Maxima and crocea's secrete the byssus...that's the fibrous protein that anchors it to rock. (Same as what you find on mussels..where its called the "beard".) Derasa's secrete a smaller, weaker one. So maxima's and crocea's are the ones that are supposed to prefer the rock work, as that is where they would be found in nature. Crocea's secrete an acid that helps them bore into the rock itself. However, a tank isn't nature, and as I've said, in my experience (can't speak for anyone else) at least crocea's seem perfectly happy sitting in the sand bed. If you are going to go bare bottom, definitely put the clam on top of something. If it attaches to the bottom itself, it will be very difficult to move it, if you ever need to. That byssus holds tight and can easily tear the tissues if the clam is moved, and that's it for the clam.