The Christmas Tree Worm (Spirobranchus giganteus) seems to thrive everywhere I go diving--worldwide.  They come in a multitude of colors, yet are the same species.  Normally I find them attached to rocks or dead coral, but these two struck me as interesting because they were in a colony of living (and feeding) coral.  I don't understand their anatomy at all, but would be interested in learning.  Large fish can graze right past them without disturbing them, yet when I shine a strong light on them from 20' away they retract immediately.  Clearly they are sensitive to changes in light intensity and water movement.  But then why do they ignore most fish?  These worms are in a different family from the feather dusters--they are Serpulidae, whereas feather dusters are Sabellidae.