The sun goes down and the polyps start to pump themselves full of
water to extend their tentacles to catch the animal plankton which comes
up at night from the deeper water layers to eat the plant plankton which
has grown in the light during the day (Bonaire).
Half an hour later all polyps are fully extended. Clearly
visible are the ridges of little stinging cells on the tentacles used
to paralyze the plankton. These stinging cells are like small harpoons
which shoot out when a plankton animal touches the "trigger hair".