Taxonomy & naming
Pethia erythromycter was described by Kullander and Fang in 2008, when it was placed in the then-current genus Puntius — its original combination was Puntius erythromycter (Kullander & Fang, 2008). The species was subsequently transferred to Pethia, a genus erected by Pethiyagoda, Meegaskumbura and Maduwage in 2012 to accommodate a clade of small, typically spotted or plain-bodied barbs centred on the Indian subcontinent and Sri Lanka. The Catalog of Fishes (Eschmeyer, CAS) records the current valid combination as Pethia erythromycter (Kullander & Fang, 2008), with the parenthetical authority marking the transfer out of Puntius.
The genus Pethia was carved out of the historically sprawling Puntius following molecular and morphological revision that showed the old genus to be polyphyletic. Within Pethia, P. erythromycter sits geographically at the northeastern edge of the genus's range — Myanmar rather than the core Indian or Sri Lankan homeland — and the Indawgyi basin population appears to be the only known occurrence of the species.
Morphology
A very small barb, P. erythromycter reaches a maximum recorded standard length of 1.5 in. The body is moderately deep and laterally compressed in the typical Pethia fashion, with large scales and a relatively blunt snout. The overall coloration is pale silver to straw-yellow with a faint lateral sheen, and males develop a conspicuous red to orange-red flush concentrated on the snout and the lips — the "lipstick" character from which both the common name and the species epithet are drawn.
Sexual dimorphism is evident: males are slimmer and carry the diagnostic red snout, while females are deeper-bodied, rounder when gravid, and lack the vivid nasal flush. The fins are otherwise unexceptional, without the elongated or colourful extremities seen in some congeners.
Habitat
Lake Indawgyi in Kachin State, northern Myanmar, is the primary known range of P. erythromycter. The lake is the largest natural freshwater lake in Myanmar and sits in a broad, low-lying basin surrounded by forest and wetland; it drains southward via the Indaw Chaung into the Irrawaddy system. The lake is shallow in its margins and is characterized by extensive beds of submerged and emergent vegetation — Vallisneria, Elodea and the filamentous alga Spirogyra are all recorded — over a soft clay substrate.
Water conditions at Indawgyi are broadly near-neutral and moderately soft, with temperatures in the subtropical to warm-temperate range reflecting the lake's elevation (~509 ft above sea level) and its latitude in northern Myanmar. Seriously Fish records pH around 7 and hardness of approximately 90 ppm (around 5 dGH), making this cooler and softer water than many lowland tropical barb habitats. The species is benthopelagic and is associated with the vegetated shallows rather than open water.
Feeding
Pethia erythromycter is an omnivore in the wild, browsing among the dense aquatic vegetation of Lake Indawgyi for small invertebrates — insect larvae, tiny crustaceans, worms — as well as algae, diatoms and plant material. The lipstick barb's small size and association with plant beds are consistent with a gleaning feeding strategy, picking food items from leaf surfaces and the soft substrate.
In the aquarium the species accepts a broad range of foods: fine-grade flakes and small pellets form a workable staple, enhanced by small live and frozen foods such as micro-worm, daphnia and baby brine shrimp, along with algae-based supplements. Varied feeding maintains condition and brings out the male's nasal colouration.
Mating
Like all barbs, P. erythromycter is an egg-scattering, non-guarding spawner. There is no nest construction, no pair bond, and no parental care of any kind. Males in spawning condition intensify their red nasal colouration and actively pursue gravid females through vegetation and across the substrate, driving them in short bursts until the pair press together and release eggs and milt simultaneously.
No territory is held beyond the brief act of courtship, and multiple males may pursue the same female. The adhesive eggs fall among plants or onto the substrate immediately, and the adults take no further interest in them — actively eating any they encounter.
Breeding
In the aquarium, P. erythromycter can be bred following the standard approach for small egg-scattering barbs. A separate spawning tank is recommended: shallow, with soft and near-neutral water around pH 7 and a temperature of 72–75 °F reflecting the cooler conditions of Lake Indawgyi, planted with fine-leaved vegetation or fitted with a spawning mop, and with a mesh or fine gravel layer over the bottom to protect the eggs from the adults.
Well-conditioned adults — fed on a variety of live and frozen foods over several days — can be introduced as a pair or a small group with males outnumbering females. Spawning typically occurs in the morning, with the female releasing adhesive eggs that scatter among the plants and fall out of reach. Because both sexes eat their own eggs readily, the adults should be removed after spawning. Eggs hatch in roughly 24–36 hours and larvae become free-swimming a few days later, when infusoria and then newly hatched brine shrimp can be offered.
In the aquarium
The lipstick barb is an uncommon but rewarding aquarium fish, suited to smaller tanks given its diminutive size. It is a peaceful shoaler that should be kept in groups of at least six to ten individuals; when well-grouped, males display continuously to one another, cycling up their red snouts in social competition without inflicting harm. Males kept singly or in pairs become withdrawn or may direct attention to tankmates.
The species prefers the cooler end of the tropical range — 64–72 °F is often cited — reflecting its subtropical lake origin, and it does well in near-neutral, moderately soft water with dense planting. High temperatures and hard alkaline water should be avoided. Suitable tankmates are other small, peaceful species that share its preference for cool, vegetated tanks. The lipstick barb's miniature size and the males' distinctive red snout make it an attractive candidate for planted nano or species tanks.
Conservation
Pethia erythromycter is assessed as Least Concern (LC) on the IUCN Red List, with the assessment dating to 2010. The species is associated with Lake Indawgyi, a lake of considerable size within a relatively well-preserved watershed in Kachin State, and no evidence of major population decline was identified at the time of assessment.
Nonetheless, the species' apparent restriction to a single lake system makes it inherently vulnerable to any localised threat. Lake Indawgyi and its surrounds have faced pressure from gold mining, agricultural encroachment and human settlement, and water quality in parts of the lake has been adversely affected by sedimentation and pollution. The area has also been subject to the broader disruption associated with prolonged civil conflict in Kachin State. Monitoring of the lake's fish community remains limited, and the lipstick barb's actual population status at present is poorly quantified. Collection for the aquarium trade appears to be low-volume.