S.rhombeus has been spawning at least twice in captivity. The best known spawning was the one in at Duisburg Zoo in Germany, 1963.
In 1969, Gedaschke presented his breeding report with additional information by Dr.Gewalt, the Zoo's director.
Wolfgang Schulte described it in his work "Piranhas in the aquarium".
The specimen had been classified as Serrasalmus niger and had been obtained from the Netherlands in 1955. At that time, they were fry of 1-1.5" length.
They were placed into a display tank of 1200 US gallons (4500 litres) with roots providing hiding places. The tank was densily planted with Cryptocoryne species, Echinodorus, Aponogeton and Vallisneria.
Starting with 20 specimen, the number was reduced to six by cannibalism.
In 1963, the specimen were 8 years old and measured 10-13" (26-32cm) and had a body height of 5-8" (12-20cm). They had spawned on several occasions, but without any resulting fry.
All eggs that were removed, grew mycotic (covered with fungus).
In the evening of August 20th 1963, between 7 and 8 pm, eggs were shedded again. The 1500-1600 eggs were deposited near dense moss near the root of a tree.
Half of them were syphoned and divided over 4 aerated all-glass tanks that were all hung in the display tank to maintain the temperature.
The four tanks had different containts:
Tank 1 had display tank water with methylene blue
Tank 2 had display tank water with methylene blue and 50% distilled water
Tank 3 had 95% distilled water, 5% display tank water and Cilex breeding aid
Tank 4 had display tank water only
The developments was described as below:
August 21 - about the same quatity in all tanks had grown mycotic, all mycotic eggs were removed with a pipette
August 22 - around 1 pm, the first embryonic movements were seen in tank 4. Th ehatching soon occured and around half past 3 pm, the same happened in the other tanks.
That same evening all living embryos or fry were taken out with a pipette and put into an all-glass tank of 95% distilled water, 5% tank water and Cilex breeding aid.
August 23 - about 750 living fry were transferred to a tank without Cilex. Water was kept at 25 degrees C (77F). It was crystal-clear, filtration with a PS flat filter over glass wool and charcoal
August 25 - eyes were clearly discernible with jerky, reeling movements. Body length was about 7-8mm
August 27 - the temperature was raised to 28 degrees C (82F) and the aeration was increased. The fry stood facing the current, yolk sac still clearly visible
August 28 - Fontinalis (moss) was added to the tank and individual fry attached themselevs to it. Around 11 pm, all fry swam free
August 29 - most of the fry were still hanging in the moss. Around half past 6 pm, the first feeding attempt was done with brine shrimp. Around 8pm, all fry were lying at the bottom.
Around 10pm, they were all swimming very lively around in the dark.
August 30 - about 50/60 fry were swimming around, the rest was lying on the bottom. It was not possible to tell if they were still resorbing the yolk sac or had allready fed actively.
Around 10pm in the darkness, they were very lively, after the light had been switched on they lay motionless in a corner. Around 11pm, they were fed with cyclops and small daphnia
August 31 - around 2pm and a quarter to 10pm, the fry were offered small daphnia again and these were undoubtedly swallowed
September 1 till September 8 - the fry attained a length of about half an inch (10-12mm)
September 15 - the fry were transferred to a tank of 20x14" (50x35cm) and counted.
There were 655 of them, measuring between 15 and 20mm.
On october 1st 1964 the fry were about a year old and 20 of them were put into the display tank with the adult fish which didn't cause any problems. They were swimming side by side.
The adult fish spawned again in 1964 and 1965. The spawn was left in the tank however and was all devoured.
From 1966 on, the young fish measured 6" (15cm) length and had been reduced in number due to cannibalism, mainly attacks by the older fish. The exact number was hard to tell for they hid themselves.
Piranhas in the aquarium - W.Schulte