Taxonomy & naming
Tyttobrycon xeruini was described by Jacques Géry in 1973. The genus Tyttobrycon was established to accommodate a small number of minute characins from the Amazon basin that could not be comfortably placed in the large genera then available; the name combines the Greek tyttos, meaning small or tiny, with brycon, a well-known characid genus name itself derived from the Greek bryko, to bite or gnaw — reflecting the fishes' general characid affinity rather than any particular feeding behaviour.
The species epithet xeruini refers to the rio Xeruini, a right-bank tributary of the rio Nhamundá in the state of Amazonas, Brazil, from whose vicinity the type material was collected. Eschmeyer's Catalog of Fishes (Catalog of Fishes, Fricke, Eschmeyer & Fong) recognises Tyttobrycon xeruini Géry, 1973 as the valid name. Because the species was described directly in Tyttobrycon, where it remains, no parenthetical is applied to the authority.
The broader placement of Tyttobrycon within Characidae has been discussed in various phylogenetic contexts. The genus has at times been associated with stevardiin and related small-characin lineages, but its precise relationships remain incompletely resolved; the family Characidae as currently construed is a very large and still-changing grouping, and Tyttobrycon represents one of its many small Amazonian genera known primarily from a limited number of specimens.
Morphology
Tyttobrycon xeruini is a small, slender characid, reaching only a few centimetres in standard length — a truly tiny fish even within the small-characin guild of the Amazon. The body is compressed and elongate, with the general proportions typical of small characids: a moderately pointed snout, small terminal mouth, and the single adipose fin behind the dorsal that is a hallmark of the family.
Coloration in preserved specimens, which underpin most descriptions, is silvery with a faint lateral stripe; details of live coloration are poorly documented in the literature. The caudal fin may carry a small dark marking. Like many small characids the species displays a humeral spot, though its prominence can vary. Meristic and morphometric data given in the original description remain the primary morphological reference; detailed revision of the genus as a whole has not been widely published.
Sexual dimorphism is not well characterised in the literature. As in many small characids, ripe females may appear deeper-bodied than males when gravid, but consistent external characters distinguishing the sexes at rest have not been formally described for this species.
Habitat
The type locality of Tyttobrycon xeruini is the rio Xeruini drainage, a tributary system entering the right bank of the rio Nhamundá in the state of Amazonas, Brazil. The Nhamundá is a blackwater or clearwater river of the central Amazon basin, flowing northward to join the Amazon near the border of Amazonas and Pará states.
Like many small characids of such Amazonian blackwater systems, T. xeruini likely inhabits slow-moving or still, tannin-stained water with low conductivity and acidic pH — conditions typical of the igapó (flooded forest) and marginal stream environments associated with these drainages. Specific water-parameter data for the collection site have not been published in accessible literature; available data are sparse.
The species has not been widely reported from locations beyond its type locality drainage, and survey coverage of this region remains limited. It is not known whether the species occurs across a broader range within the Nhamundá basin or is genuinely restricted to a small part of it.
Feeding
No published dietary study specific to Tyttobrycon xeruini is available. Based on its small size, jaw morphology, and the general feeding ecology of small characids from Amazonian blackwater streams, the species is presumed to be a micropredator or omnivore, taking small invertebrates — microcrustaceans, aquatic insect larvae, and zooplankton — as well as fine organic matter and possibly small amounts of plant or algal material.
Small characids of comparable size in similar habitats are frequently observed feeding in the water column and at the surface film, taking terrestrial invertebrates that fall onto the water. Whether T. xeruini exhibits any specialised feeding behaviour is unknown from available sources.
Mating
Reproductive behaviour has not been described for Tyttobrycon xeruini in published accounts. By analogy with the broad small-characid pattern, the species is presumed to be an egg-scattering open spawner: males likely pursue females, and spawning probably takes place among fine-leaved aquatic vegetation or over substrate, with adhesive or semi-adhesive eggs broadcast and left without parental care.
Many small Amazonian characids show opportunistic spawning tied to seasonal water-level changes and associated shifts in temperature and conductivity, with the flooding season triggering spawning migrations into inundated forest margins. Whether T. xeruini follows such a seasonal pattern has not been documented.
Breeding
No captive breeding records for Tyttobrycon xeruini have been published in accessible hobby or scientific literature. The species is essentially unknown in the aquarium trade and has not been bred in aquaria under documented conditions.
By inference from closely related small characids, breeding would likely involve conditioning adults with live or frozen foods, providing a dimly lit tank with fine-leaved plants or spawning mops, and offering soft, slightly acidic water to simulate blackwater conditions. Eggs would be expected to be adhesive or semi-adhesive, scattered among vegetation without parental care, and vulnerable to being eaten by the adults. Fry, if obtained, would be tiny and require infusoria or similarly fine first foods. These details remain speculative in the absence of documented breeding attempts.
In the aquarium
Tyttobrycon xeruini is essentially absent from the aquarium hobby and is not available in the commercial trade. No care records have been published. The species is known primarily from museum specimens and the original taxonomic description.
Should it become available, its requirements would likely mirror those of other small blackwater characids from Amazonian tributaries: soft, acidic, tannin-stained water in the low-to-mid 20s Celsius, a densely planted or botanicals-decorated aquarium that replicates the low-light, leaf-litter environment of its native streams, and very small live or frozen foods. Its diminutive size would make it unsuitable as a tankmate for anything but the smallest peaceful species, and it would probably show best in a species-only or biotope aquarium.
Data sparse — specific care parameters have not been documented in available sources.
Conservation
Tyttobrycon xeruini has not been assessed by the IUCN Red List; its status is Not Evaluated (NE). No targeted conservation studies have been published for the species. Its restricted known range in the rio Xeruini drainage of the Nhamundá basin places it in a region that is subject to the broad deforestation and land-use pressures affecting the Brazilian Amazon, though the specific impact on this species is unknown.
Given the paucity of records and the limited survey effort in its type-locality area, the species' true distribution and population status cannot be evaluated without further fieldwork. It does not appear in the ornamental fish trade in any significant volume, so collection pressure is not a known concern at present. The absence of an IUCN assessment reflects a lack of data rather than a finding of security.