That clam should be well over 12" end to end. I last saw it last year (that reef tank, somewhere in the range of 3-5,000 gallons, is on one end of the coral lab I used to work in). The tank was set up by Paul Clarkson, it's amazing what he did in there, isn't it?
Len, is that righteously fat mandarin in there (I swear that thing was better than 4")?
Also, Kristy, the aquarist who cares for the leafy dragons, received an award last year from the AZA for her work breeding them!