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Cnidarians
“The guys with the stinging
All About Cnidarians
• Evolution of multicellular animals with
tissues that perform special functions
• Cnidarians: Stinging animals
• Mostly marine
• 10,000 known species
• 2 body forms: medusa and polyp
Anatomy
A. Polymorphism- Cnidarians have
more than one body form:
1. Polyp
2. Medusa
• The life history of some cnidarians
includes both polyp and medusa stages.
Others spend their entire lives as either
polyp or medusa
medusa
• Free floating form which is transported
by water currents, mouth with
surrounding tentacles are positioned
downward
Medusa
• Umbrella shape
• Tentacles around mouth
• Motile, Free-swimming
polyp
• Sessile, attached form with mouth and
tentacles positioned upward
Polyp Form
• Tube with tentacles
around the mouth
• Sessile
Coral polyp
Polyp (sea anemone)
Polyp (Hydra)
Cnidarians
Cnidarians actually exhibit
radial symmetry
in which similar body
parts can be grouped
around a central axis.
• Here we also observe a
large jump on the
evolutionary scale:
tissues that perform
specific functions.
• Animals with radial symmetry look the
same from all sides and have no head,
front, or back.
• They do, however, have an oral surface,
where the mouth is, and an aboral
surface on the opposite side
• See figure 7.6 page 119
Radial Symmetry
Radial
Compass jellyfish
tissues
• Two layers present
• Epidermis covers body surface (external)
• Gastrodermis lines internal body cavity and is
specialized for digestion
• There is also a narrow, gelatinous middle
layer, mesoglea,that usually doesn’t contain
cells. Forms “bell” of medusae
Two Tissue Layers
1. EPIDERMIS –
OUTER LAYER
2. GASTRODERMIS –
STOMACH CAVITY
MESOGLEA – JELLY MATERIAL
BETWEEN TISSUE LAYERS
ABUNDANT IN JELLYFISH TO
HELP THEM FLOAT
Cnidocytes
• Stinging cells
• Nematocyst capsules located on
tentacles
• These are used for protection and
feeding
Cnidocytes-Stinging Cells
• Within the cnidocyte is the
nematocyst (which is like a
capsule)
• The capsule has the stinging
structure which is a hollow
thread (Filament) with barbs
• Most contain a toxin
• Very small, but discharged in large
numbers
• Usually the sting is only strong
enough to kill zooplankton or tiny
fish
• But there are exceptions!
• Discharged nematocysts,
stinging cells used for
defense and prey
capture, have been the
plague of more than fish.
• Stings from certain
jellyfish have resulted in
death in a matter of
hours, especially for
infants.
Discharging nematocyst
• http://www.youtube.com/watch?v=6zJiB
c_N1Zk
Jellyfish stings
• http://www.youtube.com/watch?v=Tp38DUjUnM
• 1:01
• http://www.youtube.com/watch?v=SP6T
B8kiZB0
Digestive and nervous
systems
• Digestive system is incomplete (sac-like
with mouth only)
• Centrally located mouth surrounded by
tentacles
• Tentacles capture and handle food
• Mouth opens into gut where food is
digested
• Nerve net throughout body coordinates
movements
• Some jellyfish also have sensory cells
and contractile cells
One Opening System
FOOD IN
Food enters through the
mouth and is digested in the
cells.
Wastes are excreted
through the mouth.
Sperm and eggs are released
through the mouth.
WASTES,
GAMETES
OUT
Response
•
•
•
•
No nervous system
No brain
Nerve net around mouth
Nerves cover the body but do not tell the
difference between body parts.
Respiration
• Via diffusion
• Body is two cell
layers thick
Habitat
Aquatic:
• Most are
Marine
• A few are
freshwater
Feeding
1. Carnivores
(predators)
2. Process of feeding
Lion’s mane eats
a. Tentacles sting prey with
another jelly!
nematocysts
b. Tentacles grab prey
c. Prey pulled into mouth
3. Prey moved into gastro-vascular cavity (GVC)*
4. GVC makes enzymes, breaks down food, extra-cellular
digestion
5. Undigested food moves back out of mouth
Process of feeding
3. Prey stuffed into gastrovascular cavity (GVC)*
4. GVC makes enzymes, extracellular digestion
5. Undigested food back out
mouth
*incomplete digestive tract (no anus)
Locomotion
A. Medusa- motile, free-swimming
B. Polyps- sessile, attached to hard
substrate
Exceptions:
1. Hydra tumbles on tentacles
2. Sea anemones glide on pedal
disc
Reproduction
1. Asexual
budding
2. Sexual
a. Medusae release sperm & eggs
b. Larvae are free-swimming
Sexual reproduction
• Medusa is normally the sexual stage
with epidermal gonads. Eggs and
sperm released from medusa
• Zygote develops into swimming larva
called a planula
• Planula settles on bottom to form colony
• Eventually, new medusa are formed
Planula
Life Cycle of a Typical Cnidarian
• Alternation of Generations
• One form is the polyp (a cylinder with
the closed end attached to the substrate
and the open end with the mouth and
tentacles directed upward)
• The other form is the medusa, freeswimming, with the mouth underneath
(like a jellyfish)
Life Cycle of a Typical Cnidarian
Hermaphrodite
sexual
asexual
Alternation of Generations
Cnidarian life cycle
• http://www.youtube.com/watch?v=ct9Ky
Lmnu0I
• http://www.youtube.com/watch?v=K9cF
bJ8qYWg
Phylum Cnidaria
The Classes of Cnidaria are:
• Anthozoa- Sea Anemones and Corals
• Hydrozoa- Hydra
• Scyphozoa- “true” jellyfish
• Cubozoa- “box-shaped” jellyfish
• Ctenophora- non stinging Cnidarians:
 Comb Jellies (covering separately)
Class Anthozoa
•
•
•
•
•
•
•
All marine “flower animal”
Corals, anemones, sea fans/whips, sea pansy
Colonial polyps that normally lack a medusa stage
Sting organisms around them
Can move by crawling along the substrate
Corals secrete calcium carbonate “shells”
Most coral species possess symbiotic algae within
body tissues called zooxanthellae
Anthozoa- sea fan
Anthozoa- anemone
Sea anemone
Giant Sea Anemone
Anthozoa- coral
Sun Coral
Brain Coral
Anemone Video
• http://www.youtube.com/watch?v=NFO
4DwEkkr0
Homework
• Research the relationship between
clown fish and anemones
• http://video.nationalgeographic.com/vid
eo/animals/invertebrates-animals/otherinvertebrates/clownfish_amonganemon
es/
Class Scyphozoa
• True jellyfish. All marine
• Free swimming large medusa forms
with polyp only in reproductive life
• They move by rhythmic contractions of
the bell, but cannot fight against
prevailing water currents
• Many with powerful stings
Scyphozoa
National geographic jellyfish
• Dance of the jellyfish
• http://video.nationalgeographic.com/vid
eo/animals/invertebrates-animals/otherinvertebrates/dance-jellyfish-eorg/
• 1 minute
National geographic
• http://video.nationalgeographic.com/vid
eo/animals/invertebrates-animals/otherinvertebrates/jellyfish/
Super Jellies
• http://www.youtube.com/watch?v=NbpB
5F9CcLc
• 3:31
Jellyfish video
• http://www.youtube.com/watch?v=B3fa
XiTZdjo
• 3:43
• http://www.youtube.com/watch?v=pimIb
TqJLZc
• 11:08
Homework
• The case of the killer Cnidarians page
122
• Research most deadly jellyfish
• Irukandji
• Carukia barnesi
Killer Jellyfish
• http://www.youtube.com/watch?v=GK_
Cl_54Qh8
• http://www.youtube.com/watch?v=Ml7b
O021DMk
• http://www.youtube.com/watch?v=rQ2W
YbJSvTk
worksheet
• “Killer Cnidarians” (Hoyle)
Life cycle of Scyphozoa
Largest known jellyfish
• Lion’s Mane Jelly
• bell of a lion's mane jellyfish can be
over 8 feet across
• tentacles can reach over 100 feet, and
they have many of them - the lion's
mane jellyfish has eight groups of
tentacles, and there are 70-150
tentacles in each group
Lion’s Mane Jellyfish
• Encountering a lion's mane jellyfish probably won't be
lethal, but it won't be fun, either. A lion's mane
jellyfish sting usually results in pain and redness in
the area of the sting. The sticky tentacles of a lion's
mane jellyfish can sting even when the jellyfish is
dead, so give lion's mane jellyfish on the beach a
wide berth. In 2010, a lion's mane jellyfish washed
ashore in Rye, NH, where it stung 50-100
unsuspecting bathers
Class Hydrozoa
• Mostly polyp forms with reproductive
medusa
• Freshwater and marine
• Physalia
• Portuguese man of war
• hydra
Hydrozoa
Hydra
• Exist only as polyps
Portuguese man of war
National geographic
portuguese man of war
• http://animals.nationalgeographic.com/a
nimals/invertebrates/portuguese-manof-war/
National geographic port. Man
of war
• http://www.youtube.com/watch?v=RBd
CpcapB0s
Ecological Role
A. Predators and prey
B. Neurotoxins in medical research
C. Coral – jewelry, building, reefs
(surfing!)
D. Coral reefs - habitat for many different
species, great biodiversity, protect
coastline
E. Symbiosis with other organisms
Cnidarians Documentary
• http://www.youtube.com/watch?v=AjMWOeM3uA
• 9:17