Interesting to see little blue dots if it is a derasa, that does make it look vaguely giga-ish, but the shell and the way the mantle sits looks more derasa-ish to me. I babysat a gigas about a year ago, you'd think I could remember how scutey the shell was, but noooooooooo. I can't remember at all. Argh!
Anyhow, what I really meant to point out to you is you may want to pry off that vermetid snail (tubeworm thingy) on the shell to the left there. They multiply like crazy and will drive you (and your corals, and your clams) nuts with the little spiderweb fishing nets they use to catch passing detritus.