Taxonomy & naming
Nannostomus anduzei was described by Fernandez and Weitzman in 1987. The type locality is Laguna Provincial, approximately 12 mi north of Puerto Ayacucho in Amazonas State, Venezuela (5°50'N, 67°30'W), in the upper Orinoco drainage. The valid name is recorded in Eschmeyer's Catalog of Fishes as Nannostomus anduzei Fernandez & Weitzman, 1987.
The genus Nannostomus belongs to the family Lebiasinidae, subfamily Pyrrhulininae, and encompasses the pencilfish — slender, often boldly striped characiforms that occupy slow, heavily vegetated, or leaf-littered soft-water habitats across tropical South America. N. anduzei is the smallest member of the genus, distinguished from all congeners by its extremely small adult size (maximum 0.5 in SL) and its restricted distribution in the upper Orinoco and Negro systems.
Morphology
Adults of Nannostomus anduzei reach only 0.5 in standard length, making them among the smallest characiform fishes known — a peer-reviewed cytogenetic study confirmed the species as one of the tiniest Neotropical fishes by absolute size. The body is pencil-shaped and slender, as is typical of the genus, with the compressed lateral profile and pointed snout characteristic of Nannostomus.
Colouration follows the general pencilfish bauplan: a dark, often iridescent mid-lateral stripe runs from the snout to the caudal fin base, contrasting with a pale or silvery flank above and below. Sexual dimorphism is present: it has been documented via both external coloration and gonadal morphology in cytogenetic collections from the Negro River basin in Brazil, though detailed description of the colour differences is limited in available literature.
Habitat
Nannostomus anduzei occupies two disjunct drainage systems: the upper Orinoco basin in Venezuela (type locality near Puerto Ayacucho) and the rio Ereê drainage within the rio Negro basin of Brazil. In the Negro system, specimens have been collected from Zamula stream near Barcelos, Amazonas — a peat-swamp forest stream draining into the Negro River and characterised by highly acidic blackwater conditions.
This is classic Neotropical blackwater habitat: shallow, heavily shaded streams and lagoons stained brown to amber by humic and fulvic acids leaching from waterlogged forest soils and decaying leaf litter. The water is extremely soft and ion-poor, with reported parameters of 72–82 °F, pH 5.0–7.0, and hardness in the range of 18–90 ppm. Structurally, the habitat features dense submergent and marginal vegetation, submerged wood, and deep leaf-litter beds — all of which provide the microhabitat refuges that a fish of 0.5 in body length requires.
Feeding
Nannostomus anduzei is a micropredator, feeding on tiny invertebrates in the wild — zooplankton, micro-crustaceans, insect larvae, and similar minute organisms that share its blackwater habitat. Its extremely small mouth and body size constrain it to prey of corresponding scale; it cannot process food items that most community fish accept without difficulty.
In the aquarium, this translates to a dependence on very small live or frozen foods: newly hatched Artemia (brine shrimp) nauplii, micro-worms, grindal worms, and Daphnia nauplii form the core diet. Appropriately sized dried and prepared foods can supplement, but live and frozen items are essential to maintain condition. Feeding this species well requires sourcing or culturing live food regularly — a practical consideration for anyone assessing whether they can keep it responsibly.
Mating
Nannostomus anduzei is an egg-laying species with no parental care. Like other pencilfish it is an egg-scatterer that spawns among fine-leaved aquatic plants or equivalent dense vegetation, where the adhesive or semi-adhesive eggs are deposited and then abandoned by the adults. Sexual dimorphism is documented in the species, suggesting that mate selection and courtship displays occur, though detailed field or aquarium observations are limited in the published record.
As a typical Nannostomus, spawning in captivity is associated with thickly planted tanks or the presence of fine-leaved mosses and spawning mops that simulate the dense submerged vegetation of the natural habitat. Males may display lateral colour intensification to females, as is common in the genus.
Breeding
Breeding Nannostomus anduzei in captivity presents challenges proportional to its size. The species will spawn in heavily planted aquaria maintained with the soft, acidic water that mirrors its native blackwater habitat — pH in the 5.5–6.5 range and very low hardness are advisable to condition adults and improve egg viability. Eggs are scattered among fine-leaved plants and are adhesive; the adults do not guard or tend them and will consume both eggs and fry if not separated.
Fry are extremely small at hatching — proportionally even more delicate than those of larger Nannostomus species — and require infusoria, paramecia, or very fine commercial liquid fry foods as first foods before they can accept micro-worms or Artemia nauplii. The combination of tiny fry, exacting water conditions, and the need for live foods at every stage makes this a demanding species to raise successfully and one better suited to a dedicated breeding setup than to a general community tank.
In the aquarium
Nannostomus anduzei is a fish for the specialist. Its minute size makes it vulnerable to the currents of standard filtration, the competition of larger tankmates, and the difficulty of delivering appropriately sized food in a community context. A dedicated species tank or a nano aquarium with very gentle flow, dense planting, and a dark substrate or leaf-litter layer to replicate blackwater forest-stream conditions is the appropriate setting.
Water chemistry must reflect the species' acidic, ultrasoft origins: pH 5.0–7.0 and very low hardness (target below 90 ppm). Temperature of 72–82 °F is acceptable, with the lower end of this range often preferable for long-term health. The species is best kept in a small group rather than as isolated individuals; a school of six or more allows natural behaviour and display. Because of its size and specialist requirements it is infrequently available in the trade; hobbyists who obtain it should be prepared to invest in live-food cultures and close observation.
Conservation
Nannostomus anduzei has not been formally assessed for the IUCN Red List and currently carries a status of Not Evaluated (NE). Available information suggests no major conservation threats have been identified, and collection for the ornamental fish trade is not considered to pose significant concern for wild populations at present.
Nonetheless, the species occupies a restricted range in two separate drainage systems in Venezuela and Brazil, and its dependence on intact blackwater peat-swamp forest habitats ties its long-term outlook to the conservation of those ecosystems. Deforestation and drainage of peat-swamp forest for agriculture and cattle ranching continue to be pressure points across the Orinoco–Amazon interface. Given that no formal assessment has been conducted, the species' true conservation status remains uncertain.