Apognophos - I agree with your example for flounders and all relevant species like that example.
Here is one from your article that does do it on purpose,
Self-decoration [ edit ]
Self-decorated sea urchin
Further information: Decorator crab
Self-decoration is employed by animals in different groups, including decorator crabs, which attach materials from their environment, as well as living organisms, to camouflage themselves. For example the Japanese hermit crab, Eupagurus constans, has the hydroid Hydractinia sodalis growing all over the shell that it lives in. Another hermit crab, Eupagurus cuanensis, has the aposematic orange spongeSuberites domuncula which is bitter-tasting and not eaten by fish. [20]
Similarly, Sea urchins use their tube feet to pick up debris from the bottom and attach it to their upper surfaces. They use shells, rocks, algae and sometimes sea anemones. [21]
But those behaviors in the above example could be an evolution, they learned to hide themselves. However what wouldn't make sense is if how those creatures above are hiding themselves, they suddenly developed the ability to change their bodies to appear that way vs putting those things on them. That's the question.