Taxonomy & naming
Melanorivulus megaroni was formally described by Wilson J. E. M. Costa in 2007, originally under the genus Rivulus; the authority is therefore cited in parentheses as (Costa, 2007) to indicate the subsequent transfer to Melanorivulus. The genus Melanorivulus was established to accommodate a clade of South American rivulines formerly scattered across the broadly defined Rivulus, following molecular and morphological revisions that dismembered the old, polyphyletic genus. Eschmeyer's Catalog of Fishes (CAS) records Melanorivulus megaroni (Costa, 2007) as the valid combination.
The species epithet megaroni refers to the Megaroni River or the broader Megaroni territory, a reference to the indigenous Kayapó lands within which the type locality lies — the Parque Indígena do Xingu in the state of Mato Grosso, Brazil. Specimens from the original description were collected from a small stream crossing road BR-080, approximately 4 mi east of the Xingu River, at roughly 10°43' S, 53°00' W. Additional material includes a male 1 in SL catalogued under UFRJ 9416, collected in the Canarana area of Mato Grosso.
Within Melanorivulus, this species belongs to a group of Xingu drainage endemics characterised by high geographic restriction and significant inter-species morphological diversity. The original description reports meristics of 34–36 scales in the longitudinal series, 7 pelvic-fin rays, and 28–31 caudal-fin rays — a suite of characters that distinguishes it from close congeners in the drainage.
Morphology
Melanorivulus megaroni is a small killifish; the original description records a maximum standard length of approximately 1 in SL, with the holotype and type series falling within that range. A male specimen catalogued under UFRJ 9416 measured 1 in SL, indicating that males can reach close to 1 in. The body form is typical of non-annual Melanorivulus: moderately elongate, somewhat cylindrical anteriorly, with a slightly compressed posterior region. The caudal fin in males typically shows the pointed or rounded profile characteristic of the genus, with 28–31 rays.
Colouration in life follows the Melanorivulus pattern for Xingu drainage species — males display patterned flanks with iridescent scales and colour in the unpaired fins, while females are plainer and more cryptically marked. Precise colour descriptions from the field are not available in the sources consulted; the coloration of preserved type material is documented in the original description. The relatively large scale count (34–36 in the lateral series) and the 7 pelvic-fin rays are diagnostically important meristic characters that separate this species from sympatric congeners.
Habitat
The type locality of Melanorivulus megaroni is a small stream crossing road BR-080 approximately 4 mi east of the Xingu River, within the Parque Indígena do Xingu, Mato Grosso, Brazil. This places the species firmly within the southern Amazon basin, in the lowland to sub-plateau transitional zone of the Xingu drainage. The stream is typical of Melanorivulus habitat: shallow, marginal, and exposed — the genus characteristically occupies aquatic environments less than 8 in deep, including roadside ditches, seepages, and the vegetated margins of small forest streams where sunlight reaches the water surface.
These habitats in the Xingu catchment typically carry warm, soft, mildly acidic water reflecting the highly weathered, lateritic soils of the Brazilian Cerrado and transitional forest. No specific water chemistry data (pH, hardness, conductivity) were recorded for the type locality in the sources available, but conditions consistent with other Melanorivulus sites in Mato Grosso would be expected: temperatures in the range of 72–82 °F, low conductivity, and pH trending slightly acidic. Aquatic vegetation and leaf litter at the stream margins provide both spawning substrate and cover from predators.
Feeding
No dietary studies specific to Melanorivulus megaroni exist in the published literature. Based on its body form, size, and genus ecology, it is reasonable to characterise it as a micro-predator of invertebrates — targeting small aquatic and terrestrial insects, insect larvae, microcrustaceans, and zooplankton that fall onto or occur near the water surface in its shallow stream-margin habitat. This feeding strategy is standard across non-annual South American rivulines of comparable body size occupying similar shallow, productive microhabitats.
In captivity, Melanorivulus species of this size class readily accept small live and frozen foods including Daphnia, Artemia nauplii, micro-worms, and finely chopped chironomid larvae. Small dry foods of appropriate granule size are typically taken once the fish is settled. Feeding twice daily in modest quantities suits the species' small stomach capacity and helps maintain water quality in the shallow, lightly filtered setups that suit the genus.
Mating
Melanorivulus megaroni is a non-annual killifish, a fact that fundamentally shapes its reproductive behaviour. Unlike the annual killifishes of seasonally desiccating pools — which race through a compressed life cycle and bury diapausing eggs in substrate mud — non-annual rivulines live in permanent or near-permanent water and reproduce continuously or semi-continuously throughout the warm season. Pairs engage in repeated, low-investment spawning events rather than a single exhaustive burst.
Mating involves brief courtship displays in which the male positions himself alongside or slightly above the female, displaying his coloration. Spawning occurs over fine-leaved plants, root masses, or fibrous spawning substrate near the water surface, where individual adhesive eggs are deposited and fertilised over days to weeks. Eggs are small, clear to slightly amber, and adhere to the substrate rather than falling to the bottom. Males of non-annual rivulines are typically polygynous in nature, moving between multiple females in structured shallow habitats where territories are loosely defined by vegetation patches.
Breeding
In the aquarium, Melanorivulus megaroni can be bred using the methods standard for non-annual Melanorivulus: a small, well-planted tank with Java moss, fine-leaved plants, or a synthetic spawning mop positioned near the surface, with a pair or a small group (one male, two or three females) housed together at 75–81 °F. Eggs are deposited individually and should be collected every few days; they can be incubated in a small container of aquarium water at the same temperature, or left in a finely planted setup where the density of vegetation provides some protection from egg-eating adults.
Incubation at around 77 °F typically runs two to three weeks for non-annual rivuline eggs of this size class, though no species-specific incubation data are published for Melanorivulus megaroni. Fry are small at hatching but free-swimming and able to take micro-foods (infusoria, commercial fry powder, Paramecium) from the outset; they graduate to Artemia nauplii within a week. Growth is relatively steady for a small rivuline, and juveniles can reach sexual maturity within two to three months under good conditions. Adults should be fed live or frozen invertebrates regularly to sustain spawning condition.
In the aquarium
Melanorivulus megaroni is suited to a small, dedicated setup rather than a community aquarium. A tank of 10–15 US gal with a tight-fitting lid (all killifishes are escape risks) and dense planting at the surface — floating plants, fine-leaved stem plants, Java moss — replicates the shallow, vegetated stream margins of its native habitat and gives the fish both spawning sites and security cover. Subdued lighting is preferred; direct overhead light without shading tends to make the fish nervous and suppresses natural behaviour.
Water conditions should reflect the soft, warm, mildly acidic chemistry of its Xingu basin origin: temperature 73–81 °F, pH 6.0–7.0, low hardness, low conductivity. Gentle filtration with a sponge filter maintains water quality without creating the strong current that would be inappropriate for a species from still, shallow pool margins. The species should not be mixed with larger or more aggressive fish; it is best kept with its own kind or with peaceful dwarf species of a similar size.
As with most Melanorivulus, the fish is not widely available in the trade and is primarily maintained by specialist killifish keepers and hobbyists associated with national killifish associations. Those seeking stock should contact the American Killifish Association (AKA) or equivalent European bodies, where species from the Xingu drainage occasionally circulate through member fish lists.
Conservation
FishBase records Melanorivulus megaroni as Least Concern (2018), reflecting the IUCN assessment for this species. The type locality — a small stream within the Parque Indígena do Xingu, a large indigenous protected area in Mato Grosso — provides a degree of institutional protection unavailable to many small-stream Xingu endemics. The park's legal status restricts logging, agricultural conversion, and road development within its boundaries, limiting some of the major freshwater habitat threats that affect unprotected watersheds in the southern Amazon.
Nevertheless, Melanorivulus megaroni occupies a very restricted, single-drainage range within the Xingu basin, and the shallow, marginal microhabitats it depends on are inherently vulnerable: minor changes in hydrology, sedimentation load, or riparian vegetation can eliminate the shallow, plant-fringed stream margins that the genus requires. The wider Xingu basin faces ongoing pressures from the expansion of soy and cattle agriculture along its southern margin, road network expansion (road BR-080, the type locality, is a deforestation frontier corridor), and the long-term hydrological legacy of the Belo Monte dam complex on the Xingu River main stem. Monitoring of this and other single-locality Melanorivulus endemics as the Xingu's tributaries come under increasing agricultural pressure would be a prudent conservation measure.