Taxonomy & naming
Melanorivulus nigromarginatus was first described by the German aquarist and ichthyologist Hermann Meinken in 1952 under the name Rivulus nigromarginatus, from material collected in the Corrente drainage of Goiás, Brazil. For most of the twentieth century it sat within the broadly circumscribed genus Rivulus, a catch-all for South American rivulines that has been progressively dismembered as molecular and morphological analyses have resolved the group's true phylogeny.
In 2018 Wilson J.E.M. Costa formally transferred the species to Melanorivulus in a paper published in Zoosystematics and Evolution (volume 94, number 1, pages 17–27), which simultaneously described three new species from the rio Paraná system of the central Brazilian Cerrado. Costa's revision placed nigromarginatus in the 'pictus group' of Melanorivulus — a clade of small, slender rivulines from the Paraná and adjacent drainages. The authority is therefore written as (Meinken, 1952) in parentheses to indicate that the species was described in a different genus from the one it currently occupies.
Eschmeyer's Catalog of Fishes treats the valid name as Melanorivulus nigromarginatus (Meinken, 1952). Note that the genus Laimosemion, in its broader modern usage, has been applied to portions of this same South American rivuline assemblage in some classifications; regardless of the generic assignment used, the specific epithet nigromarginatus and the Meinken 1952 authority remain fixed. The holotype is deposited as UFRJ 8434 in the collection of the Universidade Federal do Rio de Janeiro.
This fish was long listed as Laimosemion nigromarginatus; Eschmeyer's Catalog of Fishes places the species in Melanorivulus, and the current combination Melanorivulus nigromarginatus is followed here.
Morphology
Males of Melanorivulus nigromarginatus reach a maximum of 1.4 in standard length; females are smaller, typically topping out at about 1.2 in SL — modest dimensions even by killifish standards. The body form is slender and elongated, consistent with the rivuline body plan: a laterally compressed but not deep profile, a slightly upturned snout, and a rounded caudal fin.
The diagnostic feature from which the species takes its name is the striking black marginal stripe along the distal edge of the anal fin in males — nigromarginatus derives directly from the Latin for 'black-edged'. The flanks carry the colourful spotting or barring typical of the pictus group, and the overall patterning is more vivid in dominant males. Females are cryptically coloured, lacking the intense anal-fin margin and most of the lateral ornamentation seen in males.
Fin-ray counts reported for the species are: dorsal soft rays 10–11, anal soft rays 13–15, and a vertebral count of 30–32 — characters that, combined with colouration, distinguish nigromarginatus from its closest congeners in the Corrente drainage. The relatively high anal-fin ray count is a feature of the pictus group within Melanorivulus.
Habitat
The known distribution of Melanorivulus nigromarginatus is extremely restricted: two small, adjacent streams in the middle section of the rio Corrente drainage, specifically in or near the Lajeado stream (a tributary of the rio Corrente) at Itarumã municipality, Goiás, Brazil (approximately 18°49'45"S, 51°22'55"W). The rio Corrente is itself a tributary of the upper Paraná basin. This locality sits in the Cerrado, Brazil's vast tropical savanna-woodland biome, where watercourses alternate between perennial gallery-forest streams and more seasonal drainage networks.
As a non-annual species, Melanorivulus nigromarginatus inhabits permanent or near-permanent water bodies — shallow, typically clear to slightly tea-stained streams running through gallery forest. These habitats are generally characterised by soft, acidic water with low mineral content, abundant aquatic and riparian vegetation at the margins, and a fine leaf-litter or sandy substrate. Water temperatures in the Cerrado interior fluctuate seasonally; dry-season minima can be cool relative to Amazonian lowlands, while warm-season maxima under direct sunlight push higher.
The hobbyist literature associates the type locality (Itarumã, Goiás) with the standard collection point for this species. Its apparent confinement to just two streams gives it a range among the smallest of any rivuline in the Paraná basin, with obvious implications for its resilience to disturbance.
Feeding
Like most small South American rivulines, Melanorivulus nigromarginatus is a micro-predator in the wild, taking invertebrates at or near the water's surface and in the mid-water column. The typical diet of stream-dwelling Melanorivulus species consists of small insects and their larvae, worms, microcrustaceans, and other zooplankton, with opportunistic feeding on whatever invertebrate prey is seasonally abundant.
In the aquarium the species is easy to feed. It accepts small live and frozen foods — Drosophila, artemia nauplii, micro-worms, cyclops, and small bloodworm — and adapts to good-quality dried micro-pellets and flake once settled. Feeding near the surface or mid-water column is natural; these are not bottom-foragers. Variety in the diet, and inclusion of live or frozen foods, maintains the colouration and conditioning needed for successful breeding.
Mating
Melanorivulus nigromarginatus is a non-annual, plant-spawning killifish — its reproductive biology is fundamentally different from the annual, peat-burying killifishes that dominate hobbyist popular culture. The species inhabits permanent water and has no need for diapausing eggs; instead, pairs deposit small numbers of adhesive eggs over fine-leaved aquatic plants, floating plant roots, or artificial spawning mops over an extended period rather than in a single spawning event.
Male courtship involves lateral display — the male aligns alongside the female, spreading and vibrating the fins to show the black-margined anal fin and any flank colouration at maximum advantage. Receptive females follow the male into dense vegetation or the fibres of a spawning mop, where eggs are individually fertilised and adhere to the substrate. Because spawning is continuous over days or weeks rather than a discrete seasonal event, breeding pairs can produce a steady trickle of eggs when maintained in good condition.
Breeding
The non-annual plant-spawning mode means that eggs of Melanorivulus nigromarginatus incubate in water, with no diapause required and no peat storage. Hobbyist experience with this and closely related Melanorivulus species indicates an incubation period of roughly two to three weeks at typical aquarium temperatures before free-swimming fry emerge — considerably faster than the multi-month dry-storage required for annual killifishes.
Eggs are best harvested by pulling or replacing the spawning mop weekly and incubating the eggs separately in shallow water with a drop of methylene blue to suppress fungal infection. Alternatively, a heavily planted spawning tank with few predators can be left to run, with fry removed as they are noticed. Newly hatched fry are small but large enough to take artemia nauplii immediately; micro-worms and paramecium also work as first foods. Growth is moderate and the fish reach sexual maturity at several months of age.
Maintaining a ratio of two females per male, or keeping the sexes in a group with multiple females, reduces the intensity of male pursuit and improves female condition. The species is short-lived relative to many aquarium fish — as with most small killifishes, two to three years is a reasonable lifespan under good aquarium conditions — so a steady production of fry is the practical approach for anyone wishing to sustain the species in the collection.
In the aquarium
Melanorivulus nigromarginatus is a niche-interest species kept primarily by killifish enthusiasts within organisations such as the American Killifish Association, where it is catalogued as a Class 2 fish. Its small size — under 1.6 in — means it can be housed in modest aquaria of 8–16 US gal, although a longer tank is preferable to a tall one to provide horizontal swimming space.
Water conditions should reflect the soft, moderately acidic conditions of its Cerrado stream habitat: a temperature range of approximately 72–79 °F, pH in the 6.0–7.0 range, and low to moderate hardness. Subdued lighting, a dark substrate, and floating plants or surface cover encourage natural behaviour and enhance the colouration of males. Filtration should produce gentle flow; the species is not adapted to strong current. A tight-fitting lid is essential, as with all killifishes, since these are capable jumpers.
The species does well in a species-only setup or with very small, peaceful companions that will not compete aggressively for food. Mixed-killifish tanks are possible with appropriate size-matching. Because the wild population is so small and the species is not commercially farmed at scale, specimens in the hobby are typically obtained through AKA member trades or specialist breeders; maintaining and propagating the species in captivity has genuine conservation relevance for a fish with such a restricted natural range.
Conservation
Melanorivulus nigromarginatus has not been formally assessed by the IUCN Red List and carries a status of Not Evaluated (NE). Given what is known of its range — apparently confined to two small adjacent streams in a single municipality of Goiás state — it would almost certainly qualify for a threatened category if a formal assessment were conducted. Highly localised single-drainage endemics in the Cerrado are among the most at-risk freshwater fish on Earth.
The Cerrado is one of the most biodiverse and simultaneously most threatened biomes in South America. Agricultural conversion — primarily soybean, cotton, and cattle production — has destroyed or degraded the majority of the original Cerrado, and the remaining gallery forests and riparian corridors that support stream-dwelling killifishes are under continuous pressure from deforestation, conversion to irrigated cropland, agrochemical run-off, and water extraction for irrigation. The rio Corrente drainage in Goiás lies in an agricultural frontier zone and has experienced significant habitat transformation in recent decades.
For a species whose entire known range occupies two small tributaries, a single land-use decision, drainage project, or pollution event could eliminate the wild population entirely. The presence of this species in the collections of killifish enthusiasts — accessed through AKA networks and hobbyist breeders — therefore represents a meaningful ex-situ buffer, even if it cannot substitute for in-situ habitat protection. A formal IUCN assessment, tied to a search for the species across similar Corrente-system tributaries, would be a valuable first step toward understanding the true extent of its range and vulnerability.