Many people on this site have a great deal more experience with clams than I have. I am sure they will answer you, as well.
However, I kept a dersa under 440 of VHO in a 55 gallon reef for over a year. I kept it about 1/2 up in the tank on the rockwork. I feed the tank DT every other night. The clam looked good, it open up and expanded it mantel fully. It lived. However, when I move it to my new 75 gallon reef with 2 X250/6500 MH and 440 VHO supplemental lighting the clam has lived and grown! The clam has been under the new lighting for 6 months and it has almost 1/2 inch of new growth.
Just my experience.
Good luck,
Connie