Poll's bichir

Poll's bichir （Polypterus polli）（圖片來源：John P Friel，採用 CC BY 4.0 授權。）

Poll's bichir (Polypterus polli) is a member of the class Actinopterygii, order Polypteriformes, and family Polypteridae. It is a predatory fish commonly found in the aquarium trade. Because of its strongly primitive appearance and body structure that differs markedly from most other bony fishes, it is often referred to as a “dinosaur fish” in the aquarium market.

Morphology

Like other members of Polypteridae, Poll's bichir possesses one of the family's most distinctive characteristics: multiple separated dorsal finlets. These small finlets arranged along the back are among the most important diagnostic features used to identify bichirs. In this species the number of dorsal finlets typically ranges from five to seven, and the anal fin contains fourteen soft rays. The lower jaw protrudes slightly, which also serves as an important species-level diagnostic trait.

Compared with most modern ray-finned fishes, bichirs retain a number of ancestral features characteristic of early actinopterygians. One notable example is the presence of ganoid scales. These scales consist of several layers: an outer highly mineralized ganoine layer, a cosmine-like dentine layer, a vascular layer containing pulp cavities, and an inner isopedine layer formed by parallel bone plates.

Close-up of the head of Polypterus polli（圖片來源：John P Friel，採用 CC BY 4.0 授權。）

The ancestral habitat of bichirs consists of densely vegetated swamps and floodplain waters. Their cylindrical body shape and flattened head are well suited for navigating complex benthic environments. In addition, because many swamp and lake habitats in Africa experience pronounced wet and dry seasons, bichirs have evolved respiratory adaptations convergent with those seen in lobe-finned fishes. These include functional spiracles and a gas bladder that acts as an accessory respiratory organ. Such adaptations allow them to survive in poorly oxygenated waters and even temporarily leave drying pools in search of more suitable habitats.

Adult Poll's bichirs are generally brownish in color. A dark brown stripe runs from the snout to the region beneath the eye, and another extends from above the upper lip toward the mouth. Along the dorsal surface of the body are ten darker longitudinal bands that extend downward toward the belly. The ventral surface is yellowish brown. The caudal fin bears irregular black spots, and the other fins display darker markings. In terms of body size, Poll's bichir is considered a medium-sized predatory fish, reaching a maximum length of approximately 32 cm.

Growth and development

Members of Polypteridae differ from most fishes in that their larvae possess external gills. These structures resemble the external gills seen in amphibian larvae. The evolutionary reason for this unusual convergence remains unclear. However, it is known that these external gills gradually regress and disappear as the fish grows.

Native Range and Habitat

Poll's bichir is distributed in the middle and lower Congo River basin in Africa. It inhabits streams, swamps, and slow-moving waters with dense vegetation. These fish are typically found in freshwater rivers and sluggish waterways. As ambush predators, bichirs prefer habitats with low water velocity, shallow areas rich in vegetation, or substrates composed of mud and sand. When dissolved oxygen levels decline, they can supplement oxygen intake through aerial breathing using their lung-like gas bladder.

Aquarium Conditions

Poll's bichir is a medium-sized predatory fish. It is generally recommended to keep individuals singly and avoid housing them with small fishes or large aggressive predators. If community housing is attempted, it should only be done with large, peaceful species.

Aquarium size should be determined according to the adult body size. A single individual should be provided with at least a 300–500 liter aquarium with a minimum length of 120 cm, allowing sufficient room for turning and movement. If multiple individuals are kept together, a larger custom aquarium is required.

Bichirs are extremely powerful jumpers. For this reason, aquaria must be equipped with a secure but well-ventilated lid to prevent individuals from jumping out when stressed or while surfacing for air.

In terms of aquascaping, bichirs are classic ambush predators that prefer dimly lit or sheltered spaces. Driftwood, caves, dense plants, or floating vegetation can be arranged to provide hiding places. The substrate should consist of sand or fine gravel. Strong direct lighting should be avoided because it may induce stress.

Water parameters should be maintained at temperatures between 24–28°C with neutral to slightly alkaline conditions (approximately pH 6.0–7.8). Water flow should be gentle, and strong currents should be avoided. Because bichirs are benthic carnivores that produce significant food waste and organic debris, frequent water changes are required. Regular partial water changes of approximately 20–30 percent per week are recommended, depending on water quality. Efficient filtration is essential, ideally combining biological filtration with mechanical filtration.

Feeding Conditions

Bichirs are bottom-dwelling carnivores. In nature they feed primarily on small fishes, crustaceans, worms, insects, and other aquatic invertebrates. In captivity they readily accept sinking carnivore pellets or frozen foods such as bloodworms, shrimp, and fish flesh. A varied diet is recommended to maintain balanced nutrition.

Adult fish are typically fed once per day or every other day, depending on activity levels and appetite. Juveniles require more frequent feeding, usually one to two times per day. Feeding is best performed in dim lighting or during evening hours, as bichirs are largely nocturnal and feed more actively under low-light conditions.

Breeding Conditions

Bichirs are egg-laying fishes that produce scattered eggs. The eggs are slightly adhesive and attach to aquatic plants or spawning substrates. A single spawning event typically produces several dozen to several hundred eggs, depending on the species.

Fertilization occurs externally. The male wraps around the female during spawning and assists in releasing sperm to fertilize the eggs. Incubation usually lasts approximately three to seven days, depending on water temperature. Newly hatched larvae initially feed on planktonic or very small benthic prey, and young larvae may possess external gills during early development.

In the aquarium trade, Poll's bichir is rarely bred reliably in captivity. Most individuals available commercially are still wild-caught or originate from occasional breeding events. For aquarists attempting to induce reproduction, it is recommended to provide abundant aquatic plants or spawning mops, maintain stable water conditions, and simulate seasonal environmental changes such as fluctuations in temperature or water level. During incubation, antifungal precautions may be necessary, for example the careful use of methylene blue. Newly hatched larvae should be provided with microscopic live foods such as protozoa, small rotifers, or newly hatched brine shrimp.

Preventing Invasive Spread

Poll's bichir is native to Africa and is a highly adaptable predatory fish. If released into non-native environments, it may become an invasive species and pose ecological threats. These fish may prey on the eggs and larvae of native fishes and compete with local species for food resources, potentially disrupting native populations.

Aquarists must therefore follow the principle of “no abandonment and no release.” Under no circumstances should bichirs be released into natural waterways. If an owner can no longer care for the fish, it should be rehomed to another aquarist or transferred to an aquarium facility willing to accept it.

Author: Rodrigo

References:

1. Gosse, J.-P. (1988). Révision systématique de deux espèces du genre Polypterus (Pisces, Polypteridae). Cybium, 12(3), 239–245.

2. Moelants, T. (2010). Polypterus polli. The IUCN Red List of Threatened Species 2010: e.T182302A7854569. https://doi.org/10.2305/IUCN.UK.2010-3.RLTS.T182302A7854569.en (Retrieved September 13, 2025).

3. Froese, R., & Pauly, D. (Eds.). (2025). Polypterus polli Gosse, 1988. In FishBase. Retrieved September 13, 2025, from https://www.fishbase.se/summary/Polypterus-polli.html

4. GBIF Secretariat. (2025). Polypterus polli Gosse, 1988. GBIF Backbone Taxonomy. Retrieved September 13, 2025, from https://www.gbif.org/species/2346585

5. Evans, S. (n.d.). Polypterus polli (Poll's bichir). Polypterus.info. Retrieved September 13, 2025, from https://www.polypterus.info/p_polli.htm

6. Schäfer, F. (2023, April 28). Polypterus polli — species account. Aquarium Glaser. Retrieved September 13, 2025, from https://www.aquariumglaser.de/en/02-living-fossils/polypterus-polli-2/

7. Moritz, T., & Britz, R. (2019). Revision of the extant Polypteridae (Actinopterygii: Cladistia). Verlag Dr. Friedrich Pfeil.