Taxonomy & naming
Melanorivulus dapazi was described by Wilson J. E. M. Costa in 2005 as Rivulus dapazi, one of seven new species assigned to the R. punctatus species group in that paper. The type locality is the floodplains of the Rio Tenente Amaral, upper Rio São Lourenço drainage, which forms part of the upper Rio Correntes system within the broader Rio Paraguay basin of central Brazil. The authority is therefore written in parentheses — (Costa, 2005) — to indicate that the species has since been moved to a new genus.
The genus Melanorivulus was erected by Costa in 2006 to accommodate a clade of South American rivulines previously housed in the catch-all genus Rivulus, and Rivulus dapazi was transferred accordingly. Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & Fong) recognises the valid combination as Melanorivulus dapazi (Costa, 2005). The species remains in the informal R. punctatus group, a cluster of small, comparably patterned rivulines from the Cerrado and adjacent biomes of central Brazil.
The specific epithet dapazi honours Ricardo Campos da Paz, the Brazilian ichthyologist who collected the type series and first brought the species to the attention of researchers. The American Killifish Association lists the older combination Rivulus dapazi as the primary synonym recognised in the hobby literature, and JAKA articles from 2011 and 2019 document the species under that name.
Morphology
Melanorivulus dapazi is a slender rivuline of modest proportions: FishBase records a maximum of 1.5 in standard length (SL) for males, with females somewhat smaller at around 1 in SL, placing it firmly among the dwarf members of the genus. The lateral-line scale count runs to 34–35 scales. Meristic data from the original description and subsequent hobbyist records give dorsal-fin ray count around 10–11, anal-fin rays approximately 14–15, and a dorsal-to-anal fin origin ratio placing the dorsal well posterior — the pre-dorsal length averages around 74.6% SL — and a relatively shallow body depth of approximately 23.5% SL, giving the fish a pencil-slim profile characteristic of the genus.
Sexual dichromatism is pronounced in the typical rivuline fashion. Males are distinguished by a vivid orange distal stripe along the anal fin with a black ventral outline, and the caudal fin shows a marginal dark orange band with a black ventral edge — the combination that provided the key diagnostic characters separating this species from congeners in the 2005 description. The body base colour in males follows the earthy brownish-olive tones typical of Cerrado rivulines, with iridescent scaling that intensifies in display. Females are cryptically marked, lacking the bright fin patterning, and are reliably identified by this absence in conjunction with the smaller size and rounder abdomen when gravid.
Habitat
The native range of Melanorivulus dapazi is restricted to the upper Rio Correntes drainage of the Rio Paraguay basin, Mato Grosso do Sul, Brazil. Collection localities on record include sites near Sonora, Corrego do Burro, Itiquira, and Rio Comprido — all within the Cerrado biome, the vast tropical savanna of central Brazil characterised by seasonally dry grassland, gallery forest along watercourses, and a deep, permeable soil that supports a rich network of small streams and seepage-fed pools.
The streams that this species inhabits are typical Cerrado watercourses: often shallow, clear to lightly tannin-stained, with sandy or fine-gravel substrates, moderate flow in the wet season, and a tendency to contract dramatically during the dry season between May and September. The IUCN assessment describes the type locality specifically as a Cerrado-typical stream surrounded by soybean and corn plantations — an agricultural matrix that has replaced much of the native savanna in Mato Grosso do Sul, isolating stream habitats as linear remnants in a crop-dominated landscape.
No detailed water-chemistry data for the species' natural waters have been published, but Cerrado headwater streams in the upper Paraguay system are generally soft, slightly acidic, and low in dissolved minerals — conditions consistent with the granitic and sandstone substrates of the central Brazilian plateau. Temperature in these streams tracks the strongly seasonal Cerrado climate, with cold dry-season nights and warm wet-season days.
Feeding
No specific dietary study of Melanorivulus dapazi has been published. Like other small Melanorivulus rivulines of the Cerrado, it is assumed to be a generalist micro-predator in the wild, feeding on small invertebrates encountered at and near the water surface — aquatic insect larvae, microcrustaceans, small worms, and terrestrial insects that fall onto the water. This surface- and mid-water feeding mode is characteristic of the genus and aligns with the torpedo-shaped body and upturned mouth angle seen in the species.
In captivity, hobbyists maintain the species on a varied diet of small live and frozen foods — microworms, Drosophila, baby brine shrimp, Artemia nauplii, and frozen bloodworm or daphnia — with prepared micropellets accepted by acclimated individuals. Given the small adult size, all foods should be appropriately fine; large flake or pellet foods are not suitable. Regular feeding with live or frozen invertebrates promotes the best colour and conditioning, particularly in males.
Mating
Melanorivulus dapazi is a non-annual killifish. Unlike the annual rivulines of temporary pools that bury eggs in substrate mud to survive a complete dry season, this species inhabits permanent or near-permanent Cerrado streams and reproduces as a plant or mop spawner: the pair deposit adhesive eggs on fine-leaved aquatic vegetation, roots, or a floating spawning mop over an extended period of days or weeks, with no diapause phase required in the eggs.
Male courtship in rivulines of this group is persistent and can be vigorous: the male displays lateral coloration toward the female, spreading the distinctive orange-and-black anal and caudal fins to advertise condition, and drives her toward cover or a spawning substrate. Multiple eggs are deposited per session, and spawning activity continues throughout the breeding season — which in non-annual species corresponds to the warm wet season but is not strictly bounded by it. Males housed together can become aggressive; hobbyist experience with the R. punctatus group recommends introducing fish carefully after any separation to prevent injuries.
Breeding
In the aquarium, Melanorivulus dapazi is bred in the same manner as other small non-annual South American rivulines. A small tank of around 2.5 US gal is adequate for a pair or trio; dark substrate (peat moss or fine dark gravel) and dim lighting reduce stress and promote natural behaviour. A floating spawning mop of dark wool or Java moss provides the adhesive spawning surface the fish seek.
Eggs are deposited singly and can be collected by hand from the mop every few days — checking every 3–4 days is a common schedule. Collected eggs are incubated in a small container of tank water treated with a low dose of acriflavine to suppress fungal infection, at around 75–79 °F; hatching typically occurs within approximately 14 days at this temperature range, consistent with hobbyist records for the R. punctatus group. Fry are tiny but free-swimming from hatching and accept infusoria, paramecia, and newly hatched Artemia nauplii from the outset. The species is comparatively long-lived for a rivuline: reports indicate lifespans of up to three years in captivity under good conditions, a reflection of its non-annual life history in permanent water.
In the aquarium
Melanorivulus dapazi is an attractive and manageable species for hobbyists with experience keeping small South American killifish. The small adult size (under 1.5 in) means a species tank of 2.5–5.5 US gal accommodates a breeding pair or trio comfortably, though a slightly larger tank reduces male aggression. Water chemistry should broadly approximate the soft, slightly acidic conditions of Cerrado headwater streams — a pH around 6.0–7.0, low to moderate hardness, and temperatures in the range of 72–79 °F suit the species well; it does not require extreme conditions.
The tank should be well covered (rivulines jump readily) and planted, or furnished with floating mops and surface cover, to give the fish shelter and confidence and to provide spawning sites. Gentle filtration — a small sponge filter — keeps water clean without creating turbulent current. Males display best and spawn most readily when kept in pairs or single-male trios, separated from visual disturbance; cohabitation with other active or fin-nipping species is not recommended.
The species appears in the hobbyist literature primarily through the American Killifish Association, with JAKA articles from 2011 and 2019 documenting keeping and breeding records. It is not commonly traded commercially and is more typically exchanged among specialist killi keepers. A diet rich in small live and frozen invertebrates — Drosophila, baby brine shrimp, microworms — maintains the vivid male colouration that makes the species worth keeping.
Conservation
The IUCN Red List assessed Melanorivulus dapazi as Data Deficient (DD) on 7 November 2018, a status reflecting the species' known occurrence at only a handful of localities and the absence of systematic surveys across the upper Rio Correntes drainage. The type locality — a Cerrado stream surrounded by soybean and corn plantations in Mato Grosso do Sul — encapsulates the core threat facing this fish: the Cerrado is one of the world's most threatened biotic zones, with the majority of its original savanna and grassland cover converted to agriculture over the past half-century, and Mato Grosso do Sul sits at the heart of Brazil's expanding soy belt.
The IUCN assessment notes that the surrounding area was unexplored at the time of evaluation and the species may occur at additional nearby localities, which is why a Data Deficient rather than a threatened listing was assigned — insufficient evidence rather than demonstrated security. No conservation programmes targeting this species are known, and it is not recorded from any protected area. Primary research needs identified in the assessment are delimitation of the full distributional range and surveys of population size and trends.
The pressures on its Cerrado stream habitat are ongoing: agricultural intensification continues to expand, riparian vegetation along small watercourses is frequently cleared for grazing or cultivation, and pesticide and fertiliser runoff from adjacent fields degrades water quality in the very streams this species depends on. For a small-bodied, potentially single-drainage endemic, even moderate habitat modification can translate to significant population decline. Until targeted surveys establish whether Melanorivulus dapazi persists across a broader range than currently documented, its conservation status remains genuinely uncertain.