Taxonomy & naming
Roeboides dayi was described by the Austrian ichthyologist Franz Steindachner in 1878, based on material from South America; the parenthetical authority reflects the fact that the species was subsequently moved from the genus in which Steindachner originally placed it. Eschmeyer's Catalog of Fishes (CAS) is the governing authority for the valid combination Roeboides dayi (Steindachner, 1878).
The genus Roeboides is placed within the family Characidae, a vast assemblage of New World freshwater fishes. Roeboides and its close relatives are sometimes grouped under the informal concept of scale-eating characins, though the behaviour is facultative rather than obligate in most members, including R. dayi. The specific epithet honours Francis Day (1829–1889), a British physician and ichthyologist who published extensively on Indian and other tropical fishes; Steindachner named several species in his honour.
Morphology
Roeboides dayi is an elongate, laterally compressed fish reaching a reported maximum of 5 in standard length. The body is deep relative to most slender characins, with a notably convex dorsal profile and a more or less straight ventral line. The scales are relatively large and reflective, giving the living fish a silvery, semi-translucent appearance — a common trait in open-water characins that spend time mid-water in turbid or slightly tea-coloured rivers.
The jaws are distinctive and functionally significant: they bear small, pointed teeth capable of rasping scales from the flanks of other fishes, and the gape and jaw musculature are accordingly robust for a fish of this size. As is typical in the family, a small adipose fin is present. Juveniles and subadults lack the well-developed scale-scraping dentition of older fish, reflecting the documented ontogenetic shift in feeding behaviour documented for the species.
Habitat
The species occupies rivers of the Atlantic drainage across a range spanning the extreme northeast of Panama (San Blas region) through the Atrato River basin of Colombia and into the Magdalena–Cauca river system — the largest river drainage on the Caribbean slope of South America. It is classified as benthopelagic, meaning it moves between mid-water and near-bottom zones rather than being strictly tied to the substrate.
FishBase records a preference for slightly turbid standing or slow-moving water, and the species is noted as potamodromous, undertaking seasonal migrations within freshwater. Water parameters recorded for its range are 72–82 °F, pH 6.0–7.5, and general hardness around 10–20 dH, conditions broadly representative of the lowland and foothill rivers of northwestern Colombia and adjacent Panama.
Feeding
Roeboides dayi is a facultative scale-eater, a trophic strategy that sets it apart from the vast majority of aquarium fishes. Adults approach the flanks of other fishes and rasp off scales with their specialised dentition, consuming scales as a protein-rich food source. This behaviour is opportunistic rather than exclusive: individuals also take invertebrates, and scale-feeding is one component of a broader omnivorous diet.
The feeding strategy shifts markedly through ontogeny. Juveniles rely primarily on aquatic insect larvae and microcrustaceans — prey appropriate to their smaller gape and undeveloped scale-scraping teeth — and graduate toward scale-eating as they mature and the jaw morphology develops. FishBase assigns the species a trophic level of 3.5, consistent with a mix of invertebrate prey and animal material. Spawning activity reportedly peaks in the wet season, suggesting feeding opportunities and breeding condition are tied to river hydrology.
Mating
Detailed behavioural observations of mating in Roeboides dayi under natural conditions are sparse in the published literature. Like the large majority of characins, the species is an egg-scatterer with no parental care; males likely pursue ripe females, and eggs are released and fertilised in open water or among fine-leaved vegetation without any nest construction or guarding behaviour.
FishBase notes that spawning occurs year-round with a peak corresponding to the wet season, suggesting that rising water levels, temperature shifts, or the flush of invertebrate prey associated with seasonal flooding serve as spawning cues — a pattern seen in many Neotropical characins of similar size and habit. Whether males establish any transient hierarchy or display to females is not documented in available sources.
Breeding
Roeboides dayi is rarely kept or bred in the aquarium hobby, and a detailed captive-breeding account does not appear in accessible hobbyist literature. Based on the pattern expected for a free-spawning characin of this size and habit, breeding likely follows the standard Characidae model: egg-scattering over fine-leaved plants or open substrate, small adhesive or semi-adhesive eggs, no parental involvement, and adults that will consume eggs given the opportunity.
Any serious attempt at captive breeding would therefore follow the general protocol for similarly sized, egg-scattering tetras: condition the fish on a varied diet including live or frozen invertebrates, use a heavily planted or moss-filled spawning tank, and remove the adults once spawning is complete. Water conditions mimicking the species' natural range — soft to moderately hard, slightly acidic to neutral, and warm — would be the reasonable starting point. Fry would be expected to require very small live foods initially.
In the aquarium
Roeboides dayi is an uncommon species in the ornamental trade and is primarily of interest to specialist collectors of unusual characins. Its scale-eating habit is the central practical challenge: it cannot reliably be housed with other species without risk of damage to tankmates' flanks, and this alone places it outside mainstream community fishkeeping. A species-only or carefully chosen species-specialist setup is the only responsible approach.
For those willing to accommodate its biology, R. dayi requires a spacious aquarium — appropriate for a fish reaching nearly 5 in — with good filtration, some water movement, and conditions in the soft-to-moderate hardness range at 72–82 °F and pH 6.0–7.5. In the absence of documented captive husbandry reports, care guidance should be treated as provisional. The species' potamodromous habit and seasonal feeding peaks suggest it may benefit from a cyclical variation in temperature and diet to stay in condition over the long term.
Conservation
Roeboides dayi is assessed as Least Concern (LC) on the IUCN Red List, with the assessment recorded in 2019. The species occupies a relatively broad range across the Magdalena–Cauca drainage — the most extensive river system on the Caribbean slope of the northern Andes — together with the Atrato basin and coastal rivers of northeastern Panama, and no range-wide population declines sufficient to trigger a threatened category have been identified.
Nevertheless, the Magdalena–Cauca system is one of the most impacted river basins in South America, subject to extensive agricultural runoff, mining discharge (particularly gold mining in the Atrato), deforestation, and hydrological alteration. These pressures do not currently threaten R. dayi at a species level given its breadth of occurrence, but they affect habitat quality for riverine fishes across the region. The Atrato basin and its biodiversity were formally recognised by Colombia's Constitutional Court in 2017 as a subject of rights, a legal development that may improve longer-term watershed governance.