Must be the skull structure of bonytongue fishes, both arowana and arapaima get drop eye, but it is never seen in other large predators like gars, channa, pike or peacock bass.
I am not saying they cannot get a drop eye because of a head Trauma.
But i dont think this is the main reason for dropeye.
I think its mostly from a fat deposit behind the eye.
I think we will never no for sure what is the main reason for drop eye.
You both make valid points -- and in fact the 2 issues might be inter-related: as boomerk9 suggests Aro skull structure might make them more susceptible to drop eye, and then fatty deposits (overfeeding) might increase the chances significantly.
Of course it raises the question if an Aro has never had any head trauma, can it still get DE from feeding too much? But this is hard to test -- I think it would be nigh on impossible to find an Aro in a tank that has
never jumped and hit the lid at least a couple of times.
My Blue Base jumped out of her tank when she was about 10" and landed on a hard tile floor 5 feet down (and then tried 'swimming' away). Luckily she survived with only a few lost scales -- but who knows if this could cause some DE issue way later down the road...establishing cause/effect in this hobby can sometimes be frustratingly difficult as we all know.