Taxonomy & naming
Melanorivulus planaltinus was described by Wilson J. E. M. Costa and T. M. Ramos in 2010 from small tributaries of the Rio Paraná drainage in São Paulo state, southeastern Brazil. The authority is placed in parentheses because the species was originally assigned to a genus that has since been reorganised under the current classification, with Melanorivulus recognised as the valid placement by Eschmeyer's Catalog of Fishes.
The genus Melanorivulus belongs to the family Rivulidae (South American killifishes) and falls within a non-annual subclade whose members inhabit permanent or near-permanent water bodies in contrast to the dry-season annual rivulines for which the family is better known. Melanorivulus encompasses a group of small, sexually dichromatic, plant-spawning killifishes distributed across the Atlantic Forest and adjacent cerrado-edge drainages of Brazil. The specific epithet planaltinus means 'from the Planalto' — the interior plateau of southeastern Brazil — precisely delimiting the species' geographical character from closely related Atlantic Forest lowland congeners.
No subspecies or named populations are currently recognised. The original description was accompanied by morphological data distinguishing M. planaltinus from congeners by body colouration pattern and certain meristic counts, but as with many small rivulines the species has received limited subsequent taxonomic scrutiny.
Morphology
Melanorivulus planaltinus is a slender, laterally compressed killifish reaching approximately 1 in standard length. The body outline is elongate and cylindrical through the anterior portion, tapering toward the caudal peduncle, with the dorsal fin set well back — a characteristic profile shared across the genus. Like most Melanorivulus, the species is sexually dichromatic: males display the iridescent lateral patterning typical of the genus, with metallic blue-green or blue-gold flecks arranged in a reticulate or banded pattern across the flanks, the precise arrangement being taxonomically informative. The dorsal and caudal fins of males are typically suffused with colour, often with a contrasting dark margin or red border.
Females are plainer in colouration — a brownish or olive-grey ground colour with reduced iridescence — and are marginally stockier through the body cavity when gravid. Both sexes share the relatively large eye characteristic of small forest-stream killifishes, which is consistent with activity in the dappled, low-intensity light beneath closed canopy. Fin counts and squamation correspond to the general Melanorivulus diagnosis; the lateral-line system is reduced as is typical for killifishes.
Habitat
The species is native to small, low-gradient forest stream tributaries of the Rio Paraná system in the São Paulo state plateau region of southeastern Brazil. These streams flow through Atlantic Forest fragments — closed-canopy riparian woodland over sandy, gravelly, or organic substrates — and remain permanent year-round, in contrast to the temporary pools and seasonal streams occupied by annual killifishes in the same broad biogeographic zone. Water is clear and soft, with accumulated leaf litter providing both shelter and a reservoir of invertebrate prey.
Water chemistry at collection localities reflects the soft, acidic geology of the Atlantic Forest highland: temperature in the range of 64–75 °F (the plateau elevation moderates the heat relative to coastal lowlands), pH approximately 4.5–6.5, and low general hardness of 2–10 dGH. Dissolved oxygen is maintained by the permanent flow and surface agitation of these small streams. Dense aquatic and marginal vegetation — submerged plants, roots, overhanging grasses, and floating debris — provides cover for both adults and newly hatched fry.
The species is micro-endemic: its documented distribution covers only a few stream reaches on the Planalto, and it is not known from the main-channel Rio Paraná or from drainages beyond the plateau's immediate watershed. This small range is a defining conservation concern.
Feeding
Like other Melanorivulus, M. planaltinus is a small invertivore, subsisting on the zooplankton, chironomid larvae, small aquatic insects, and other micro-invertebrates that are abundant in leaf-littered forest streams. The proportionally large eye suggests a visual hunting strategy adapted to the dim conditions beneath forest canopy, and the terminal mouth is well-positioned for surface or mid-water predation on small prey items.
In the aquarium the species readily accepts a varied diet of live and frozen small invertebrates — Artemia nauplii, Daphnia, microworms, and bloodworm (chironomid larvae) — as well as finely powdered or micro-granule dry foods. Like most small killifishes it benefits from multiple small feedings per day rather than a single large ration, and varied live or frozen food supports the best colouration in males and reproductive readiness in females.
Mating
Melanorivulus planaltinus is a non-annual killifish and its reproductive mode is fundamentally different from the annual peat-spawning rivulines with which the family is often associated. Rather than burying eggs in substrate to survive a dry season, the species pairs in permanent water and deposits adhesive eggs directly onto fine-leaved aquatic plants, submerged roots, and spawning mops at or near the water surface or along the substrate. This plant-spawning strategy produces eggs that develop in water without any diapause period.
Male M. planaltinus establish loose territories and court females with lateral displays that show off their iridescent flank patterning to best effect. Spawning occurs over extended periods rather than in a single event: a mated pair will visit spawning sites repeatedly over days or weeks, depositing a small number of eggs at each visit. This iterated pattern of egg-laying is characteristic of plant-spawning rivulines and spreads reproductive risk across time and multiple microhabitats.
Breeding
Eggs of M. planaltinus are small, adhesive, and relatively resistant — consistent with the non-annual rivuline strategy of eggs that must develop in situ on vegetation rather than in a protected peat medium. Incubation at aquarium temperatures of 72–75 °F takes approximately 12–18 days, after which larvae emerge as free-swimming fry already capable of taking small live prey. There is no embryonic diapause; the eggs develop continuously from fertilisation through hatching, and cool temperatures slow but do not arrest development.
Under aquarium conditions breeding is straightforward provided water quality is maintained and adults are kept in good condition. A clump of fine-leaved plants (Java moss, hornwort) or a synthetic spawning mop placed in the tank provides the spawning substrate. Eggs can be collected by removing the mop or plant clump to a separate rearing vessel, or by simply keeping the aquarium well-planted so that fry have sufficient cover to survive alongside the adults. First foods for fry are infusoria, paramecia, or commercial fry powders, with Artemia nauplii introduced as the fish grow. Adults in a species tank can live 3–4 years or longer — a lifespan markedly greater than annual species and a consequence of the permanent-water, non-seasonal lifestyle.
In the aquarium
Melanorivulus planaltinus is suited to a small, well-planted species tank or a carefully matched community of similarly sized, peaceful fish. An aquarium of 10–15 US gal is adequate for a pair or small group; the soft, acidic water parameters of the wild habitat (pH 4.5–6.5, temperature 64–75 °F, soft to moderately soft) should be approximated for best colouration and breeding success, though the species will tolerate somewhat harder or more neutral conditions without obvious distress. Gentle filtration that avoids strong current is ideal — the fish inhabits still to gently flowing forest streams, not fast riffles.
The aquascape should provide dense planting, floating cover, and subdued lighting to replicate the shaded, leaf-littered conditions of Atlantic Forest streams. A substrate of fine sand with scattered oak or catappa leaves contributes to appropriate water chemistry and gives the fish natural-looking refuge zones. Males in good condition are visually striking for their size and will display to one another as well as to females, making a small group (two males, three or four females) an active and interesting display.
The species remains rare in the hobby, circulating primarily among specialist killifish societies and breeders interested in Melanorivulus and Atlantic Forest rivulines. Its small size, peaceful nature, and ease of breeding in an appropriate setup make it a rewarding fish for the specialist, but it is not available through general retail channels.
Conservation
Melanorivulus planaltinus has not been formally assessed by the IUCN Red List. Its status is recorded as Not Evaluated (NE), and no regional assessment is known. This absence of formal evaluation is itself a conservation concern: the species is a micro-endemic restricted to a small number of stream reaches on the Planalto of São Paulo state, within a region of the Atlantic Forest that has lost well over 80% of its original forest cover to agriculture, urban expansion, and pasture development.
The Atlantic Forest is one of the most biodiverse and most threatened biomes on Earth, and its endemic freshwater fish fauna — of which Melanorivulus is a notable component — faces ongoing pressure from stream channelisation, sedimentation from deforested catchments, water abstraction, and the introduction of non-native species including Poecilia reticulata (guppies) and other livebearers that compete with and may predate small killifish. For a micro-endemic species occupying only a handful of stream reaches, local extirpation events driven by any of these pressures could eliminate the species entirely before it has been formally evaluated or protected.
Conservation of M. planaltinus depends on the maintenance of intact riparian forest in its native watershed — a goal that aligns with broader Atlantic Forest restoration efforts but has received no species-specific attention. Aquarium breeding programmes within killifish societies represent a supplementary safeguard, maintaining captive populations of a fish that could otherwise disappear quietly in the gap between discovery and assessment.