Taxonomy & naming
Macropodus erythropterus was described by Jörg Freyhof and Fabian Herder in 2002, in a revision of the genus Macropodus published in the journal Ichthyological Exploration of Freshwaters. The type locality is a hill stream near Quyêt Tiên, in Quảng Trị Province, central Vietnam (approximately 17°11.54′N, 106°37.53′E). Eschmeyer's Catalog of Fishes treats the name as valid and the authority as Freyhof & Herder, 2002; because the species was described directly in Macropodus, the authority is not parenthetical.
The species sits within the paradisefish group of the genus Macropodus, which includes the widespread and familiar M. opercularis as well as M. spechti and several other southeast Asian relatives. Its relationship to M. spechti has been a source of ongoing discussion in the literature: some workers regard the two as insufficiently distinct, while others — following Freyhof and Herder's original diagnosis — maintain them as valid species on the basis of consistent differences in colouration and meristics. Current Catalog of Fishes treatment recognises M. erythropterus as a valid species.
Morphology
Macropodus erythropterus reaches approximately 2.5 in standard length (FishBase), with Seriously Fish giving a maximum standard length of 3 in. Like other members of the genus it is laterally compressed with an elongate body, relatively large fins, and the extended soft rays in the dorsal, anal, and caudal fins that are especially pronounced in mature males.
The species is distinguished from congeners primarily by its colouration in life. The dorsal fin and the dorsal area of the body are a striking reddish or brown-red in life, the caudal fin similarly suffused with red, and the area above the anal fin carries an iridescent blue-green sheen. The body is crossed by approximately 10–12 dark vertical bars. The opercular spot — a dark blotch behind the gill cover that is prominent in M. opercularis and several relatives — is pale or effectively absent in M. erythropterus, which is one of the cleaner diagnostic characters separating it from similar species. Males develop extended soft rays in the dorsal, anal, and caudal fins and are larger than females at equivalent ages; females are less brightly coloured.
Habitat
Macropodus erythropterus is known from the Giang River system and associated drainages in Quảng Bình and Quảng Trị Provinces of central Vietnam, a region of hill streams draining toward the South China Sea. The type locality and most records place it in small, heterogeneously structured hill streams with overhanging riparian shrubs, submerged root masses, and dense marginal vegetation — the kind of sheltered, complexly structured marginal habitat that is characteristic of the genus.
Water chemistry records from Seriously Fish for this region span a wide range: temperatures of 68–86 °F, pH 6.0–8.0, and hardness of 90–357 ppm, reflecting the variability of small hill-stream systems that may shift markedly between wet and dry seasons. As a labyrinth fish, M. erythropterus possesses the suprabranchial accessory breathing organ that allows it to extract oxygen from air gulped at the surface, an adaptation that gives it tolerance of warm, still, or oxygen-depleted marginal water that would stress purely gill-breathing fishes.
Feeding
In the wild Macropodus erythropterus is carnivorous, feeding opportunistically on small invertebrates in the stream margin — insect larvae and similar small prey are the staple. Like other paradisefishes it is an active, visually oriented predator in its marginal microhabitat, taking food at mid-depth and at or near the surface.
In captivity the species accepts a broad range of small live and frozen invertebrate foods readily: Daphnia, Artemia (brine shrimp), white worm, and grindal worm are all reported as eagerly taken. Quality dried foods are accepted as a dietary base, but condition and colour are best maintained with regular live or frozen supplements.
Mating
Macropodus erythropterus is a bubble-nesting labyrinth fish. The male constructs and maintains a raft of mucus-bound air bubbles at the water surface, typically anchored to or built beneath floating or emergent vegetation. Courtship follows the typical osphronemid pattern: the male displays intensely to the female, spreading fins and presenting the red-suffused dorsal area, before the pair enters the clasping embrace characteristic of labyrinthfish spawning.
Beyond breeding condition, M. erythropterus is described as a peaceful species that can be maintained as a pair or in a compatible group. During the breeding period the male becomes territorial around his nest and will defend it, but directed aggression outside that context is not a primary concern for well-managed groups.
Breeding
Conditioning with live and frozen foods and a moderate rise in water temperature are the primary triggers for spawning. The male builds his bubble nest at the surface, often incorporating floating plants or plant fragments; spawning then occurs in the typical osphronemid embrace, with the pair releasing several hundred eggs that are gathered and placed into the nest by the male.
The male guards the nest and tends the eggs and newly hatched fry, which is normal behaviour for bubble-nesting Macropodus. Fry are tiny at hatching and require infusoria or comparable very fine first foods initially, progressing to microworm and freshly hatched Artemia nauplii as they grow. A closely fitting cover is important during the early weeks of fry development: labyrinths fishes must be able to gulp warm, moist air at the surface for the labyrinth organ to develop properly, and a cold draught can cause high fry mortality.
In the aquarium
Macropodus erythropterus is only occasionally available in the trade, where it sometimes appears under the common name 'red-backed paradisefish' and has on occasion been misidentified as a colour form of M. opercularis or M. spechti. For keepers who do locate it, its requirements are broadly similar to those of the genus: a well-planted aquarium with areas of dense marginal cover, floating plants to support bubble-nest construction, and gentle filtration that does not create heavy surface turbulence.
The wide water-chemistry range recorded from its natural streams (pH 6.0–8.0, temperatures 68–86 °F) suggests reasonable adaptability, and a moderately warm, neutral to slightly acidic aquarium suits it well. It is peaceful enough for a species or pair aquarium and can be kept with other small, non-aggressive fishes, though males may quarrel with one another if space is limited. Surface access is non-negotiable: as a labyrinth fish it must be able to breathe atmospheric air, and the aquarium should always have a gap at the surface covered by a snug lid to keep the air layer warm and humid.
Conservation
Macropodus erythropterus is assessed as Data Deficient (DD) on the IUCN Red List, assessed in August 2010. The Data Deficient designation reflects a genuine lack of distributional and population data rather than a determination that the species is not at risk: a fish with a restricted range in hill streams of central Vietnam, a region that has experienced considerable habitat change, may well face pressures that have not yet been formally quantified.
Its apparent scarcity in the wild and the limited survey attention given to small Macropodus species in this part of Indochina mean that the true population status remains uncertain. For aquarists, the species' rarity in the trade and tendency to be confused with similar congeners underline the value of careful identification and, where captive stock is available, of maintaining it in culture — a small contribution toward ensuring that any future reassessment has captive populations to draw on.