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Totally makes sense to me. The one Dendronephthya that I ever had for any length of time opened and closed quite regularly, and the rhythm definitely appeared to be driven more by food availability than by light exposure. And your comment also had me thinking about the many dendronephthiid coral photos I've seen taken in the wild under full sunlight. But yeah, regarding the expelled zooxanthellae in your tank... Do you have a fairly high powered microscope? Would be interesting (and relatively easy) to test diel density flux by looking at some morning and evening water samples.Further musing to Ken over at Hydrospace LLC's comments...
After rereading this I realized that there is a way for small polyped NPS corals to receive the benefits of Symbiodinium DHA, namely through ingestion.
This small reef aquarium has a rather dense assortment of various coral and other photosynthetic organisms for it's size, which regulate the density of their symbiotic partners via expulsion into the local environment. I've noticed that this Scleronephthya is often retracted during most of the light period (even though it is not in high light), but then typically expands prominently when the light cycle is ramping down and then for a many hours thereafter.
So perhaps the coral is not light shy, but simply saving energy by not expanding it's polyps until the photosynthetic organisms start to expel their excess zooxanthellae towards the end of the light cycle.
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