Taxonomy & naming
Parecbasis cyclolepis was described by Carl H. Eigenmann in 1914 in his contribution to the Contributions to the Zoology Laboratory of Indiana University, published as Indiana University Studies No. 20. The type locality is São Antonio do Rio Madeira, Brazil, and the holotype is deposited at the Field Museum of Natural History (FMNH 56677, ex CM 5495), with paratypes at FMNH 56678 (ex CM 5496).
The genus Parecbasis is monospecific — Parecbasis cyclolepis is its only member. Catalog of Fishes (Eschmeyer) records the valid name as Parecbasis cyclolepis Eigenmann, 1914. Some sources have placed this genus within the family Acestrorhamphidae, a grouping erected for certain Amazonian characins, while others retain it within the broader Characidae; the top-level family is given here as Characidae following current broader usage. The species belongs to the order Characiformes, the large, predominantly Neotropical order that includes piranhas, tetras, and headstanders.
Morphology
Parecbasis cyclolepis is a small, elongate characin reaching a maximum reported standard length of 3 in. Like many small Amazonian characins it is laterally compressed with a fairly streamlined profile, reflective silvery flanks, and the rounded, cycloid scales that give the species its epithet.
Detailed meristic and morphometric data are sparse in the accessible literature beyond the original description, which is typical for many mono- or oligotypic Amazonian genera that have not been the subject of modern morphological revision. The fish likely shares the general characin body plan common to small pelagic species of similar size in the basin — a single dorsal fin, an adipose fin, a forked caudal, and the usual complement of fin rays and lateral-line scales characteristic of the subfamily. Data on colouration in life are not well documented in the available sources.
Habitat
Parecbasis cyclolepis is native to the Madeira-Mamoré River basin, one of the largest tributaries of the Amazon system, draining the eastern slopes of the Andes across Bolivia, Brazil, Colombia, and Peru. The Río Mamoré forms the border between Bolivia and Brazil before joining the Beni River to become the Madeira, which ultimately enters the Amazon. The river system carries warm, often sediment-laden brownish water through lowland tropical forest.
FishBase records the species as a freshwater, pelagic inhabitant, consistent with occupancy of slow-moving backwaters, flooded ponds, and smaller tributary streams within the basin — habitats typical of small characins in this region. Regional water parameters for small characins in the Madeira-Mamoré basin are typically warm (approximately 72–82 °F), slightly to moderately acidic to neutral (pH roughly 6.0–7.5), and of low to moderate conductivity, reflecting the soft, often tea-coloured waters of the Amazonian lowlands.
Feeding
No detailed dietary study of Parecbasis cyclolepis appears in the accessible literature. As a small, pelagic characin from Amazonian slow-moving and standing waters it almost certainly feeds opportunistically on small invertebrates — aquatic insect larvae, microcrustaceans, and other zooplankton — as well as any small surface-fallen prey that becomes available, which is the typical foraging guild of comparably sized Amazonian characins.
In the aquarium, such species generally accept small live and frozen foods (Artemia nauplii, Daphnia, micro-worms, finely crushed flake or micro-pellets) readily. A varied diet suits fish from this ecological group, and regular small feedings are preferable to infrequent large ones.
Mating
No specific observations of reproductive behaviour in Parecbasis cyclolepis have been documented in the available literature. By analogy with ecologically similar small Amazonian characins, the species is presumed to be an egg-scattering, open-water or plant-spawning fish with no parental care — the typical reproductive mode across the Characiformes.
Spawning in related small characins typically involves the pursuit of females by males in open water or among fine-leaved vegetation, with adhesive or semi-adhesive eggs deposited over vegetation or substrate. Seasonal flooding in the Madeira basin likely synchronises reproductive activity, as it does for the majority of Amazonian fishes, though no direct evidence for P. cyclolepis is available.
Breeding
Captive breeding of Parecbasis cyclolepis has not been documented in the hobbyist or scientific literature accessible for this account. Given its probable status as an egg-scatterer with no parental care, standard protocols for small Amazonian characins would apply as a starting point: conditioning adults with varied live and frozen foods, using soft, slightly acidic warm water (around 75–81 °F, pH 6.0–7.0), and providing fine-leaved spawning vegetation or a spawning mop. Eggs and adults should be separated promptly after spawning, and fry would require very fine first foods.
The species is rarely if ever traded in the ornamental hobby, so practical captive-breeding reports are essentially absent. Aquarists working with wild-caught specimens should document any successful spawning, as such records would be genuinely useful for the scientific community.
In the aquarium
Parecbasis cyclolepis is essentially unknown in the ornamental trade and is not a species most aquarists are likely to encounter outside of specialist biotope collections or direct importation from scientific field collections. Its modest size (3 in) and presumed peaceful disposition make it theoretically suitable for a well-planted Amazonian biotope aquarium alongside other small characins, smaller cichlids, and Corydoras species from the Madeira basin.
For aquarists who do obtain specimens, a quiet tank with soft, slightly acidic to neutral water, subdued lighting, dense planting or driftwood for cover, and gentle filtration would best approximate its natural habitat. As a pelagic species it likely prefers open swimming space alongside areas of cover. Data on compatibility, feeding behaviour in captivity, and water-parameter tolerances beyond the regional norms are not available from the current literature.
Conservation
Parecbasis cyclolepis is assessed as Least Concern (LC) on the IUCN Red List. The Madeira-Mamoré River basin is a large, still largely intact river system, and the species is distributed across Bolivia, Brazil, Colombia, and Peru — a range broad enough to buffer against local threats. No major population declines have been documented.
That said, the Madeira basin faces ongoing pressure from hydroelectric dam construction — the Santo Antônio and Jirau dams on the Madeira River have already altered flow regimes and sediment transport, with consequences for the fishes of the middle and upper basin. Deforestation in the catchment affects water quality and the lateral connectivity between the main channel and the flooded-forest habitats that Amazonian fishes depend on. The Least Concern assessment reflects current knowledge but does not imply the species is free from longer-term threat as the basin develops.