Stony corals belong to the Phylum Cnidaria which also includes gorgonians, jellyfish, hydrozoans, and sea anemones. All Cnidarians contain a special type of stinging cell (cnidoblast). The cnidoblasts are microscopic capsules which, when triggered by touch, chemicals or changes in salinity, eject a barbed filament with venom used for prey capture and defense. Stony corals are distinguished from other Cnidarians by the presence of a hard skeleton made of calcium carbonate (limestone). This skeleton supports and protects the delicate living coral polyps.
The polyp looks like a small sea anemone or an upside down jellyfish, a watery sack of flesh with a ring of tentacles around a central mouth. Each polyp fits in a cup in the skeleton called the corallite. All of the polyps of one colony are connected by tissue.
Coral polyps have algae living within their tissues that provide food in the form of sugar the algae make by photosynthesis. These algae, called Zooxanthellae, give the coral animals their color.
In addition to giving the coral its color and some of its food, Zooxanthellae precipitate calcium carbonate from the sea water by manipulating the acidity of the water. The calcium carbonate makes the hard coral skeleton.
The overall form of the coral varies among species but can also vary within a species. The shape and ornamentation of the corallite, the small cup that houses each polyp, is species specific and, sometimes must be closely examined for proper identification.
All polyps of a particular coral colony are derived from a single individual by asexual budding but the very first individual is the result of sexual reproduction. Corals launch their spawn to drift with the plankton. The resulting larvae, if lucky, find substrate to settle on and form their own colonies.
Some coral species are classified as hermatypic, that is they grow in massive colonies and are responsible for the basic structure of the reef. While the coral lives it increases in size by budding off new polyps and in thickness by each polyp continuing to lay down more calcium carbonate underneath. The living part of the reef is restricted to the thin covering of polyps on the surface.
Fire Coral is not a true coral but a hydrozoan. The polyps protrude through pores rather than sitting in corallites. The tentacles have cnidoblasts that contain a potent venom that stings like “fire”. In most cases, it is annoying but not dangerous.
Coral is the foundation of the reef, a habitat for countless organisms. Tiny polyps and algae create the largest living structures on the planet. Corals even provide food for some of the reef’s inhabitants.
I hope I have given you an overview of the structure and variety of corals so that you might take a second look at a seemingly “bare rock”. When you are diving or snorkelling notice what living coral looks like so that you can avoid bumping it or breaking it. If you have to steady yourself or stand up, make sure you look first.