Taxonomy & naming
Priocharax phasma was described in 2024 by Mattox, Lima, Britz, Souza & Oliveira in a paper published in Vertebrate Zoology titled "Two new miniature species of the fish genus Priocharax from the Rio Tapajós and Amazonas drainages, Pará, Brazil." The type locality is a series of floodplain lakes on two river islands — Ilha Marimarituba and Ilha Nazareth — situated upstream from the mouth of the Rio Tapajós near Santarém, Pará state, Brazil. Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & Fong) is the governing authority for the valid name.
The genus Priocharax belongs to the family Characidae and is endemic to the freshwaters of Brazil and the Orinoco basin of South America. Most species in the genus are confined to nutrient-poor, acidic, dark-stained blackwater habitats across the Amazon and Orinoco drainages; Priocharax phasma is notable within the genus as a species associated with the lower Tapajós and Amazonas drainages, and published data suggest it may deviate from the strictly blackwater affinity shown by most congeners. The genus is characterised by extreme miniaturisation — members are among the smallest vertebrates on Earth by standard length — and by the developmental truncations (paedomorphosis) that accompany small adult body size in characids.
The specific epithet phasma is from the Latin for ghost or phantom, an apt name for a fish whose transparent body renders it all but invisible to the naked eye.
Morphology
Priocharax phasma reaches a maximum recorded size of 0.5 in standard length, placing it among the smallest freshwater fishes in the world. Like other members of the genus, it is an extreme miniature: the body is highly compressed laterally, slender, and — most strikingly — completely transparent, with no appreciable melanophore pigmentation. This degree of transparency means that in life the internal organs and skeleton may be visible through the body wall, and the fish becomes practically invisible against the leaf litter and tannin-stained water of its floodplain habitat.
The anatomy of miniature characids like Priocharax is shaped by the paedomorphic processes associated with small adult size: fin-ray counts, squamation, and cranial ossification may all be reduced relative to larger relatives. Trophic level data from FishBase place P. phasma at 3.0, consistent with feeding on small organisms. Beyond these details, morphological data on this species remain sparse in the accessible literature given its very recent description.
Habitat
Priocharax phasma is known from floodplain lakes on the islands of Ilha Marimarituba and Ilha Nazareth, located in the Rio Tapajós drainage upstream from the confluence with the Rio Amazonas near Santarém, Pará, Brazil. These are low-lying, seasonally flooded Amazonian várzea and igapó lake environments typical of the lower Tapajós-Amazonas confluence zone.
The broader Priocharax genus is strongly associated with low-nutrient, acidic, blackwater systems across the Amazon and Orinoco basins; however, published notes on P. phasma suggest it may depart from this pattern, potentially occupying habitats with somewhat different water chemistry than the strictly acidic blackwaters favoured by most congeners. The floodplain-lake habitat in the Tapajós-Amazonas confluence area is characterised by shallow, often heavily vegetated waters with abundant leaf litter, submerged roots, and organic debris, which would provide both cover and invertebrate prey for a fish of this size. Distribution data are limited to the type locality and immediate vicinity.
Feeding
FishBase records a trophic level of 3.0 for Priocharax phasma, consistent with feeding on small zooplankton, micro-crustaceans, small insect larvae, and other tiny invertebrates. This is the expected diet for a highly miniaturised characid of this size — at 0.5 in maximum, the gape limits prey to organisms of only a fraction of a millimetre in diameter.
Beyond the trophic-level estimate, detailed dietary data for P. phasma are not available in the accessible literature. By analogy with other Priocharax species and similarly miniaturised Amazonian characids, it likely forages at or near the substrate and in the water column among leaf litter and fine organic debris, picking off micro-invertebrates. In an aquarium context — should the species ever enter captivity — ultra-fine live foods such as rotifers, infusoria, and early-instar nauplii would be necessary.
Mating
No specific mating or reproductive behaviour data have been published for Priocharax phasma. As a characid, the species almost certainly follows the family's general pattern of egg-scattering with no parental care, dispersing small adhesive or semi-adhesive eggs among fine-leaved vegetation, roots, or leaf litter in the floodplain-lake habitat.
Miniaturised characids often have reduced fecundity relative to larger relatives as a consequence of small adult body size, but the specific clutch size, spawning seasonality, and courtship details of P. phasma remain undescribed in the accessible literature. The seasonal flooding dynamics of its várzea and igapó habitat likely entrain a reproductive cycle tied to rising and falling water levels, as is common across Amazonian small characids.
Breeding
No captive breeding records or detailed reproductive biology data for Priocharax phasma are available. The species was formally described only in 2024, and there are no known reports of it being maintained in aquaria. By analogy with the family Characidae, it would be expected to scatter small, adhesive eggs among fine-leaved plants or leaf litter with no parental investment, the eggs hatching into very small larvae that require microscopic first foods.
The extreme miniature size of P. phasma would make captive breeding exceptionally challenging even if the fish were available to hobbyists: rearing larvae at this scale demands live micro-foods and highly controlled conditions. At present, breeding information should be considered absent pending future study.
In the aquarium
Priocharax phasma is not currently available in the aquarium trade and is unknown in captivity. Its extremely small adult size — 0.5 in SL — combined with its recent scientific description and restriction to a small number of floodplain lakes in the Brazilian Amazon mean it has not entered the hobby. Even if it did, maintaining and breeding a fish of this size would demand specialised micro-food culture, very fine filtration, and precise water chemistry replicating the soft, warm, low-pH conditions of the lower Tapajós drainage.
For the present, P. phasma is strictly a species of scientific interest: a remarkable example of vertebrate miniaturisation and an illustration of the extraordinary diversity still being discovered in Amazonian freshwaters as recently as 2024.
Conservation
Priocharax phasma has not been assessed by the IUCN Red List and carries a status of Not Evaluated (NE) as of 2026. Given that it was described only in 2024, no formal conservation assessment has yet been conducted.
The species is known only from floodplain lakes on two river islands in the lower Rio Tapajós system near Santarém, Pará, Brazil — a restricted range that, by definition, makes any population sensitive to habitat disturbance. The lower Tapajós and its confluence with the Amazon face ongoing pressures from deforestation, agricultural expansion, and hydrological alteration. As a newly described species from a specialised floodplain microhabitat, P. phasma merits formal IUCN evaluation to establish a baseline for monitoring. No specific conservation measures are currently in place.