Polypterus retropinnis – Green Congo Bichir
“if I had discovered only this species in Egypt, it would compensate me for the pains usually involved in a long journey”. These are words of scientist Geoffry Saint Hilaire who was participating in a scientific expedition organised by Napoleon along with the invasion of Egypt. Sitting next to the Nile river he discovered a 1 meter-sized green pre-historic fish, which had been known since the time of the Pharaos. Locals called the fish something like Bee-sheer, probably phonetically spelt by the French as Bichir. In 1802 this species was described as Polypterus bichir.
The current family Polypteridae are the sole survivors of the ancient group Polypteriformes, dating from the late Cretaceous period, 100-60 million years ago, referenced as the last phase of domination of dinosaurs. The distribution of Polypteriformes covered an area stretching current Africa and South America.
Polypteridae (common name bichir’s) are one of the most current primitive ray-finned fish. Overall distribution covers freshwater systems of Africa, mostly 10 degrees north and 10 degrees south of the equator. Fossil records of Chad point out that the current species already lived during the late Miocene (5.3 million years ago). The fossil is said to be very similar to the current P. Bichir and P. endlicheri, species that are still found in that area nowadays.
Evolution may seem to take place in slow-motion concerning this ancient fish. However, under influence of geographical distribution and different habitats, different species and variants arose. The family Polypteridae consists of 2 genera:
- Erpetochthys, consisting only one species: E. Calabaricus
- Polypterus, consisting of 13 species and several variants
Last twenty years the existing description of the species was subject to a lot of confusion among ichthyologists, struggling to fit in variants that were newly discovered. Amended insight led to structural changes in classification, distinguishing 3 groups of Polypterus:
- Senegalus group including Erpetoichthys. Species within these group have a larger upper jaw or of similar length. As a result, the mouth opens to the lower side, equipping these species for feeding on (dead) food from the bottom. This upperjaw group consist E. calabaricus, P. senegalus, P. palmas, P. delhezi, en P. polli.
- Weeksii group is another upper jaw group with a larger upper jaw or of similar length. Members of this group are P. weeksii, P. ornatipinnis, P. retropinnis, P. teugelsi en P. mokelembembe.
- Bichir and Endlicheri group with a larger underjaw than the upper jaw, resulting in a mouth that opens to the upside. These species are more specialised in hunting living animals, such as amphibians, aquatic insects and fish. To this group belong P. bichir, P. ansorgii en P. endlicheri. Underjaw species tend to grow considerably larger than upper jaw species.
Bichirs have a cylindric body shape, similar to that of a reptile. The body is covered by multilayered scales, rhomboid in shape, with an outer layer of ganoine a middle layer of dentine a layer of isopidine and an inner dermal, cosmine bony layer and are referred to as ganoid scales. They have 7 to 16 triangle formed fin dorsal din (Polypterus means many fins), which make a dragon-like appearance. Pectoral fins are placed exactly where you would expect legs on a lizard. The swim bladder is modified to a primitive lung, enabling the fish to take supplementary gulps of air. Combined with the gills a useful adaption to swamps with low oxygen content. Even when the oxygen level is sufficient, Polypterus will gulp air from the surface on a regular basis. Young specimens have external gills, that are clearly visible and are similar to the salamander. While growing these external gills disappear. The scales prevent Polypterus from drying out quickly when out of the water. In dense conditions Polypterus can survive out of the water for up to 2 days, using their pectoral fins to crawl over land.
The eyesight of Polypterus is poor. Food is found by smell. Most of the time when they smell food they follow the general direction of the smell. Often they stumble upon the food, smell again and swallow. Sometimes they hoover past or over the food source to discover they just missed it, and simply hoover back. Even blind specimens are able to find their food. Polypterus have two bony plates in the underjaw which help to crush food. In the natural habitat, a death roll is observed when the fish stumbles on bait to large to swallow. Like crocodiles, the fish bites into the bait and pivots to tear off pieces of meat.
Males are distinguished by the thicker anal fin that is about twice as wide as the anal fin of a female and is also more muscular. This is the only reliable way of sexing specimens. However, the differences appear at a length of 15-20 cm only. Some species are sexually mature after 2 years, the larger species after -8 years. Female specimens grow generally larger than males and have a somewhat larger head.
Polypterus retropinnis is a relatively small Bichir, that grows considerably slower than other species. The maximum size is 10-30 cm. The shape of the body is more slender than most other species. It is quite similar to Polypterus mokelembembe, but Polypterus retropinnis has a greener glow all over the body. Pectoral fins are also green.
Polypterus retropinnis is found in swamps, flooded areas and small rivers of West Africa. The area of distribution consists of Nigeria, Cameroon, Congo, Ghana, Liberia, Ivory coast, Togo, Gabon, Benin and CAR.
Polypterus are opportunistic ambush predators that mostly feed on bait. Generally, they move slowly and cautiously. Most of the time they hide. Often they get more active in dusk or at night, although there are large differences in terms of activity within the species. The larger underjaw species tend to be more active, with less tendency to hide.
There are variations concerning activity, curiosity and voraciousness among the species. However, there are also variations per specimen. Aquarists therefore often have different experiences when it comes to keeping a species in an aquarium.
In general, Polypterus is peaceful towards other fish species, as long it is clear that tankmates do not fit in their mouth. Smaller fish up to 2/3 of their own size can be considered food, especially torpedo-shaped fish. As natural fish hunters, especially the underjaw species can swallow large prey and are able to open their mouths almost the diameter of their bodies. Polypterus is also tolerant to conspecifics of their own size. However, there will be minor territorial conflicts. On the other hand, there are reports from aquarists from individual specimens that will not accept conspecifics or even other fish.
Polypterus is not a good choice for combining with aggressive tankmates, like breeding cichlids. The Polypterus will hide then and stop feeding.
Polypterus has poor eyesight and find their food by smell and coincidence. Most of the time the food is approached slowly until stumbled upon and then suddenly swallowed. Sometimes they hoover past of over the food source, to detect their fault and hoover back to where the smell is strongest. This clumsy sighting can raise an expectation that this is not an efficient hunter at all and to the misplaces conclusion that smaller and faster tankmates might be safe.
Fish are seldom hunted actively (only some larger underjaw species are known for hunting fish actively). Appearances are deceptive, as a Polypterus is able to attack prey from an ambush with A considerable burst of speed. If this tactic is not effective there is also dusk or night to catch prey with relative ease.
Polyterus can produce sound. A rhythmic knocking sound is only noticeable using equipment and is made when specimens meet each other while spreading the dorsal fins. The sound proved to be strongest in the frequency range of 100-200 Hz, with a maximum of 3 pulses a second. The volume decreases when specimens take a distance from each other. Groaning can be heard without equipment when specimens are chased away. It is not know how these sounds are mechanically produced.
Despite it is a rudimentary predator, for many aquarists Polypterus have a touching appearance, caused by the dragon-like appearance, the slow way of moving and the funny pectoral fins. Many aquarists pet their Polypterus by feeding them by hand.
In the aquarium, Polypterus can get 10-15 years old.
A common sight is a floating Polypterus at the surface, preferably resting on aquatic plants of a piece of wood. Some specimens show this behaviour on a regular basis, while other species only show this when they are relatively new. Most of the time it is a sign that the Polypterus does not feel comfortable (yet). Often a Polypterus that is introduced in an aquarium was quite active in the shop and hangs only at the surface in the new aquarium. It seems a natural defensive posture. In a juvenile stage, they are most vulnerable to predation when they have to burst to the surface for a gulp of air. To prevent such dangerous trips to the surface they choose to stay near the surface. This behaviour is especially common for the species from the upper jaw group.
Apart from this, a Polypterus that is stressed too much, often will excrete a more slimy coat on its body.
Launching itself forward in an uncontrolled manner
A defensive reaction when the Polypterus feels threatened. It seems passive and at rest and suddenly launches itself forward by stretching its tail and knocking several objects on its way. Since Polypterus have a sensitive lateral line system all over the body it is possible they detect disturbing sounds like a slamming door in the house of a person entering the room.
Balancing on its tail
One unique behavioural feature is the balancing act on its tail. In this ambush position, they can overlook the area and attack in case of an opportunity.
With a full stomach, the Polypterus bends his body from left to right, suppressing its muscles in order to repack the food in the stomach. Comparable to how we overload a suitcase.
Just like we can observe with crocodiles, Polypterus can carry out death rolls. When a bait is too large to swallow, they bit in it and rotate to tear off pieces of meat. However, aquarists will not see this behaviour very often since it makes no sense to feed larger food than fish can swallow.
Breaking the surface
If a specimen breaks the surface (other than gulping air) then it is a sign of the start of the mating season
Bending the anal fin (only males)
The muscled anal fin is important for courtship. When the males become sexually mature, they instinctively show off by bending their anal fin. With the bent anal fin, they are able to catch eggs, fertilize them, and release them.
If a male is stalking a female then it is part of the courtship.
Polypterus can use their dorsal fins to scratch the sides of their body. While scratching they bend their muscled body so they can reach further.
Probably the yawning is an exercise to keep the mandibular joints healthy and flexible or to reposition them after swallowing a meal.
Lay next to or upon each other
Especially in an aquarium with less hiding place or too bright light this behaviour is seen. Often some Polypterus are piled up in a corner. This can be seen as just an attempt to hide. They feel secure next to each other, underneath or simply in a position where they cannot see their surroundings.
Bury their head
This behaviour is only observed from P. Endlicheri, P. Delhezi en P. Weeksii.
It can be used to hide when it feels threatened, but also as an ambush hunting technique. They cover themselves in the sand, while only their eyes remaining visible.
An aquarium with an open place for swimming room combined with many hiding places with aquatic plants, rocks of driftwood. Somewhat dimmed lighting will result in a Polypterus that feels more comfortable and is more active. Especially young or new introduced specimens will appreciate spots where they can rest next to the surface
Polypterus are muscled, powerful fish that like to hide in vegetation. Aquatic plants should be strongly attached. Young specimens can be kept in an aquarium sized 80 cm. While growing – depending on the species – a larger aquarium will be required. It is important to close the aquarium very well. With their muscled body they are able to lift the lit of the aquarium or squeeze themself through small openings. These animals are true escape artists and most of the fish kept in aquariums end their life next to the aquarium. Last but not least it is important to leave an air gap between de lit and the water surface, so the Polypterus is able to gulp air from the surface.
Polypterus is quite resistant against a variety of fish diseases, like Ich. However, they do not cope with medication very well. The suggestion is to half the advised dosage of this medication.
Often, wild-caught specimens (most Polyterus in the trade) can carry the parasite Macrogyrodactylus polypteri. In the aquarium, this parasite will proliferate very fast and will find the fish soon. Most speciments will not survive this event. When buying Polypterus a quarantine aquarium might be considered. Also, it is possible to kill parasites with a salt bath during transfer. Mix 5 teaspoons of common salt (NaCI) per 3,8-litre water. Add 1/3 of the solution in the transfer bag, wait for 10 minutes and add the next 1/3. Pay attention to the fish, and take it out of the water in case of the first signs of stress. Otherwise, Cover the bag, so it is dark and wait another 15 minutes. Common salt does not influence the PH. Do not use aquarium salt or sea salt, since this influences PH level, which is in fact the biggest (fatal) cause of stress and acclimatisation issues. Seems the saltwater dip cause stress, take the fish out of the water.
Since Polypterus can be tamed in aquariums, it is tempting to feed it by hand. Watch out. The last dorsal fins are razor-sharp and an escaping Polypterus knows how to use them.
In general, Polypterus is not aggressive towards other fish of similar size. Suitable tankmates are:
- Other Polypterus species
- Erpetoichthys calabaricus
- Pimelodus blochii
- Auchenoglanis occidentalis
- Parauchenoglanis macrostoma
- Sturisoma aureum
- Synodontis alberti
- Synodontis angelicus
- Synodontis pleurops
- Synodontis decorus
- Synodontis eupterus
- Synodontis notatus
- Synodontis brichardi
- Synodontis nigrita
- Synodontis flavitaeniata
- Synodontis soloni
- Heteropneustes fossilis
- Bagrichthys hypselopterus
- Metynnis argenteus
- Distichodus notospilus
- Phenacogrammus interruptus
- Myleus schomburgki
- Bryconaethiop microstoma
- Macrognathus aculeatus
Cichlids (avoid breeding pairs, because of the level of aggression)
- Cichlasoma bimaculatum
- Tilapia joka
- Astronotus ocellatus
- Cichlasoma severum
- Pelvicachromis pulcher
- Satanoperca acuticeps
- Aequidens pulchrus
- Gymnogeophagus balzanii
- Cichlasoma spilurum
- Mesonauta festivus
- Aulonocara nyassae
- Gymnogeophagus balzanii
- Aequidens geayi
- Steatocranus casuarius
- Cichlasoma callolepis
- Aulonocara jacobfreibergi
- Cleithracara maronii
- Aequidens portalegrensis
- Uaru amphicanthiodes
- Ctenopoma acutirostre
- Ctenopoma oxyrhynchus
- Ctenopoma kingsleyae
- Balantiocheilos melanopterus
- Leptobarbus hoevenii
- Puntius denisonii
- Barbodes schwanefeldii
- Phractolaemus ansorgei
- Apteronotus albifrons
- Gnathonemus petersii
- Pantondon bucholzi
- Xenomystus nigri
Polypterus does not combine with territorial or otherwise aggressive fish species. Also, they are not a good combination with Channa’s (Snakeheads) and fish of the Labeo family. It is reported that combining with mouthsucking catfish can cause a problem since they can stalk them to graze the body of the Polypterus and destroy it.
Polypterus is carnivorous. In the natural habitat, they prey on small fish, aquatic insects, crabs, shrimp and amphibians. In the aquarium, they are no selective eaters. Suitable food is:
- mosquito larvae
- Pieces beefsteak
- Fish meat (avoid salmon because of its high concentration of oil
- Mealworms (avoid popped ones, since they can cause constipation)
- Feeder fish (avoid goldfish because of the high concentration of copper, fat and hormones)
- Dried pallets
Variation in the menu is important for good health.
There is no information available on breeding Polypterus retropinnis. Probably it has never been bred in captivity yet. It is likely that breeding is similar to that of other Bichirs.
The most difficult part is to trigger courtship. In the natural habitat, courtship is triggered by seasonal changes, like rain season, resulting in a change of temperature, water parameters, and air pressure.
In breeding farms, courtship is often forced by the use of hormones. Maybe an even more animal unfriendly method is to add 1 drop of natriumjodide (KJ) 1% solution to 100 litres of water. In no case more! Mentioned is to close the lit of the aquarium very well since the fish will become very restless.
Often courtship is observed after a regular change of water. To imitate natural conditions keep the fish at a dry season like a temperature of 28-30 degrees Celsius and a pH around 6.0. Simulate rain season by a drop of temperature to 16-21 degrees C and PH7. After this add some distilled water and raise temperature to 26-28 degrees Celsius. If all parties are ready this should initiate mating. Other sources mention the addition of sea salt with trace elements.
Courtship starts when de male breaks the water surface several times, bending its body, showing its spread fins and slowly descend towards the female. Approaching from behind the male contacts the female by some subtle headbutts. De female lies motionless when the male stimulates the fallopian tube of the female with his anal fin. With the anal fin, the male also catches the 300-1000 eggs, fertilizes them and release them. This process will repeat for several days. There is no parental care. After 4 days (temperature of 26 degrees C) the eggs will hatch. After 3 or 4 days the larvae start free swimming.
In order to breed several tanks are needed. One for courtship and one for breeding. Use for courtship a sandy substrate and fine-leaved plants like Taxiphyllum barbieri. Hopefully, the eggs are laid within these plants.
If courtship has taken place, move the plants to the other tank. No substrate is needed here. Keep temperature up to 27-18 degrees C and a PH of 6.5 and 7. After 60-9- hours the larvae will hatch. The first 6-7 days, feeding is not needed since the larvae will feed on their yolk-sac. After this, they start free-swimming and should be fed baby artemia or chopped mosquito larvae. During this phase unfortunately it is hard to prevent reduction of numbers because of fightings and cannibalism.
P. de Pijper
Biology of fishes (Quentin Bone/Richard H. Moore)
Last Updated on 7 October 2021 by John
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