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Thayeria sp. Guaranta do Norte is a small South American characid currently traded under an informal species designation, associated with the municipality of Guarantã do Norte in northern Mato Grosso, Brazil. Its shape recalls the penguin tetras of the genus Thayeria, with active swimming and a tendency to remain in groups in the middle part of the aquarium. This form is uncommon in the aquarium trade, making it interesting to aquarists who value geographic forms and distinct characid populations. Stable water, open swimming space, and a varied diet made of suitably small foods are central to its successful care.

General description of the species

Thayeria sp. Guaranta do Norte is a geographic form of the genus Thayeria, in the family Characidae. Like other members of the genus, it is a peaceful, schooling, mainly midwater fish. Its behavior is most natural when kept in a group of at least six individuals, in an aquarium with enough length for swimming. This form is uncommon in the trade and should be kept away from very boisterous or much larger fish. Identification should not be based only on general body shape, since several Thayeria can look similar and coloration changes with age, stress, and maintenance conditions.

Geographic origin and biotope

Reference country: Brazil. The name Guarantã do Norte refers to a municipality in northern Mato Grosso, a region linking Amazonian forest environments with southern Amazon drainage systems. The fish is associated with tropical, forest-influenced streams, where sunlight may be filtered by bankside vegetation. In an aquarium, soft to moderately mineralized water, effective filtration, and gentle to moderate circulation are preferable to strong current. Roots, branches, and floating plants can reproduce the shaded character of this type of habitat and provide shelter without removing the open swimming areas the fish needs.

Precise location and local name

The known geographic reference is Guarantã do Norte in Mato Grosso, Brazil. This designation should be treated primarily as a collection or provenance locality and should not automatically be converted into a river name. No widely established Brazilian vernacular name is associated with this form; aquarists generally use the locality name, Guarantã do Norte, together with the genus name. Keeping this geographic reference attached to the fish is valuable because it helps preserve population traceability and reduces the risk of mixing different Thayeria forms from separate regions.

Appearance and physical characteristics

Thayeria sp. Guaranta do Norte has a slender, laterally compressed body, a forked caudal fin, and a generally oblique swimming posture, a trait often associated with Thayeria. The body is silvery to grayish, with reflections that vary according to lighting and condition. The fins may show yellow, red, or dark tones, but their intensity depends on the stock and aquarium conditions. Its slender body profile and schooling behavior are among its easiest features to observe. Adult size remains modest, making it suitable for a carefully planned community aquarium rather than only very large systems.

General diet

In nature, Thayeria sp. Guaranta do Norte likely feeds on small prey and animal material found in the water column. In aquaria, it accepts quality dry foods, but its diet benefits from regular portions of frozen or live foods. Offer small amounts of fine micro-pellets, daphnia, brine shrimp, and bloodworms, once or twice daily depending on group size. Food should be small enough to swallow easily and quickly. A varied diet supports general condition, coloration, and breeding readiness while reducing uneaten food that could damage water quality.

Essential data

• Native river name: southern Amazon basin, Guarantã do Norte locality
• Local name used: Guarantã do Norte
• Origin of the scientific name: Thayeria is the genus name; sp. indicates an undetermined species within the genus
• Etymology of the full name: Thayeria honors the Thayer family; sp. means species
• Common/vernacular names: Guarantã do Norte penguin tetra
• Family: Characidae
• Subfamily: not specified
• Name pronunciation: thay-REE-ah sp. - goo-ah-RAN-ta doh NOR-teh
• Discovery: not specified
• Maximum adult size: approximately 5 cm
• Water temperature and pH: 23 to 28 °C; pH 5.5 to 7.2
• Stream appearance and surrounding vegetation: shaded tropical water, riparian vegetation, roots, and submerged woody debris
• Typical tankmates: peaceful small characids, calm bottom-dwelling fish, and small loricariids
• Temperament and behavior: peaceful, schooling, active in midwater
• Water depth: shallow to moderate in areas near the banks
• Sexual dimorphism: mature females are usually rounder; differences are subtle
• Bottom structure and flow: sand, leaf litter, and submerged wood; low to moderate flow

In your aquarium

To succeed with Thayeria sp. Guaranta do Norte, choose a mature, covered aquarium that is long enough for natural schooling movement. Keep it in a group of at least six fish, with clean, stable water that is slightly acidic to neutral. Dim lighting provided by floating plants, roots, and open swimming areas will improve security and coloration. Avoid aggressive or excessively fast tankmates, provide fine and varied foods, and acclimate new fish slowly. This approach respects its schooling behavior and helps offset its limited availability in the aquarium trade.