This is interesting. I have some zoanthus that acually only open 100% when lights are off.
zoas don't need a lot of light, so probably b/c during day they hide b/c lights are too strong. they also like to feed at night so opening up at night is probably a feeding response.
unfortunately the interaction between light, polyp open/close, color, and coral nutrition need is a very complex web of cause and effect and as far as i know current research is not 100% clear on what's the causal relationship between these factors. what is known is the below relationships, how you interpret them to determine the reaction between light and open/close depends on what your specific coral is doing and how you observe its behavior
1) corals maintain a balance between the nutrient it generates via photosynthesis vs capture via polyp, and can regulate this to a degree if there is an imbalance via different mechanism
2) coral can regulate the zoox concentration as a response to available light. since most zoox is brown, too much concentration is whats commonly causing corals to "brown out". conversely too much light it will expell the zoox, causing bleaching
3) Zoox also reproduce based on available no3/po4 (its an algae), so when tank is too dirty, it wil also brown out corals, or bleach out corals, depending on how much the coral can tolerate the zoox instability.
4) the "good" colors - the color pigmentation protein, functions as a "sunblock" so its triggered when there is a lot of light. the flurescent proteins further serves a purpose of converting spectrums that zoox can't use in the spectrums that it can use. so that's triggered when there's a abundance of the correct spectrum of light.
5) if there is too much light that exceed the maximum that the coral can handle via these natural countermeasures listed above, it will shrivel and hide to protect itself
6) if there is too low light that falls below the minimum that the coral can handle via these natural countermeasures, it will also shrivel to preserve its energy.
7) coral open up to maximize more light capture when light is high. They also open up to maximum food capture, when there's not enough light. This seems contradictory to 5) and 6) but really its all just about the level. since opening up takes energy, they only do that if it's within this safe range.
These factors help me understand why some corals "open" when the high is high and "close" when light is low, while others seem to do just the opposite. Based on the above, the "sweet spot" is the range that's below the maximum it can handle, but above the normal level that it currently needs to maintain normal functions b/c the extra light will boost the "sun block" and "spectrum converter" production - giving the corals those "pop" colors.
obviously this is all generalizing, the tricky part is different corals have not only different needs, but also different "size" of the "safe range". This is where you need to do research on specific corals to see what kind of light it needs and place it accordingly. An accurate par reading is a start
