I have noticed these "warts" on my favia from time to time. They seem to go away after a day or two, so I've never been concerned.
This morning it occurred to me that perhaps this is new polyps forming? The "warts" appear in different places, and never are there scars or other marks of injury. Other LPS seem to create a second mouth before the polyp divides, but I can not find any polyps with more than a single mouth on this favia.
If you look closely, you can see four of these forms. Anyone got the answer for me?
I believe it to be a form of asexual reproduction called Extra-tentacular budding. My Baby's bottom favia does it all the time. The Intra-tentacular budding is where the polyps split. "Extra" is when the coral grows by forming polyps outside the walls or between the existing polyps.
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