The
Mushroom Coral is a solitary, free living coral polyp. It is not attached
to the rock underwater. Usually in the form of a round or oval disc,
this coral was used by the ancient Hawaiians to sand and polish their
canoes. They called it "Ko'a-kohe." It fit perfectly in the
palm and is an excellent abrasive.
This
image (taken at night) is an extreme close-up of the colorful feeding
tentacles extended between the septa (ridges).