Red bellies can get really large, up to 16” in some cases. They’ll typically max out around 12”. The minimum I’d do for a school is a 135, and if they get really large probably a 180.
Red bellies don’t typically wait in one position, they slowly move around in the open.
Thank you for the response - I decided to do some reading on shoals and the reason behind that behaviour and I’m finding varied results. You may be well aware of it already but I’m still learning ..
One site says they “coordinate” to take out larger prey while other sites say they stay in groups out of fear from being eaten by larger predators.
...a recent investigation of the red-bellied piranha found no support for cooperative hunting and suggested that an important function of shoaling behaviour in the species is defence against predation”..
(Magurran & Queiroz 2003)
Maybe both reasons are true ? However to support that claim - the piranha has quite a few predators ranging from river dolphins, caimans, birds of prey, and fish like the Payara aka Vampire fish. Experiments using wild-caught piranhas supported the hypothesis that individual perception of risk, as revealed by elevated ventilatory frequency (opercular rate), is greater in small shoals.” (Magurran & Queiroz 2003). In other words Piranhas in a small shoalare more “nervous” in their behaviour, breathing much faster than those in a large shoal – just like scared humans hyperventilating.
ChefAlex quote “I had kept red belly piranhas for over 10 years and have never seen or heard of a school of red bellies eating each other.”
I find this to be fascinating as further reading reported that tests conducted in the flooded waters of the Brazilian Amazon revealed that the sexually mature RBP’s would occupy the central zone of the polygon connecting to the edges and the non-reproductive individuals showed more motivation to feed. No evidence of intraspecific aggression occurred during this time that the test was conducted. (Anne E. Magurran and Helder L. Queiroz)
Piranha Behaviour - Supporting your claim that you mentioned in a previous post about RPB’s are unlikely to prey on each other despite their portrayals of being known for their cannibalism.
Now in Aquariums, RBP do not need to worry about swooping aquatic birds, caiman, or river dolphins, at least I hope not - but I can understand the importance of shoaling as it makes them feel safer in a group and perhaps less stressed as explained further by Helder Queiroz and Anne E Magurran - “.. exposure to a simulated predator attack by a model cormorant demonstrated that resting opercular rates are regained more quickly by piranhas in shoals of eight than they are in shoals of two. Together, these results show that shoaling has a cover-seeking function in this species.”
And finally to respond to your last post about schools and aquarium sizes as large as 180 - unfortunately that would not be feasible for me at this time as the cost for a setup like that and an adequate filtration system to support that would run to about 3 grand after taxes, (in my countries currency.) Not including heaters, decor etc I appreciate the advice but that’s something I’m going to have to weigh and consider. And I only live in a basement apartment right now..
I learned a lot on these forums so far, and I want to personally thank you all for responding, and being kind, yet forthright on my setup. I assure you that I will be making some changes.
Cheers.
