Taxonomy & naming
Polypterus retropinnis was described by the French zoologist Léon Vaillant in 1899 and is placed by the Catalog of Fishes in the genus Polypterus, family Polypteridae, order Polypteriformes. The Polypteridae — the bichirs and the reedfish — are the most basal living lineage of the ray-finned fishes (Actinopterygii), a branch that split from all the others deep in the fossil record; they are genuine living fossils, retaining ancient features the rest of the ray-finned fishes lost long ago. The genus name Polypterus means "many fins," for the row of separate dorsal finlets running down the back that gives every bichir its saw-toothed silhouette.
The species epithet retropinnis is built from the Latin retro ("backward") and pinna ("fin"), a name generally read as referring to the fish's fins being set unusually far back along the body relative to other Polypterus. In the trade it is sold simply as the West African bichir, sometimes as the retropinnis bichir, and it is one of several similar-looking West and Central African species in the genus; FishBase notes that its distinctive iris pattern — alternating creamy-white patches and black dots — together with a finlet count of seven to nine, is a useful character for telling it apart from close relatives.
Morphology
Polypterus retropinnis has the classic bichir body plan: an elongated, roughly cylindrical body, faintly flattened beneath, sheathed from head to tail in thick, interlocking ganoid scales — rhombic bony plates capped with a hard, enamel-like layer that together form a flexible suit of armour. Along the back runs a row of seven to nine separate dorsal finlets, each a short spine trailing a small membranous flag, rather than a single continuous dorsal fin. The paired pectoral fins sit on fleshy, muscular lobes that let the fish prop itself up and haul across the substrate almost like a pair of stubby limbs.
One of the species' more useful field marks is its eye: FishBase describes the iris as patterned with alternating creamy-white areas and black dots, a look distinct enough to help separate it from other West and Central African bichirs in hand. Maximum recorded size is modest for the family — FishBase gives 13.4 in total length, and Seriously Fish notes individuals to about 13.8 in — making it one of the smaller members of the genus. Like all bichirs it carries a pair of rudimentary, sac-like lungs behind the gills and breathes air gulped at the surface; its eyesight is weak, and it hunts chiefly by smell.
Habitat
Polypterus retropinnis has one of the broader ranges among the West and Central African bichirs, recorded across a swathe of Upper and Lower Guinea and into the Congo basin: Seriously Fish lists it from Liberia, Ivory Coast, Ghana, Togo, Benin and Nigeria in the west, through Cameroon, Gabon and the Central African Republic, to the Republic of the Congo and the Democratic Republic of the Congo, and FishBase likewise places it across the Congo basin and West Africa more broadly. This is a considerably wider distribution than many of its closest relatives, which tend to be narrower-range endemics.
Across that range it favours slow, warm lowland waters — marshes, swamps, seasonally flooded forest and grassland, and small rivers — rather than fast, open channels. These are exactly the low-oxygen, often stagnant habitats where a pair of lungs pays off: a fish that can rise and gulp air at the surface can persist in swampy water that would suffocate a gill-only fish, and can even make short overland movements between shrinking pools as the water recedes.
Feeding
Like the rest of its family, Polypterus retropinnis is a nocturnal, ambush-hunting carnivore that relies on smell rather than sight. Its eyes are poor, but its nose is acute, and it cruises slowly along the bottom after dark, locating prey by scent before engulfing it with a quick sideways snap of the jaws. In the wild it takes aquatic insect larvae, worms, crustaceans, small molluscs, amphibians and any small fish it can catch — the operating rule for every bichir is that if it fits in the mouth, it is food.
In the aquarium it accepts meaty foods readily: earthworms, prawn, mussel, and whole frozen or live foods such as bloodworm, with many individuals eventually learning to take sinking pellets. Because it hunts by smell and moves deliberately, it can be crowded out at feeding time by faster, more visually driven tankmates, so is often best fed after dark or with food dropped close to where it is resting.
Mating
Bichir reproduction is built around external fertilisation and a distinctive courtship apparatus rather than any pairing bond. As in other Polypterus, the male is expected to cup his thickened, muscular anal and pelvic fins into a shallow pouch beneath the female as she releases a few eggs at a time; the male fertilises the eggs within this cradle, and the small, adhesive eggs are then scattered onto plants and the substrate. The sequence is repeated many times over an extended spawning bout.
Sexual dimorphism in the genus is chiefly this spawning equipment: males carry a broader, thicker anal fin than females, most pronounced in breeding condition. Beyond the act of spawning itself there is no pair bond and no territory defended — courtship and mating in Polypterus retropinnis have not been documented in detail in the aquarium, but the pattern is expected to follow the well-studied behaviour of its close relatives.
Breeding
Breeding has not been formally documented in home aquaria for this species. Based on its wild ecology and the pattern shared across the genus, it is presumed to be an egg-scatterer that spawns during the rainy season, when rising, softer, more acidic water simulates the seasonal flood pulse of its swamp and small-river habitats. As in other bichirs, there is no parental care at all — adults will eat both their own eggs and any fry they encounter, and the small adhesive eggs are left to fend among fine-leaved plants.
Where spawning has been observed closely in the family, eggs typically hatch in three to four days into larvae that carry feathery external gills behind the head, becoming free-swimming roughly three days later as those gills are absorbed. Deliberate captive breeding of Polypterus retropinnis is rare, and — unlike the Senegal bichir and, to a lesser extent, the ornate and barred bichirs, which are now farmed commercially — most individuals in the aquarium trade remain wild-caught.
In the aquarium
Polypterus retropinnis is a manageable, mid-sized bichir for a keeper prepared for a dedicated predator tank rather than a community fish. Seriously Fish recommends a minimum of around 53 US gal, with floor space mattering far more than height — a long, wide footprint of soft sand lets it move and rest naturally. Furnish the tank with driftwood, smooth rockwork and plants to provide daytime cover for what is fundamentally a nocturnal hunter, and keep the lighting dim.
As with every bichir, the lid is not optional: this species is a determined escape artist that will exploit the smallest gap, and a fish found dried out on the floor is the most common way keepers lose one — the tank needs a tight, gap-free cover while still leaving an air pocket at the surface for it to breathe. It is also a genuine predator that will swallow any tankmate small enough to fit in its mouth, so companions must be too large to eat and calm enough not to harass it; otherwise it is a hardy, undemanding fish whose ganoid armour makes it tougher and more forgiving of medications than scaleless catfishes. Because it is still mostly wild-caught, careful acclimation and quarantine are worthwhile.
Conservation
Polypterus retropinnis is assessed as Least Concern, a status corroborated by FishBase, Seriously Fish and the U.S. Fish & Wildlife Service's federal species reference. Its comparatively wide distribution across Upper and Lower Guinea and into the Congo basin — spanning Liberia, Ivory Coast, Ghana, Togo, Benin, Nigeria, Cameroon, Gabon, the Central African Republic and both Congos — means it is not confined to a single vulnerable watershed, and it faces no species-wide threat at present.
As with other widespread West and Central African freshwater fishes, local populations can nonetheless come under pressure from wetland drainage, deforestation and habitat conversion within its swamp and floodplain habitat. Because most individuals in the aquarium trade are still wild-caught rather than farm-bred, collection for the ornamental trade is a factor worth monitoring even though it is not currently flagged as a driver of decline.