drainage, and FishBase confirms the distribution as the swamp system adjacent to the córrego da Queixada, río dos Bois drainage, río Paraná basin — no other confirmed collection localities are recorded in the published literature or in the AKA database.
Feeding
No detailed dietary studies have been published for Melanorivulus illuminatus specifically. Its body plan — the upturned mouth, the small size, the preference for shallow surface-layer habitat — is consistent with the feeding ecology observed in other small rivuline killies, which typically take small invertebrates including aquatic insect larvae, microcrustaceans, and terrestrial insects that fall onto the water surface.
In aquarium conditions the species proves easy to feed. It Rains Fishes reports acceptance of standard small live and frozen foods. Artemia nauplii (baby brine shrimp) are recommended both for growing fry and for conditioning adults before spawning; adult mosquito larvae, Daphnia, and small frozen bloodworm are suitable as part of a varied diet. The small adult size (under 1 in) means food particle size must be appropriate — overly large offerings are ignored or wasted, and only suitably scaled invertebrate-based foods maintain the colour intensity of males.
Mating
Melanorivulus illuminatus is a non-annual killifish. It inhabits permanent or near-permanent water — the swamp system at the type locality retains water outside dry-season extremes — and it does not produce diapausing eggs buried in substrate. Instead, the mating and spawning system is that of a plant- or mop-spawner: pairs court and deposit adhesive eggs on fine-leaved vegetation, roots, or fibrous spawning mops near the water surface or mid-water, with the female releasing a small number of eggs per encounter over an extended period of days or weeks.
Courting males display the full blue-silver and red-dot livery to females, and competition between males in group settings would be expected given the strong sexual dichromatism. The It Rains Fishes care sheet confirms the spawning-mop methodology and the protracted daily egg-laying pattern. Because egg production is continuous rather than a single burst, a pair or small group can be left with a permanent mop and eggs harvested on a regular schedule, making the species tractable for hobbyist breeders comfortable with regular checking and transfer of eggs.
Breeding
Under aquarium conditions, a spawning pair or small group of Melanorivulus illuminatus deposits three to eight eggs per day on floating Java moss, fine-leaved plants, or synthetic spawning mops. The eggs are adhesive and incubate directly in water — no peat, no diapause — hatching in approximately 14 to 20 days at the temperatures typical of a cerrado-matched aquarium setup. The It Rains Fishes database records both the daily egg count and the incubation period from its own breeding trials.
Fry are small at hatching but immediately free-swimming and capable of taking Artemia nauplii from the first day. Growth is steady; the It Rains Fishes source records that young fish reach sexual maturity at approximately four to five months of age. An interesting environmental note from the same source: incubating eggs at lower pH produces a higher proportion of females — a sex-determination response to ambient water chemistry that may have significance in the wild if natural water conditions shift seasonally. Eggs and young fry should be reared separately from adults to avoid predation. Because this is a small-bodied, non-annual species, adults are comparatively long-lived relative to annual killies and will continue to spawn over the course of months or years with proper care.
In the aquarium
Melanorivulus illuminatus is well-suited to the dedicated killifish keeper. The species is small, peaceful, and — once settled — straightforwardly kept in a modest aquarium. It Rains Fishes recommends a minimum tank size of 2.5 US gal for a pair, though a slightly larger tank of 5.5–10 US gal provides better stability and more space for a small group and a permanent spawning mop. A dark substrate (peat moss or fine dark gravel) enhances both the comfort of the fish and the visual impact of male colouration; dim or diffuse lighting is preferable to bright overhead illumination.
The single non-negotiable in housing this species — as with virtually all killifishes — is an extremely well-fitted lid. Melanorivulus illuminatus is an accomplished jumper, and any gap in the cover will result in losses. Soft, mildly acidic water matching the type-locality conditions (pH around 5.9, low hardness, temperature in the range of 68–79 °F) is optimal, though the species is reportedly tolerant of a modest range around these parameters. Water quality must be maintained through regular small changes; stagnation in a small tank rapidly degrades conditions. The species is not aggressive and can be kept with other similarly sized, soft-water killies in a dedicated tank, though mixed-species setups require care to prevent hybridisation.
Melanorivulus illuminatus has a small but loyal following among killifish enthusiasts. The American Killifish Association lists the species, and collection locality records from Montividiu, Goiás — the single known collection point — appear in hobbyist records. Because the wild population is assessed as Vulnerable with a highly restricted range, captive-bred stock should be prioritised over any wild collection, and maintaining pure, uninterrupted strains from the original collection locality is encouraged by the AKA and similar hobbyist organisations.
Conservation
The IUCN Red List assessed Melanorivulus illuminatus as Vulnerable (VU) in 2018. The assessment reflects the species' exceptionally restricted range: it is known from a single drainage system — the córrego da Queixada in the río dos Bois, an upper-Paraná tributary — within the cerrado of Goiás state, Brazil. The wild habitat is a shallow swamp system with interconnected pools at an elevation of 805 metres. This type of cerrado wetland has been under severe pressure for decades from the expansion of large-scale row-crop agriculture (soy, sugarcane) and cattle ranching across the Brazilian interior.
The cerrado has lost over half its original extent to agricultural conversion, with ongoing deforestation and drainage of the seasonal and semi-permanent wetlands on which endemic species like M. illuminatus depend. Because the species occupies a small swamp adjacent to a minor watercourse, even a single drainage event, a change in local land use, or contamination from adjacent agricultural land could extirpate the entire known population. No formal protected area is documented at or near the type locality. The IUCN assessment year is 2018 and the status has not been superseded as of this writing.
The American Killifish Association notes the Vulnerable status and promotes awareness of the species' conservation situation among hobbyists. Captive populations maintained by AKA members and killi clubs internationally represent an important insurance population for this fish. Hobbyists are encouraged to propagate M. illuminatus from captive-bred stock, maintain strain integrity by keeping records of origin, and avoid wild collection — the species' entire known wild range is a single small swamp in a degraded agricultural landscape.