I got these guys a while ago but never shared anything about them even though I've wanted to. What follows is more or less a rehash of what's on wikipedia.
Ossubtus - Wikipedia
en.m.wikipedia.org
Ossubtus xinguense is the sole species in the genus Ossubtus in the family Serrasalmidae. It is strictly rheophilic and restricted to the rapids in the Xingu river and confirmed in the Volta Grande do Xingu but possibly occurs further upstream as far as Sao Felix do Xingu. Primarily a herbivore but macroinvertebrates have also been found in an analysis of 10 specimen's diet. In captivity shrimp, bloodworms, brine shrimp, daphnia and vegetable matter are readily consumed. Interestingly according to the wiki the females are the dominant sex and will often seek caves and guard it against all other fish. The hypothesis being it's in an attempt to find a suitable breeding location. Very cool a SD/Pacu that thinks it's a cichlid. It is parasitized by Anphira xinguense only known from Ossubtus xinguense where it lives in the fish's gills. Their intestines also are commonly infested with Rondonia rondon in what is thought a possible symbiotic relationship. Many Ossubtus are also found to have black spot disease. From the little I gleamed online it doesn't seem to be harmful to the infected fish.
Now on to my own experiences. I got three and I have a feeling they're different sexes as the anal fins are slightly different between them. Aggression is more intense then I've experienced with most other SD's. I can't speak of in comparison to Pacus as I've only had two Pacus a long time ago and their growth was ridiculous, so I rehomed them pretty fast. My experience may also be because I only have three of them but they are mixed with other SD's currently M. Fasciatus, M. Lippinocottianus? and Myleus Setiger? Sorry I'm not positive of the I.D.s of the Lippo and Setiger as there are things that give me pause in their looks versus what they were sold to me as. The Setiger are fairly aggressive amongst themselves and the other SD's but I know I've got a male and 2 females which might be affecting the dynamic of their interactions. The Ossubtus despite being smaller change up the pecking order constantly. The Setiger going from chaser to chased when it comes to the Ossubtus. Luckily it's not a constant thing but occasionally something sets things off and a quick back and forth dance happens. It can be shocking because their attack movements are weird. They attack from under and over each other darting back and forth at one another which results in occasional loud sudden splashing.
As for food the Ossubtus readily took to pellets with no issue. The Setiger are skittish eaters dashing to the surface snatching pellets and making quick u-turns to grab more which also causes considerable splashing. The Ossubtus are aggressive eaters but much more confident so much less dramatic. Another cool fish that studies you or is just constantly looking for food. If I'm in eyesight they're watching me and are quick to respond to my approach. They even respond to me looking at the tank. If my face turns to look they move to the front to see if food is incoming. They seem more aware of my looking at them then other fish. I don't know maybe I just like them so much I'm giving them more credit than they deserve. Then again, the more intricate social structure such as the female being the dominant and more aggressive sex and established territories like rocks and caves for potential breeding may mean a higher intelligence than typical of some other Serrasalmidae? Meh, I'm probably grasping at straws.
They're unfussy as far as husbandry as far as I can tell. While being rheophilic I haven't seen any struggling or stress in my tank. The tank they're in I don't believe has a lot of flow but there is a canister with spray bar, two HOB's(low ph out of tap etc so I have crushed coral in filter bags in the HOB's to stabilize it) and one powerhead. Most of the current if not all is focused at the surface of the water. My own personal preference to maximize surface agitation and promote good gas exchange for a hopefully higher oxygen concentration.
Now a pic that is not mine but may give a more accurate representation of Ossubtus xinguense
credit for pic: Rudolf Wildekamp submitted by: Benedetta Spelta website: biotopeaquariumproject
