Taxonomy & naming
Serrapinnus sterbai was described by Axel Zarske in 2012 in the journal Vertebrate Zoology (vol. 62, pp. 3–17). The holotype (UFRJ 8516, 1.5 in SL) came from aquarium specimens originating in the Araguaia region of central Brazil. The species is placed in Serrapinnus Eigenmann, 1915, a genus of small characids characterized in part by modified anal fin rays in males — a feature that led Zarske to note that the unusual anal fin morphology of this species may eventually warrant recognition at the generic level, though it currently remains in Serrapinnus under Eschmeyer's Catalog of Fishes.
The specific epithet honours the German ichthyologist and aquarist Dr. Günther Sterba (1922–2014), renowned for his comprehensive references on freshwater fishes. In the trade the fish has circulated as the 'Mint Tetra', referencing the hint of iridescence on the flanks, and also as the '7-Ray Tetra', a reference to the seven anterior anal fin rays forming the saw-like keel diagnostic of adult males. The family assignment follows Catalog of Fishes: Characidae.
Morphology
Adults reach approximately 1.5 in standard length (FishBase), with trade specimens typically in the 1–1.5 in range. The body is slender and fusiform, typical of small Serrapinnus. A prominent black lateral stripe runs from the supracleithrum to a defined dark spot at the base of the caudal fin — a pattern reminiscent of several related small characids but combined here with a distinctive iridescent sheen on the flanks that gives rise to the 'mint' trade name.
The most diagnostic feature of sexually mature males is the seven anteriormost anal fin rays, which are modified and thickened to form a saw-like keel along the lower caudal peduncle. This structure, uncommon in the genus, is absent or less developed in females and is the basis for the species' informal '7-Ray Tetra' moniker in the hobby. Otherwise the fin arrangement is standard for the genus: an adipose fin is present, and the caudal fin is forked.
Habitat
Serrapinnus sterbai is native to the Araguaia–Tocantins drainage of central Brazil, a large river system draining the cerrado and transitional forest zone of Goiás, Tocantins, and Mato Grosso states. FishBase records the distribution specifically within the Rio Tocantins drainage. Aquarium Glaser lists the origin more broadly as the Araguaia/Tocantins basin.
The cerrado waterways this species inhabits are typically warm, often relatively clear with moderate flow, and subject to seasonal variation in water level typical of the Neotropical savannah drainage systems. Specific water-chemistry data for wild-caught specimens are not available in the published record, but the broader Tocantins system is characterised by soft to moderately hard, slightly acidic to neutral water, warm year-round. Aquarium specimens are reported to be adaptable and undemanding with respect to water parameters.
Feeding
Like other small Serrapinnus, S. sterbai is an omnivore with a trophic level of 2.6 (FishBase), indicating a diet low on the food web and composed primarily of small invertebrates and plant material. In its natural habitat it almost certainly feeds on microcrustaceans, insect larvae, small worms, and organic detritus in the mid-water and near the substrate.
In the aquarium the species is reported to be undemanding and readily accepts a variety of prepared and live foods. A varied diet that includes high-quality micro-pellets or fine flake, supplemented with frozen foods such as Daphnia, baby brine shrimp, or Cyclops, suits it well and maintains condition in a school. Its small adult size means foods must be appropriately sized; coarse granules are unsuitable.
Mating
Serrapinnus sterbai is an egg-scattering characid, and like other small schooling tetras it practises open spawning with no parental care. Males are distinguished by the modified anal fin ray keel, which may play a role in courtship or copulatory contact during spawning; the functional significance of this unusual structure has not been formally studied in a behavioural context.
Courting males likely display alongside females in the water column, pursuing receptive individuals among fine-leaved vegetation or open water. The spawning act involves brief pairing in which adhesive or semi-adhesive eggs are scattered among plants or permitted to fall to the substrate. As with nearly all small tetras, the adults take no interest in the eggs or fry after spawning.
Breeding
Captive breeding of Serrapinnus sterbai follows the general pattern for small schooling characids: a conditioned group of adults will spawn over fine-leaved plants such as Java moss or over spawning mops placed in a well-planted tank. Slightly warmer water and protein-rich conditioning foods help bring fish into breeding condition.
Eggs are small and adhesive, deposited among plant leaves or mop fibres. The adults will consume eggs given the opportunity, so separating the spawning medium or moving the breeders after spawning is advisable. At typical tropical temperatures eggs hatch within 24–48 hours and fry become free-swimming a day or two later. Initial foods must be very fine — infusoria or commercial fry-feeding preparations — graduating to newly hatched brine shrimp as the fry grow. Detailed captive-breeding reports for this species specifically are limited in the published literature, but its close relatives provide a reliable template.
In the aquarium
Aquarium Glaser describes Serrapinnus sterbai as an undemanding, lively schooling fish well suited to a general community aquarium. It does best in a group of eight or more individuals, which encourages the natural schooling behaviour and the inter-male interactions that display the modified anal fin structure of the males. A tank of 23.5–31.5 in or more in length provides adequate swimming space for an active school.
Water conditions should be warm (72–82 °F is a reasonable range) and clean, with good filtration and regular water changes. The species tolerates a range of pH and hardness and does not require specialist soft-water conditions. A planted aquarium with some open swimming area and gentle to moderate flow suits it well. Tankmates should be similarly sized, peaceful fish; larger or aggressive species will stress the school. The relatively small adult size and undemanding nature make S. sterbai an attractive option for the planted community tank or a single-species biotope representing the cerrado rivers of central Brazil.
Conservation
Serrapinnus sterbai is assessed as Least Concern (LC) on the IUCN Red List, assessed in 2018. The Araguaia–Tocantins drainage is a large river system, and the species is not believed to face the level of habitat threat that would qualify it for a higher category at the time of assessment.
Nonetheless, the broader Tocantins system has been heavily affected by hydroelectric development, most notably the Tucuruí Dam, one of the largest hydroelectric projects in the world, which has altered the hydrology, sedimentation, and fish community composition across a large portion of the drainage. Small characids dependent on specific microhabitat conditions may be locally affected by such large-scale alterations even where the species as a whole remains abundant elsewhere in the basin. The LC status reflects the current state of knowledge and should not be read as indicating the Tocantins fish fauna is without pressures.