African Knifefish breeding - what we know

This is an article I wrote on another forum, thought may as well put it here.

African knife fish, Xenomystus Nigri, are popular aquarium fish hailing from the congo and other tropical areas of Africa. Despite their wide popularity and adaptability to aquarium conditions there is very little information on their breeding habits or behaviours, leading to presumably all of the fish in the hobby being wild caught. I hope to assemble some of this information for everyone.

Actual documentation of spawning is extremely rare. One member kept a group in a 300+ gallon tank many years ago as a clean up crew with his larger fish. He observed that there were some smaller individuals within his tank, much smaller than the others. A likely indication that they may have bred within his aquarium. Other reports are basically unheard of.



Quick disclaimer, I had to translate some parts of articles so the English is not 100% on these. Sorry about that

What do we know of the behaviour?

Luckily there are more reports of them enagaging in courtship behaviour, and it is interesting indeed.

My first source, a study Reproductive styles of Osteoglossomorpha, found that after the injection of hormones the pairs The pairs indeed were seen
swimming very closely to each other during day time. In fact, double of the
concentration of the regular hormonal injection was given on the last day (day 415) of
the trial to all of the selected specimens, however still nothing happened
A German aquarist gave a similar but more detailed account of the behaviour

Well I see once in a while, like these animals on the side together but not head to head. The male puts his head in the rear flank of a female here and stubs it more often with a bobbing motion.

There is a similar position, no matter whether male or female, and the animals start loud barking, where they squeeze air from their swim bladder into the anterior intestine. This bark is easily audible and has been reported to awaken sensitive people from their sleep.
Somebody uploaded videos of similar behaviour:



Barking is commonly associated behaviour of courting knifefish.

A third article gave some minor detail on wild spawning:
In the wild, the females lay 150-200 eggs in a shallow well, where they are then fertilized by the male. The male takes up tending to the eggs and he fans the eggs constantly with fresh water.

This coincides with seriouslyfish's writeup:
the pair produce characteristic barking sounds during courtship. The male will then guard the brood of 150-200 eggs which are laid on the surface of wood or rocks and will tend them by fanning them with his fins. Apparently, the eggs hatch within 10 days

This means they likely set up a spawning site and defend it until the fry have hatched

Colour changes have also been noted during summer heating periods.

The biology of spawning:

The article Reproductive styles of Osteoglossomorpha gives the most comprehensive descriptions and images of the reproductive biology of xenomystus nigri and will be used as my source of reference in this article. Here is a summary of the initial process:
The first 16 (9♀, 7♂) fish arrived on 30 March 2009 and comprised a range in total
length of 85–120 mm. The fish, originating from the Niger Basin, were
purchased from a wholesale dealer. This group was placed in a tank measuring
120 x 75 x 60cm and one third of the water was regularly changed once a week. The first tank was furnished with black polyethylene shreds as hiding substrates. Two weeks upon arrival, some of the fish were infected with the ectoparasite Ichthyiophthirius (known as white spot disease). Medicinal treatment was performed for
three weeks, although three heavily infected fish died. The remaining fish recovered well. Acclimation was continued in a disinfected tank for about 60 days.

The second group was purchased on 6 May 2009 fr
om a wholesale dealer consisting of 10 specimens(9♀,1♂) with a total length of 128–173 mm. The specimens were placed into a tank measuring 90 x 60 x 53cm, which was also supplied with black polyethylene shreds and two PVC tubes as hiding places. Acclimation period lasted about three months. . Fish were fed once daily with live Chaoborus sp. and/or frozen chironomids. Selected specimens with swollen bellies received several hormonal injections.

Three tanks (90x60x53cm) were furnished with several PVC tubes, stones and woods. Each of tanks comprised one male and female. The selected specimens were separated for two weeks before the injection. Immediately after the injection, all specimens were returned into their previous tanks.
Click to expand...
Female knives were observed with swollen bellies between days 34-70 but no spawning occured, even with triple hormone dosage. The ovaries and testes are positioned in the front of the body just behind the head

This female died during the experiment and was heavily berried at that time:



And an underdeveloped male that also died during the experiment:


The females started swelling at a greater rate when the conductivity dropped, but no spawning happened. The conductivity ranged widely and the pH was shifted between 6.6-8.2 over the months but nothing happened still.

Fry are said to be fairly large at hatching and can start on baby brine shrimp or similar organisms.

How to tell the difference?
Also from the same article they explain the sexing differences:


This would mean I own 2 males, the gonad is very reduced on males so look carefully. It seems to become more pronounced as they mature.

Only presumed females were observed with eggs so I would believe it to be accurate.

To summarise:
Xenomystus nigri appear to be breedable with the reports of courtship and swelling of eggs, but the exact trigger of spawning has yet to be found and Is something I would like to look into. Hopefully one day somebody will be able to crack the enigma that is preventing their spawning in captivity.

What now?
Breeding African knife fish will be a major breakthrough for the hobby, and would allow us access to captive bred specimens. Good news for the wild populations and the hobby as a whole, one day I will hope to see CB specimens in the shops.

For anyone aspiring to breed these fish here is a summary of things that appear to be important:

1. Low conductivity in the water.
2. Frequent feeding of live and or frozen foods.
3. A large share of hiding spaces for security.
4. Softer water with a pH between 5.5-7.5
5. A sandy substrate or wood as spawning sites.
6. Temperature of 26-28 celsius (79-82F).
7. A mixed group of males and females.
8. A good attitude and perseverance

List of references and photos
http://edoc.hu-berlin.de/dissertationen/yanwirsal-honesty-2013-05-16/PDF/yanwirsal.pdf
http://www.seriouslyfish.com/species/xenomystus-nigri/
https://www.aquaristikfreaks.de/fishbase/Xenomystus_nigri.html
https://www.aquariumforum.de/threads/149885-xenomystus-nigri
http://wasser-landexoten.de/afrikaner/westafrika/xenomystus-nigri-der-afrikanische-messerfisch/
http://www.aqua4you.de/fischart1054.html


I hope this article is interesting or helpful for all those interested in knife fish.

 

Thanks @Hendre, very interesting article.

I guess you have African knifefish?

How about a thread on yours?

 

I have 2 males, I'll make a thread once I get all my new fish and sell the ones I'm not wanting to keep

 

BGK have been repeatedly spawned in aquariums. African knives on the other had I have only found two sources of confirmed breeding and one is no longer available. The other was done by an MFK member but he doesn't have details