An individual coral polyp is a sack with a ring of tentacles around a central mouth. It sits in a limestone skeletal case. Many hard corals have brownish unicellular algae (zooanthellae) living within the tissues of the coral. This algae uses the sun and carbon dioxide (photosynthesis) to produce food which is then consumed by the coral polyps.

Soft corals are similar in that they have the same algae (zooanthealle), but probably get more of their nutrients from the water column. Instead of a limestone skeleton, the colonies of animals live on a supporting stem consisting of fleshy tissue. This often resembles a tree.

When the current is flowing soft corals in the South Pacific extend their tentacles to feed, creating a spectacular display of color.