They will do that sometimes. In the wild they get positioned all over the place. One thing regarding how they position themselves is always a constant though. They always make sure they get enough exposure to light, so they never stay put under overhangs and stuff like that where no light reaches. That is another sign of proof that they get most of their nutrients from photosynthesis. Your friend named Chris is definitely on the right train of thought, although t5ho don't need to be overdriven to keep even the most light needy clams.