Mottled bichir

Mottled bichir（Polypterus weeksii）。（圖片來源：Stan Shebs，採用 CC BY-SA 3.0 授權。）

The mottled bichir (Polypterus weeksii) 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 highly primitive appearance and body structure that differs markedly from most other bony fishes, it is often referred to in the aquarium market as a “dinosaur fish.” In Chinese-speaking markets it is commonly called “Sai dinosaur,” while in English it is usually known as the Weeksii bichir or mottled bichir.

Morphology

In terms of morphology, the mottled bichir possesses one of the most distinctive traits of the family Polypteridae: multiple separated dorsal finlets. These small dorsal fin spines are among the most important features used to identify bichirs. Individuals typically have about 9–11 dorsal finlets and 10–14 anal fin rays. The upper and lower jaws are approximately equal in length, which also serves as a diagnostic characteristic within the species.

Unlike most ray-finned fishes, bichirs retain many traits associated with early actinopterygians. One example is their distinctive ganoid scales. These scales consist of several layers, including a heavily mineralized outer ganoine layer, a cosmine-like dentine layer, a vascular layer containing pulp cavities, and an inner isopedine layer composed of parallel bone plates.

Because their natural habitat consists of vegetation-rich swamp environments, bichirs have evolved cylindrical bodies and flattened heads that allow them to navigate complex bottom habitats. In order to survive in wetlands and lakes that experience pronounced wet and dry seasons, these fishes have also independently evolved respiratory adaptations similar to those found in lobe-finned fishes, including spiracles and a gas bladder capable of functioning in air breathing. These features allow them to temporarily leave drying pools and move in search of more suitable water bodies.

Mottled bichir juvenile（圖片來源：ginawalsh，採用 CC BY-NC 4.0 授權。）

In coloration, adult mottled bichirs typically display a grayish-brown body with a beige underside. The back often has a faint dark green tint. The sides of the body show irregular dark patterns running along the length of the fish, while the fins display marbled markings and dark spots. In terms of size, the mottled bichir is considered a medium-sized predatory fish, reaching a total length of up to about 54 cm.

Growth and development

Members of the family Polypteridae differ from most fishes in their early development. Juvenile bichirs possess distinctive external gills that resemble those seen in amphibian larvae. The evolutionary reason for the appearance of these external gills remains unclear. However, it is known that these structures gradually regress and disappear as the fish matures.

Native Range and Habitat

The mottled bichir occurs in the middle basin of the Congo River in Africa. It inhabits streams, swamps, and slow-moving waters with dense vegetation. These fishes mainly occupy freshwater rivers and gently flowing habitats within the region. As ambush predators, bichirs prefer areas with slow currents, shallow margins, abundant vegetation, and substrates composed largely of mud or sand. When dissolved oxygen levels decline, they are able to supplement oxygen intake by breathing atmospheric air through lung-like respiration.

Aquarium conditions

The mottled bichir is a medium-sized predatory fish, and it is generally recommended to keep individuals singly. It is not advisable to house them with small fish or with large aggressive predators. If mixed-species housing is attempted, it is best done with large, relatively peaceful fish.

Aquarium size should be determined according to the adult size of the fish. A single individual typically requires at least 300–500 liters of water, with a tank length of at least 120 cm to allow sufficient space for turning and movement. If multiple individuals are to be housed together, a significantly larger aquarium is required. Bichirs are also strong jumpers, so tanks should be fitted with well-ventilated but secure lids to prevent individuals from leaping out due to stress or while surfacing to breathe.

In terms of tank design, bichirs are classic ambush predators that prefer dimly lit or sheltered areas. The aquarium should include driftwood, caves, dense vegetation, or floating plants to create shaded hiding places. A substrate composed of sand or fine gravel is recommended. Direct strong lighting should be avoided, as it may cause stress.

For water conditions, a temperature range of 24–28°C is suitable, with water that is neutral to slightly alkaline (approximately pH 6.5–7.8). Water flow should remain gentle, and strong currents should be avoided. Because bichirs are bottom-dwelling carnivores that often produce uneaten food and organic waste, effective filtration is essential. Frequent water changes—such as 20–30% weekly or adjusted according to water quality—are recommended. A combination of biological and mechanical filtration is ideal.

Feeding Conditions

Bichirs are bottom-dwelling carnivores. In nature they primarily feed on small fish, crustaceans, earthworms, insects, and aquatic invertebrates. In captivity they 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 every other day, depending on activity level and appetite. Juveniles require more frequent feeding, typically one to two times per day. Feeding is most effective under dim lighting or during dusk and nighttime periods, as bichirs tend to be more active and responsive to food in low-light conditions.

Breeding Conditions

The mottled bichir is an egg-laying species that releases scattered eggs. The eggs are slightly adhesive and attach to plants or other structures. A single spawning event may produce several dozen to several hundred eggs, depending on the species and individual.

Fertilization occurs externally. During spawning, the male wraps around the female and assists in releasing sperm. Eggs usually hatch within approximately 3–7 days, depending on temperature. Newly hatched larvae initially feed on planktonic or very small benthic prey, and early larvae may possess external gills.

Within the aquarium hobby, the mottled bichir is considered a relatively uncommon species, although it is occasionally maintained by collectors and specialized suppliers. Documented cases of captive breeding are limited. For those attempting reproduction, providing abundant plants or spawning mops, maintaining stable water conditions, and simulating seasonal environmental changes such as shifts in temperature or water level may help trigger spawning. During incubation, antifungal treatment (such as the careful use of methylene blue) may be helpful, and newly hatched larvae should be provided with very small live foods such as protozoans, micro-rotifers, or newly hatched brine shrimp.

Preventing Invasive Spread

The mottled bichir is native to Africa and is a highly adaptable carnivorous fish. When released into non-native environments, it can potentially become an invasive species that threatens local ecosystems. It may prey on the eggs and larvae of native fishes and compete with them for food resources, thereby affecting the survival of indigenous fish populations.

For this reason, aquarium keepers must follow the principle of never abandoning or releasing captive animals into the wild. Under no circumstances should mottled bichirs be released into natural waterways. If a keeper is no longer able to maintain the fish, it can be transferred to another aquarist or donated to an aquarium facility willing to accept it.

Author: Rodrigo

References:

1. Boulenger, G. A. (1898). On the species of Polypterus. Annals and Magazine of Natural History (Series 7), 2(11), 416–420. (Original description of Polypterus weeksii).

2. Froese, R., & Pauly, D. (Eds.). (2025). Polypterus weeksii Boulenger, 1898. In FishBase. Retrieved September 13, 2025, from https://www.fishbase.se/summary/Polypterus-weeksii.html

3. Moelants, T. (2010). Polypterus weeksii. The IUCN Red List of Threatened Species 2010: e.T182301A7854552. https://doi.org/10.2305/IUCN.UK.2010-3.RLTS.T182301A7854552.en

   (Retrieved September 13, 2025).

4. GBIF Secretariat. (2025). Polypterus weeksii Boulenger, 1898. GBIF Backbone Taxonomy. Retrieved September 13, 2025, from https://www.gbif.org/species/2346594

5. Evans, S. (n.d.). Polypterus weeksii (Weeks' / mottled bichir). Polypterus.info. Retrieved September 13, 2025, from https://www.polypterus.info/p_weeksii.htm

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