Looking at the mouth and jaws, and body of any animal, be it big cat, or water buffalo, or Parachromis or killifish seems to signify its food preference.
And fish are as varied as any land animal.
Just the upturned mouth of many livebearers or killis seem to indicate insects or their larvae
And competition in the habitat determines what a fish or mammal evolves to eat .
Habitat where fruit falls, be it land or water, will create a situation for seed dispersers to take advantage of, whether Pacu in Brazil, or Vieja or similar species in Central America, or monkeys on land or birds.
Eventually my bocourti reduced the papyrus below to nothing but root nubs, after they were introduced to the tank, the Paratilapia first housed with them, never touched the plants.
And though my Ptychochromis will sift sand all day, they barely notice the much smaller swordtails housed with them.
There are cichlids from Lake Barumbi mbu (Pungu maclareni) that exist almost exclusively off freshwater sponges, while others like Stomatepia pindu, from the same lake have a mouth that shows it is obviously predator.
or Theraps lentiginosum a predator, but only on prey small enough.