What size clam were you interested in? Everything in j21kickter's post is right on the money depending on the size you are buying.
If it is less then say...2 inches you will need to feed it. If it is more mature, and say more then 2...even 3 inches to be safe, you would most likely not have to feed it as it will live on soley the light.
BTW, what kind of metal halide lighting do you have?