Ok, so this is what I've got so far...
All corals have some mechanism to remove sediment and debris from their surfaces. Larger polyps exhibit 'twitching' mechanisms, whereas small polyped corals tend to have a full retraction of the polyp, leaving a relatively smooth skeletal surface which currents can easily blow sediment off of. This same response is also triggered by irritants like little red bugs so, if your polyps are continually not opening, it is most likely the turbidity of your water. Though it makes me wonder because my turbidity is also pretty high and my polyps couldn't be hanging out any further. No little bugs or AEFW?
I also found that corals with poor health (for various reasons) would tend to not extend their polyps.
But this is all stuff we kinda already knew....
In terms of potassium, corals definitely have a response to it as noted by several researchers but no one has really offered a difinitive answer on the matter. I did find that potassium DOES regulate pressures within coral membranes but I can't actually say that is what is causing the polyp extension. The explaination offered by a number of researchers that K is a limiting nutrient and that corals exhinited better health on its addition. They observed (cut and paste directly from the document):
- better growth of all photosynthetic organisms,
- more polyps at SPS,
- intensified branching of SPS,
- less „asthenic“ growth of branching SPS,
- intensified colors of colored corals,
- often a higher demand of macronutrients, due
to the increased primary production.
Unfortunately, no one knows why yet...
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