[lcstorc]

Well no matter how many times I re-read the descriptions of both clams there is just not any way to make even a decent id based on the information given.
Based on the picture alone, I think it is a Crocea.
It actually looks very much like a Crocea in my tank except mine has a blue/green ring around the outside.
But inhalent syphon, shell shape, scutes, better definition in the mantle etc could easily convince me otherwise.
Derasa usually has a more striped appearance often with a thin ring around the edge of the mantle. Often they are a variation of creams and browns with a ring of blue or purple.
Croceas are very commonly found in blue or green but can be any color. They can have stripes spots or rings.

One of the biggest difference between these two clams is on the inhalent siphon.

Below are excerpts from "Giant Clams in the Sea and the Aquarium" by James Fatherree about the inhalent siphon of both.

"The Crocea inhalent siphon is ringed with numerous but very small tentacles which may be very finely branched or oddly shaped."


"The Derasa inhalent siphon is is almost always ringed with prominent tentackes that have numerous branches. However sometimes they'll have a mix of these larger, fancier tentacles and smaller unbranched and pointed ones, and then there are a few that may have only tentacles that don't branch at all. Oftentimes, some of their tentacles had an odd sort of club shaped ends as well."

If we throw the other types of Triacnids into the mix it becomes even more difficult to tell without a lot more information.

So, can we get more detailed pics?

I will say most of all though it looks like a happy clam. I'm not sure how much PC light you have or how far away the clam is but it does look happy.