Taxonomy & naming
Pyrrhulina stoli was described by Marinus Boeseman in 1953 in Zoologische Mededeelingen (Leiden), volume 32, number 1, page 14. The holotype is RMNH 20469, deposited in what is now the Naturalis Biodiversity Center in Leiden. The type locality is the Marowini River basin in Suriname, a coastal river system draining into the Atlantic on the northeastern coast of South America.
The valid name, as maintained in Eschmeyer's Catalog of Fishes, is Pyrrhulina stoli Boeseman, 1953. The species sits within the subfamily Pyrrhulininae of the family Lebiasinidae — a group that also contains the closely related genera Copella, Copeina, and Nannostomus (the pencilfish proper). CAS records indicate the species occurs across coastal South American rivers including Suriname, Guyana, Peru, and Venezuela, though the extent of its range remains imperfectly documented given limited sampling of interior stream systems.
Morphology
Pyrrhulina stoli reaches a maximum recorded length of 2.5 in standard length (SL) for males or unsexed specimens, placing it at the smaller end of the Pyrrhulina size range. Like others in the genus, it is a slender, fusiform fish adapted to life near the water surface. The body is elongated, the mouth strongly upturned for surface feeding, and the unpaired fins relatively small.
Coloration in the genus Pyrrhulina typically involves a dark lateral stripe running from the snout through the eye and along the flank, overlaid on a pale golden or silvery body, with orange to red tints in the fins of breeding males. Sexual dimorphism, while not as stark as in some related genera, is present: males tend to be more intensely colored and develop the exaggerated fin rays associated with display, while females are plainer and deeper-bodied when gravid.
Habitat
In Suriname, P. stoli occurs in small streams and backwaters, often in company with Pyrrhulina filamentosa and Copella species. These are typically slow-moving or still, shallow waters with dense emergent and overhanging vegetation — the kind of microhabitat where leaf litter accumulates on soft substrates, dissolved-oxygen levels may fluctuate, and tannins from decaying plant matter stain the water tea-brown.
The species favors areas where overhanging vegetation provides both cover and potential spawning surfaces. Slow-moving, heavily vegetated banks with the water surface accessible — and only 4–8 in deep in the spawning zones — are characteristic haunts. These conditions define much of the flooded forest edge and stream-margin habitat across Suriname's coastal plain and lower river basins.
Feeding
Pyrrhulina stoli is a surface and near-surface feeder, its upturned mouth well suited to taking insects and invertebrates from the water film. In the wild the diet consists of small terrestrial invertebrates that fall onto the surface, as well as aquatic insects and their larvae from the upper water column — a pattern typical of the genus and subfamily.
In the aquarium the species readily accepts a range of small live and frozen foods: Daphnia, fruit flies (Drosophila), small mosquito larvae, and finely crushed flake. Its preference for surface feeding means that sinking pellets are less useful than foods that float or drift in the top centimeter of the water column. Regular provision of live or frozen invertebrates helps maintain condition and color, particularly in breeding fish.
Mating
The Pyrrhulina reproductive strategy is one of the more remarkable in the family Lebiasinidae. Males court females with fin displays and pursue them to the underside of overhanging leaves, broad plant blades, or occasionally the glass panel of an aquarium wall. The pair spawns on this surface above the waterline, depositing adhesive eggs that cling out of the water.
The male then guards the clutch and performs the defining behavior of the genus: he repeatedly splashes water upward onto the eggs using rapid tail strokes, maintaining the moisture necessary for development while also oxygenating the eggs. This splash-guarding behavior continues until the eggs hatch and the larvae fall into the water. Females produce multiple small batches — typically 8 to 10 eggs per spawning event, with total production across a breeding period reaching roughly 150 to 200 eggs — rather than releasing a single large clutch.
Breeding
Breeding Pyrrhulina stoli in captivity requires a shallow aquarium or breeding vessel with very low water level — approximately 4–8 in — so that the male can readily reach overhanging spawning surfaces with his splashing. Broad-leaved floating plants, or a glass cover with slight condensation, serve as natural spawning sites; some breeders use strips of plastic sheeting positioned just above the water surface.
The water should be soft and slightly acidic, reflecting the species' Surinamese stream habitat: pH in the range of 6.0 to 7.0, low conductivity, and temperatures in the mid-to-upper 20s Celsius. After spawning, the male should be left to guard the eggs while the female is removed to prevent predation of the clutch. Hatched larvae fall to the water and initially hang near the surface; they require very small first foods — infusoria or commercial liquid fry preparations — before graduating to newly hatched brine shrimp nauplii as they grow. Because the male actively tends the clutch, interference during the guarding phase should be minimal.
In the aquarium
Pyrrhulina stoli is an undemanding species for the hobbyist willing to provide the correct shallow, vegetated setup. A species tank or pairing with calm, similarly sized South American fishes is preferable: its surface-oriented lifestyle and preference for still or gently moving water make it poorly suited to turbulent community aquaria. Other Lebiasinidae, small Apistogramma, or peaceful bottom-dwelling corydoras are reasonable companions.
The tank should be tightly covered — all Pyrrhulina are capable jumpers and will exit through any gap. A heavily planted aquarium with floating vegetation providing shade and cover, soft dark substrate, and low light complements the species' natural habitat and encourages natural behavior. Water changes with slightly cooler, soft water can trigger spawning. Because the fish is relatively uncommon in the trade, captive-bred specimens are valued, and a successful breeding setup will be rewarding for any keeper interested in the unusual reproductive biology of the subfamily.
Conservation
Pyrrhulina stoli is assessed as Least Concern (LC) on the IUCN Red List, with the assessment dated 2020. The species is distributed across coastal river systems in Suriname and has a broader range that extends into Guyana, Venezuela, and Peru according to current records, providing a degree of range security that distinguishes it from more restricted lebiasinids.
Suriname's interior rivers remain among the least disturbed river systems in South America, with substantial areas of intact forest catchment. The primary conservation concerns for small-stream specialists in the region are deforestation, gold-mining activity affecting water quality, and changes to riparian vegetation — pressures that are present but have not yet driven significant declines in P. stoli's population. Continued monitoring of coastal plain stream communities in Suriname and adjacent countries is the most relevant conservation action for the species.