West African Bichir

West African bichir（ Polypterus retropinnis）。（圖片來源：Ooi Hong Liang，採用 CC BY-NC 4.0 授權。）

Polypterus retropinnis is a member of the class Actinopterygii, order Polypteriformes, and family Polypteridae. It is a predatory fish commonly encountered in the aquarium trade. Because of its primitive appearance and body structure that differs markedly from most other bony fishes, it is widely referred to as a “dinosaur fish.” In Chinese-speaking markets it is commonly known as the Sai green dinosaur, while in English-speaking markets it is usually called the West African bichir.

Morphology

Polypterus retropinnis possesses the defining feature of the family Polypteridae: a series of separate dorsal finlets. These small, individual finlets are one of the most important characteristics used to identify bichirs. In this species, the number of dorsal finlets typically ranges from 7 to 9, and the anal fin contains 12 to 15 soft rays. The upper and lower jaws are approximately equal in length, which is also an important trait used for distinguishing species within the group.

Unlike most ray-finned fishes, bichirs retain many features associated with early actinopterygian fishes. One notable example is their distinctive ganoid scales. These scales consist of several layers, including a heavily calcified ganoine surface layer, a dentine-like cosmine layer, a vascular layer containing a medullary cavity, and an inner isopedine layer composed of parallel bone plates.

Because their natural habitat consists of vegetation-rich swamp environments, bichirs have evolved a cylindrical body and a flattened head that allow them to navigate complex bottom habitats. In addition, the swamps and lakes they inhabit often experience pronounced seasonal fluctuations between wet and dry periods. To cope with these conditions, these fishes have convergently evolved spiracles and a specialized swim bladder capable of functioning in air breathing, somewhat similar to the respiratory systems found in lobe-finned fishes. This adaptation enables them to temporarily leave drying pools during droughts and search for more suitable aquatic habitats.

In terms of coloration, adult Polypterus retropinnis typically have a brown body with dark green tones on the head and dorsal region. Several branching, thick dark longitudinal bands occur along the dorsal side of the body. The muscular base of the pectoral fins bears dark patches, and all fins display darker patterning. In overall size, this species is considered a medium-sized predatory fish, reaching a maximum length of approximately 34 cm.

Growth and Development

Members of the family Polypteridae differ from most fishes in that their juveniles possess distinctive external gills, giving them an appearance somewhat similar to amphibian larvae. The evolutionary reason for this unusual trait is still not entirely clear. However, it is known that these external gills gradually regress and disappear as the fish grows.

Native Range and Habitat

Polypterus retropinnis is distributed in central Africa, particularly in the middle Congo River basin and the Ogooué River drainage. It inhabits streams, swamps, and slow-moving waters with dense vegetation. These fishes are primarily found in freshwater rivers and sluggish aquatic environments.

As ambush predators, bichirs prefer habitats with slow currents, shallow waters, abundant vegetation, and muddy or sandy substrates. When dissolved oxygen levels decline, they are able to supplement oxygen intake by breathing atmospheric air.

Housing Requirements in Captivity

Polypterus retropinnis is a relatively large predatory fish. It is generally recommended to keep individuals singly and avoid housing them with small fish or other large aggressive predators. If community housing is attempted, compatible species should be large and relatively peaceful.

Aquarium size should be determined according to the adult size of the fish. A single individual typically requires at least a 300–500 liter aquarium with a minimum length of about 120 cm to provide sufficient turning space. If multiple individuals are kept together, a substantially larger aquarium is necessary.

Bichirs are powerful jumpers. Aquaria should therefore be equipped with a secure but ventilated lid to prevent the fish from escaping during stress or while surfacing for air.

In terms of aquascaping, bichirs are classic ambush predators that prefer dark or sheltered areas. The aquarium should include structures such as driftwood, caves, dense vegetation, or floating plants to provide hiding spaces. A substrate of sand or fine gravel is recommended. Strong direct lighting should be avoided, as excessive brightness may cause stress.

For water conditions, a temperature range of 24–28°C is recommended. The water should be neutral to slightly alkaline, typically around pH 6.5–7.8. Water flow should be gentle, using mild circulation or filtration. Strong currents are not suitable for this species.

Because bichirs are bottom-dwelling carnivores, they tend to produce considerable leftover food and organic waste. Frequent water changes are therefore necessary, typically around 20–30% per week or adjusted according to water quality. An efficient filtration system combining biological and mechanical filtration is strongly recommended.

Feeding Conditions

Polypterus retropinnis is a bottom-dwelling carnivore. In the wild it feeds mainly on small fish, crustaceans, earthworms, insects, and other aquatic invertebrates. In captivity it can also accept sinking carnivorous pellets and frozen foods such as bloodworms, shrimp, or fish flesh.

A varied diet is recommended to maintain balanced nutrition. Adult individuals are usually fed once per day or once every two days, depending on their activity level and appetite. Juveniles require more frequent feeding, typically once or twice daily.

Feeding is best carried out under dim lighting or during dusk and nighttime, as bichirs are primarily nocturnal and feed more actively in low-light conditions.

Breeding Conditions

Polypterus retropinnis is an egg-laying species that releases scattered eggs with slight adhesiveness. The eggs attach to aquatic plants or other surfaces. A single spawning event typically produces several dozen to several hundred eggs, although this varies among species.

Fertilization occurs externally. The male wraps around the female and assists in releasing sperm during spawning. The eggs usually hatch within approximately 3 to 7 days, depending on water temperature.

During early development, the larvae feed primarily on planktonic or very small benthic prey and may possess external gills during the juvenile stage.

Although captive breeding of bichirs has been recorded in the aquarium hobby, successful reproduction and rearing of larvae require specialized equipment and experience. Dedicated breeding tanks, protection of eggs and larvae, and careful management of larval food are often necessary.

If breeding is attempted, providing abundant plants or spawning mops may help facilitate egg deposition. Stable water conditions and simulated seasonal changes, such as adjustments in temperature or water level, may also stimulate spawning behavior. During incubation, antifungal precautions such as the careful use of methylene blue may be beneficial. Newly hatched larvae should be provided with microscopic live foods such as protozoans, rotifers, or newly hatched brine shrimp.

Preventing Invasive Spread

Polypterus retropinnis is native to Africa and is a highly adaptable carnivorous fish. When released into non-native environments, it may become an invasive species that threatens local ecosystems. These fishes may prey on the eggs and juveniles of native fish and compete with indigenous species for food resources, potentially affecting native fish populations.

For this reason, aquarists must follow the principle of responsible fishkeeping: never abandon or release aquarium animals into natural waters. Under no circumstances should Polypterus retropinnis be released into the wild. If an owner can no longer keep the fish, it should be transferred to another aquarist or surrendered to an aquarium facility willing to accept it.

Author: Rodrigo

References:

1. Froese, R., & Pauly, D. (Eds.). (n.d.). Polypterus retropinnis summary. FishBase.

   
   

2. FishBase. (n.d.). Field guide: Polypterus retropinnis.

   
   

3. SeriouslyFish. (n.d.). Polypterus retropinnis — West African bichir: care & habitat.

   
   

4. Polypterus.info. (n.d.). Polypterus retropinnis species page.

   
   

5. Eschmeyer, W. N. (CalAcademy). (n.d.). Catalog of Fishes: Polypterus retropinnis.