Taxonomy & naming
Melanorivulus kayabi was described by the Brazilian ichthyologist Wilson J.E.M. Costa in 2007, with the original description published in Ichthyological Exploration of Freshwaters (volume 18, number 4, pages 345–350). The type locality is the Arinos and Teles Pires drainages of the Tapajós river basin in central Brazil, within the territory of the Kayabi (also spelled Kaiabi) indigenous people — a name reflected directly in the specific epithet kayabi.
The authority is given in parentheses as (Costa, 2007) because the species was not placed in Melanorivulus when first described; it was originally referred to the genus Rivulus, a broadly circumscribed catch-all for New World rivulines that was progressively dismembered as molecular and morphological revisions clarified the phylogeny of the group. Melanorivulus Costa, 2006 now serves as the valid generic home for this and related species in the Tapajós drainage region. Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & Van der Laan) is the authority for the valid combination Melanorivulus kayabi (Costa, 2007).
The species is affiliated with the broader South American rivuline assemblage — a diverse, paraphyletic grade of killifishes spanning the continent's Atlantic drainages, Amazonian tributaries, and Guiana Shield rivers. Within Melanorivulus, M. kayabi is distinguished by its meristics (31–33 scales in longitudinal series, 13 pectoral-fin rays, 28–31 caudal-fin rays) and by the chevron-like red flank markings described below.
Morphology
Melanorivulus kayabi is a small, slender-bodied killifish. FishBase records a maximum of 1 in standard length for males and 1 in SL for females, making this one of the more compact species in the genus. The body form is typical of the rivuline grade: laterally compressed, elongate, with the dorsal fin set well back toward the caudal peduncle and the anal fin positioned symmetrically beneath it — a body plan suited to resting and manoeuvring in shallow, densely vegetated margins.
The most striking feature is the colouration of the male flank, which carries a series of red markings arranged in a chevron-like pattern with the vertex of each chevron pointing ventrally. This pattern overlies a ground colour of olive-brown to greenish-tan, giving the fish a jewel-like quality under good lighting. Females are more subdued in colour — the flanks are plainer — but females of M. kayabi are noted for bearing white spots on the dorsal and caudal fins, a feature that aids sex identification in a group where dimorphism can be subtle. Both sexes have the relatively large eye typical of killifishes, and the mouth is terminal and upturned, reflecting a surface-oriented feeding habit.
Habitat
Melanorivulus kayabi is restricted to the middle and upper Tapajós river basin in central Brazil, specifically the Arinos and Teles Pires drainages — both major right-bank tributaries of the Tapajós. The Tapajós itself is a large southern tributary of the Amazon, draining a geologically ancient landscape on the southern margin of the Amazon basin.
Within this basin the species occupies shallow portions of streams and swamps, with recorded water depths of 0–1 ft. The water is clear rather than blackwater, consistent with the drainage characteristics of the central Brazilian plateau, and is slightly acidic: FishBase records a pH range of 5.7–6.0 for this species' habitat. These are permanent or near-permanent water bodies rather than seasonal floodplain pools, which is consistent with the species' non-annual life history.
The shallow, clear-water stream and wetland habitats it favours are typically shaded by gallery forest and carry fine-leaved aquatic vegetation or submerged roots along the margins — the structural complexity that non-annual killifishes require both for spawning substrate and for cover. The Kayabi indigenous territory, which lends the species its name, lies in the upper Teles Pires drainage in what is now the Serra do Cachimbo region of Mato Grosso and Pará states.
Feeding
Like most small rivuline killifishes, Melanorivulus kayabi is an opportunistic micro-predator. The upturned, terminal mouth and the species' preference for shallow surface-associated habitats suggest a diet focused on small invertebrates at and near the water surface: insects and their larvae, small crustaceans, worms, and other invertebrates that fall onto or emerge from shallow stream and swamp margins.
In captivity the species is straightforward to feed. It accepts live and frozen small invertebrates — Daphnia, Artemia nauplii, small chironomid larvae (bloodworm), and Tubifex — along with appropriately sized dried foods once conditioned. Regular feeding with varied, predominantly animal-protein-based foods maintains colour and condition and supports successful reproduction. Because of its small adult size, food particles must be appropriately scaled: large pellets or whole frozen foods should be broken down before offering.
Mating
Melanorivulus kayabi is a non-annual killifish and a plant or mop spawner — it does not bury its eggs in mud or leaf litter and does not undergo embryonic diapause. Instead, pairs spawn repeatedly over days and weeks in permanent water, depositing individual adhesive eggs on fine-leaved plants, submerged roots, Java moss, or the fibres of artificial spawning mops near the surface or substrate. This is the characteristic reproductive mode of rivuline killifishes that live in permanent stream habitats rather than ephemeral seasonal pools.
Males display to females with spread fins and intensified colour, driving the pair into the spawning substrate where a brief embrace releases a single egg at a time. Reports from hobbyists indicate a production of approximately 5–15 eggs per day, laid continuously over the breeding period. Eggs are amber-coloured and approximately 0.5 in in diameter — relatively large for the size of the fish, which is typical of non-annual rivulines that invest heavily in individual egg quality rather than producing vast numbers of diapausing eggs.
The fish are noted as excellent jumpers and active, and males can be persistent in courtship. In a community killifish setup, a surplus of cover — floating plants, dense mops, fine-leaved vegetation — reduces aggression and gives females refuge from overzealous males.
Breeding
Breeding Melanorivulus kayabi follows the standard protocol for non-annual, plant-spawning rivulines. A conditioning period on varied live and frozen foods brings both sexes into breeding condition; pairs or trios (one male, two females) can then be moved to a dedicated spawning aquarium as small as 2.5 US gal, furnished with one or more dense spawning mops of dark wool or nylon, or clumps of fine-leaved plants such as Java moss.
Eggs are deposited individually over weeks and can be harvested by removing and replacing the mops at regular intervals — every five to seven days is a common practice. Harvested eggs are incubated in shallow water at room temperature or in moist peat moss; at typical tropical temperatures hatching occurs within two to three weeks, though exact incubation times vary with temperature. Unlike annual killifish eggs there is no requirement for a dry-incubation phase or any simulation of a dry season — the eggs develop continuously in water.
Fry are relatively large at hatching and accept Artemia nauplii as a first food from the outset. Growth is steady and the fish reach sexual maturity within a few months; captive lifespan is approximately three years, which is longer than annual killifish but typical for a small non-annual rivuline. The species is described as easy to breed and well-suited to killifish beginners willing to work at small scale with acidic, clear water.
In the aquarium
Melanorivulus kayabi is well suited to small, species or genus aquaria and is considered an accessible entry point into the Melanorivulus genus for killifish hobbyists. A tank of 12–15.5 in (2.5–5.5 US gal) is sufficient for a pair or trio, though larger tanks with planted margins allow the fish to display more natural behaviour and reduce stress. The species is an accomplished jumper and a tight-fitting lid is essential — any gap in the cover will eventually result in a fish on the floor.
Water chemistry should reflect the slightly acidic, clear conditions of the Tapajós drainage: pH in the range of 5.7–6.5, soft to moderately soft water, and a temperature somewhere in the tropical range consistent with central Brazil (approximately 72–79 °F is a reasonable target based on the drainage). Filtration should be gentle — a small sponge filter creates minimal current while maintaining water quality — and regular partial water changes keep the water clear and well-oxygenated without sharp chemistry swings. Fine-leaved plants, floating plants for surface cover, and dark substrate improve the fishes' confidence and bring out male colouration.
Melanorivulus kayabi has been maintained and bred in the killifish hobby since its description, with the American Killifish Association documenting it in its journal. Its small size, manageable breeding approach, and attractive chevron-pattern colouration make it appealing to specialist hobbyists, though it remains uncommon in the wider aquarium trade and is typically obtained through killifish society exchanges or specialist breeders.
Conservation
The IUCN Red List assessed Melanorivulus kayabi as Least Concern, with the assessment dated 7 November 2018. This status reflects the species' occurrence across a reasonably large portion of the Tapajós drainage — spanning both the Arinos and Teles Pires sub-basins — and the fact that parts of its range fall within or near the Kayabi indigenous territory, which historically provided a degree of de facto protection from large-scale habitat conversion.
Nevertheless, the Tapajós basin has been under accelerating pressure in recent decades. Deforestation for soy agriculture and cattle ranching has been extensive across Mato Grosso, the state from which both the Arinos and Teles Pires originate, and associated sedimentation and stream warming threaten the clear, shallow forest-stream habitats that M. kayabi requires. The Teles Pires drainage was also the site of major hydroelectric development in the 2010s — the Teles Pires hydroelectric dam complex altered hydrology and fragmented fish populations along the main channel, with consequences for tributary stream species not yet fully quantified.
As a small-bodied, habitat-specialist rivuline with a range tied to a single river basin, M. kayabi is intrinsically vulnerable to any localised degradation of clear-water forest streams within the Tapajós system. The Least Concern listing is based on data as of 2018; continued monitoring and reassessment in the context of ongoing agricultural and hydroelectric development in the region would be warranted.