Taxonomy & naming
Elachocharax junki was described by Jacques Géry in 1971, originally under the genus Geisleria; the Catalog of Fishes (Eschmeyer, California Academy of Sciences) now lists Elachocharax as the currently valid combination. It sits in the family Crenuchidae, subfamily Characidiinae — the "darters," a lineage of small, bottom-associated characins that includes Characidium, Melanocharacidium and the more familiar sailfin characins Crenuchus and Poecilocharax in the sister subfamily Crenuchinae. The type locality is the Rio Novo, in the Madeira basin, Territorio Guaporé, Brazil, at approximately 8°46'S, 63°30'W.
The species epithet honours Dr. Wolfgang Johannes Junk, a German-born limnologist and botanist whose research on Amazonian floodplain ecology (the várzea and igapó systems) has been foundational to understanding the seasonally flooded habitats where fish like this one live. The genus Elachocharax itself groups a small number of similarly diminutive characidiin darters, several of which — like E. junki — are known mainly from type material and scattered survey records rather than dedicated field studies. Buckup's 1993 comprehensive taxonomic review of the characidiin fishes remains the key reference clarifying the genus.
Morphology
Elachocharax junki is genuinely tiny even by nano-fish standards: FishBase records a maximum published length of just 1 in standard length, and aquarium specimens typically run smaller still, often under 0.5 in. Like other characidiin darters it has an elongated, slender body built for perching rather than for sustained swimming, with a relatively straight ventral profile and a small, forward-set mouth suited to picking tiny prey items off leaf litter and detritus rather than engulfing anything of size.
Colouration in this genus tends toward translucent to pale straw tones with a dark lateral stripe or series of blotches running the length of the body — a pattern that, combined with its habit of resting motionless on the substrate, likely helps it blend into leaf litter and dappled light. Detailed, verified colour-pattern description specific to E. junki is not established in the sources available here, and given the extreme size of the fish, fine morphological detail is best confirmed against a current specimen rather than assumed from congeners.
Habitat
Elachocharax junki is known from the Madeira and Negro river drainages of the Amazon basin, with records from Brazil and Guyana. Its habitat, per the type description and subsequent records, is slow-flowing water over a sandy bottom, typically among floating plants, driftwood and accumulated leaf litter — the classic structural cover that characidiin darters use both to hide from predators and to ambush the tiny invertebrates they eat.
Specific water chemistry data — temperature and pH readings taken at collection sites — are not established in the sources available here. Given its range within the Madeira and Negro systems, which include both whitewater and blackwater tributaries, and given that most Amazonian characidiin darters are documented from warm, soft, acidic conditions typical of leaf-litter and slow-backwater habitats, a cautious, unverified assumption of warm and acidic water is reasonable, but this article does not assert specific figures the cache does not support.
Feeding
Elachocharax junki is a micro-predator, not a grazer or a detritivore. In the wild it takes tiny aquatic invertebrates, insect larvae and micro-crustaceans picked from leaf litter and sediment, darting forward to seize prey and then returning to a resting perch — the classic "dart and rest" foraging style that gives the darters their common name.
In the aquarium this translates directly: standard flake food is frequently ignored, both because of its size and because it does not trigger the prey-capture response of a sight-hunting micro-predator. Keepers need to offer live or frozen foods scaled to a fish under 1 in — microworm, baby brine shrimp nauplii, vinegar eels, grindal worm and similarly tiny fare — introduced where the fish is actually perching rather than broadcast across open water it will not chase into.
Mating
No courtship behaviour specific to Elachocharax junki is documented in the sources available here. Within the Crenuchidae more broadly, the sailfin characins (Crenuchus spilurus, Poecilocharax species) court and spawn in caves or among fine root tangles and floating plant mats, with the male taking up a defended site and the female entering briefly to lay a small clutch; Characidium darters, by contrast, are egg-scatterers with no defended site. Elachocharax sits taxonomically closer to Characidium within the characidiin darters, but this article does not extrapolate a specific courtship pattern for E. junki beyond what is actually confirmed, given how sparse the published record is for this exact species.
Breeding
Breeding biology for Elachocharax junki is not established in the sources available here — this is a case where honesty matters more than a tidy narrative. What can be said with more confidence is the pattern across its closest relatives: the crenuchid sailfin characins (Crenuchus, Poecilocharax) are cave- or leaf-spawners in which the male tends a small clutch for a brief period after the female departs, a genuinely breedable pattern at nano scale that hobbyists have replicated with tight leaf litter, root tangles or coconut-shell caves and very soft, acidic water. Characidium darters, the other branch of the subfamily, scatter eggs over the substrate with no parental care at all.
Where E. junki falls between those two patterns has not been confirmed in captivity or in the field as far as the sources checked here show. Prospective breeders should treat any attempt as genuinely experimental: dense leaf litter and fine-twigged cover, soft and acidic water, and abundant infusoria-sized first foods for any fry that do appear are reasonable starting points drawn from its darter relatives, not a documented protocol for this species specifically.
In the aquarium
Elachocharax junki is about as far from the piranha stereotype as a characin gets: at roughly 1 in, it is a true nano fish that needs a tank built around its scale rather than a general community setup. A well-aquascaped nano tank with fine sand, driftwood, leaf litter and floating plants replicates the structure it uses in the wild and gives it the perches and cover it needs to feel secure enough to actually be visible rather than hiding constantly.
Because it is a micro-predator that often ignores flake, feeding is the single biggest husbandry challenge: live or frozen foods sized for a sub-1 in mouth are close to mandatory for long-term condition, and tankmates need to be equally diminutive and non-aggressive — other tiny characins such as Nannostomus (pencilfish), Carnegiella (hatchetfish, for the water column above it) or Corydoras pygmaeus for the substrate layer are commonly suggested companions that will not outcompete or intimidate it at the feeding site. Given its size, a species-only or very carefully stocked quiet nano tank will show off its natural dart-and-perch behaviour far better than a busy community aquarium where it is likely to simply vanish among faster, larger fish.
Conservation
Elachocharax junki has not been assessed by the IUCN Red List and carries a status of Not Evaluated. This is unsurprising for a fish this small and this poorly surveyed — it is known mainly from type material and scattered collection records in the Madeira and Negro drainages rather than from any dedicated population study, so there is no scientific basis at present for assigning it a threat category one way or the other.
As with many small-range or poorly documented Amazonian fish, the honest conservation statement is simply that data is sparse: the broader pressures on Amazon basin freshwater habitats — deforestation, sedimentation and hydrological change tied to damming and land-use conversion — are relevant background context for any species restricted to specific tributary systems, but this article does not claim a specific threat level for E. junki beyond what has actually been assessed, which at present is nothing.