Yes, exactly, Mark426. That's why you always seem to get a few people saying they've successfully kept a clam under PCs. If you ask some pointed questions, you usually find out they've only had the clam a few months. . .or less. I'm sure there are exceptions to the rule, but I'm also sure there aren't very many.
An experiment was done many years ago. I can't remember a lot of the specific details, but basically clams were kept in sterile salt water (i.e. no phytoplankton, bacterioplankton, etc.) with good lighting. Although they didn't really thrive, they were otherwise pretty healthy. Next they used substandard lighting but fed heavily. The clams didn't do so well in that part of the experiment. I don't know if they used PCs for the second phase. It was probably even less intense lighting, but the point is, light is extremely important for a clam's health and survival.