Not all giant clams belong to the Genus Tridacnea. There are also several species that belong to the genus Hippopus. The two most commonly seen ones in the trade are Hippopus hippopus and Hippopus porcelanis. I myself have a H. porcelanis. They are generally cheaper than most Tridacnea spp. clams, but they still comand a nice price, and are stil very much photosynthetic. I would say that the bivalves you are looking at are cold water species, as I have dived those out millions of times in the Mediteranean. However, I have never seen them in the Pacific, Carribean or any other Atlantic location for that matter. With that said, that does not mean that they will not adjust to the temperatures of a reef tank. What I would suggest, is going to your local library, keying it out to a genus AND species, and then doing a little research on them. If they are the species that I have seen, they will perish unless they are burried in soft mud. So if they are a cold water species that needs to be burried in soft mud, you may be looking at a too specialized animal for a reef tank.
Andrej