General biological information on corals describing different types
of corals, their structure, and how corals form coral reefs.
In brief, corals are a group of organism related to sea anemones
and sea jellies. These groups of soft-bodied animals make up the
phylum Cnidaria. A primary characteristic of all Cnidarians is that
they have tentacles with stinging cells called nematocysts. Unlike
many Cnidarians, corals are a sessile organism, meaning that they
are attached to the substrate, or base. Most species of coral are
colonial, though some are solitary. One coral colony is made up of
anywhere from several to thousands of individuals known as polyps.
All coral polyps have the same basic body structure including
tentacles with stinging cells that surround a mouth at the top of
the central body cavity.
There are two general categories of coral:
Hard Corals are corals that have six tentacles or multiples of
six (i.e. 6, 12, 18, 24…). They secrete a rigid skeleton made of
calcium carbonate. The polyps live on the surface and within the
top few centimeters of the skeleton. Hard corals are also called
"stony corals" and corals that form reef structures also referred
to as hermatypic corals.
Octocorals are corals that have eight tentacles. Most octocorals
secrete a flexible skeleton that consists of a central core made of
a protein called gorgonin and outer layer called the rind. The
polyps are embedded in the rind. Octocorals are also called
gorgonians and 'soft corals.
Hard corals come in a variety of growth forms. Some examples of
these growth forms are branching corals, plate or sheet-forming
corals, encrusting corals, and massive or boulder corals. Each
polyp produces its own skeletal material and sits in a pitted area
of the skeleton known as a corallite. For some species of coral the
corallites are fused together to form elongated ridges and valleys.
Many species with fused corallites are commonly called brain corals
because of their appearance, whereas many species with discrete
corallites are commonly referred to as star corals.
Although growth rates differ between different coral growth
forms and species, hard corals generally grow very slowly. Hard
coral growth is highly dependant on sunlight, warm temperatures
ranging from 20oC (68oF) to 29oC
(84oF) and water quality (clear, low nutrient water is
ideal) and is therefore varies greatly from location to location
and limited to shallow tropical and subtropical waters. In Florida,
branching corals can grow as much as 10 centimeters per year,
whereas massive coral colonies grow about one centimeter per year.
Massive coral colonies like the star and brain corals pictured
above can grow to be several meters across and can be several
hundred years old.
Octocorals come in two general
growth forms, encrusting and branching. The branching growth form,
however, has many variations. Examples are sea rods, sea plumes,
sea whips, and sea fans. While most octocorals have a flexible
skeleton with a central core made of a protein called gorgonin,
some have a central core made of tightly bound or fused calcareous
known as spicules. These octocorals are not as common and are
typically not as flexible.
Though growth rates in octocorals
are not as well studied as those of stony corals, the more common
octocoral species in Florida have been found to grow in height
about two centimeters per year.
Many hard corals, and all hermatypic corals, share a symbiotic
relationship, a relationship between two dissimilar
organisms, with microscopic algae called zooxanthellae. These
algae give stony corals their vivid color. Zooxanthellae create
nutrients through photosynthesis and provide important resources to
the coral polyp in exchange for a home within the polyp's living
Some octocorals share a symbiotic relationship with
zooxanthellae, which is similar to that of hard corals, however,
many do not. Octocorals that do not have zooxanthellae get their
colors from pigments in the coral polyp or the outer layer of the
Both of these groups of corals can use their tentacles to
capture food (plankton). Although acquiring energy in this manner
is more common among octocorals.
Hard corals and octocorals can reproduce both sexually and
Sexual reproduction in corals results in the start of a new
coral colony and is accomplished by spawning or brooding. In
spawning, gametes (sperm and egg cells) are released into the water
column where they meet and form larvae which settle to the bottom
and become new coral colonies. In brooding gametes meet and develop
into larvae within a coral polyp and are then released into the
water column to settle and become new coral colonies. Coral species
that spawn do so only once per year. This generally happens in one
mass-spawning event that typically occurs at the end of the
Asexual reproduction in corals is accomplished either by
fragmentation or by budding. Because corals are colonial animals,
when their skeletons are broken up or fractured the resulting parts
can continue to grow as new colonies. This method of reproduction
is common among branching corals that can break into pieces from
the intense physical stress of storms. These pieces then grow into
new colonies. In this way large stands of branching coral can be
produced. Budding is the process where one coral polyp splits into
two. This is how a coral colony grows from one individual polyp to
Hydrocorals and Black Corals
Two additional groups of corals are the hydrocorals and the
black corals. These two groups of corals are also Cnidarians and
have the same basic body structure as stony corals and
Hydrocorals are found in all the same habitat types that stony
corals are found in. The most common kinds of hydrocorals are the
fire corals. These corals form hard calcium carbonate skeletons
just like stony corals, however their skeletal structure and
arrangement of polyps is much different from that of stony corals.
While fire corals grow their own skeletal structures they are also
known to encrust substrate and other living organisms and are
commonly observed overgrowing sea fans. Fire corals have unusually
strong stinging cells and can sting snorkelers and divers who brush
up against them.
Black corals are generally thought to live in deeper waters, but
several species do live in shallow areas. These types of corals are
similar in appearance to octocorals and form a skeleton made of
protein, however the skeletal structure of black corals is hard and
inflexible. Black coral polyps are not imbedded in the skeletal
structure as with other types of corals. Because of this the dead
skeleton of black corals has a smooth glossy finish which has led
to the cultivation of these corals for jewelry and other trinkets.
While these slow growing types of corals used to be common in some
areas, they are now rarely seen due to overharvesting.
Black Coral (overgrown by algae)
Coral reefs form over thousands of years as successive
generations of corals grow on top of each other. When one stony
coral colony dies other new stony coral colonies settle and grow on
the former colony's remains. Although stony corals produce much of
the calcium carbonate that forms reefs structures, other organisms
such as algae and mollusks also produce considerable amounts.
Coral-like organisms that form calcium carbonate skeletons have
existed on earth for 450 million years. Coral reefs similar to the
ones we know today have existed since the age of the dinosaurs.
They are present over large areas of the tropical and subtropical
oceans and, like rainforests, are one of the most diverse and
highly productive ecosystems on the planet. Though coral reefs are
widespread, they comprise less than 1 percent of the world's oceans
area. Despite this, nearly 25 percent of all marine animals use
coral reefs for some part of their life cycle, including
approximately 7,000 species of fishes, and countless species of
algae and invertebrates.
The living coral reefs of Florida began forming about 6,000
years ago when sea levels stabilized following the last ice age. In
Florida, well-developed coral reef communities exist from Palm
Beach County southwest to the Dry Tortugas. The two types of coral
reef structures off of south Florida are patch reefs and bank
reefs. Patch reefs up to several hundred meters across with several
meters of vertical relief are generally found between the numerous
islands and the outer bank reefs of the Florida Keys. Bank reefs
make a continuous line of coral reefs stretching from southeast
Florida to the Dry Tortugas several kilometers offshore. Bank reefs
in Florida have grown into distinct spur in groove structures, with
elongated sections of reef separated by sand spurs.
In Florida there are four general types of coral habitat.
habitats are generally close to shore and are dominated by
octocorals, sponges, and algae with low stony coral coverage and
low stony coral species diversity.
Patch reef habitats are usually characterized
by high diversity of both octocorals and stony corals. A mature
patch reef is roughly circular and the perimeter is buttressed by
large boulder corals.
Shallow offshore reef (Bank Reef) habitats are
high-energy environments with large branching corals. Expanses of
fire coral, Millepora comlanata, and the zoanthid, Palythoa
caribaeorum, are also common.
Deep offshore reef (Bank Reef) habitats are
inhabited by a diverse array of benthic (bottom dwelling) organisms
including Giant barrel sponges, Xestospongia muta,
octocorals, and stony corals.
Coral Species in Florida
On Florida's coral reefs there are approximately 50 species of
hard corals. The most common hard coral species are Boulder Star,
Montastrea annularis, Great Star, Montastrea
cavernosa, Massive Starlet, Siderastrea siderea,
Mustard Hill, Porites astroides, and Grooved Brain,
Colpophyllia natans. Two branching species, Staghorn,
Acropora cervicornis, and Elkhorn, Acropora
palmate, were formerly abundant in the Florida Keys, but have
experienced large scale die-offs over recent years.
There are about 54 species of octocoral in southern Florida.
Identifying octocorals by species is difficult and often requires
the use of a microscope. The most common octocorals in Florida are
Bent Sea Rod, Plexaura flexuosa, Spiny Sea Fan,
Muricea muricata, Slimy Sea Plume, Pseudopterogorgia
Americana, Common Sea Fan, Gorgonia ventalina, and
Warty Sea Rod, Eunicea calyculata.
Unless otherwise noted, all images are credited to the Fish
and Wildlife Research Institute (FWRI), Florida Fish and Wildlife
Conservation Commission (FWC)