Nocturnal radial corallite polyp extension really doesn't mean anything, axial corallite polyp extension does, the axial (branchlet/branch tip) is usually only partially extended during the photoperiod, on some species, it wont be extended at all, in the dead of night however/wee early hours of the morning you will see the axial corallite polyp fully extended on happy colonies, the axial polyp of many acropora species has significantly less symbiodinium sp, (zooanxthelle) per mass of tissue than that of radial corallite polyps, this is because these polyps primary purpose is for food capture, not photosynthesis.
A stressed acropora will generally not feed/expend valuableenergytowardsfood item capture, which is why nocturnal axial corallite extension to be one of the best indicators of stability with acropora, esp new pieces.
When I worked at a wholesale /importer facility, we would have acropora colonies coming from halfway across the world and for the first few nights, despite good radial corallite daytime PE, they wouldnt show nocturnal axial polyp extension, after 3-4 days of being stabilized, we would start observing nocturnal axial corallite extension and thus the colony was deemed stable and ready to ship, frag etc etc.
Just food for thought.
