Polypterus bichir

Polypterus bichir（圖片來源：Matthew Inabinett，採用 CC BY 4.0 授權。

Polypterus bichir belongs to 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 widely known in the aquarium market as a “dinosaur fish.” In Chinese-speaking markets it is commonly called the Nile dinosaur bichir or crocodile bichir, while in the English-speaking aquarium trade it is generally referred to as the Nile bichir.

Morphology

In terms of external morphology, Polypterus bichir possesses the most characteristic feature of the family Polypteridae: a series of separated dorsal finlets. These dorsal finlets are among the most important diagnostic traits used to identify bichirs. Individuals typically have about 14–18 dorsal finlets, 13–16 anal fin rays, and 46–54 lateral line scales. The upper and lower jaws are approximately equal in length, which also serves as an important trait for species identification.

In addition, unlike most ray-finned fishes, bichirs retain several primitive characteristics associated with early actinopterygians. One example is their distinctive ganoid scales. These scales consist of a heavily calcified outer ganoine layer, a cosmine-like dentine layer beneath it, a vascular layer containing pulp cavities, and an inner isopedine layer formed by parallel bony lamellae.

Because their natural habitat consists of vegetation-rich swamp environments, bichirs have evolved a cylindrical body and a flattened head suited for navigating complex bottom habitats. In order to survive in swamps and lakes where wet and dry seasons alternate, these fishes have also convergently evolved spiracles and a gas bladder capable of assisting air breathing, structures somewhat similar to those found in lobe-finned fishes. This allows them to temporarily leave drying pools during the dry season and search for more suitable bodies of water.

In terms of coloration, adult individuals are typically olive green with a white underside. The lateral line scales possess grooves, and the sides of the body display three dark longitudinal stripes. The dorsal and pectoral fins show green coloration with pale yellow patterning. As for body size, Polypterus bichir is considered a large predatory fish, and individuals may reach a total length of about 72 centimeters.

Growth and development in the family Polypteridae differ from those of most fishes. Juveniles possess distinctive external gills that resemble those of amphibian larvae. The evolutionary reason for the presence of these structures is still unclear. However, it is certain that the external gills gradually regress and disappear as the fish develops.

Native Range and Habitat

Polypterus bichir is distributed in Africa, particularly within the Nile River basin, Lake Chad, and nearby river systems. It inhabits streams, swamps, and slow-moving waters with dense vegetation. These fishes primarily occupy freshwater rivers and gently flowing environments.

As an ambush predator, the species tends to inhabit areas with slow currents, shallow banks, abundant vegetation, and muddy or sandy substrates. When dissolved oxygen levels decline, it can supplement oxygen intake by breathing air through its lung-like gas bladder.

Aquarium Environmental Conditions

Because bichirs are large predatory fish, it is generally recommended to keep them individually. They should not be housed together with small fish or other aggressive large predators. If mixed housing is necessary, they may be kept with large and relatively peaceful fish species.

Aquarium size should be determined according to the adult body size of the fish. A single individual usually requires at least 300–500 liters of water, with a tank length of at least 120 centimeters so that the fish has enough room to turn. If multiple individuals are to be housed together, a substantially larger aquarium will be necessary. It should also be noted that bichirs are powerful jumpers. The aquarium should therefore be equipped with a secure but ventilated lid to prevent individuals from escaping due to stress or when surfacing for air.

In terms of aquarium layout, bichirs are typical ambush predators that prefer dim or sheltered spaces. Decorations such as driftwood, caves, dense plants, or floating vegetation can be arranged to provide cover. The substrate is best composed of sand or fine gravel. Direct exposure to strong lighting should be avoided, as it may cause stress.

Polypterus bichir（圖片來源：TVRGolf，CC0 1.0 公共領域。）

Regarding water conditions, it is recommended to maintain a temperature of 24–28°C and water that is neutral to slightly alkaline (approximately pH 6.5–7.8). Filtration should produce gentle water flow rather than strong currents. Because bichirs are bottom-dwelling carnivores that produce considerable food debris and organic waste, relatively frequent water changes are required, such as replacing 20–30% of the water each week or adjusting according to water quality. A highly efficient filtration system combining biological and mechanical filtration is recommended.

Feeding Conditions

In feeding behavior, bichirs are bottom-dwelling carnivores. Their diet mainly consists of small fish, crustaceans, earthworms, insects, and aquatic invertebrates. In captivity they can also accept sinking carnivorous pellets or frozen foods such as bloodworms, shrimp, and fish meat. A varied diet is recommended to maintain balanced nutrition.

As for feeding frequency, adult individuals are typically fed once per day or every other day, depending on activity level and food intake. Juveniles require more frequent feeding, usually once or twice daily. Feeding is best conducted in dim lighting or during evening and nighttime hours, as bichirs are largely nocturnal and tend to feed more actively under low-light conditions.

Breeding Conditions

Polypterus bichir is an egg-laying species that produces scattered eggs. The eggs are slightly adhesive and will attach to plants or other surfaces. 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.

The eggs typically hatch in about 3–7 days, depending on water temperature. In the early stages, the larvae feed primarily on planktonic or very small benthic prey. Juveniles may possess external gills during early development.

Although captive breeding of bichirs has been achieved in the aquarium hobby, the breeding and larval-rearing stages require specialized equipment and considerable experience. This often involves the use of dedicated breeding tanks, protection of eggs and larvae, and careful management of larval food. If breeding is attempted, providing abundant plants or spawning mops, maintaining stable water quality, and simulating seasonal changes such as temperature or water-level fluctuations may help induce spawning. During the incubation period, antifungal treatment such as the careful use of methylene blue may be applied. Newly hatched larvae should be supplied with very small live foods such as protozoans, rotifers, or newly hatched brine shrimp.

Preventing Invasive Spread

Polypterus bichir is native to Africa and is a highly adaptable carnivorous fish. When released or abandoned in non-native environments, it can easily become an invasive species and threaten local ecosystems. These fishes may prey on the eggs and juveniles of native fish species and compete with them for food resources, potentially affecting the survival of indigenous fish populations.

For this reason, aquarium keepers must follow the principle of not abandoning or releasing captive animals into the wild. Under no circumstances should bichirs be released into natural waters. If an owner is no longer able to keep the fish, it can be transferred to another aquarist or rehomed through aquarium shops or institutions willing to accept it.

Author: Rodrigo

參考資料

1. Froese, R.; Pauly, D. (2017). "Polypteridae". FishBase. Retrieved 18 May 2017.

   
   

2. Diallo, I. (2020). "Polypterus bichir". IUCN Red List of Threatened Species. 2020: e.T182155A58329182. doi:10.2305/IUCN.UK.2020-3.RLTS.T182155A58329182.en. Retrieved 30 November 2024.

3. Van Der Laan, Richard; Eschmeyer, William N.; Fricke, Ronald (11 November 2014). "Family-group names of Recent fishes". Zootaxa. 3882 (1): 1–230. doi:10.11646/zootaxa.3882.1.1es.

   
   

4. Moritz, Timo; Britz, Ralf (2019-07-23). Revision of the extant Polypteridae (Actinopterygii: Cladistia). DE: Verlag Dr. Friedrich Pfeil. doi:10.23788/ief-1094.