Phylum Cnidaria
General Background Review
What are the characteristics of cnidarians?
• Organisms that make up phylum
cnidaria include:
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Soft and hard corals
Sea anemones
Hydroids
Sea fans
Jellyfish
#1-Define radial symmetry and explain how it applies to phylum
cnidarian.
#2-What are the two body forms of organisms in the phylum Cnidaria?
• Insert figure 5-31a
• Insert figure 5-31b
• Cnidarians take one of two body forms:
• Polyp-organisms that are attached such
as corals and anemones are polyps
• Medusae-organisms that are free
swimming such as jelly fish
• Both of them are radially symmetrical
(which means they can be divided in half
though the middle in any direction.)
#3-What is the significance of nematocyst cells
in Cnidarians?
• Most people are wary of jellyfish stings and
rightfully so. Most species of cnidarians have the
ability to defend themselves with nematocysts.
• A nematocyst is like a coiled dart waiting to be
discharged.
• Most of the cnidarians also secrete a toxin
when they release their nematocysts.
• Only a few of the 10,000 species of cnidarians
are harmful to humans
• Reason being, is most cnidarians lack the
nematocyst strength to penetrate our tough
skin.
• Insert figure 5-32
#4-What are zooxanthellae and why are they
important?
• What is the relationship
between corals and
algae?
• Recall from our discussion
about micro- algae.
Dinoflagellates have a have a
symbiotic relationship with
species of hard and soft
coral.
• During the day the
dinoflagellates
photosynthesize using the
nutrient wastes form the
animal corals and releasing
excess carbohydrates to the
coral.
• At night the coral animals
extending tentacles to capture
drifting plankton.
#5-Identify the common names of organisms in class
anthozoa, hydrozoa and scyphozoan. Compare and
contrast their characteristics.
• Kingdom Animalia, Phylum Cnidaria, Class Anthozoa include all the
hard and soft corals and anemones
• The soft corals like anemones do not have a calcium carbonate
skeleton. They cover the surface of the reef and act as the
“decoration”.
#5-Identify the common names of organisms in class
anthozoa, hydrozoa and scyphozoan. Compare and
contrast their characteristics.
• A specifically harmful species of cnidarian that looks like a hard coral is called Fire coral, it actually
belongs to class Hydrozoa.
• What makes hydrozoans different is they alternate between polyp and medusa stage in their life cycle.
• Fire coral gets its name because if you touch it you, it might well remind you of touching fire
• It hurts like a &!#@$!
• Other hydrozoans of notoriety include the infamous Portuguese man-of-war, although it looks like
a jelly fish it is in fact a siphonophore. This is one cnidarian to be wary of as its pneumatocysts
can puncture human skin and the toxin can be life threatening.
• Siphonophores are the longest animals on the planet, some species reach lengths of 40 meters (131
feet).
#5-Identify the common names of organisms in class
anthozoa, hydrozoa and scyphozoan. Compare and
contrast their characteristics.
• Jelly fish belong to class scyphozoa.
• They can range from coin size to more than a meter (3 feet across).
• Jelly fish swim by contracting their bell (the rounded part of their body). Despite
the ability to swim, they are planktonic drifting with the currents.
• They feed primarily on plankton, catching it in their tentacles. Blooms in jelly fish
are not uncommon following seasonal phytoplankton blooms.
• One species of jelly fish to be wary of (if you ever dive the great barrier reef
anyways) is the box jellyfish. People that come into contact with this jellyfish
often die within minutes.
What are the characteristics of cnidarians?
• Insert figure 5-42
• Last but not least are organisms that look like
jelly fish, but are not actually jelly fish.
• Ctenophores have bag like bodies similar to
jellyfish, however, they have cilia that they
use for locomotion
• Cilia are short hair like structures similar
to flagella.
• Comb jellies lack stinging cells and are often
bioluminescent.
#6-What abiotic factors are important for
coral reefs?
• When coral animals get stressed out
they can actually kick out their algae
partners. Since the algae gives the
coral its color, the coral becomes
“bleached”.
• Good water quality is absolutely
necessary in order for coral animals to
survive.
• Increased nutrients (called
eutrophication) in the water cause algal
blooms which block the light for the
symbiotic algae in the coral tissues.
• Other factors that influence coral
communities include water
temperature. Fluctuations in water
temperature can stress the coral out
causing them to kick the symbiotic
algae out of their tissues.
• Changing salinity and pH also are
known to stress the coral out and cause
them to bleach.
Calcium Carbonate-Limestone
• The skeleton that corals produce are
made out of calcium carbonate, also
known as limestone.
• The limestone accretes (gathers) and
physically build the reef.
• Eventually the reaches the ocean’s
surface where it will stop growing.
• Climate change influences sea level.
As sea level drops it exposes the reef.
Florida is the result of a exposed coral
reef after sea level dropped.
• Reefs can form as Atols (top), fringing
reefs (right along shore) (middle) or
barrier reefs (offshore) (bottom)