In the wild they attach to rocks from the free floating plankton stage. As they grow they slowly breakdown the surrounding rock, creating a depression. If you google "images of Crocea in the wild" or "Tridacna in the wild" you can probably find images of this. Both Maxima and Crocea do this, but it is not necessary to provide this in the home aquarium. I do however feel that Crocea do better higher in the rock work in a stable spot where they can not fall, after they have been acclimated to the lighting. I have an image of my Crocea and it's placement in the rock work, where after 2 years it did make a slight depression.