designed, manufactured and just plopped right on the organism?
Fossil record doesn't show they just showed up fully formed, one day...

I think I would have to go with some sort of biological reason for them. Stress is probably not the answer since there really wouldn't be any mechanical stress on them while just floating around in the oceans.

There are several theories about the function of these intricate shell designs....that they evolved to control fluid flow, for example....

Using the word "evolve" is not science; it is junk science.
A scientific working for this sentence would be:
"There are several theories about the function of these intricate shell designs....that they are designed to control fluid flow, for example....

So when bacteria develops resistance using the word evolves is junk science?
Your faith is clouding your reason.