Taxonomy & naming
Pterobrycon myrnae was described by William A. Bussing in 1974 on the basis of specimens collected from Quebrada Aguabuena, a stream in the Río Rincón drainage about 1 mi west of the town of Rincón on Costa Rica's Osa Peninsula, at an elevation of roughly 98 ft. Bussing placed it in a new genus, Pterobrycon, erected to accommodate this species' uniquely elaborated flank scales — a character unlike anything else in the Characidae at the time.
The species belongs to a group once informally called the glandulocaudines, characids characterised by caudal-gland structures and various specialisations associated with internal fertilisation and sperm transfer. Catalog of Fishes (Eschmeyer, Fricke & van der Laan) records the valid name as Pterobrycon myrnae Bussing, 1974. The genus name combines the Greek pteron (wing or fin) with brycon (a reference to the characid genus Brycon), alluding to the wing-like expanded scales. The specific epithet myrnae is a patronym, though the identity of Myrna is not stated in published accounts accessible from the cache.
Morphology
Pterobrycon myrnae is a slender, laterally compressed tetra reaching approximately 1.5 in standard length in males and up to roughly 2 in in females, with aquarium specimens sometimes reaching 3 in total length. The body is elongate for a characid, suited to life in flowing water, and carries the family's typical lateral line.
The defining feature of the species is the row of highly modified scales on the flanks of adult males. These scales are enlarged and their free tips are extended into paddle-like projections that can be raised and fanned outward during courtship, functioning as visual signals. Males are also more colourful and smaller than females: the male shows extended fins and scale ornamentation, while females are a plain blue-grey and notably larger. This reversed size dimorphism, with females the larger sex, is characteristic of the glandulocaudine breeding system.
Habitat
The species is restricted to Pacific-slope river drainages of the southern Costa Rican lowlands, including the Río Jicote, Río Rincón, and Río Esquinas systems of the Osa Peninsula, from sea level to an elevation of approximately 262 ft. These are streams of the tropical wet lowland forest — one of the most rain-saturated forest types on Earth — characterised by dense canopy cover, abundant leaf litter, and variable flow.
Reported water conditions are warm and relatively soft to moderately hard: temperatures of 73–79 °F, pH 6.0–7.0, and hardness up to about 15 dH. Current ranges from low to high depending on season and stream position. The type locality, Quebrada Aguabuena, is a shaded forest creek at 98 ft elevation. This combination of restricted drainage range and specialised lowland-forest habitat makes the species particularly sensitive to land-use change.
Feeding
Pterobrycon myrnae is insectivorous in the wild, taking small invertebrates associated with the forest-stream environment — aquatic insect larvae, terrestrial insects falling from overhanging vegetation, and similar prey. The shaded forest creeks it inhabits receive abundant allochthonous input, and the fish are adapted to exploiting this surface and near-surface drift.
In the aquarium the species accepts a range of small live and frozen foods — Daphnia, brine shrimp, bloodworm, and mosquito larvae — and will take good-quality small flake and micro-pellets once settled. Live and frozen foods are preferable for conditioning, and a varied diet brings out both condition and the subtle iridescence of the male's scale ornamentation. Feed in small amounts suited to the fish's small mouth.
Mating
Courtship in Pterobrycon myrnae is among the most specialised in the family. Males use their enlarged, paddle-tipped flank scales as visual display structures, raising and fanning them to attract and stimulate females — a behaviour analogous to semaphore signalling, which gave rise to the common name Semaphore Tetra. The males are smaller and more ornate than females, and the dynamic of the pairing reflects the internal-fertilisation system: the male delivers a spermatophore, a packet of sperm the female stores internally.
Females are able to store viable sperm for extended periods after a single mating, fertilising subsequent batches of eggs without further contact with a male. This sperm-storage capacity is an adaptation shared with related glandulocaudine groups and has practical implications for captive breeding: a female separated from males can continue to produce fertile eggs.
Breeding
Fertilisation is internal, distinguishing Pterobrycon myrnae from the typical egg-scattering tetra. After receiving the spermatophore the female attaches fertilised eggs to substrates — fine-leaved plants, roots, or similar surfaces. The eggs are adhesive. Sexual maturity is reached at approximately six months of age in aquarium conditions.
Breeders report that females can continue to lay fertile eggs for some time after the last contact with a male, given the sperm-storage mechanism. As with most characids there is no parental care, and adults should be removed from the eggs after spawning to prevent predation. The fry are small and require appropriately fine first foods. The combination of internal fertilisation, sperm storage, and substrate-attached eggs makes this species biologically unlike most other tetras commonly kept in the hobby.
In the aquarium
Pterobrycon myrnae is a peaceful species suited to a species tank or a community of similarly small, non-aggressive fish. It is a fast-running brook species by origin, and benefits from moderate to good water flow and well-oxygenated conditions. A tank with some current, shaded areas, and fine-leaved plants or spawning mops provides an appropriate environment. Water should be soft to moderately hard and slightly acidic to neutral (pH 6.0–7.0, up to ~15 dH), held at 73–79 °F.
Keeping the species in a small group with at least one male allows observation of the remarkable courtship display. Males kept together will compete and display, making the paddle-scale behaviour readily visible. The species is rare in the hobby and most specimens derive from dedicated breeders rather than wild collection; sourcing captive-bred fish is strongly preferred given its Endangered status and restricted range. Aquarists willing to invest effort in its specific water and feeding requirements are rewarded with behaviour found in almost no other tetra.
Conservation
Pterobrycon myrnae is assessed as Endangered (EN) on the IUCN Red List under criterion B1ab(iii), assessed in May 2019. The listing reflects an extremely limited extent of occurrence — a small cluster of Pacific-slope drainages on the Osa Peninsula of Costa Rica — combined with ongoing habitat degradation driven by deforestation, agricultural conversion, and alteration of the lowland forest-stream environment on which the species depends.
The Osa Peninsula retains significant forest cover relative to much of Costa Rica and includes the Corcovado National Park, which provides some protection to headwater streams within its boundaries. However, lowland streams in the 33–262 ft elevation band occupied by this species fall largely outside the most strictly protected zones, and forest clearance for agriculture and cattle ranching remains a pressure. The species is not commercially traded at scale; hobbyist interest and captive-breeding programmes represent the main insurance population outside the wild. Given its internal-fertilisation biology, captive maintenance is feasible for dedicated keepers.