Cnidarian
The phylum Cnidaria (Coelenterata in some texts) includes both solitary and colonial organisms
that have radial and/or bilateral symmetry. Typical cnidarians alternate each generation
between a fixed polyp stage and a free living medusoid stage. Most cnidarians are considered
carnivores because of their ability to actually catch food with their stinging cells called
nematocysts. Some groups, particularly the reef-corals employ photosynthetic algae
(zooxanthellae) within their tissues in a symbiotic relationship to aid in supplying food needed for
their rapid growth.
The cnidarian classes Anthozoa (corals) and Hydrozoa have calcified skeletons of aragonite and
calcite and a good fossil record, whereas the long fossil record of the class Scyphozoa (jelly fish)
is comprised mostly of molds and casts. Class Octocorallia is not well represented in the fossil
record because of its poorly calcified skeletons. The general form of coral colonies may be quite
similar in unrelated anthozoans (e.g., some colonial Tabulates and Scleractinians) because form
represents a basic response to long-term environmental conditions (i.e., limiting factors such as
light, turbidity, and especially wave and current energy).
Cnidarians are a diverse group of aquatic animals. More than 9,000 species are part of the Phylum Cnidaria, and
all species are aquatic. Cnidarians are widespread in marine habitats and less common in fresh water.
This interesting group of invertebrates includes many charismatic
organisms such as hydras, sea fans, jellyfishes, sea anemones,
corals, and the Portuguese man-of-war. Cnidarians all have some
type of specialized stinging cell organelle.
Cnidarians' bodies typically take one of two forms: the polyp or
the medusa. While the polyp form is adapted for a sedentary or
sessile lifestyle, the medusa form is adapted for floating or freeswimming. Sea anemones and corals (class Anthoza) are all
polyps. True jellyfishes (class Scyphozoa) are all medusae, though
some have a polyp larval stage. Notably, some hydroids
(class Hydrozoa) alternate between polyp and medusa forms
throughout their live
Polyps
The polyp is a sessile form which attaches to the sea floor and
often forms large colonies. The polyp structure consists of a basal
disc that attaches to a substrate, a cylindrical body stalk, inside of which is the gastrovascular cavity, a mouth
opening located on the top of the polyp, and numerous tentacles which radiate out from around the edge of the
mouth opening.
Some cnidarians remain a polyp for their entire life, while others pass through the medusa body form. The more
familiar polyp cnidarians include corals, hydras, and sea anemones.
The polyp is the basic body form of a coral animal. It is essentially a round animal with a mouth in the middle and a
ring of tentacles around the mouth. The tentacles possess stinging cells (called nematocysts) and can be used to
sting, paralyze, and catch prey. The prey is wiped off into the mouth and digested internally in a one-way digestive
tract (there is no anus). Undigested material must be regurgitated through the mouth. When the animal is
disturbed, or not feeding, it may close up, withdrawing its tentacles but the circular outline of the polyp is still there
with its mouth in the middle.
Corals secrete a hard skeleton, called a corallite, under their skin. Each polyp
secretes a hard, circular corallite (made of
calcium carbonate). This circular corallite is
attached at the bottom and has thin walls,
starting at the outer circle, radiating toward the
middle (but leaving room for the central mouth
cavity). The thin (but numerous) radiating walls
are
called 'radiating septa.' The corallite is usually
permanently attached to the solid surface upon
which it lives (except in species like mushroom coral). The numerous radiating septa
cause the corallite to be extremely dense and strong. The coral animal can add calcium carbonate to its corallite and
extend it upward, keeping its living tissues in the uppermost part of the corallite, leaving a hard, permanently attached,
base beneath.
http://www.marinebio.net/marinescience/04benthon/crani.htm
Symbiosis
Whenever two organisms of different species exist in close physical contact to the benefit of both
organisms, that's symbiosis. Symbiosis can occur between animals, plants, fungi or any combination
thereof. Each organism contributes something that benefits the survival of the other, and in turn receives a
survival benefit of its own.
Some symbiotes are so closely intertwined that it's difficult to tell where one organism ends and the other
begins. And in the case of plant/animal symbiotes, it can be difficult to tell whether the organisms are
plants, animals, or a little bit of both.
Symbiotes aren't cartoon animals living and working together in perfect harmony. Most symbiotes have no
idea that they're helping another creature. They're just surviving in whatever way works best for them, an
instinctive behavior driven by natural selection.
You probably didn't realize that you're a symbiote yourself. Or that life on Earth probably wouldn't exist
without symbiosis. Or that symbiosis might have been responsible for the evolution of multicellular life. Or
that some scientists think the entire planet is one giant symbiotic organism. Sometimes symbiosis is
pretty weird.
http://science.howstuffworks.com/life/evolution/symbiosis.htm
Zooxanthellae
Most reef-building corals contain photosynthetic algae, called zooxanthellae, that live in their tissues. The corals
and algae have a mutualistic relationship. The coral provides the algae with a protected environment and
compounds they need for photosynthesis. In return, the algae produce oxygen and help the coral to remove
wastes. Most importantly, zooxanthellae supply the coral with glucose, glycerol, and amino acids, which are the
products of photosynthesis. The coral uses these products to make proteins, fats, and carbohydrates, and
produce calcium carbonate (Barnes, R.D., 1987; Barnes, R.S.K. and Hughes, 1999; Lalli and Parsons, 1995;
Levinton, 1995; Sumich, 1996). The relationship between the algae and coral polyp facilitates a tight recycling of
nutrients in nutrient-poor tropical waters. In fact, as much as 90 percent of the organic material
photosynthetically produced by the zooxanthellae is transferred to the host coral tissue (Sumich, 1996). This is
the driving force behind the growth and productivity of coral reefs (Barnes, 1987; Levinton, 1995).
In addition to providing corals with essential nutrients, zooxanthellae are responsible for the unique and
beautiful colors of many stony corals. Sometimes when corals become physically stressed, the polyps expel their
algal cells and the colony takes on a stark white appearance. This is commonly described as “coral bleaching”
(Barnes, R.S.K. and Hughes, 1999; Lalli and Parsons, 1995). If the polyps go for too long without zooxanthellae,
coral bleaching can result in the coral's death.
Because of their intimate relationship with zooxanthellae, reefbuilding corals respond to the environment like plants. Because their
algal cells need light for photosynthesis, reef corals require clear
water. For this reason they are generally found only in waters with
small amounts of suspended material, i.e., in water of low turbidity
and low productivity. This leads to an interesting paradox—coral reefs
require clear, nutrient-poor water, but they are among the most
productive and diverse marine environments (Barnes, 1987).
Tiny plant cells called zooxanthellae live within
most types of coral polyps. They provide the coral
with foods resulting from photosynthesis.
http://oceanservice.noaa.gov/education/kits/corals/coral02_zooxanthellae.html