I agree with sammy,
The clam that is typically know for being the least light needy is the squasmosa. I have one in my 29 gallon tank under PC lights. I have had it just over a year and it is doing okay, but not great. PC lights are not as intense as MH lights, thus they cannot penetrate the water as deep as MH. Therefore, in a shallow tank, pc MIGHT be ok for sqausmosa and possibly derasa clams. But in a deeper tank, the light will not reach them well.
Also, since squasmosa and derasa clams like to be on the substrate, you can not raise them higher in the tank. They will usually jump down, and injure themselves or your coral.
BTW, I am upgrading to MH because of my clam