Cnidaria phylum

Phylum Cnidaria
 All aquatic, mostly marine and some freshwater.
 Acoelomate
 Exoskeleton chitinous or calcareous.
 Body wall diploblastic with two cellular layers- outer epidermis and inner
gastrodermis which forms a gastrovascular cavity with a gelatinous acellular
mesoglea in between. In advance types, it is triploblastic.
 gastrovascular cavity or coelenteron is a single internal cavity into which mouth
opens and it is lined with gastrodermis
 Two basic forms of individual-
Polyp: Cylindrical form which attach bases to substratum
Medusa: Flattened, mouth down version of the polyp. Moves freely
 Mouth of polyps and bell margin of medusae often encircled by short and slender
tentacles.
 No respiratory, circulatory and excretory system
 Anus is absent
 Peculiar stinging cell organelles or nematocysts with one or both body layers help for
adhesion, food capture and offence and defense.

Polymorphism
 Occurrence in the same species of more than one type of individuals, which differ in form and function, is known as
polymorphism (Gr., polys=many + morphe=form).
 It is an important feature of single or colonial hydrozoan-which included in hydrozoa class.
 Two basic forms of individual-
 Polyps :
- It has a tubular body
- Mouth is surrounded by tentacles at one end
- Other end is usually attached by a pedal disc to the substratum.
- Sessile
- Part of the benthic community
 Medusae:
- Bowl or umbrella-shaped body which is called bell with marginal tentacles
- Mouth centrally located on a projection of the lower concave surface
- Free-floating or motile, move by weak contractions of body
- Part of the plankton
 Polymorphism ensures an efficient division of labour between the several individuals.
 Different functions are assigned to different individuals, rather than to parts or organs of one individual
 Polyps are concerned with feeding, protection and asexual reproduction
 Medusae are concerned with sexual reproduction

Patterns of polymorphism
Dimorphic:
 Simplest and commonest pattern of polymorphism
 The colonies which bear only two types of individuals are called dimorphic and
the phenomenon is termed dimorphism
 It is shown in many hydrozoan colonies like Obelia, Tubularia, Campanularia
etc.
 They have only two types of individuals- Gastrozooids or hydranths and
gonozooids or blastostyles
 Gastrozooids are concerned with feeding and gonozooids concerned about
sexual reproduction.
Trimorphic:
 Some colonies such as Plumularia which possess three types of individuals
are called trimorphic
 The three types of individuals- Gastrozooids, gonozooids and dactylozooids.
 Dactylozooids are functionally non-feeding and defensive polyps bearing
batteries of nematocysts

Polymorphic:
 Coelenterates having more than three types of individuals are
called polymorphic.
 The colony of Hydractinia has five types of polyps which perform a
specialized function.
 The five types of individuals-(i) Gastrozooids for feeding, (ii) spiral
dactylozooids for protection, (iii) long sensory tentaculozooids with
sensory cells, (iv) skeletozooids as spiny projections of chitin and (v)
gonozooids or reproductive individuals, bearing male or female
gonophores or medusae for sexual reproduction.

Physical structure
Aurelia : A Jelly-fish:
Shape: Umbrella-shaped body
- four red or purple horseshoe-shaped gonads on its upper surface of body
- Four long and narrow oral lobes hanging downwards from lower surface
- Its circular body presents a convex aboral or exumbrellar surface and a concave oral or
subumbrellar surface.
1) Manubrium, mouth and oral arms
2) Nematocysts
3) Subgenital pits
4) Gonads

5) Lappets and rhopalium:
-The circular margin of umbrella or bell is broken into 8 lobes or notches
-In each notch, there are two delicate leaf-like processes, called the marginal
lappets.
- A small sensory organ lies between lappets, called the rhopalium or tentaculocyst
6) Marginal tentacles: It bears batteries of stinging cells or nematocysts.
7) Velarium

Nutrition
 Cnidarians are carnivores
 They eat small aquatic animals such as insect larvae, crustaceans (Cyclops,
Daphnia etc.), annelid worms, young fishes, tadpoles, nematodes etc.
 The gastrovascular cavity exists as 1 opening for food intake and the elimination
of waste
 There is no system of internal transport, gas exchange or excretion; all these
processes take place via diffusion

Stinging Organelles
 Cnidarians are predators which have tentacles
possessing batteries of special cells called
Cnidocytes or nettle cells.
 Prey capture is enhanced by use of specialized
stinging cells called cnidocytes located in the
outer epidermis.
 Each cnidocyte is armed with a stinging
structure called a nematocysts.
 The undischarged nematocyst is composed of
a long coiled thread
 When triggered to release, either by touch or
chemosensation, the nematocyst is released
from the cnidocyte and the coiled thread is
discharged.
 When triggered, these cells shoot out a
discharge thread which can entangle and/or
poison it's intended prey

Reproduction
Budding Regeneration

• Gametes develop in gastrodermis of gastric pouches; eggs and sperm are shed
through mouth
• Fertilized eggs develop into a planula larva; settles on substrate and develops into
a polyp - scyphistoma
• Scyphistoma produces a series of polyps by budding - strobila
• The polyps undergo differentiation and are released from the strobila as free
swimming ephyra
• Ephyra matures into an adult jellyfish

Class 1. Hydrozoa
(Gr., hydra, water+ zoon, animal)
• Most varied and derived of the cnidarian groups
• Freshwater or marine e.g. Hydra sp.
• Only polyps or both asexual polyps and sexual medusae
present
• Medusa with true velum
• Mesoglea noncellular
• Sex cells shed directly on outside
• Most have typical dimorphic life cycle
• Examples of polyp-only forms e.g. Hydra
• Examples of medusa-only forms e.g. Polycolpa

Class 2. Scyphozoa
• Typically thought of as jellyfish
• Exclusively marine
• Most have typical dimorphic life cycle
• Medusa stage is dominant. Polyp stage reduced or
absent
• Majority of life cycle spent in medusa form
• Medusa without distinct velum
• Mesoglea extensive, gelatinous with fibre and cells
• Sex cells released in digestive cavity
e.g. Aurelia

Lion’s Mane Jelly (Cyanea capillata)

Class 3. Anthozoa
 Exclusively marine
 All polyps, no medusae.
 Gastrovascular cavity subdivided by 8 or more septa or mesenteries
 Mesenteries with nematocysts and gastrodermal glands
 Mesoglea stout and cellular
 Includes sea anemones and corals
 Corals build calcium shells to protect themselves
 Have symbiotic relationship with algae.
 Can build extensive masses which can form land masses
 e.g. Metridium

Class Anthozoa – Typical Polyp Form

Coral
• Coral is actually an animal.
 Coral animals or corals are marine, mostly colonial, polypoid
cnidarians
 Most of the corals belong to the class Anthozoa and a few to the
class Hydrozoa of phylum cnidaria.
 Corals are sessile animals and are the builders of the reef!
 They are different from most other cnidarians because they have no
medusa stage in their life cycle.
 The body unit of the animal is a polyp. A single coral animal is
called a coral polyp.
 They typically live in a secreted skeleton of their own. Their
calcareous or horny skeleton is also commonly known as coral

Structure of coral polyp
 A typical coral polyp is a small organism about 10 mm long and 1 to 3 mm in
diameter.
 Solitary coral polyp is much larger –up to 25 cm in diameter.
 A basal disc is absent
 Basal region of polyp is surrounded by a calcareous exoskeleton.
 Over many generations, the colony thus creates a large skeleton that is
characteristic of the species.
 Central mouth opening or oral disc bears numerous tentacles in several
rows around an elongated, oval or circular mouth.
 Pharynx is short.
 Mesenteries are restricted to the upper part of the coelenteron.
 Mesenterial filaments contain only one glandular lobe bearing nematocysts
 Living polyps are found only on surface layers of coral masses

Structure of coral skeleton
 Each polyp has a cup-like shape
 There is no skeleton inside the polyp itself. Instead, the polyps sit on top of an
external skeleton that is made from the polyp's secretions.
 The skeletons of corals are secreted by the lower portion of the polyp.
 The cup shaped calcareous exoskeleton of coral is known as corallite which
is sereted by the epidermis of an individual stony coral polyp
 composed of aragonite, a crystalline form of calcium carbonate
 In colonial coral, corallites of individual polyps fuse together to form a skeletal
mass, called corallum
 concave depression that houses the polyp or The inner surface of the
corallite is known as the calyx.

 The walls surrounding the corallite are the theca
 The skeletal plates that radiate into the calyx from the wall
called septum (sometimes called scleroseptum) and these
plates extend outside the corallite wall called costae
 Inner ends of scleroseptum are fused to form an irregular
central skeletal mass or columella.
 In colonial coral, the skeletal material between walls of
adjacent corallites is called coenosteum and the layer of
living tissue that covers the coenosteum or skeleton of the
coral is called coenosarc

 As long as the colony is alive, a polyp will lift off its base and
secrete a new floor to its cup by depositing calcium carbonate,
forming a new basal plate above the old one.
 Over time, a series of floors builds up below the living polyps,
resulting in a thickening and lateral expansion of the coral.
 When polyps are physically stressed, they contract into the calyx
so that virtually no part is exposed above the skeletal platform.
 In colonial species, when the corallites each have a surrounding
wall, the colony is said to be plocoid. When the walls are tall and
tubular, the colony is phaceloid, and when several polyps share a
common wall, the colony is cerioid. Sometimes the polyps are in
valleys on the surface of solid corals, they are then known as
meandroid

Polyps Outside!Polyps Inside
Tiny, little animals!

Feeding
 Corals feed on a variety of small organisms, from microscopic zooplankton to small fish by using
stinging cells of tentacles
 The polyp's tentacles catch or immobilize or kill prey using their nematocysts.
 They can scavenge drifting organic molecules and dissolved organic molecules
 Most corals only extend their polyps and tentacles at night when zooplankton is most abundant,
but some corals (especially soft corals) keep their polyps open throughout the day.
 most corals obtain the majority of their energy and nutrients from the symbiotic relationship with
photosynthetic unicellular dinoflagellates that live within their tissues. These are commonly known
as zooxanthellae and the corals that contain them are zooxanthellate corals. e.g. Symbiodinium
 The algae produces energy-rich sugars and fats via photosynthesis and provides energy to the
corals. This helps the coral grow and produce its skeleton faster than a coral without the
zooxanthellae.
 In return, the algae have a safe place to live within the coral tissue and the algae uses the coral's
carbon dioxide and waste nutrients for growth.
 Such corals require sunlight and grow in clear, shallow water, typically at depths shallower than
60 metres (200 ft).
 The zooxanthellae also gives the coral its color.
 Other corals do not rely on zooxanthellae and can live in much deeper water, surviving as deep
as 3,000 metres (9,800 ft). e.g. Lophelia

Class Anthozoa – Hard Coral
• They are called “hard" or “stony” because they have rigid
calcium carbonate skeletons
• They are also members of the Subclass Hexacorallia and
order Scleractinia
• The polyps produce a skeleton composed of calcium
carbonate to strengthen and protect the organism which
are deposited by the polyps and by the coenosarc, the
living tissue that connects them.
• composed of aragonite, a crystalline form of calcium
carbonate
• stony corals relying on their hard skeleton and cnidocytes
for defence against predators
• The polyps of stony corals have six-fold symmetry
• The tentacles are cylindrical and taper to a point
e.g.: Pocillopora damicornis, Pocillopora elegans, Favia bestae

Class Anthozoa – soft corals
• They are called "soft" because they do
not consist of rigid calcium carbonate
skeletons
• They are also members of the Subclass
Octocorallia and order Alcyonacea,
known as the Octocorals.
• there is no stony skeleton but the
tissues are often toughened by the
presence of tiny skeletal elements
known as sclerites, which are made
from calcium carbonate.
• Composed either of a fibrous protein
called gorgonin or of a calcified material
• The polyps of soft corals have eight-fold
symmetry i.e. polyps with eight tentacles
and eight mesentaries.
• The polyps of soft corals have eight
tentacles

 Form complex tube-like skeletal structures
 soft corals generally relying on chemical defences in the form of toxic
substances present in the tissues known as terpenoids
 The polyp tentacles of soft corals have numerous side-branches, or
pinnules, which give the polyps a feathery look
 most soft corals thrive in nutrient-rich waters with less intense light
e.g. Dendronephthya klunzingeri, Alcyonium sp., Clavularia sp., Lemnalia
sp.

 Stony corals fall into two main ecological groups
 Reef-forming or hermatypic corals, which mostly contain zooxanthellae;
 Non-reef-forming or ahermatypic corals, which mostly do not contain zooxanthellae
 Hermatypic corals are mostly colonial corals which tend to live in clear, oligotrophic, shallow
tropical waters;
 form large colonies
 they are the world's primary reef-builders.
 Ahermatypic corals are either colonial or solitary and are found in all regions of the ocean and
do not build reefs.
 are solitary or form small colonies
 Some live in tropical waters but some inhabit temperate seas, polar waters, or live at great
depths, from the photic zone down to about 6,000 m (20,000 ft)
 They thrive at much colder temperatures and can live in total darkness, deriving their energy
from the capture of plankton and suspended organic particles.
 The growth rates of most species of non-zooxanthellate corals are significantly slower than
ooxanthellate corals
 the typical structure for these corals is less calcified and more susceptible to mechanical
damage than that of zooxanthellate corals
 Examples of ahermatypic corals are soft corals, black corals, gorgonians, precious corals

Soft Corals – Class Anthozoa

Reproduction - Sexual
• Corals can be both gonochoristic (unisexual) and hermaphroditic
• Spawning techniques
 Broadcast Spawners
 Brooders
Broadcast Spawners
 About 75% of all corals "broadcast spawn" by releasing gametes—eggs and
sperm—into the water to spread offspring.
 The gametes fuse during fertilization to form a microscopic larva called a planula,
typically pink and elliptical in shape.
 Planula larvae disperse and settle on hard substrate in clear shallow water and
begin producing a tiny calcium skeleton.
 A typical coral colony forms several thousand larvae per year to overcome the
odds against formation of a new colony.
 Generally all corals spawn on the same night.
 Corals rely on environmental cues such as temperature change, lunar cycle, day
length, and possibly chemical signaling to determine the proper time to release
gametes into the water.

Brooders:
 Brooding species are most often ahermatypic corals (not reef-building)
in areas of high current or wave action.
 Brooders release only sperm, which is negatively buoyant, sinking on
to the waiting egg carriers who harbor unfertilized eggs for weeks.
 After fertilization, the corals release planula that are ready to settle

Reproduction - Asexual
• Two main ways:
– Budding (Extratentacular budding)
• The polyps bud off a from the side creating a new polyp
– Fission (Intratentacular budding)
• Polyps split into half through the tentacles creating the new polyp
• New colonies can also form by fragmentation
– Broken pieces of a colony fall off and reattach themselves to the substrate
• Advantage is that the new colony is more likely to succeed than Planular Larvae
• Disadvantage is that the new colony is cloned and therefore susceptible to the
same diseases as the parent colony

Coral reef
 Coral colonies grow continuously in size by
budding of polyps and often form extensive
masses known as coral reefs
 Principle builders of coral reefs are stony
corals and coralline algae and Foraminiferan
protozoa also take part in the formation.
 Coral reef are also called as ‘Rainforests of the
Sea’

Zooxanthellae
Coral polyp
Coral colony
Corals
Coral
Reef
Zooxanthellae
Coral polyp
Coral colony
Corals
Coral
Reef

Kinds of coral reefs
• Fringing reefs:
 This coral reefs are lying close to the shores of some volcanic island
or part of some continent are termed fringing reefs
 A shallow water channel-50 to 100 m
 Between the reef edge and shore
 Composed of coral sand, mud, dead and living coral colonies and
other animals
• Barrier reefs:
 They are located some distance away from the shore
 Lagoon separate the reef from the island
 Lagoon is 10 to 50 fathoms deep and Suitable for navigation
• Atoll:
 Also termed as coral island or lagoon island
 Ring-like or horse-shoe shaped reefs that surrounds a laggon
 May be complete or broken by a number of channels of which only a
few are navigable

Why are coral reefs important?
• Habitat: They are home to
33% of all known fish species.
•Nursery: And a nursery ground
for over 25% of all marine
species.
Photo by J. RandallPhoto by Dee Wescott
Photo by MacGillivray Freeman Films
• Habitat: They are home to
33% of all known fish species.
•Nursery: And a nursery ground
for over 25% of all marine
species.

Reefs are home to a large variety of organisms, including
fish, seabirds, sponges, cnidarians (which includes some types of corals
and jellyfish), worms, crustaceans (including shrimp, cleaner shrimp,
spinylobsters and crabs), mollusks (including
cephalopods), echinoderms (including starfish, sea urchins and sea
cucumbers), sea squirts, sea turtles and sea snakes.

Fish– Some fish feed on small animals living near the coral or on the
coral itself; other fish, including some sharks cruise the perimeter of the
coral reef.
Algae– Though a vital part of reef life, overfishing and excess
nutrients from onshore can lead to algae encroachment, where
algae can outcompete and kill the coral
Organisms that are part of Reefs include:
Life on a Reef
1
2

Seabirds– Coral Reef systems provide habitats for seabird species, many
of which are endangered. The short-tailed albatross has only 2200
surviving species
Cnidarians– organisms like jellyfish with specialized cells called
cnidocytes. Cnidocytes are used used mainly for capturing prey which
ranges from the size of plankton to animals larger than themselves.
:
Life on a Reef, Continued
Item
3
Item
4
5
Other inhabitants–Sea snakes and land based reptiles
(crocodiles, lizards) feed on fish and their eggs.

Why are coral reefs important?
• Food: they are a food source
for millions of people.
• Tourism: coral reefs attract
tourists from all over the
world.
•Income: they provide millions of
dollars of income annually for
people living by coral reefs.
•Medical Research: coral reefs have
the potential to be used as medical
cures to treat cancer, heart disease,
HIV and arthritis among others.
• Protection: they protect 20% of
the world’s coast from wave erosion.
• Food: they are a food source
for millions of people.
• Tourism: coral reefs attract
tourists from all over the
world.

Why do we have to care about Corals?
• Ecological value: corals sustain rich marine biodiversity.
(ex. Shelter for some animals, food for other animals)
350 million coastal people rely directly on coral reefs for
their food and survival.
• Economical value: tourism, fishery industry
(ex. Divers, tourists, and food supply
Over 20 million scuba divers visit coral reefs each year.
• Environmental value: they provide protection for us.
(ex. Breaking storm wave, tsunami, typhoon, erosion
and flooding)

Environmental requirements
• Physical environment
– Temperature of 25-31oC
– Salinity of 34-37 ppt
– Sunlight
– High Water Transparency
– Good water circulation
• Biological environment
– Oligotrophic, highly stratified water column

Threats to coral reef systems
• There are two types of threats to coral
reefs, anthropogenic and natural
• Overpopulation
• Unsustainable fisheries
• Destructive and non-sustainable fishery
practices
• Coastal development
• Global climate change
• Coral bleaching – socio economic
impacts, reef based tourism and
fisheries
• Coral mining – construction, lime
industry, ornamental purposes
• Pollution – agriculture, coastal
development
• Sedimentation - deforestation

Threats from Nature
• Unusually strong waves such as those from a
hurricane
• Water temperature changes
• Dramatic changes in saltiness of water
• Predators, such as snails and crown of thorns
starfish
• Overgrowth of algae

Crown of Thorns
• The crown-of-thorns sea star, Acanthaster planci, is a
large, multiple-armed starfish (or seastar) that usually
preys upon hard, or stony, coral polyps (Scleractinia).
• The crown-of-thorns sea star receives its name from
venomous thorn-like spines that cover its upper surface,
resembling the Biblical crown of thorns. It is one of the
largest sea stars in the world.
• Eats coral
• Aggregations can remove 95% of coral cover
• May result in collapse of remaining skeleton
• Pheromone controlled aggregated spawning
• Recovery takes at least 12 years

What are the man-made threats to
coral reefs?
An example of coral bleaching.
• Global warming leading
to coral bleaching.
• Runoff of chemicals and
nutrients from landla
• Rubbish including marine
debris
• Overfishing
• Physical damage from
tourists and fishermen
• Pollution from untreated
sewage and oil.
• Sedimentation
• Ocean acidification

Threats from Humans
• Pollution
– Sediments block light from zooxanthellae
– Chemicals either poison corals or allow too much
algae to grow
• Power plants
– Filter water and kill fish and plankton
– Releasing hot water kills organisms

More Threats from Humans
• Deforestation
– Causes erosion which clouds the water
– Burning of trees could be a factor in climate change
• Destructive fishing
– Blasting with dynamite
– Cyanide poison
– Boats running aground, anchors
– Overfishing

• The main threat to coral reefs from climate warming is coral bleaching,
however there are other threats from climate warming such as ocean
acidification and rising sea levels.
• Pollution from land activities such as farming and land development increases
the nutrient concentration in the water. Elevated nutrient concentrations
result in a range of impacts on coral communities, and under extreme
situations can result in coral reef community collapse. Elevated nutrient
concentrations affect corals by promoting phytoplankton growth, which in turn
supports increased numbers of filter feeding organisms such as tubeworms,
sponges and bivalves that compete with coral for space. Macroalgal blooms
can also result under enhanced nutrient regimes and macroalgae may
overgrow coral structures, out-competing coral for space and shading coral
colonies to critical levels.
• Sedimentation causes problems for the coral by decreasing available light and
smothering the coral and other surfaces.
• Over fishing removes keystone species from the coral reef ecosystem upsetting
the delicate balance.
• Unsustainable tourism leads to increasing pollution and damage to the coral.

1) Use of gears and mesh sizes not sanctioned by
government
2) Use of gears and mesh sizes not sanctioned
within the fisherfolk community…
3) Use of gears that destroy the resource base
4) Use of gears such as dynamite or sodium cyanide
that do all of the above and even endanger the
fisherfolks themselves”

Climate change
• Potential impacts on coral communities
– Changes in water temperature
– Increases in CO2 concentration
– Changes in solar irradiation (if cloud cover
changes)
– Sea level rises leading to drowning of reefs
– Changes in surface run-off (sedimentation)
– Changes in land-use patterns leading to increased
reef exploitation

What is Coral Bleaching?
• Coral Bleaching = Corals which
lost their symbiotic algae appear
whitish.
• Corals are compelled to expel the
algae because of its toxin when
they are under stressful condition.
• Unusual high water temperature
is thought as the main cause of
the mass bleaching event in
1997~98.
• Some species can survive
bleaching but the aftereffect
includes slower growth, fragile
body and higher risk of disease.

Coral Bleaching
• First described in 1984
• Multiple re-occurrences at same sites
• New sites impacted during 1990s
• Many known triggers
– Temperature (especially increases)
– Solar radiation (especially UV)
– Combination of UV and temperature
– Reduced salinity
– Infections

Effects of bleaching
• Loss of symbiontic algae (Zooxanthellae) algae by:
– Degradation In situ
– Loss of algae by exocytosis
– Expulsion of intact endodermal cells containing algae
• Resulting impacts
– Vary between species, and even parts of the same colony
– Loss of sensitive species (especially Acropora spp.)
– Recovery slow and highly variable between sites

The Problems
• A large (and growing) number of people
are dependent on coral reefs
• Management of a multispecies fishery is
extremely complex, and often fails
• Terrestrial development may destroy
coastal reef systems
• Global climate change may exert new
pressures

How can we protect Corals from
bleaching?
• Not to touch corals physically.
• Stop destructive actions such as dynamite fishing,
over coastal development causing sedimentation.
• Have an interest on coral reef and take actions to
spread knowledge.
• Proper instruction for any people trying to play
around coral reefs.

Reef Survival
Future Solutions:

Marine Protected Areas (MPA’s)
• Marine Protected Areas describe
areas or regions which have been
placed under some restrictions in the
interested of protecting the
environment.
•MPA’s do not necessarily restrict all
human activity, but instead place
limitations on what activities are
allowed, for example fishing, fishing
seasons, catch limits, etc.
•MPA’s include a variety of reefs, but
may not be a sustainable solution for
the future.

• Integrating coastal zone management
• Effective environmental laws
• Educating policy makers and the public about
how reef habitats should be maintained
• Promoting environmentally sound practices
for land use
• Minimizing illegal fishing and developing
sustainable fisheries
• Developing disaster strategies

Here is a good example of a live coral.

Reefs grow when calcium
containing sediments are
deposited in spaces between coral.
As encrusting coraline algae
“glues” the sediments together,
new “live rock” is formed.

Once this Halimeda(calcareous green algae) dies, 95% of what
remains will be sediment and real estate for new coral
colonies.
Coral benefits from the death of other organisms.

To this….
Temperature increases and competition from red algae have killed
much of this coral reef.

Disease is another major factor limiting growth. Here is
an example of elkhorn coral (Acropora palmata) infested
by “white band disease.”

Food Webs: Same concept, more complexity

It’s up to Us to Save Coral Reefs
Be Responsible.

Cnidaria phylum

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