Cnidaria
Cnidaria


The phylum Cnidaria includes over 9,000
species of aquatic, radially symmetrical
animals which have specialized stinging
organelles called nematocysts.
They include the jellyfish, box jellyfish, sea
anemones, fire corals, sea pens and hard
corals.
Cnidaria


Cnidarians are diploblastic having only two
well-defined germ layers (ectoderm and
endoderm).
Cnidarians are the simplest animals
equipped with nerve cells which are
arranged into a nerve net, but there is no
central nervous system.
Nerve net



The nerve net is a diffuse nervous system.
Nerve impulses are transmitted between cells by
the release of neurotransmitters from vesicles,
which carry the signal across the synapse (gap)
between cells.
Unlike “higher animals” impulses can travel in
both directions along a nerve because many
synapses have vesicles on both sides.
Nerve net


In cnidarians there is no brain, but in some
medusae there are multiple nerve nets.
For example, in Scyphozoan jellyfish there is a
fast-conducting nerve net for coordinating
swimming movements and a slower net to
coordinate movements of tentacles.
Cnidaria: digestion


Cnidarians have an internal body cavity,
the gastrovascular cavity, but no one-way
gut. Food enters and waste exits through
the same opening, the oral cavity.
Digestion takes place extracellularly within
the gastrovascular cavity.
Cnidaria: Body wall


The body wall that surrounds the
gastrovascular cavity has an outer
epidermis and an inner gastrodermis.
In between these two layers is a
gelatinous layer of mesoglea, which may
contain elastic fibers or be stiffened with
spicules or flexible proteins.
Figure 13.02
Fig. 7.2
Body forms

Cnidaria have one of two basic body forms



Polyp
Medusa
In some groups one or other body form is
used exclusively, but in others the two
forms are used in a single life cycle.
Polyp and medusa


The polyp or hydroid form is adapted to a
sessile existence and the medusa form to
a free-floating or pelagic life.
In both cases radial symmetry is favored
because stimuli and food are equally likely
to come from all directions.
Foraging strategy

Cnidarians are sit-and-wait predators
rather than pursuit predators. They wait
for prey to drift or blunder into their
tentacles, which in the case of jellyfish,
are almost invisible.
Polyp and medusa

Polyps and medusae may look quite
different, but are basically inverted
versions of each other.
Figure 13.02
Fig. 7.2
Polyp and medusa


Both polyps and medusae are equipped
with tentacles around the oral cavity.
The tentacles are equipped with
cnidocytes that contain stinging
nematocysts, which are used to kill prey.
Cnidocytes



Cnidocytes contain cnidae which are stinging
organelles (the most common of which is the
nematocyst).
The cnida is a small capsule made of chitin that
contains a coiled, often barbed, filament.
The capsule is covered by a lid, which has an
associated trigger mechanism.
Figure 13.03
Figure 7.3
Cnidocytes


When the trigger mechanism is tripped
the cnida is expelled at high velocity into
the prey.
In many cases a toxin is injected, but in
others, the cnida entangles or sticks to the
prey.
Cnidocytes

Nematocysts of most cnidarians are not
harmful to humans, but the stings of some
(e.g. Portugese Man-of-war) are painful or
even fatal (certain box jellyfish).
Figure 13.05
7.5
Movement of medusae


The medusal form of cnidarians is pelagic and
the medusa can move by rhythmically
contracting and pulsing its bell, which expels
water and drives the medusa upwards.
Most cnidarians are relatively weak swimmers,
but cubozoans (box jellyfish) swim strongly.
Youtube videos Mudusae in aquaria


http://www.youtube.com/watch?v=gSq72
gkTdH4&NR
http://www.youtube.com/watch?v=0ANt1l
LDtQw
What do jellyfish eat?


Jellyfish prey on anything small that their
tentacles can kill.
They eat the planktonic larvae of various
organisms, crustaceans, small fish, fish
eggs and other jellyfish.
What eats jellyfish?


Jellyfish are eaten by a variety of animals
including other jellyfish, tuna, sharks,
swordfish and turtles.
Turtles, which face numerous other
threats, frequently starve as a result of
eating plastic bags that they mistake for
jellyfish.
What eats jellyfish?


Populations of all predators of jellyfish
have decreased dramatically in the past
100 years as a result of overfishing and
mortality as a result of by-catch.
Many shark populations have declined by
>90% and some by as much as 99%.
Many turtle populations have been pushed
to the brink of extinction
Jellyfish populations


Jellyfish populations have increased
dramatically in many oceans.
Reduced predation has certainly played a
role in that increase.
Jellyfish populations


It has been speculated that jellyfish
numbers may have increased as a result
of overfishing of various fish stocks.
Because jellyfish consume the same food
as many adult and larval fish they may be
benefiting from the reduction in
competition.
Jellyfish populations


Jellyfish populations also benefit from the
fact that they are more tolerant of low
oxygen conditions and high nutrient ocean
conditions.
Anoxic (no/low oxygen) conditions and
dead zones as a result of fertilizer
pollution and associated plankton blooms
are common in the oceans today.
Jellyfish populations


As much as 1% of the oceans (especially
in relatively enclosed bodies of water) are
seasonal dead zones.
These occur when high levels of fertilizer
runoff lead to blooms of phytoplankton.
When these die their decomposition
removes oxygen from the water and this
kills a wide variety of organisms.
Jellyfish populations


Jellyfish, however, can thrive low oxygen
conditions.
In addition in high nutrient conditions with
lots of phytoplankton smaller harder-tosee zooplankton become more common
which favors jellyfish over fish predators
because jellyfish hunt by touch not sight.
Jellyfish populations


Because they grow quickly jellyfish blooms
have become widespread.
Such blooms have led to the fish farms
being wiped out by jellyfish, widespread
beach closures, and the collapse of
fisheries in the Black Sea.
Jellyfish populations


There is considerable concern that
increasing jellyfish populations may result
in permanently changed marine
ecosystems.
With populations of their predators
reduced, larger jellyfish populations
because they consume so many larval fish
and eggs, may suppress the recovery of
fish stocks permanently.
Polyps



Most polyps have tubular bodies and there is a
mouth surrounded by tentacles.
The tentacles capture prey which is then
transferred to the gastrovascular cavity and
digested there.
Polyps are sessile and are attached to the
substratum by a pedal disk.
Youtube Anemones

http://www.youtube.com/watch?v=O7_Is
X-WZoc
Life Cycles


In cnidarian life cycles polyps and
medusae play different roles. Life cycles
differ among groups, but usually a zygote
develops into a planula larva, which is
free-swimming.
This larva settles and develops into a
polyp.
Life Cycles



The polyp may reproduce asexually and
generate other polyps.
Or as in the Hydrozoa (hydras) and Scyphozoa
(jellyfish) polyps bud off medusae.
These medusae are generated asexually, but
each medusa is either male or female and
produces gametes, which are shed into the
water and produce zygotes beginning the life
cycle again.
Figure 13.09
7.9
Life cycle of Obelia, a marine hydroid.
Life Cycles


In the Anthozoa (sea anemones and
corals) there is no medusa stage and all
individuals are polyps.
Both asexual and sexual reproduction take
place in Anthozoa. Gametes are
produced, but new individuals can be
budded off too.
Classes of Cnidaria

There are 4 classes of Cnidarians




Class Hydrozoa: Hydras, colonial hydrozoans
inc. Portuguese man-of-war.
Class Scyphozoa: most of the larger jellyfish
Class Anthozoa: Sea anemones, hard corals,
sea fans and sea pens
Class Cubozoa: box jellyfish. Small group once
considered an order of Scyphozoa
Hydrozoa


Most Hydrozoa are marine and colonial,
but Hydra a freshwater hydrozoan is
common in the U.S. and often seen in
biology classes.
Hydra is only about one inch long and has
typical polyp form. It has no medusoid
stage.
Pink-hearted hydroid (Hydrozoa)
Porpita (a colonial
Hydrozoan)
Hydra (Hydrozoa)
Colonial Hydrozoa


Individual polyps in colonial forms are
referred to as zooids and specialize in
particular tasks.
The commonest are feeding polyps:
gastrozooids. These capture and partially
digest prey before emptying the food into
the common gastrovascular cavity.
Colonial Hydrozoa


In colonial Hydrozoa the epidermis, mesoglea,
and gastrodermis are all continuous making it
difficult to tell where one individual ends and the
next begins.
Most colonial Hydrozoa are also surrounded by a
non-living supportive protein-chitin envelope
secreted by the epidermis and called the
perisarc.
Figure 13.09
7.9
Life cycle of Obelia, a marine hydroid.
Colonial Hydrozoa



Reproductive polyps (gonangia) produce medusae that
leave the colony and produce gametes.
These medusae are usually quite small (no bigger than a
few cm). Unlike scypohozoan medusae the edge of the
bell projects inwards forming a lip or shelf called a
velum, which reduces the size of the opening of the bell.
Muscular contractions and relaxations alternately fill and
empty the bell moving the animal by a form of jet
propulsion.
Colonial Hydrozoa: Physalia


Some hydrozoans known as
siphonophores (including the well-known
Physalia the Portuguese man-of-war) form
floating colonies.
The colony includes several forms of
modified medusae and polyps.
Colonial Hydrozoa: Physalia


Physalia has a large float (a modified
polyp) which is filled with carbon dioxide
and this acts as a sail.
There are multiple different polyps that
hang beneath the float including feeding
polyps, reproductive polyps and long
stinging tentacles.
Figure 13.14
Physalia Portugese Man-of-War
Fig 7.12
Scyphozoa



The class Scyphozoa includes most of the large
jellyfish and the medusa is the dominant life
stage. They are entirely marine.
Most are 2-40 cm in diameter, but a few species
(including Cyanea next slide) may be 2 meters in
diameter with 60 meter tentacles.
Scyphozoans bells differ from those of
Hydrozoans in that they do not have a velum.
Scyphozoa

Bells vary in shape from helmet-like to shallow
saucers.

Many bells have a scalloped edge and the
notches contain sense organs called rhopalia.

Rhopalia include statocysts that assist with
balance, other sensory cells that sense
chemicals and in some cases simple eyes (called
ocelli)
Figure 13.16
Giant
Jellyfish
Cyanea
capillata
Fig 7.14
Scyphozoa

Most Scyphozoans are pelagic, but in one
unusual order the medusa attaches to
seaweed.
Figure 13.19
Thaumatoscyphus hexaradiatus
sessile medusa
Scyphozoa




Scyphozoans have a typical medusa with a large bell and
long tentacles. The mesoglea is thick (hence the name
jellyfish).
They feed on all sorts of small animals from protozoa to fish,
which are stung and captured by the tentacles and
transferred to the gastrovascular cavity.
The gastrovascular cavity is complex in structure with 4
gastric pouches that connect with a series of radial canals
and join a ring canal that run around the outside of the bell.
The complex gastrovascular cavity allows nutrients to
circulate around the whole animal.
Figure 13.17
Moon
Jellyfish
Aurelia aurita
Gastric pouches
Radial canals
Ring canal
Scyphozoa



In Scyphozoa the sexes are separate (gonads are located in
the gastric pouches) and fertilization occurs inside the
gastric pouch .
Zygotes may be brooded or released into the water and
they develop into a ciliated planula larva.
The planula larva attaches to a substrate and develops in a
series of stages into a strobila, which buds a medusa-like
ephydra that grows into an adult medusa.
Figure 13.18
Aurelia (moon jellies;
Scyphozoa)
Lion’s Mane jellyfish
(Scyphozoa)
Cubozoa


The cubozoans are a small entirely marine group
in which the medusa is the dominant form
(polyps are either inconspicuous or unknown)
The bell of cubozoans is almost square in section
(hence “cube” ozoans and box jellyfish), it is not
scalloped, and there is a flattened structure at
the base of each tentacle called a pedalium,
which facilitates identification
Box jellyfishes (Cubozoa)
Pedalium
Cubozoa

Cubozoans are very strong swimmers (like
Hydrozoans they possess a velum-like structure
called a velarium that enhances propulsion) and
they are very effective predators, mainly on fish.

They produce highly toxic venom and often have
very long tentacles.

Their rhopalia each contain 6 eyes as well as
other sense organs.
Chironex fleckeri (box jellyfish) has tentacles
that can be 10 feet in length More than 100
people have died from stings in the past century in
northern Australia.
Box jellyfish video

http://www.youtube.com/watch?v=uIf0kR
pkQ_0
Anthozoa


Anthozoa (“flower animals”) are polyps with a
flowerlike appearance. They are entirely marine
and there is no medusa stage.
The Anthozoa includes three groups:



Zoantharia: sea anemones, hard corals
Octocorallia: sea fans, sea pansies, sea pens, soft
corals
Ceriantipatharia: tube anemones and thorny corals. A
small group with few species.
Orange sea pen
(Octocorallia)
Sea anemones (Zoantharia)
Figure 13.21
Anthozoa: Zoantharia: sea
anemones and hard corals


Have a hexaramously symmetrical
bodyplan (based on multiples of 6). In
contrast octocorallians are based on
multiples of 8.
Tentacles are simple tubular structures in
contrast to octocorallians which have
featherlike tentacles.
Anthozoa: Zoantharia: sea
anemones and hard corals



Sea anemone polyps are much larger and
heavier than hydrozoan polyps.
Usually they are colorful and may be up to 4
inches in diameter.
Sea anemones are cylindrical with a crown of
tentacles arranged in one or more rings around
the mouth.
Figure 13.21
Fig 7.19
Sea anemones
Anthozoa: Zoantharia: sea
anemones and hard corals



Sea anemones are carnivorous and feed on fish
or any other suitably sized prey.
Sea anemones depending on the species may be
hermaphroditic or have separate sexes.
Zygote develops into a ciliated larva that settles
and becomes a polyp. Asexual reproduction by
fission also occurs.
Anthozoa: Zoantharia: sea
anemones and hard corals


Hard corals (or scleractinian corals) are
effectively miniature sea anemones that
live in calcareous cups that they
themselves secrete from their epidermis.
The cup is made of calcium carbonate
(CaCO3) and the coral can retreat into it
when threatened.
Figure 13.27
Hard coral polyp
Anthozoa: Zoantharia: sea
anemones and hard corals


When contracted most corals are very
difficult for fish or other predators to
extract.
The skeleton is secreted entirely below the
living tissue and so is an exoskeleton.
Coral polyps
Anthozoa: Zoantharia: sea
anemones and hard corals


In colonial corals the skeleton may
become massive over the years with living
coral occupying only a thin sheath of
tissue on the surface.
The gastrovascular cavities of polyps are
all connected through this tissue.
Anthozoa: Zoantharia: sea
anemones and hard corals


The patterns formed in coral rock are caused by
the growth patterns of the coral and the
arrangement of polyps.
For example, in brain corals, the polyps are
arranged in rows. The rows are well separated,
but the polyps that make up each row are very
close together and their cups merge. As a
result, the skeleton of the colony looks like a
human brain with valleys separated by ridges.
Brain coral (Anthozoa)
Anthozoa: Octocorallia



Octocorals all have 8 pinnate (feathery) tentacles and include
soft corals, sea pens, sea fans, and gorgonians.
They are colonies of polyps connected to each other by a
coenchyme (consisting of mesoglea, spicules and connecting
tubes) and forming elaborate branching structures.
The gastrovascular cavities of polyps in colonies are connected
by tubes called solenia. The solenia run through a gelatinous
mesoglea, which is enclosed by epidermis.
Figure 13.31
Polyps of an
octocorallian coral
7.26
Anthozoa: Octocorallia



Octocorals have an endoskeleton in which
stiffening elements are secreted into the
mesoglea.
In some there is a central supporting rod
of protein (gorgonin) or calacareous
spicules that runs through the coenchyme.
(Coral jewelry is made from such rods
taken from the red coral Corallium.)
Anthozoa: Octocorallia

The coupling of spicules (sometimes fused
together) with the stiff, but flexible protein
gorgonin (similar to keratin) provides
enough structural support for large fanlike or branched colonies of octocorals to
develop.
Figure 13.33b
Sea fan (gorgonian)
Figure 13.33a
Red gorgonian
Soft corals


Some octocorallians lack the central
supporting rods and are very soft.
These soft corals have fleshy bodies that
contain calcareous spicules in the
mesoglea.
Figure 13.32
7.27
Soft coral (Octocorallia)
Anthozoa: Octocorallia


Octocoral colonies may be in the form of
mats or ribbons, feather-like or clusters of
vertical branches.
Often they are brightly colored: red,
orange, yellow or purple.
Figure 13.22a
Orange
sea pen
(Octocorallia)
Fig 7.20
Coral Reefs



Coral reefs are found in shallow waters in the
tropics.
They are calcareous structures and what makes
them unique as geological structures is that they
are formed by some of the organisms that live
on them, specifically reef-building corals and
coralline algae.
They are the largest living structures on the
planet.
Coral Reefs


Reef-building corals contain symbiotic algae
(zooxanthellae) that supply a significant part of
the coral’s energy in exchange for protection
and access to light.
These algae require light for photosynthesis and
so reef-building corals can live only in clear
waters less than 100m deep (and most species
occur in much shallower waters).
Coral Reefs

Many other cnidarians on reefs including
octocorallians, sea anemones and
hydrozoan corals also have zooxanthellae
and are similarly restricted in their vertical
distribution.
Coral Reefs


In addition to light, reef building corals
require warm water where the average
minimum temperature is at least 20ºC.
As a result of their narrow tolerances,
coral reefs are found only in waters
between 30º N and 30º S of the equator.
Coral Reefs

Coral reefs are restricted to the Caribbean,
Indian Ocean and tropical Pacific.

Because of their narrow tolerances coral reefs
are absent from much of the Atlantic.

Water tubidity, because of the sediment carried
by large rivers limits corals along the east coast
of South America and west coast of Africa (cold
water currents also restrict corals off Africa).
Types of Coral Reefs

Three general types of coral reef can be
recognized.



Fringing reefs: are the commonest type and project
into the sea directly from the shore.
Barrier reefs: are separated from adjacent land by a
lagoon. Great Barrier Reef is the longest at >1000
miles on NE coast of Australia.
Atolls: rest on summits of submerged volcanoes.
Usually circular/oval with a central lagoon. Parts of
the reef platform may emerge as islands.
Shumann Island, Papua New Guinea (fringing reef).
Pohnpei Atoll Micronesia
Coral Reefs



A reef platform (the layer of coral rock) may
extend considerably below the current photic
zone (reefs more than 1000m deep are known).
How is this possible?
Growth of reef platforms occurred as a result of
changes in sea level or subsidence of the
substratum.
Coral Reefs


In the late Pleistocene sea levels were
about 120m below current levels when
lots of water was locked up in ice sheets.
As the Earth warmed between 18,000 and
7,000 years ago sea level increased by
about 1cm/year (very fast). Corals simply
grew upwards as the sea levels rose.
Coral Reefs


Reef platforms of great thickness are due
to subsidence. Most atolls sit over
volcanic seamounts that have subsided
and as they have sunk corals have kept up
with rate of subsidence.
Charles Darwin was the first to figure this
out.
Coral Reef Zonation



Coral reefs show considerable zonation depending on
exposure to wave action.
The seaward side of reef rises from the depths to just
below surface and may be gently or steeply sloped.
Large domed or columnar corals occur between 10 and
60m depth.
Usually the reef front is not a smooth wall but rather a
series of finger-like projections. This pattern disperses
wave energy.
Coral Reef Zonation


The reef crest is where the reef
approaches closest to the surface. There
is high wave stress here and corals such
as elkhorn corals predominate.
Behind the reef crest is a reef flat which is
quite protected and contains smaller,
delicate branching corals.
Figure 13.34
13.34
Coral Reef Diversity


The waters in which corals are found are
nutrient poor. The clear, blue color of the
water is a tip-off to this.
Productive waters have a lot of
phytoplankton. As a result, they are
usually green and light is absorbed
quickly.
Coral Reef Diversity


Despite being in nutrient poor water, coral
reef ecosystems are some of the most
productive marine environments.
This is because the populations of algae
and symbiotic zooxanthellae carry out a
huge amount of photosynthesis and so
form the basis for an extensive food web.
Coral Reef Diversity


Besides cnidarians large numbers of
sponges, molluscs, clams, tunicates, and
bryozoans live on the reef.
In addition, sponges, clams, and some
worms bore into exposed coral.
Coral Reef Diversity


The huge numbers of holes and crevices
offer shelter to shrimps, crabs, worms,
molluscs, fish and other animals.
All of these smaller animals attract large
number of predators including fish, turtles,
and sharks.
nudibranch
Puffer fish
Blenny
Green Sea turtle
Coral reef, Indonesia
Schooling jack fish




Nature Conservancy Video clips
http://www.nature.org/joinanddonate/resc
uereef/explore/video.html
Kimbe Bay
Belize fish spawning
Threats to coral reefs



There are numerous threats to coral reefs.
These include nutrient enrichment from
sewage and agricultural runoff and overfishing
of herbivorous fish, which result in heavy algal
growth.
In addition, sediment resulting from
deforestation reduces water clarity and covers
corals.
Threats to coral reefs


Global warming from increased levels of carbon
dioxide also threatens reefs because when water
becomes too warm corals expel their
zooxanthellae (coral bleaching) and die.
In addition, higher carbon dioxide levels in the
atmosphere are lowering marine pH levels
making the water more acidic. This makes it
harder for corals to produce calcium carbonate
and may also dissolve corals.



Nature Conservancy Video clips
http://www.nature.org/joinanddonate/resc
uereef/explore/video.html
Palau
Youtube corals reefs


Truk Lagoon 2:30
http://www.youtube.com/watch?v=ju6QxK6FJM&mode=related&search=
Coral reef,
Komodo Island
National Park,
Indonesia
Phylum Ctenophora



The phylum Ctenophora is a small phylum of fewer than
100 species of comb jellies all of which are marine.
Comb jellies are named for the 8 rows of short, comb-like
plates of long cilia they beat in order to move. Most
ctenophores are free swimming.
Beating of the cilia in each row begins at the aboral end
and all plates beat in synchrony to move the ctenophore.
An organ called the apical sense organ coordinates
beating of the comb rows.
Figure 13.35a
7.28
7.30
Figure 13.35b
Phylum Ctenophora


Ctenophores are quite similar to cnidarians in many ways: but there are a
number of differences:
Similarities: both have:






nerve net
diploblastic with thick gelatinous mesoglea
Pelagic, transparent floating predators, slow moving
Single oral cavity
Tentacles solid not hollow
Differences:





Biradially symmetrical rather than radially symmetrical.
Ctenophores lack nematocysts, have colloblasts
Ctenophore cells are multiciliated
Cnidaria swim by jet propulsion, ctenophores by beating of combs
Like cnidarians ctenohores have no anus but possess anal pores, small
openings to the outside from the gastrovascular cavity.
Figure 13.36a
Phylum Ctenophora
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Many ctenophores possess two long tentacles that are
covered with adhesive cells called colloblasts, not
nematocysts as in the cnidarians. However, one species
of ctenophore does carry nematocysts, which it appears
to obtain from cnidarians it eats.
Unlike cnidarian tentacles, those of ctenophores can be
retracted into pits or sheaths.
In ctenophores without long tentacles the body is
covered with colloblasts and the whole surface is used to
trap prey. Small tentacles transfer prey to the mouth.
http://www.marlin.ac.uk/images/taxonomy_descriptions/Ctenophora.jpg
Phylum Ctenophora
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Ctenophores can be major predators of larval
fish and other zooplankton such as crustaceans.
The introduction of Mnemiosis leidyi an invasive
species of comb jelly into the Black Sea about 25
years ago caused the collapse of the local
anchovy fishery.
The comb jellies consumed fish eggs and larvae
as well as competed with fish for zoo plankton
Figure 13.38
Ctenophore video clips
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http://www.oceanfootage.com/stockfoota
ge/Comb_Jelly_Footage/owner%3Dekovac
s//?DVfSESSCKIE=8cdc73b166081cdf4f03
3fe0c520befaa7ddb171