At the simplest level, these coelenterate organisms can be
described as countless tiny polyps (simple gelatinous “sacs” with an opening on top, ringed by
tentacles) consolidated into a hard and fragile calcareous structure built by the organisms themselves: an
exoskeleton (external skeleton) whose often bizarre but always functional shapes create the scenery
of the reef.
Endowed with a horny and elastic skeleton rather than a rigid and calcareous one, gorgonians (commonly known as sea fans) usually tend to develop at medium and greater depths, far from the action of waves. As they are plankton-feeding animals, the colony always grows perpendicular to the current flow to optimize its exposed surface, which sometimes exceeds 2 m (6.5 ft) in diameter. It is not uncommon to find sea fans growing toward the sea floor or horizontally because the current, rather than light, determines the colony’s orientation. The so-called sea whips of the Juncella genus also belong to the gorgonian group. The Alcyonacea (soft corals) and gorgonians both belong to the Octocorallia subclass, whose colonies’ polyps all have eight pinnate tentacles each.
There are hermatypic corals which are able to build reefs slowly as the colonies expand
and propagate themselves, and ahermatypic corals, whose flexible framework does not lend itself to
composite structures (like gorgonians, whose skeleton is composed primarily of a horny substance, and the
so-called soft corals).
Zooxanthellae are generally associated with the first kind; these are
unicellular, symbiotic algae that live in the cells of coral polyps, averaging a million for every cubic
centimeter of coral. Zooxanthellae furnish the polyps with caloric substances such as sugars and aminoacids
through photosynthesis (which explains the importance of ambient light).
At the same time, they remove potentially harmful compounds like carbon dioxide,
which could dissolve the colonies’ limestone skeleton by converting it to carbonic acid
when it comes into contact with water (this explains why the excessive pumping of carbon
dioxide in the Earth’s atmosphere by man is possibly endangering the very structure
of coral reefs in this very moment).
An individual polyp embodies the living part of madreporic or stony coral colonies. Once distended, it looks like a pouch with a mouth on top, encircled by a crown of tentacles. Nematocysts found on the crown can often deliver a powerful sting, even to humans. The skeletal component at the base of the polyp into which the sac collapses during daylight hours is called the calyx. Its growth determines the colony’s rate of growth. Branching corals can grow as much as 30 cm (12 in) a year, while spherical colonies confine themselves to a few millimeters (fractions of an inch) per year. Observed at night and up close, coral polyps not only offer images of exceptional beauty but also valuable details for recognizing the species to which the colony belongs.
Coral polyps (picture them as tiny sea anemones)
are not limited to using the
zooxanthellae to procure nutrients. Indeed, they are endowed with extremely efficient weapons that allow them
to paralyze and capture suspended microorganisms (plankton) that are suspended in the water column and carried
by the current, especially at night.
These humble and often overlooked organisms are found as
both colonies and individuals. They are filtering animals, like sponges
and bivalves, that ingest water through an orifice on the upper siphon,
filter it through a simple pharynx, and finally expel it through the lower
siphon. It is thought that ascidians, commonly known as sea squirts, may
be the direct ancestors of vertebrates despite their primitive character.
At the simplest level, these coelenterate organisms can be
described as countless tiny polyps (simple gelatinous “sacs” with an opening on top, ringed by
tentacles) consolidated into a hard and fragile calcareous structure built by the organisms themselves: an
exoskeleton (external skeleton) whose often bizarre but always functional shapes create the scenery
of the reef.
Zooxanthellae are generally associated with the first kind; these are
unicellular, symbiotic algae that live in the cells of coral polyps, averaging a million for every cubic
centimeter of coral. Zooxanthellae furnish the polyps with caloric substances such as sugars and aminoacids
through photosynthesis (which explains the importance of ambient light). 
Coral polyps (picture them as tiny sea anemones)
are not limited to using the
zooxanthellae to procure nutrients. Indeed, they are endowed with extremely efficient weapons that allow them
to paralyze and capture suspended microorganisms (plankton) that are suspended in the water column and carried
by the current, especially at night.
