Taxonomy & naming
Cope described this species in 1870 in the Proceedings of the American Philosophical Society, placing it in the genus Holotaxis. Eschmeyer's Catalog of Fishes (CAS) records the valid name as Pyrrhulina melanostoma, with the feminine ending correcting Cope's original masculine form in accordance with the International Code of Zoological Nomenclature; the species epithet and its variants are often seen interchanged in the literature and in aquarium references. Syntypes from the original collection have not been located.
The species sits within the subfamily Pyrrhulininae of the family Lebiasinidae, the pencilfish-and-pencilfishes clade of the Neotropical freshwater fish order Characiformes. Pyrrhulina is the largest genus in Pyrrhulininae, comprising laterally compressed, generally surface-dwelling species of slow and sluggish waters across South America. Its relationships within the genus have received only intermittent systematic attention, and the group is likely in need of a comprehensive revision.
This fish was long listed as Pyrrhulina melanostomus; Eschmeyer's Catalog of Fishes places the species in Pyrrhulina, and the current combination Pyrrhulina melanostoma is followed here.
Morphology
Pyrrhulina melanostoma is a small, slender, moderately laterally compressed fish typical of its genus. Like congeners it is unlikely to exceed about 2.8 in in total length, with most individuals smaller. The body is elongated and streamlined; the mouth is terminal to subtly upturned, an adaptation for surface feeding. The dorsal fin is set well posterior, and the caudal fin is rounded to slightly truncate.
Colouration in the genus generally runs to a pale to brownish silver ground colour overlaid with a bold dark lateral stripe running from behind the eye to the caudal peduncle, sometimes accompanied by scattered dark flecks or a faint mid-lateral row of spots. Specific colour notes for P. melanostoma are sparse in the available literature; field and aquarium accounts of closely related species suggest the pattern can intensify under good conditions and dims noticeably in stress or pale water. The specific epithet melanostomus (from Greek, meaning 'black mouth') presumably refers to pigmentation around the mouth in the type specimens.
Habitat
The type locality is Pebas (Pevas), a town on the Amazon River in the Loreto region of northeastern Peru, and the species is recorded from Ecuador, Peru, and Bolivia, indicating a range across the upper and western Amazon basin. Within this area it inhabits the warm, soft, and slightly to moderately acidic blackwater and clearwater streams, river margins, and flooded-forest channels characteristic of the western Amazon lowlands.
As is typical for Pyrrhulina, this species favours sheltered conditions: slow-moving or still backwaters, heavily vegetated banks, shallow forest pools, and flooded areas where emergent and floating vegetation provides cover near the surface. Water temperatures in these habitats generally fall in the range of 72–82 °F, and pH is typically soft and acidic, around 5.5–7.5. Conductivity is low, reflecting the mineral-poor soils of the Amazon basin.
Feeding
Pyrrhulina species are carnivorous surface hunters. Their upturned mouths and dorsal-set eyes are adapted for intercepting small prey at or just below the water surface. In the wild the diet consists of terrestrial and aquatic invertebrates — insects, small larvae, and other soft-bodied invertebrates that fall onto or congregate at the surface film — as well as small aquatic crustaceans and zooplankton.
In captivity the species accepts small live and frozen foods: fruit flies (Drosophila), mosquito larvae, small bloodworm, brine shrimp, and Daphnia are all suitable. Prepared foods may be accepted once the fish is settled, but offering a varied diet that includes live or frozen invertebrates keeps the fish in good condition. Feeding at or near the surface respects the species' natural foraging posture and reduces competition with deeper-dwelling tankmates.
Mating
Pyrrhulina species are substrate spawners that deposit adhesive eggs on leaves, flat rocks, or glass surfaces — a contrast with the egg-scattering habit of many characins. Males select and defend a spawning site, courting females with lateral displays and short chases. A receptive female is led to the chosen substrate, where the pair spawn in a series of short passes, depositing a small number of eggs at a time.
Males exhibit territorial behaviour around the nest site and may be aggressive toward rival males during the breeding period. The courtship and spawning sequence of congeners suggests that the male takes the lead in site selection and nest tending, with pair bonds forming briefly for each spawning bout. The degree of sexual dimorphism in P. melanostoma is not well documented; in related species males are often slightly larger and may show brighter or more contrasting pigmentation when in breeding condition.
Breeding
Like other Pyrrhulininae, P. melanostoma is a substrate-spawning species whose male guards the eggs after fertilisation. Eggs are deposited directly onto a flat surface — in aquarium settings a broad plant leaf, a piece of slate, or the aquarium glass will serve — and adhere in a compact cluster. Clutch sizes in related Pyrrhulina species are typically in the range of 150–200 eggs.
The male fans and guards the eggs until hatching, driving off other fish. Incubation at tropical temperatures generally takes two to four days, after which the larvae are mobile within a day or two of hatching. The fry are tiny and require the finest first foods — infusoria, rotifers, or commercial microalgae-based liquid fry food — before graduating to newly hatched brine shrimp nauplii. The male's guarding behaviour diminishes once the fry are free-swimming. A species-only or quiet, fine-planted breeding tank is strongly advisable, as the male's nest-guarding instinct is quickly overwhelmed by tankmates.
In the aquarium
Pyrrhulina melanostoma is a specialist's fish rather than a mainstream aquarium species: it is rarely traded and almost never discussed in hobbyist literature under this name, though related Pyrrhulina species such as P. spilota and P. vittata do appear periodically in the hobby. Its care requirements follow the genus closely. A quiet, well-planted aquarium of 23.6 in or longer, with soft, slightly acidic water (pH 5.5–7.0, temperature 75–81 °F), a dark substrate, floating plants to diffuse light, and little surface agitation provides appropriate conditions.
As a surface-oriented fish it should be kept with calm, non-competitive tankmates that occupy lower or mid-water zones. Small corydoras, dwarf cichlids such as Apistogramma, and peaceful bottom-dwellers are compatible. The species is a capable jumper and a tight-fitting lid is essential. A species-only setup is ideal for breeding attempts. Water quality must be stable and clean; frequent small water changes with conditioned, slightly acidic water are better than large, infrequent ones for maintaining the soft-water conditions the fish prefers.
Conservation
Pyrrhulina melanostoma has not been formally assessed by the IUCN Red List, and FishBase notes the species as Not Evaluated. The absence of an assessment reflects limited research attention rather than confirmed abundance. The western and upper Amazon basin, from which this species is known, is a region of continuing habitat pressure from deforestation, agricultural expansion, and river modification, though the species' relatively wide range across Ecuador, Peru, and Bolivia suggests it is less immediately vulnerable than strictly localised endemics.
No specific conservation measures are in place for P. melanostoma. Given its obscurity in the hobby and the paucity of survey data, baseline population information is lacking. A formal IUCN assessment drawing on collection records and habitat mapping would be a necessary first step toward understanding whether any conservation intervention is warranted.