Well there are only two clams kept in captivity that can out grow tanks but leaps and bounds. Sorry bobby crocea is not one of them they max out about 6 to7 in. Squamosa gets big upto 18" but looks nothing like that. maxima Largest I have ever heard of was 18" The two that it could be as far as siz would be a:Gigas which has a completly different shell shape than that one or b: Deresa. There you have it folks a 100% answer. How do I know I have had 2 like that about 10" pass though my hands in the pass month. I get alot of clams in. Nice clam though it will keep you tank clean and suck you dry for cal.