Scleractinians, stony corals, hard corals, true corals, hexacorals
Scleractinans are anemone-like animals with a hard, calcareous skeleton on the outside. Each individual animal is called a polyp. Polyps may be widely spaced or close and sharing skeletal walls, or the polyps may be joined in rows and occupy grooves in a round skeletal mass. Polyps have six, or multiples of six tentacles.

Hard corals obtain most of their nutrients from resident zooxanthellae, but they also capture prey by stinging. The energy provided by sybionts means that shallow water corals grow much faster than deep water species. Coral skeletons are white, brownish colours come from the zooxanthellae, whereas blue and lavender colours come from pigments in the coral tissues.

Many species of butterfly fish feed on coral polyps as do crown of thorns starfish, parrot fish eat the whole structure.

Corals reproduce sexually in mass spawning events, the resultant larvae are then dispersed by currents before settling. Corals also reproduce asexually, that is, polyps subdivide and expand the colony. Some corals such as staghorns can establish new colonies from broken of fragments.

Reef building corals are very important in tropical waters much less significant in temperate waters. Reef forming corals generally require a winter temperature above 18C and high light levels, so that the zooxanthellae can photosynthesise.

Soft Corals - octocorals
Octocorals are so called because their polyps have eight tentacles. They occur as colonies which may be erect, encrusting or branched. Colonies are supported by sclerites - calcareous structures - the shape of which is used to identify species. Sclerites may be free within the polyp or joined to form a central axis which may also include horny material

ahermetypic - corals which lack zooxanthellae and do not contribute to reef building
corallite - the structure formed by an individual polyp within a colony
hermetypic - corals which contain zooxanthellae and contribute to reef building
meandroid - corallites forming groups within common walls to form ridges
planula - the planktonic larval stage of corals.
zooxanthellae - algae which live symbiotically within coral tissue and produce energy from sunlight .