Taxonomy & naming
Nannostomus marilynae was described by Stanley H. Weitzman and Joseph N. Cobb in 1975, in a contribution to the Smithsonian Contributions to Zoology series. The type locality is the Rio Negro in Brazil; the holotype is catalogued at the California Academy of Sciences (CAS-SU 50238). Eschmeyer's Catalog of Fishes (Catalog of Fishes, CAS) maintains the valid name as Nannostomus marilynae Weitzman & Cobb, 1975, with the authority not parenthetical because the species has remained within Nannostomus since its original description.
The genus Nannostomus belongs to the family Lebiasinidae, the pencilfish family, within the larger order Characiformes. Pencilfish are characterised by the elongated, pencil-like body form, horizontal stripe patterns, and small size. N. marilynae sits within the diverse Rio Negro radiation of the genus alongside better-known species such as N. eques and N. unifasciatus, and its distribution overlaps with several congeners in the blackwater systems of that basin.
Morphology
Nannostomus marilynae is a small pencilfish reaching a maximum of around 2 in total length per FishBase, though most individuals encountered in the hobby are in the 1–1.5 in standard length range. The body is slender and elongated, consistent with the pencilfish bauplan.
The species carries the characteristic horizontal banding of the genus: a dark mid-lateral stripe runs from the snout to the base of the caudal fin, with a lighter stripe above it. Coloration is generally subdued compared to some of the more flamboyant pencilfish — the overall impression is of a quietly-marked, olive-toned fish. Caudal fin markings are present in a typical Nannostomus arrangement. Sexual dimorphism is mild; females are fuller-bodied, particularly when gravid, and males may show slightly more intense markings when displaying.
Habitat
Nannostomus marilynae is native to two river systems in north-western South America: the Rio Negro basin in Brazil (with records from the São Gabriel da Cachoeira region) and the Río Vichada in Colombia. Both drainages carry the characteristic blackwater chemistry of the region — highly acidic, extremely soft, tannin-stained water with very low conductivity and minimal dissolved minerals.
Within these systems the species occupies sluggish tributaries, flooded marginal zones, and swampy areas with dense aquatic vegetation and accumulations of leaf litter. Such microhabitats buffer flow, provide structural cover among root masses and plant stems, and support the tiny invertebrate prey on which pencilfish depend. Water parameters recorded for the species reflect this blackwater context: temperature 70–82 °F (Seriously Fish), pH 4.0–6.5, and very low hardness (18–179 ppm). In terms of husbandry the safe range is comfortably achieved with RO water blended to low GH and acidified with botanicals or peat.
Feeding
In nature Nannostomus marilynae is a micropredator, picking tiny invertebrates — small crustaceans, insect larvae, protozoans, and similar micro-fauna — from the water column and surfaces of vegetation and leaf litter. The mouth is small and upturned, suited to targeting prey items in the fine structure of aquatic vegetation rather than active pursuit of larger animals.
In the aquarium the species accepts small live and frozen foods well: Artemia nauplii, Moina, micro worms, grindal worms, and similar offerings are all taken readily. Dry foods are accepted if fine enough; small-particle nano pellets and powdered or crushed flake suit the small gape. As with other pencilfish, a varied diet with a live or frozen component keeps the fish in best colour and condition. Multiple small feedings per day suit its foraging style better than a single large meal.
Mating
Nannostomus marilynae is an egg-scattering open spawner, consistent with the reproductive strategy of the genus. Males display to females with spread fins and intensified markings, courting among fine-leaved vegetation or other spawning substrates. The species does not form pair bonds and exhibits no parental care — adults show no recognition of or attention to their own eggs after spawning.
In the aquarium, conditioning a small group together rather than isolated pairs tends to stimulate breeding behaviour. Males display most actively in the presence of rivals, and the sight of competing males brings females into spawning readiness. Because the adults will eat any eggs they encounter, removing either the eggs or the adults promptly after spawning is standard practice for breeders who want to raise fry.
Breeding
Breeding Nannostomus marilynae in captivity is reportedly possible but uncommon; the species is described by Seriously Fish as rarely bred in aquaria, though it can spawn in mature, densely planted tanks maintained under appropriate blackwater conditions. The soft, acidic water of the natural range matters more for this genus than for many tetras — breeding attempts in hard, alkaline water typically fail.
Spawning follows the typical pencilfish pattern: small, adhesive eggs are deposited among fine-leaved plants or spawning mops, with only a few eggs produced per spawning event rather than a large clutch released all at once. FishBase notes around 20–30 young produced per year, suggesting low fecundity. Eggs and fry require separation from adults; the tiny fry need infusoria or similarly fine first foods before graduating to newly hatched brine shrimp. Incubation time and fry development rate are not well documented for this species due to the rarity of captive breeding.
In the aquarium
Nannostomus marilynae enters the hobby almost exclusively as bycatch in cardinal tetra shipments from the Rio Negro region rather than as a deliberately targeted export. It is seldom traded as a species in its own right, which keeps it uncommon in retail even though its range overlaps with one of the most heavily collected tetra fisheries in the world. Wild-caught specimens are the norm.
In the aquarium the species suits a small, heavily planted blackwater setup with very soft, acidic water and subdued lighting. Java moss, floating plants, and a dark substrate or botanicals (dried leaves, seed pods) approximate the leaf-litter microhabitat and reduce stress. A quiet filter with minimal flow suits the slow-water habitat preference. The fish is peaceful and best kept in a group of six or more of its own kind; it can be combined with other small, soft-water species such as dwarf rasboras, small Corydoras, or other Nannostomus species that share its parameter requirements. Its small size and limited availability make it a fish for the specialist hobbyist rather than a community-tank generalist.
Conservation
Nannostomus marilynae is assessed as Least Concern (LC) on the IUCN Red List, with the assessment dated 21 September 2020. The wide distribution across the Rio Negro basin in Brazil and the Río Vichada in Colombia underpins this status — the species has not been identified as facing restricted range, population decline, or specific habitat threat severe enough to warrant a higher category at the time of assessment.
The species is exposed to the same broad pressures affecting all Rio Negro fishes, including the large-scale ornamental fish trade, periodic water-quality changes linked to land use in parts of the catchment, and the long-term threat of climate-driven hydrological shifts. However, the Rio Negro is one of the largest tributaries of the Amazon and remains substantially intact. The fact that N. marilynae reaches the trade only incidentally, rather than being targeted, also means it is unlikely to face the collection pressure experienced by high-demand species in the same shipments.