I think they throw off the occasional polyps, which in turn settle and start to grow, I bought a small one about 12 years ago, only lasted a month or so, was fine one day and bare skeleton the next, shortly there after I had them popping up everywhere, to this day my tank is full of little ones, when they get bigger I either give them away or turn them into reactor media, left one alone once and it grew to the size of a basketball.
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