I'm having a hard time trying to figure out what survival benefit body armor would convey to a 3 inch fish that would normally be swallowed whole by its predators.
There are a lot of defense mechanisms such as exuding poisons that give little or no defense to the individual but will protect the species. It works because it deters predators from making the same mistake twice. I think the way they explain it is that, "An chicken is an egg's way of making another chicken."