Taxonomy & naming
Roeboides margareteae was described by Carlos A.S. de Lucena in 2003 as part of a taxonomic revision and phylogenetic analysis of the Roeboides microlepis species group, published in Iheringia, Série Zoologia (93(3): 283–308). The type locality is within the Rio Parnaíba and Pindaré-Mearim drainages of northeastern Brazil. Eschmeyer's Catalog of Fishes treats it as a valid species under the authority Lucena, 2003. The study included a dichotomous key and 14 figures supporting the placement of R. margareteae as sister to R. microlepis within that group.
The genus Roeboides belongs to the family Characidae, within the diverse South American characiform radiation. Members of Roeboides are distinguishable among characids by their strongly laterally compressed bodies, the peculiar outward-projecting dentition of the premaxillary and dentary bones, and their partly lepidophagous feeding habit. No standardised common name has been established for R. margareteae in the literature or trade.
Morphology
Roeboides margareteae reaches a recorded maximum of 7.5 in standard length, placing it among the larger members of the genus. The body is strongly compressed laterally, giving the fish a deep, disc-like silhouette typical of Roeboides. The dorsal profile is more arched than the ventral profile.
The genus is most readily identified by its unusual dentition: the premaxillary and dentary teeth project anteriorly and outward, visible outside the closed mouth — an adaptation to the scale-scraping habit. The overall colouration is silvery, as is characteristic of open-water characids. Roeboides adopt a distinctive diagonal swimming posture, held with the head angled downward, which is thought to facilitate the lateral approach used when stripping scales from prey fish.
Habitat
The species is recorded from the Rio Parnaíba and Pindaré-Mearim river systems in northeastern Brazil. These are lowland tropical drainages draining into the Atlantic coastal zone. R. margareteae is characterised as a freshwater benthopelagic species, occupying mid-water to near-bottom zones.
Within its drainage systems, Roeboides species typically favour slower-moving or still stretches with dense aquatic vegetation, where they can approach other fish from cover. Reported water parameters for comparable Roeboides habitats suggest warm, moderately acidic to neutral conditions. The Parnaíba and Pindaré-Mearim basins are subject to seasonal flooding cycles typical of Brazilian northeastern drainages, which likely influence habitat availability and fish distribution through the year.
Feeding
Roeboides margareteae is lepidophagous — it feeds on the scales of other fishes, an unusual and specialised dietary strategy shared by other members of the genus. Scale-eating in Roeboides involves a swift lateral strike against the flank of a prey fish, with the outward-projecting teeth used to dislodge and ingest scales. The prey fish is not consumed whole; it survives the encounter and regenerates scales over time.
In addition to scales, Roeboides may opportunistically consume small invertebrates. The diagonal head-down resting and swimming posture facilitates the lateral-strike attack mode by presenting the dentition at an angle suitable for scraping along a prey fish's flank. This feeding strategy is demanding to replicate in the aquarium and is one of the principal reasons these fish are considered challenging to maintain in community settings.
Mating
Specific mating observations for Roeboides margareteae are not documented in the available literature. Based on the broader Roeboides and characid pattern, open-water egg scattering is the presumed reproductive mode, with fertilisation external and no pair bond formed. Males likely display to females prior to spawning, though courtship details for this species remain undescribed.
Seasonal breeding tied to the flood pulse of the Parnaíba and Pindaré-Mearim drainages is plausible — many Neotropical characids synchronise reproduction with rising water levels, when dissolved oxygen improves and food availability increases. No data on spawning substrate preferences or egg characteristics have been published for this species specifically.
Breeding
No captive breeding reports for Roeboides margareteae are documented in the accessible hobbyist or scientific literature, and the species is not established in the ornamental trade. General characid biology suggests it would be an egg-scatterer with adhesive or semi-adhesive eggs deposited among fine-leaved vegetation or over substrate, with no parental care.
The specialised lepidophagous feeding habit makes long-term captive maintenance difficult, as providing a sustained supply of donor fish for scale-feeding is impractical in most home aquariums. For this reason, captive breeding has not been pursued or documented for this species, and data on incubation period, fry size, or larval development are absent from the literature.
In the aquarium
Roeboides margareteae is not a species encountered in the mainstream ornamental trade and is generally considered unsuitable for community aquariums. The scale-eating habit means it will injure tankmates by stripping their scales, even if not otherwise aggressive, making compatible housing extremely difficult. Literature sources advise against mixing it with other fish in a standard community setting.
For the specialist keeper interested in Roeboides, a species-only or predator-type setup is the only practical approach. Reported husbandry parameters based on related species and cache data suggest temperatures of 72–82 °F, pH 6.0–7.5, and water hardness around 10–20 °dH. The characteristic head-down diagonal swimming posture is best accommodated in a tall or deep aquarium with open mid-water space. Given the scarcity of hobby experience and the feeding challenges involved, this is a fish for the specialist ichthyologist-hobbyist rather than the general aquarist.
Conservation
Roeboides margareteae is assessed as Least Concern (LC) on the IUCN Red List, with the most recent assessment completed on 7 November 2018. The Least Concern designation reflects adequate population size and distribution across the Rio Parnaíba and Pindaré-Mearim drainages of northeastern Brazil, without evidence of rapid decline that would trigger a threatened category.
The Rio Parnaíba basin faces ongoing anthropogenic pressure from agriculture, water extraction for irrigation, and land-use change in the northeastern Brazilian cerrado and transition zones. These pressures may affect freshwater habitats over the long term, though they are not currently considered to threaten R. margareteae at a population level. Continued monitoring of freshwater fish assemblages in these drainages is warranted given broader land-use trends in the region.