品種 means genera? Am I right? Because I am not sure what you are referring to.
I think Julian stated that also, it is a way corals adjust themselves into the captive systems. He is suggesting gas exchange and regulation of light could also play a role in this behavior.
Within a captive system, unless we have excellent gas exchange rate? Coral will utilize other means necessary to gain its gas requirement. The waves in nature provide far more gas exchange then we could provide in a closed environment. For those of you who dive should know the power of wave especially during tidal changes. Since we provide a constant light source without any shifting, the polyp could also played a role in UV shielding.
I was merely trying to state that the polyp extension could not and should not be perceive as an indication of coral health. If I am correct about the meaning of 品種? Then I think collected localities would be more appropriate. Corals collecting from different depths and zones might have different food, gas and light requirements. This then explains why some corals have constant polyp extension and some does not.
Andy, yes, every little thing can trigger a big discussion, isn't this why we love this hobby? Seems like the learning process never stops. Take care.