[Jim Sachs]

Most of the clams in that link are about half-closed. I think people are confused when they see them fully-open. The clams I've used in the program are common in aquarium shops. They can be quite active, opening and closing rapidly when disturbed. Just passing your hand over them to block the light is often enough to make them close up. Then they take a little longer to fully extend so that almost no shells are showing.

Update: Having a pretty good day. Yesterday I got tired of doing Lightwave simulations of the clam motion, and decided to tackle the project from the other end - the programming. Since I dread that part so much, I find that once it's done I'm a lot more motived to finish the artwork and models. Anyway, now I've got most of the programming done for one clam, and my theory will obviously work. Probably one more day to tune up the models and graphics.