I always wanted a clam. At MACNA 2002, I saw about a hundred for sale, and asked many reefers what they thought would live under 3x55w Power Compact lighting. They agreed that my best choice was a Derasa. However, they were huge! This small blue Tridacna maxima called out to me and I put it at the very top of the tank for a few months. It jumped to the substrate, and has been happy as a clam ever since - for nearly five years. I fed phytoplankton to that tank every other day.
Maxima clams can be placed on the rockwork, but the easiest choices it to place it in a clam cradle. Some are man-made, but an empty clam shell makes a great holder for a living clam. They extend their mantle during the lighting period, and retract somewhat when the lights are out. The filterfeed constantly, drawing water through the larger aperture and expelling it out of a smaller hole. Clam may occasionally spawn in captivity, filling the tank with their sperm or eggs. It can happen when the lights are on or when the lights are out, that doesn't appear to be a factor.
They do have a foot that they extend from the base to help move themselves, or they can close quickly to jet their way to a different spot. They also can secure themselves with messenterial fibers to nearby rock.
Clams will appear one color from the side view. Be sure to look down from above to see the alternate color they display. When it comes to clams, I've seen many but I'm very picky when it comes to purchasing one. I really prefer Tridacna maxima, especially the ones colored peacock-blue. My first one grew three inches in two years. Reef-safe.