Taxonomy & naming
Pyrrhulina marilynae was described by Netto-Ferreira and Marinho in 2013, with the original description published in Zootaxa. The holotype, catalog number MCP 47420, is an adult male of 1 in standard length collected from a stream crossing the MT-338 road, approximately 39 mi north of Tapurah, Mato Grosso, Brazil (12°15'29"S, 56°38'59"W). The type locality lies on the Brazilian Shield within the headwaters of the rio Tapajós drainage.
The species belongs to the genus Pyrrhulina Valenciennes, 1846, a lineage of small, surface-oriented fishes in the family Lebiasinidae — the same family that includes the pencilfishes (Nannostomus) and the splash tetras (Copella). Within Lebiasinidae the subfamily Pyrrhulininae houses Pyrrhulina and its close allies. The Catalog of Fishes (Eschmeyer, CAS) records Pyrrhulina marilynae Netto-Ferreira & Marinho, 2013 as the valid name.
The species is diagnosed by three principal characters: a dark zigzag primary lateral stripe that extends to the median caudal-fin rays, a reduced number of precaudal vertebrae compared with most congeners, and the absence of the second postcleithrum — a small bone in the pectoral girdle whose presence or absence is taxonomically informative in this group. Nine principal rays on the dorsal lobe of the caudal fin are a further distinguishing feature.
Morphology
Pyrrhulina marilynae is a tiny, slender fish reaching 1 in standard length in males — the maximum recorded length known from the original description and FishBase. Like other pyrrhulines it has a somewhat dorsoventrally flattened head and a large upturned mouth suited to taking prey at or near the water surface. The body is elongate and only moderately compressed.
The defining external feature is the dark zigzag lateral stripe, which runs from behind the eye and continues posteriorly all the way to the median rays of the caudal fin. This zigzag pattern distinguishes it from Pyrrhulina species with a straight or fragmented lateral stripe. The caudal fin bears nine principal rays on its dorsal lobe, which is a meristic character used in the original diagnosis. Overall colouration follows the typical pyrrhuline pattern of a pale or yellowish ground colour overlaid with dark pigment along the flanks, though specific colour documentation in life is sparse for this species.
Habitat
Pyrrhulina marilynae is known from the headwaters of two major Amazon tributaries — the rio Tapajós and the rio Xingu — both of which rise on the Brazilian Shield in Mato Grosso state. The type locality is a small stream in the Tapajós headwater zone, crossing a paved road in a transitional cerrado-Amazon landscape at roughly 1312–1640 ft elevation. Brazilian Shield headwater streams in this region are typically shallow, with moderate current, sandy or rocky substrate, and overhanging riparian vegetation.
No specific water chemistry parameters (temperature, pH, conductivity) have been formally published for the collection site, but the broader region's streams tend toward warm, soft, and slightly acidic to neutral conditions characteristic of terra firme forest streams on the ancient Brazilian Shield. As a surface-oriented lebiasinid, the species likely occupies the calm, shallow margins and backwaters where surface prey is available and larger predators are less prevalent.
Feeding
Like other members of the genus Pyrrhulina, P. marilynae is presumed to be a surface and near-surface predator taking small invertebrates — terrestrial insects, small aquatic insect larvae, and microcrustaceans. The upturned mouth and dorsoventrally flattened snout of pyrrhulines are adaptations to surface feeding, allowing them to intercept insects that fall onto or hatch from the water surface.
No detailed gut-content studies specific to P. marilynae are available. In captivity, congeners accept a wide variety of small live and frozen foods — Drosophila, small crickets, Artemia, and Daphnia — as well as high-quality small-sized prepared foods offered at the surface. A feeding regime that mimics surface prey items is recommended for maintaining this genus in good condition.
Mating
Detailed mating observations for Pyrrhulina marilynae specifically have not been published. Reproductive behaviour in the genus Pyrrhulina generally involves males defending territories and engaging in courtship displays toward females before spawning. Some species in the broader Pyrrhulininae — most famously Copella arnoldi — exhibit remarkable leaf-spawning behaviour, but Pyrrhulina itself typically spawns on submerged surfaces or among bottom substrate rather than above the waterline.
Males in the genus often display colour intensification during courtship. Spawning is typically preceded by a male herding or guiding the female toward the chosen spawning site. Egg deposition is on a prepared substrate, with the male providing some form of post-spawning guarding in at least some congeners — a level of parental investment unusual among small characoid relatives.
Breeding
No breeding reports for Pyrrhulina marilynae in captivity are available in the published or hobbyist literature. The genus Pyrrhulina is known to include substrate spawners that deposit adhesive eggs on flat surfaces such as leaves or stones, with the male typically guarding the clutch until hatching. This differs from the typical egg-scattering strategy common in many other small characoids.
Given its small adult size of 1 in SL, a breeding setup for P. marilynae would logically follow protocols used for other small pyrrhulines: a species tank with calm, warm, soft water, flat broad-leaved plants or smooth stones as spawning sites, and separation or monitoring to allow male guarding behaviour to proceed. Fry would be expected to be tiny at hatching and require infusoria or commercial liquid fry food as first foods. The species remains essentially unestablished in the ornamental trade, so captive breeding data are absent.
In the aquarium
Pyrrhulina marilynae is essentially unknown in the aquarium hobby. Its small adult size of 1 in SL makes it potentially suitable for a nano or species-only biotope tank replicating the warm, soft-water streams of the Brazilian Shield. Surface access is important for all pyrrhulines, as they are air-breathing surface feeders — a tightly fitted but ventilated lid is recommended since the genus is also an adept jumper.
As a fish from shield headwaters, water that is warm (approximately 75–82 °F), soft, and slightly acidic to neutral would be appropriate, though no formal aquarium chemistry data exist for this species. A biotope approach with fine sandy substrate, dried leaf litter, and low, indirect lighting would mimic the shaded forest streams of its native range. This species is not commercially available and would only be encountered through specialist breeders or scientific collections.
Conservation
Pyrrhulina marilynae is assessed as Least Concern (LC) on the IUCN Red List, assessed in 2018. This status reflects the species' occurrence across the Brazilian Shield headwaters of two large river systems — the rio Tapajós and rio Xingu — which together encompass a broad area of relatively intact interior Brazil.
Nonetheless, the Tapajós and Xingu headwater regions in Mato Grosso are subject to ongoing agricultural expansion, particularly soy cultivation and cattle ranching associated with deforestation of the Cerrado-Amazon transition zone. Headwater streams in agricultural landscapes face siltation, chemical runoff, and intermittent drying. The species' small body size and stream headwater preference make it potentially sensitive to these pressures, even if the overall population is currently considered stable. No specific conservation measures are in place for P. marilynae.