I still say it depends on the placement. I've had my crocea under PC's over a year now, and it's almost doubled in size since I got it (roughly 5" now). It's at the top of the tank about 6-7" away from the bulbs.
IMO Derasa and Squamosa do well in my tank under PC lights. We have 4 in a 75 which 2 are new and the other 2 we have had for almost 3 years now with good growth. We do monitor calcium and alk and add as needed. I believe with good care these clams will stay happy.
( imo ) no probs with t. derasa or t. squamosa with bulbs mounted parallel to each other ( four x 65watt = 260watts ) and with a mirrored reflector . if parallel they will receive light output from all tubes . lux/lumen output is constant along the length of a tube . four 65w. tubes mounted 2X2 would really only expose them to the lux/lumen output of little more than two tubes and i would be hesitant however it seems to be working for 2crazyreefers .
i'm guessing that mounting configuration is why pc's get a bad rap from people who generally use the watts per gallon method for determining suitability .
i've successfully kept t. derasa on a 4"d sand bed ( 18"d x 18"w aquarium ), under six parallel 32watt zoo med T8's for two years before replacing them with four 54watt ho T5's ( also have a t. sqamosa ) .
when i first considered a t. clam , A. Calfo told me my lighting would be o.k. for a t. derasa except the blue variety , of which i hanen't come across , other than a photo of a blue lined t. derasa in J. Sprung's INVERTEBRATES A QUICK REFERENCE GUIDE p. 124 .
I have a blue/purple maxima (i think since this is my first one) under PC's 130wats actually. It is exactly 6 inches away from the light. I feed the tank phyto once a week and cyclopz 3 times along with other stuff. Im not sure if the cyclops are the correct size or shape for the clam but it has been looking better since i first got it. I bought it from an LFS where it was subjected to about 100wats of light about 2 feet away. Its become really neon blue. Ill keep you guys updated. I feel it depends on where the clam was harvested to be able to determine what lights it will live under.
Acxtinic lighting doesn't count toward your total output wattage, so the 260 is actually 130. That really is extremely low lighting, I'd think you'd have trouble even keeping many softies, let alone any clams, just MO.