They do in the wild, but this isn't the wild. Why feed something that is potentially harmful when there are better alternatives. Replicating their natural diet is good, but only to a certain point. Even zoos don't completely follow an animal's natural diet, but rather use it as a guideline to create a better diet with better alternatives.
There's no arguing with that. Pellets are by far the best food to feed a fish. They are basically the equivalent of a one-a-day multivitamin (or something of the such). But, keeping in mind that fish are abundant throughout the amazon, why are aros jumping for terrestrial prey? Natural instincts can't be that far off, can they?
Also, don't forget that zoos can be budget motivated, too. They could be searching for alternatives for a number of reasons... possibly not always because it is what is the best. I would love for zoodiver, or someone in his profession, to chime in on this because I would really like to see what he has to say about it.
One last point I would like to make. He asked how to beef up an aro. Your suggestion of shrimp and scallops is a good one because of the protein. I think a diet of mostly pellets would be healthiest, but wont beef him up. I guess a good comparison would be a body builder that consumes thousands of calories a day to put on mass compared to a mountain biker who drinks an ensure to stay slim.
All this taken into consideration, I'm maintaining my opinion (which is just an opinion, so take it how you want) that aro's bodies are equipped and capable of processing some mammal fat and meat (differently than, say, a cichlid's), and that some would be acceptable as part of a varied diet.
Heres an interesting twist on this which slightly pertains to this discussion. In the payara forums there are discussions of the possibility of sudden death in payara being associated with our captives not eating live foods. I know this isn't the wild, but they are wild animals. With a few exceptions, most of our fish are within just a few generations of swimming in the river.
Edit: I know I said one last point halfway through, but things kept popping into my head. hahaha
