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Cnidarians
What is a cnidarian?
•invertebrates
•more than 9000 species
•jellyfishes, corals, sea anemones, and
hydras.
•worldwide
•all but a few
cnidarians are marine
What are cnidarians?
• variety of colors, shapes and sizes
- can be as small as the tip of a pencil.
• two distinct body forms during their life
cycles
Medusa
Polyp
What are cnidarians?
• polyp = sessile form of a cnidarian.
Its mouth is surrounded by tentacles.
- sea anemones, corals, and hydras.
• medusa = free-swimming form of a
cnidarian.
- jellyfish
Body Form
The Medusa
It possesses an umbrellashaped, floating body, called a
bell, with the mouth on its
underside.
Tentacles
Body Form
mouth
The Polyp
Attached to substrate,
the mouth is on the
top surrounded by
tentacles.
Body form
In cnidarians, one body form may be more
observable than the other.
In jellyfishes, the
medusa is the body
form usually observed.
http://www.masla.com/invert/moon-jellyfish.html
The polyp is the
familiar body form
of hydras.
http://www.microscope-microscope.org/gallery/Mark-Simmons/pages/hydra2.htm
Body structure
Mouth
Tentacle
• radially symmetrical
• one body opening
• two cell layers
How is this similar to
sponges?
Cavity
Inner cell
layer
Bud
Jellylike
layer
Outer cell
layer
Disc
Body structure
Mouth
• cell layers are
organized into tissues
with specific functions.
- inner layer mainly
assists in digestion
It surrounds the
GASTROVASCULAR
CAVITY
Tentacle
Cavity
Inner cell
layer
Bud
Jellylike
layer
Outer cell
layer
Disc
Body structure
• two cells layers allows easy diffusion of:
- Oxygen
dissolved in water, it can
diffuse directly into body
cells.
- Carbon dioxide /other
wastes
moves out of the body cells
directly into the surrounding
water.
Body form
Most cnidarians undergo a change in
body form during their life cycles.
Medusa
Polyp
At some point, most Cnidarians exist as
both a polyp and a medusa.
Reproduction in cnidarians
• sexual and asexual reproduction
- Sexual reproduction occurs in only one
phase of the life cycle – the Medusa
IF there is no medusa stage, then the
polyp can reproduce sexually.
Sexual Reproduction in Cnidarians
Female
Male
Eggs
Fertilization
Asexual
Reproduction
Blastula
Bud
Larva
Polyp
Both the
medusae
and
polyps are
diploid
animals.
Digestion in cnidarians
• predators
- capture or poison prey using nematocysts
A nematocyst is a capsule that contains a
coiled, threadlike tube.
The tube may be sticky or barbed, and it
may contain toxic substances.
Nematocysts are located in stinging cells
that are on tentacles.
Digestion in cnidarians
Nematocyst
before discharge
Nematocyst
after discharge
Digestion in cnidarians
Once captured by nematocysts, prey is brought
to the mouth by contraction of the tentacles.
Mouth
Polyp
Gastrovascular
cavity
Mouth
Medusa
Digestion in cnidarians
• Food enters
gastrovascular
cavity
• digestive cells release enzymes to break down prey
•undigested materials are ejected back out through
the mouth.
Diversity of Cnidarians
There are four classes of cnidarians:
• Hydrozoa
•Scyphozoa
•Cubozoa
•Anthozoa
Hydrozoa
• two groups
- hydroids (hydra)
- siphonophores (Portuguese man-of-war)
• marine animals
• branching polyp colonies formed by
budding
•found attached to pilings, shells, and other
surfaces.
Hydrozoa
The siphonophores are floating colonies
that drift about on the ocean’s surface.
The Portuguese manof-war, Physalia, is an
example of a
siphonophore hydrozoan
colony.
Each individual in a Physalia colony has a
function that helps the entire organism survive.
Anthozoa
• exhibit only the polyp
form.
• Corals
• Sea anemones
• Sea fans
Corals
• live in colonies of polyps in warm ocean waters
around the world.
• secrete protective, cuplike calcium
carbonate shelters around their soft bodies.
Colonies of many coral species build the
beautiful coral reefs that provide food and
shelter for many other marine species.
Corals
Corals that form reefs are
known as hard corals.
• soft corals do not build
calcium carbonate
structures – not reef
builders.
Corals
The living portion of a coral reef is a thin,
fragile layer that grows on top of the shelters
left behind by previous generations.
Although corals are often found in relatively
shallow, nutrient-poor waters, they thrive
because of their symbiotic relationship with
microscopic, photosynthetic protists called
zooxanthellae.
Corals
The zooxanthellae produce oxygen and food
that the corals use, while using carbon dioxide
and waste materials produced by the corals.
These protists are primarily responsible for
the bright colors found in coral reefs.
Corals
Because the zooxanthellae are free-swimming,
they sometimes leave the corals.
Corals without these protists often die.
Corals and Global Warming
• Corals are being threatened by rising ocean
temperatures and increasing acidity
• Increasing temperatures stress the
zooxanthellae and they leave the corals –
this results in coral bleaching
Corals and Global Warming
• Without the colorful zooxanthellae, the
corals appear white.
• Coral bleaching is reversible, but often does
not happen.
• At current rate, 70% of coral reefs will be
bleached in the next 20-30 years.
• At present 35 million acres of reef have
been destroyed.
http://www.wri.org/map/coral-reefs-world-classifiedpotential-threat-human-activities
Corals and Global Warming
• Corals provide habitat for 25% of marine
fish
• About 1 billion people rely on fish as their
primary food source
Corals and Global Warming
• Tourism – over a billion dollars is spent
every year in the Caribbean, Australia (3.9
billion/year), and the Pacific Islands
• 1.2 Billion in Florida each year
Corals and Global Warming
• Corals are used for pharmaceuticals
• Chemical extracts from corals have helped
create drugs to treat AIDS, Cancer, Arthritis,
Inflammatory disorders, and pain killers
• They are beneficial for patients with heart,
kidney, and liver transplants.
Corals and Global Warming
• Coral Reefs are beautiful!
• Most diverse ecosystem – “Ocean
Rainforest”
• 4000 species of fish, 800 species of coral
Scyphozoa
The fragile and sometimes luminescent bodies
of jellyfishes can be beautiful.
Some jellyfishes are
transparent, but others
are pink, blue, or orange.
The medusa form is
the dominant stage in
this class.
Scyphozoa
• musclelike cells in their outer cell layer that
can contract.
• When these cells contract together, the bell
contracts, which propels the animal through
the water.
Cubozoa
• Until recently, box jellyfish were included
in Class Scyphozoa.
• They differ from Scyphozoans in several
ways
Cubozoa
• Their bells are square-shaped, instead of
round
• They have primitive brains
• They have eyes
• They swim, not float
• They sleep
Cubozoa
• The Irukandji in Australia is one of the most
dangerous animals
• It is very small, only 2.5 cm from bell to
tentacles
Giant Jellyfish
• While Irukandji is very small, Japan has
been invaded by Nomura’s jellyfish – the
giant jellyfish.
Research:
•
•
•
•
•
What are Nomura’s Jellyfish?
Where are they found? Just Japan?
What causes them to get so big?
Are they dangerous?
Are they new? Or are they only now getting
noticed?
Giant Jellyfish
• 6.5 ft wide
and 450 lbs
• More
common in
China and
Korea
• Only recently
in Japan
Giant Jellyfish
• Not much is known, Japan studying
mating/migration habits
• Choking fishing nets
• Possible warmer seas (global warming)
causes increase in population
•
http://video.google.com/videoplay?docid=2208948115892996006&q=giant+jellyfish&t
otal=103&start=0&num=10&so=0&type=search&plindex=0
Where did they come from?
The earliest known cnidarians also date to the
Precambrian, about 630 million years ago.
The earliest coral species were not reef
builders, so reefs cannot be used to date
early cnidarians.
The larval form of cnidarians resembles
protists, and because of this, scientists consider
cnidarians to have evolved from protists.