I found this on the interwebs:
"œThat is one of the mysteries of keeping zoanthids, nominatively Zoathus spp.
Well, here is a list of things we should keep in mind:
1) Lack of light or too much light. More towards low light and low flow at the same time.
2) Sudden changes in the water chemistry, temperature and flow. Higher temperatures, specially!
3) The use of activated carbon and/or GFO. BTW I don't recommend GFO at all!!
4) Internal muscles atrophied due to lack of use, to collect food particles from water column.
Regular feeding could help PREVENT the syndrome.
5) Perhaps an internal pathogen in some cases? Don't ask me what pathogen!!
6) Natural part of their metabolism. A way to clean themselves, like pooping, or before expel zooxanthellae.
7) Irritators!!! Some times the polyps are irritated by organisms living around their bases and they develop weird shapes, like umbrella syndrome. Look for amphipods and hydroids around their bases, specially at night!!! Normally, if that is the case, it would happen with more than one polyp though!
Do I have to say that the combination of 2 or more of the above list could do it too?
Most of the time the umbrella syndrome doesn't last too long when the system is well taken care of. With the list above in mind we can see what one needs to do to help their Zoanthus to get it's natural appearance faster.
Some times just giving it a bit more water flow is all the polyps need to come back to it's natural appearance. or perhaps adjusting the temperature.
I've never seen a polyp perish because of umbrella syndrome and they normally recover from it in a couple of days or so. Normally it happens in one or two polyps, not the whole colony.
Grandis."
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