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Phylum Cnidaria
General Information:
• Cnidaria means ‘a stinging thread’
• Examples: sea anemones, corals, jellyfish,
freshwater Hydra, and the Portuguese manof-war
• Habitat: marine and fresh water
Soft Coral
Sea Anemone,
Clover Point, Victoria
B.C.
Portuguese Man Of War
Brain Coral
Corals are an
important part of BC
marine ecosystems!
•In BC, deep water corals such as
the red tree or “Gorgonian coral”
provide shelter for juvenile rockfish
such as the yelloweye rockfish or
sharpchin rockfish
Characteristics:
• All cnidarians have a basic radial symmetry and
possess only 2 layers of living tissues.
• 2 tissue layers: ectoderm and endoderm
• Contain a mesoglea
• Some possess an exoskeleton (for protection)
made up of CaCO3 (e.g. corals)
Characteristics
• Have 2 body forms:
a. Polyp (sessile) – e.g. hydra, coral, or sea
anemone
b. Medusa (free swimming) – bell shaped –
e.g. jellyfish
• Some cnidarians contain only one body
shape during their life cycle, while others
contain both during their life cycle
• When a cnidarian exhibits the 2 body forms
in its life cycle, it is called polymorphism
(‘many shapes’)
• The evolution of a primitive nervous system
sets these organisms apart from the sponges
– called a nerve net
• They contain nematocysts found on the
tentacles used for capturing prey and for
protection
• Mesoglea is sandwiched between ectoderm
and endoderm
• There are amoebocytes in the mesoglea of
cnidarians, much like in sponges
• Cnidarians possess a simple digestive
system – the gastrovascular cavity – where
digestion and circulation of nutrients occur
http://shapeoflife.org/video/cnidarians-lifemove
Sea Anemones – Class Anthozoa:
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No medusa stage
They are all marine
Polyp body form exclusively
Reproduce asexually through longitudinal or
transverse fission (like binary fission, but on a
much larger scale)
• Are primarily carnivorous – capture food using
nematocyst-studded tentacles and transfer it to a
central mouth opening
• There are around 6000 species of sea anemones
Coral – Class Anthozoa
• Colonial form of Class Anthozoa
• Can have soft and hard corals
• Hard corals secrete external CaCO3 skeletons and
are called ‘stony corals’
• Stony corals make up coral reefs (not all make
reefs though)
• Coral add about 10 kg of additional CaCO3 per m2
of reef yearly
• Soft corals do not have hard external skeletons
The Anatomy of A Coral
• Many corals have
symbiotic green algae
called zooxanthellae living
in their polyps
• Warmer ocean
temperatures can cause
these algae to die, resulting
in a phenomenon known as
“coral bleaching”
Jelly Fish – Class Scyphozoa
• Only a few hundred species – nearly all are marine
• The body is in the form of an inverted cup, with
nematocyst-studded tentacles extending
downward from the cup.
• Some jellyfish have symbiotic unicellular algae
that give them nutrients
• The life cycle of jelly fish involves the two
different body forms: polyp and medusa
A Typical Jellyfish Life Cycle
A Typical Cnidarian - Hydra
Structure:
• Polyp body form
Locomotion:
• Sessile; however, some
can somersault and glide
along while secreting mucus
Ingestion, Digestion,
Elimination
• Food is captured by nematocysts in the tentacles
by stinging or stunning prey
• Tentacles push food through mouth into
gastrovascular cavity
• Flagella line the endoderm layer and create a
current
• The endoderm cells secrete enzymes to break
down food
• Food is taken into cells and nutrients are circulated
by amoeboid cells
• Wastes are eliminated out through the mouth
The Nematocyst
Nematocysts vary in shape, but
all are adapted for stinging or
stunning prey
In the sea anemone on the right,
nematocyst aggregations are
visible as the dark pink lines
Respiration
• O2 enters the cells by diffusion
• CO2 leaves the cells by diffusion
Nervous System
• Have a nerve net within the mesoglea
• When stimulated, the nerve net will respond
by contracting / relaxing the muscles
• Have sensory cells that are sensitive to
touch, light, chemicals, and balance
Muscular System
• Latitudinal and circular muscle fibres are
found in the endo / ectoderm layers
Excretion
• Nitrogenous wastes leave the cells by
diffusion
• Wastes enter the gastrovasuclar cavity and
leave via the mouth
Reproductive System
A. Asexual
• Budding (bud breaks away)
• Regeneration (re-growth of missing parts)
A Hydra Budding
Reproductive System
B. Sexual
• Some are hermaphrodites
• Ovaries and testes grow on the side of adults
• Ovary produces an egg and remains attached to the
adult
• Testes produce the sperm which swim to the egg,
resulting in a zygote
• Zygote develops into a ball of cells called the
blastula, which develops into a ciliated larva
called a planula, which can swim to another
location, settle, and develop into a polyp adult
Ecological Importance of
Cnidarians
• Are extremely important in the ecology of tropical
and temperate oceans in the form of coral (eg
Great Barrier Reef)
• Corals provide habitats and food for many species
of fish and invertebrates
• Reefs protect the land from ocean waves
• Cnidarians form symbiotic relationships with
other organisms living among the tentacles (clown
fish and sea anemones)
The Importance of Deepwater
Corals
•Throughout the world,
many deepwater coral
reefs are being
destroyed by trawlers,
removing vital nursery
grounds for many fish
species
Cnidarians and Medicine
• Used in medical research for their chemicals
(cancer therapy)
• Stony corals are now being used in certain surgical
procedures. Because they have many pores for
blood vessels to grow through, small pieces of
coral are now being used in bone grafts,
particularly in face and jaw reconstruction, and in
arm and leg surgery
• These pieces are eventually replaced by normal
bone tissue
Evolutionary Significance
• Cnidarians are the first animals to have
organized true cell tissues
• They are therefore the first true
multicellular organisms to appear
• They are the first organisms to have muscle
tissue and nervous tissue
• Cnidarians, having radial symmetry are
thought to share a common ancestor with all
other multicellular animals