It's got an attitude and won't settle til it's happy.
I think what chewey is trying to say is:
BTA's harbor several species of zooxanthellae, which provide the majority of the energy for their growth and general well-being. They have the ability to raise or lower the populations of their zooxanthellae, or to swap out different species depending on their environment (sun intensity, water depth, availability of shade, etc). They also have the ability to physically change shape, move, or lighten or darken pigmentation in order to adapt to lighting.
Why all of these changes? As much as clown anemones need bright lighting to survive, too much light can be damaging to an anemone that is not prepared to receive it. A good analogy is to think about going to the beach in Fiji without sunscreen. If you have a really deep base tan, no issues. If you just flew down from the winter in Milwaukee, however, you are in for a world of hurt.
Anemones take a while to move from the reef to your tank. During this trip, they are often kept in holding tanks without bright lighting, and they lose much of their pigmentation and zooxanthellae. When reintroduced to a bright light environment, in many cases their first instinct is "find shade" - particularly if they are BTA's, which are the lowest light requiring of all clown anemones. They need time to adapt, and that time is dependent on factors like the brightness of the reef tank, the quality and nature of the light, the condition of the anemone when it arrives, etc. In some cases anemones can get WORSE before they get better - i.e. they may expel zooxanthellae that they already had in order to "juggle" their zooxanthellae population to better fit the conditions of their new home.
During this time, they may start to starve because they are not getting any energy from their zooxanthellae (as witnessed by shortened and stubby tentacles). Additionally, if forced out into an open bright area, they may actually get hurt or burnt by the light (similar to SPS coral bleaching when frags are added to a new tank). You can help by partially shading a new anemone when it is introduced to your tank, and by providing supplemental feedings. Sometimes anemones can adapt quickly to a new environment (in a couple of weeks). In most cases it takes a month or longer - and in the case of bleached anemones it can easily take 6 months to a year.
