I've always wondered about this too, particularly in regards potassium dosing and PE. I've always been suspect that K makes the polyps extend by a physiological response rather than their own natural accord as potassium, for most animals, regulates muscle responses and fluid pressure balances between tissues. So dosing K might just be causing them to puff up like balloons... but really I don't know that for sure, so at this point I'm talking out of my a$$.
As for some colonies having PE and some not... I am suspecting an irritant, though figuring out what that may be could take forever. I'd venture to say it's probably sand that is bothering them.
Since I've been curious about this as well, I tell ya what... I've got some time to kill today while waiting for an experiment to finish. I'll dig into some journal articles and see if I can't get some sort of definitive answer.
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