Download Anatomy of a Soft Coral Polyp

Coral Anatomy and Physiology
Drs. Foster & Smith Educational Staff
Corals are a part of the group of small aquatic animals called "Cnidarians." Cnidarians include the corals, sea anemones,
hydroids, and jellyfish. Some are motile (free-swimming), like the jellyfish and hydroids, and others are sessile (attached),
like the corals and sea anemones. Corals are a very diverse group of Cnidarians. They are made up of many tiny organisms
living together in a colony, and each individual organism is called a "polyp."
What is a coral polyp?
A polyp is a small marine invertebrate (spineless) animal. Some coral
polyps are as small as the head of a pin and others can be as large as
a foot in diameter, but the majority of them are small. A polyps' body
has radial symmetry, which means that if a line were drawn any way
through the middle of the body, it would produce two identical
halves. The body structure is simple €“ it is shaped like a tube. The
tube is closed on one end where the polyp attaches to its growing
surface. The other end of the tube is the mouth, which is used for
both taking in food and excreting wastes. Tentacles surround the
mouth for gathering food, and are usually found in multiples of six or
eight. The body tissues of polyps are extremely simple, with only two
cell layers €“ the outer epidermis, which has direct contact with the
water, and the inner gastrodermis. The space between the two cell
layers is filled with a jelly-like substance called "mesoglea."
Anatomy of a Soft Coral Polyp
The epidermis contains many different kinds of cells that perform
separate functions.
Nematocysts: These are are arrow-like barbs on the tentacles
that are filled with toxins. These toxins paralyze the microscopic crustaceans and other organisms that the polyp
feeds on, while serving to protect the polyp from attack by predators. If the toxins are strong enough, they can even
deter growth of other corals, and the stronger coral will have an advantage in competing for space to grow.
Epitheliomuscular cells: The second type of specialized cells in the epidermis are the epitheliomuscular cells that
have muscle fibers for movement and contraction of the polyp.
Sensory receptors and primitive nerve cells: These allow the organism to gather and process information about its
surroundings.
Mucus-producing cells: A very important job of some of the cells in the epidermis is to produce and secrete mucus.
These mucus cells help to clean and protect the organism by removing dirt and other particles that may settle on the
coral.
Interstitial cells: These cells may later turn into one of the specialized cells described above, or they can also play a
roll in reproduction by differentiating into sperm or egg cells.
Similar to the epidermis, the gastrodermis also has different kinds of cells and structures. There are however, only two major
types €“ glandular and epitheliomuscular.
Glandular cells: The glandular cells secrete digestive enzymes into the cavity in the center of the tube-like polyp. This
cavity is a very primitive stomach, and is called the "gastrovascular cavity." The digestive enzymes break down the
food that the organism eats into smaller pieces.
Epitheliomuscular cells: The epitheliomuscular cells function to move and contract the organism. In the gastrodermis
they also have the important role of ingesting the small particles of food that have been broken down by the digestive
enzymes. For this reason, they are known as "nutritive muscular cells." The food is then further digested inside the
cells and broken down into components the cells can use to grow and function. The nutritive muscular cells also have
flagella on them that help to mix the food inside of the gastrovascular cavity.
Mesenterial filaments: Mesenterial filaments can be found in the gastrovascular cavity. These long, thread-like
structures contain digestive enzymes, and sometimes nematocysts. The filaments can be expelled through the mouth
at the approach of food, or as offensive weapons against neighboring corals.
Zooxanthellae: In some species, the gastrodermis is also home to
zooxanthellae (zo-zan-THEL-ee), a type of unicellular algae. The
zooxanthellae live inside the gastrodermal cells of coral in what is called a
"symbiotic relationship." In this type of association, two organisms live in
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close proximity and at least one of the organisms benefit from the
relationship. In the case of coral and zooxanthellae, both organisms benefit
from their living arrangement. The zooxanthellae are photosynthetic
organisms; they capture sunlight and turn it into energy-rich compounds,
which can be transferred to the cells of the polyp. Like all plants, the
zooxanthellae need certain nutrients, such as nitrogen and phosphate, to
survive and continue photosynthesis. The concentration of these
compounds in ocean water is very small, and the zooxanthellae acquire
them from the polyp. The polyp obtains these essential nutrients through
the plankton and other food it eats. The zooxanthellae are also responsible
for the color of the coral. If the zooxanthellae die, the coral turns a white
color, called "coral bleaching," and is a very unhealthy condition. Though
the zooxanthellae and some coral polyps can survive independent of one another, it is much more efficient for them to
work together, and they are able to grow much faster if they cooperate.
How are corals classified?
There are many ways to classify corals €“ soft vs. hard; hermatypic (reef-building) vs. ahermatypic (non-reef-building); or
those with zooxanthellae vs. those without zooxanthellae €“ but the main distinction is between hard and soft corals.
What are soft corals?
Soft corals are those that do not have hard skeletons or build reefs. Common examples include sea fans, sea whips, leather
corals, and tree corals. Soft corals with zooxanthellae are often recommended for beginners in marine aquarium keeping. Only
a minority of soft corals rely on zooxanthellae, however. Since the algae need intense sunlight to photosynthesize, the corals
that do not have zooxanthellae are able to live at greater depths and in murkier water. Essentially, they can survive anywhere
there is enough plankton to sustain them. Since the soft corals do not produce the skeleton that hard corals do, their body
tissue is supported with clumps of crystallized calcite called "sclerites." The sclerites are suspended in an inorganic matrix
and give the tissues support, while still allowing a lot of flexibility. The sclerites come in many shapes and sizes, and the
shape of the sclerites is often an important clue in the identification of a soft coral.
What are hard corals?
Hard corals are those responsible for building coral reefs. There are very few hard corals that are ahermatypic. Those that are
do not contain zooxanthellae, but hermatypic corals always contain zooxanthellae. Reefs are therefore located in shallow,
clear water where there is the most direct sunlight to facilitate photosynthesis in the algae. Reefs are magnificent and beautiful
ecosystems that cover less than 0.2% of the ocean floor, yet they are estimated to support nearly 25% of all marine life. For
example, a specimen of coral measuring only 25 centimeters in diameter was found to have ten fish, at least twenty
crustaceans, several shrimp, mussels, snails, and a pair of gall crabs living in its branches. This is not to mention the many
microscopic parasites and other symbiotic organisms that were also living on or in the coral.
The polyps of hard corals make a
Anatomy of Hard
sturdy, protective shell out of calcium
carbonate. They filter the bicarbonate
and calcium ions out of the seawater,
where they are in abundance. The
lower portion of the polyp secretes the
skeleton where it is attached to a rock
or other hard surface. This process
produces a cup, called the "calyx," in
which the polyp sits. The walls
surrounding the cup are called the
"theca," and the floor is called the
"basal plate." Thin septa arise from the
basal plate and provide the polyp with
increased surface area, structural
integrity, and protection. When polyps
are physically stressed, they contract
into the calyx so that virtually no part
is exposed above the skeletal platform.
This protects the organism from predators and the elements.
Corals
Many polyps are nocturnal feeders €“ retracting into the protection of the calyx during the day and extending their tentacles
at night to feed. The calyx and polyp together are called a "corallite." The calyxes are connected to one another by more
skeletal material called "coenosteum." The polyps also have living tissue connecting them called the "coenosarc," which lies
on top of the coenosteum. The entire living tissue, therefore, lies on top of the skeleton. The polyps are able to share nutrients
with one another through the coenosarc. When one polyp obtains food, it is shared with all the others.
Reefs grow very slowly. They only grow approximately one inch per year, but that is efficient enough to surpass
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prohibited under copyright law. Site content, including photography, descriptions, pricing, promotions, and availability are subject to change without
notice. These restrictions are necessary in order to protect not only our copyrighted intellectual property, but also the health of pets, since articles or
images that are altered or edited after download could result in misinformation that may harm companion animals, aquatic life, or native species.
decomposition by natural means. Unfortunately, there are other "unnatural" factors that can tear down the reef, such as
overfishing, harmful fishing practices, damage from boat anchors, dredging, global warming, pollution, runoff and
sedimentation from deforestation, as well as other human disturbance. There are many conservation groups working to
protect these unique environments by preventing human destruction of the reefs. Organizations such as Reef Relief, MAC
(Marine Aquarium Council), Reef Check, IMA (International Marinelife Alliance), USCRTF (U.S. Coral Reef Task Force),
CORAL (Coral Reef Alliance) and CORL (Coalition of Reef Lovers) are just a few of the key groups that are providing
resources and plans of action in order to help manage and protect our beautiful coral reefs.
Coral colonies can take many fascinating shapes and sizes. They can resemble fingers, mushrooms, tree-branches, elk horns,
cups, and even brains. They can be as small as a dime or as big as a room, and come in many brilliant colors. Their
extraordinary diversity and importance to marine ecosystems makes them a life form we should enjoy, appreciate, and protect.
Coral Anatomy and Physiology - Page 3 of 3
Unauthorized use of any images, thumbnails, illustrations, descriptions, article content, or registered trademarks of Foster & Smith, Inc. is strictly
prohibited under copyright law. Site content, including photography, descriptions, pricing, promotions, and availability are subject to change without
notice. These restrictions are necessary in order to protect not only our copyrighted intellectual property, but also the health of pets, since articles or
images that are altered or edited after download could result in misinformation that may harm companion animals, aquatic life, or native species.