Here's my Derasa. Started out with a white shell and slowly it became encrusted in coraline algae and also turned dark from the excess nutrients from the fish in the tank.
Derasa Growth
Posing with other clams
My Derasa started out about the same size as the maximas in the 2nd picture and has grown quickly. I'm going to put it up for sale soon since its taking up too much of my limited sandbed.