Taxonomy & naming
Melanorivulus apiamici was described by Wilson J. E. M. Costa in 2007, the authority presented in parentheses indicating that the species was originally placed in a different genus before the modern Melanorivulus arrangement was consolidated. The Catalog of Fishes (Eschmeyer, Fricke & van der Laan) treats Melanorivulus apiamici (Costa, 2007) as the valid combination.
The species belongs to the M. pictus species group within the genus Melanorivulus — a clade of approximately eighteen rivuline killifish distributed across the Brazilian Cerrado, principally in the upper Paraná drainage. This group is characterised by a shared body plan of obliquely banded or spotted flanks and a preference for small, often shaded headwater streams. The genus Melanorivulus itself sits within the family Rivulidae, the South American rivulines, a large and diverse family of non-annual and annual killifish ranging from trinidadian swamps to Andean foothills.
The generic placement of rivuline killifish has shifted substantially over the past two decades as molecular and morphological revisions have dissected the former catch-all genus Rivulus into numerous more precisely defined genera. Costa's 2007 paper placed apiamici in its current genus and formally diagnosed it within the pictus group based on colouration and meristic characters.
Morphology
Melanorivulus apiamici is a slender, laterally compressed fish with a maximum recorded standard length of 1 in — a size typical for small Cerrado rivulines. Like other members of the M. pictus group, the species is distinguished in part by oblique rows of markings along the flanks, an arrangement reflected directly in its name (see Etymology). The dorsal profile is gently curved, the head relatively small and pointed, and the mouth terminal, suited for capturing invertebrates at or near the water surface.
Colouration follows the rivuline pattern of a pale to greenish or tan base on the flanks, overlaid with the oblique lines or rows of spots from which the species takes its name. Males are typically more vividly coloured than females, with more distinct and contrasty markings; females are plainer and rounder-bodied when gravid. Finnage in males may show iridescent highlights. Detailed published meristic data for this species are sparse in accessible literature, but the pictus-group body plan is well documented in the Pensoft Zoosystematics literature covering the broader group.
Habitat
This species is endemic to the middle Rio Paraná basin in São Paulo state, Brazil, in the region overlapping the Cerrado biome. The Cerrado — a vast tropical savanna covering roughly 2 million km² of central Brazil — hosts an extraordinary diversity of small rivuline killifish, many of them restricted to single stream systems or clusters of adjacent headwaters. Melanorivulus apiamici occupies permanent or near-permanent small streams and headwater pools within this landscape.
According to the broader M. pictus species group literature, habitat for these rivulines typically consists of small, shaded forest streams with dense aquatic vegetation, soft substrates (often clayey), and clear, relatively cool water at lower temperatures than the surrounding open savanna. Permanent water availability is the defining ecological requirement for a non-annual species: unlike annual killifish of the same region, Melanorivulus does not exploit temporary pools that dry seasonally, and its populations are tied to watercourses that persist year-round.
The Cerrado context is critical to understanding the species' ecological situation. This biome has lost more than half its original extent to agriculture — primarily soy and cattle — making it one of the world's most threatened savanna systems. The small-stream habitats that rivulines like M. apiamici depend on are particularly vulnerable to riparian deforestation, water abstraction for irrigation, and siltation from surrounding cropland.
Feeding
Melanorivulus apiamici is a carnivorous micro-predator in the manner of most small rivuline killifish. In its native streams it feeds on small invertebrates available at and near the water surface and in the water column — aquatic insect larvae, microcrustaceans, small worms, and zooplankton. The terminal, slightly upturned mouth is well adapted to surface hunting.
In aquarium conditions the species readily accepts appropriately sized live and frozen foods: Drosophila fruit flies, small bloodworm, Artemia nauplii, and micro-worms are all suitable and widely used for small killifish of this size class. Dry foods may be taken once the fish is settled, but a diet weighted toward live and frozen invertebrates maintains best condition and encourages breeding behaviour.
Mating
As a non-annual rivuline, Melanorivulus apiamici reproduces in a fundamentally different mode from the annual killifish with which it shares the Cerrado landscape. Rather than burying diapausing eggs in substrate that dries out seasonally, it is a plant or mop spawner: pairs deposit small numbers of adhesive eggs among fine-leaved aquatic vegetation, roots, or equivalent spawning surfaces over an extended period, and the eggs incubate in water without any diapause phase.
Spawning occurs as a series of individual pairings across days or weeks rather than as a single concentrated event. The male courts the female with lateral displays, showing his more vibrant colouration, and the pair position side by side at the chosen spawning site before each egg deposition. This protracted, incremental spawning strategy is characteristic of non-annual rivulines and reflects the stability of their permanent-water habitats, where there is no evolutionary pressure toward burst reproduction ahead of seasonal desiccation.
Breeding
Eggs are deposited singly or in small clusters on fine vegetation or spawning mops and are adhesive enough to remain in place until hatching. Incubation in warm water (around 75–79 °F) takes approximately two weeks, after which the fry hatch as miniature versions of the adults and are immediately free-swimming. Compared with annual killifish, whose diapausing eggs require months of dry incubation and careful humidity management, non-annual rivuline eggs are straightforwardly managed: they can be left on the plants or mop in a separate rearing vessel with gentle aeration.
Fry are tiny at hatching and require micro-sized first foods — infusoria, vinegar eelworms, or freshly hatched Artemia nauplii of appropriate size. Growth is steady in warm, clean water and the fish are relatively long-lived by killifish standards, consistent with the non-annual life-history strategy. Because the adults do not necessarily consume eggs voraciously, a densely planted spawning tank or a mop with regular removal of the mop to a separate hatching vessel are both viable approaches; experienced killifish keepers often prefer the latter to maximise yield.
In the aquarium
Melanorivulus apiamici is a small, peaceful, and undemanding killifish well suited to a quiet species tank or a community of similarly sized, non-aggressive fish. A tank of 10–15 US gal is ample for a breeding pair or trio; the addition of floating plants, fine-leaved stem plants, and shaded areas replicates the dappled forest-stream conditions of the natural habitat and encourages spawning behaviour.
Water conditions should reflect the soft, slightly acidic to neutral streams of the São Paulo Cerrado: temperature in the range of 72–79 °F, pH 6.0–7.5, and low to moderate hardness. The species does not require extreme blackwater conditions, but hard alkaline water is likely to cause long-term stress. Good filtration with gentle flow and a tight-fitting lid — killifish are accomplished jumpers — are standard requirements.
This species is not widely established in the ornamental trade and is mainly kept by specialist killifish hobbyists with an interest in South American rivulines. Those seeking it should look to killifish associations (the American Killifish Association and its European counterparts) where members maintain and exchange specimens. Its small size, calm temperament, and relatively straightforward non-annual breeding biology make it an accessible introduction to the group for aquarists new to killifish.
Conservation
The IUCN assessed Melanorivulus apiamici as Least Concern in November 2018, reflecting a judgement that the species at that time did not meet thresholds for any threatened category. However, the LC assessment must be read in its Cerrado context. The Brazilian Cerrado has lost more than 50% of its native vegetation to agricultural conversion — primarily soy cultivation and cattle ranching — and deforestation continues. Small-stream rivulines dependent on intact riparian corridors in this biome are structurally vulnerable: their habitats are fragmented, their ranges are narrow, and their populations are isolated in discrete drainage units.
Specific threats to headwater streams in the upper Paraná basin of São Paulo state include riparian deforestation (which destabilises stream banks, raises water temperature, and increases sedimentation), water abstraction for irrigation, and pesticide run-off from surrounding agriculture. Because Melanorivulus apiamici is a non-annual species tied to permanent watercourses, it cannot colonise seasonal or newly created pools and has limited ability to recolonise lost habitat. The broader M. pictus species group — the eighteen-plus species sharing this Cerrado distribution — collectively represents a conservation priority despite individual LC assessments, because the combination of restricted ranges, high endemism, and continuing habitat loss creates real extinction risk across the group as a whole.