Coral Reef 101: Stony Corals

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.

Polyps of the Great Star Coral (Montastraea cavernosa) are big enough to easily see and sometimes are expanded during the day.  They feed by capturing small organisms using the cnidoblasts in the tentacles. Food is manipulated into the mouth (seen here as a small slit in the white oral disk) which is also used to eject waste.

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.

Most coral polyps living in the Atlantic Ocean are shades of green or brown.  Sometimes the algae fluoresce making the colors intense.  Coral color can vary within a species, sometimes due to environmental conditions, particularly sunlight intensity.
You can spot these solitary corals (Scolymia sp.) under ledges or on the shady side of the reef.  This is a coral that varies considerably in color.  This bright red one was a beauty.
We were surprised to find this large (about 18″ in diameter) colony of blue Finger Coral (Porites sp.). Blue or purple Finger Coral colonies are usually limited to just a few fingers.  The camera actually could not capture the true color which was more of a royal blue.

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.

Coral bleaching occurs when the algae are expelled from the coral polyps.  Without the algae the coral animals are clear and the white skeleton is visible.  Bleaching is often a response to stress and can be temporary or may lead to the death of the colony.  This picture of this bleached Brain Coral is bluish because of the water depth it was taken at.  The fish is a Bluehead Wrasse.

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.

‘ The corallites that house the coral animals are often round with radiating sculpture that looks like a star.  Star Coral is a general term for any of these corals that have round corallites.
The corallites of the Smooth Flower Coral (Eusmilia fastigiata) are slightly elongated.  These corals live in small clusters. Many corals completely withdraw into the corallite during the day and come out at night to feed. These polyps are partially expanded.
The corallites of the Grooved Brain Coral are convoluted giving the colony a brain-like appearance. The polyps’ tentacles emerge from the corallite valleys.

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.

The Great Star Coral polyp in the middle of this photograph is in the process of budding off a new polyp.

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.

Mountainous Star Coral (Orbicella faveolata) is one of the main reef-building corals in the Atlantic Ocean.  It often forms overlapping plates like it does here at Highbourne Cay, Bahamas.
Mountainous Star Coral can also grow as mounds and boulders.
Lobed Star Coral (Orbicella annularis) is another common hermatypic  (reef-building) coral.  Each lobe is a pillar of calcium carbonate built up over the years by the thin covering of coral polyps on the surface.  In the top right background are some Mountainous Star Coral colonies. In the bottom right corner is some red Massive Starlet Coral.  The blue and orange fish is a Spanish Hogfish.
Massive Starlet Coral (Siderastrea siderea) with a black Ball Sponge.  Massive Starlet coral corallites are small and indented.  The colonies grow in a domed or spherical shape.
Part of this Symmetrical Brain Coral (Pseudodiploria strigosa) has died off showing the thin layer of living coral lying over older skeleton it has built up over the years.  Brain corals grow slowly and this coral probably has grown here for over 100 years.  The purple and yellow fish are Fairy Basslets and the yellow fish are Bluehead Wrasses.
Symetrical Brain Coral polyps are lined up in the grooves and appear to be merged together.  The arrow points to one of several polyp mouths that can be seen in this photo.  The tentacles line the sides of the valleys.
Elkhorn Coral (Acropora palmata) is a fast growing shallow-water coral that forms characteristic antler shaped fronds.  Click here for more about Elkhorn coral
This is an unusually massive Pillar Coral (Dendrogyra cylindrus) reef that can be seen during a drift dive south of Cambridge Cay in the Bahamas.  Pillar Coral is an uncommon relative of the brain corals.  The polyps are usually fully expanded during the day.
Whitestar Plate Coral (Agaricia lamarcki) forms overlapping plates on near vertical walls.  The polyps are lined up in short grooves and, in this species have bright star-like centers.  The colony at the bottom of the photo under the blue sponge has been half consumed by a disease.
Ridged Cactus Coral (Mycetophyllia lamarckiana) is related to brain corals but is smaller and not considered as a hermatypic or reef building coral.
These small plate corals are probably Sunray Lettuce Coral (Helioseris cucullata).  I am not sure what the green coral is, perhaps a Madracis sp.  The red is a sponge.
Lettuce Coral (Agaricia agaricites) can grow in encrusting, plate, knobby, or blade-like forms.  This one was a color that I do not often see.
Thinner blades distinguish Thin Leaf Lettuce Coral (Agaricia tenuifolia) from the above.  This coral can form large reefs that provide a large amount of shelter for other animals and plants.

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.

This Blade Fire Coral (Millipora complanata) looks like the Thin Leaf Lettuce Coral, however, Fire Coral can be distinguished from the lettuce coral by its smooth surface lacking in distinctive corallites.  In front of the fire coral colony is a Saucer Leaf Alga (Turbinaria sp.)
Branching Fire Coral (Millipora alcicornis) often encrusts dead sea whips and sea fans.  In this photo you can see the tentacles extending out of the pores.  Fire coral is characteristically orange or tan with white tips.
It is worth it to look into holes to find the pink and lavender branches of Rose Lace Coral (Stylaster roseus). Like other hydrozoan corals, they are hard but do not have the corallite structures of true stony corals.

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.

A large Fireworm (   ) munches down on a Finger Coral.  It has engulfed the tip of the coral in its mouth and has consumed some of the living tissue exposing the structure of the corallites.
Parrotfish graze on coral and algae with hard, sharp beak-like plates.  You can hear them crunching and the coral particles they excrete form much of the sand around the reef.  This is the Stoplight Parrotfish (Sparisoma viride).
The white marks on this Elkhorn Coral were caused by parrotfish eating it.
Dead or damaged coral is rapidly colonized by algae, gorgonians, sponges, corals and other organisms and can be the basis for a new reef.

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.




3 thoughts on “Coral Reef 101: Stony Corals”

  1. Once again, an awesome post! I really enjoy your posts and the information in both scientific and layman terms. Great job Gayle.

    Hope to see you and Bill soon.


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