The mouth is encircled by a ring of tentacles. The tentacles may be retracted; generally corals only extend their tentacles at night although this is genus specific. When the tentacles are extended they may mask some of the colony features that will aid in identification. Wafting your hand over the surface of the colony will cause the tentacles to retract, uncovering the diagnostic features. The tentacles are lined with nematocysts (stinging cells), and are used to catch food from the water column. This nutrition supplements energy gained from the symbiotic algae contained within the polyp tissue of reef building corals.
The genera Alveopora and Goniopora (p. 27 of the Coral Finder) have daytime extended polyps where the tentacles are located on the end of a long tubular polyp, giving the colony a soft coral appearance as the tissue moves back and forth in the current.