What type of lighting and tank do you have? I'm asking because unless you have 250 or 400 watt MHs you should move that crocea as high into the rock work as possible and preferably directly under a bulb. These croceas have the highest light requirement of all the Tridacna clams. Good luck with it and let us know how it does.
Aliendomain...I second what bryjo said, those are gorgeous clams! I have never seen a green clam like that... and I love the electric blue of the crocea! I am awe struck! I want one! Where did you get those?
Both of these I found local. There are a few stores that get some nice clams in metro Atlanta. The blue one at the bottom is a maxima. If you are looking for green croceas, Barry at clamsdirect gets some nice croceas like the one I have but they go fast. When I got the crocea it didn't have as much color but after three months it has grown and the color improved. This clam is only 4" from the serface of the water and my MH are 8" from there.
I have my croceas under 2 175's with 80w of no actinics. They are exactly 18" away from the bulbs, and they have turned from a blue to more of a purple in the month to month and a half I have had them. I do supplemental feed once a week, to help them stay healthy. I do not have any room for them higher up as my two sps frags occupy that space. I have a new Hamilton 10K that needs to be installed on the side opposite the clams, as the other bulb is a bit old. I can move the clams up at that time.
The are both predominately blue, and from what I understand, those seem to come from more shallow water.
Aliendomain - do you have a larger version of your pic? With the blue and green clams.. I would love to get a larger size file version of it, and if it be cool with you..turn it into a backdrop on my computer.