How about this as a defining characteristic?
When my 3-yr old jumps on the floor next to the tank, my PE's (as defined by a green slit mouth and black & white axial stripes when closed) close up ("watch the coral close their eyes, daddy"); none of the ~50 other colonies of zoas close. Since I only have one type of PE (but lots of colonies of it), I'm not sure if this is a general characteristic of PEs. Anyone else?
-Keith-