I have had clams spawn twice in my system. Once after upgrading the display tank, and once during a system crash. Each time I have over a dozen clams some bigger than a foot.
Once one clam spawns, it triggers all the other around it (in this case within my system to start spawning. With over 1000g it turned my system into a green soup. Took days to skim, carbon, filter sock some clarity back to system, and the SPS corals did not take kindly too it.
I have some friends that have had a single clam in their system spawn with no devestating effects, but it still created a mess in a moderately sized aquaria.
Dave B