Cnidaria

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Cnidaria
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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.

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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.

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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.

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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.

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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.

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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.

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Epidermis

• The epidermis contains a variety of different
  cells including mucus secreting gland cells,
  cnidocytes (stinging cells), sensory, nerve and
  epitheliomuscular cells.
• Epitheliomuscular cells contain myofibrils and
  together these form a layer of longitudinal
  muscle that can be contracted to shorten the body
  or tentacles..

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Gastrodermis

• The gastrodermis consists mainly of
  nutritive-muscular cells that contain many food
  vacuoles for digestion and also myofibrils that
  connect top form a circular muscle layer.
• There are also gland cells that secrete digestive
  enzymes.

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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.

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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.

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Polyp and medusa

• Polyps and medusae may look quite different, but
  are basically inverted versions of each other.

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Fig. 7.2
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Polyp and medusa

• Both polyps and medusa are equipped with
  tentacles around the oral cavity.
• The tentacles are equipped with cnidocytes that
  contain stinging nematocysts, which are used to
  kill prey.

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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.

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Figure 7.3
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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.

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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).

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7.5
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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.

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Youtube videos Mudusae in aquaria

• http//www.youtube.com/watch?vgSq72gkTdH4NR
• http//www.youtube.com/watch?v0ANt1lLDtQw

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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.

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Youtube Anemones

• http//www.youtube.com/watch?vO7_IsX-WZoc

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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.

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Life Cycles

• The polyp may reproduce asexually and generate
  other polyps or as in the Hydrozoa (hydras) and
  Scyphozoa (jellyfish) produce 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.

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7.9
Life cycle of Obelia, a marine hydroid.
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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.

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Classes of Cnidaria

• There are 4 classes of Cnidarians
• Class Hydrozoa Hydras, colonial hydrozoans inc.
  Portugese 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

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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.

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Pink-hearted hydroid (Hydrozoa)
Porpita (a colonial Hydrozoan)
Hydra (Hydrozoa)
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Colonial Hydrozoa

• Most hydrozoans are colonial.
• In asexual reproduction of solitary polyps a bud
  develops, forms a mouth and tentacles, and then
  drops off from its parent to grow into a new
  individual.
• In colonial forms, however, the bud remains
  attached and develops into another polyp.
  Further budding eventually leads to the
  development of a colony of connected individuals.

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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.

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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.

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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).

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Colonial Hydrozoa Physalia

• Some hydrozoans known as siphonophores (including
  the well-known Physalia the Portugese man-of-war
  form floating colonies.
• The colony includes several forms of modified
  medusae and polyps.

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Colonial Hydrozoa Physalia

• Physalia has a large float (a modified polyp)
  which is filled with gas 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.

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Physalia Portugese Man-of-War
Fig 7.12
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Scyphozoa

• The class Scyphozoa includes most of the large
  jellyfish.
• 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.

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Giant Jellyfish Cyanea capillata
Fig 7.14
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Scyphozoa

• Most Scyphozoans are pelagic, but in one unusual
  order the medusa attaches to seaweed.

Thaumatoscyphus hexaradiatus sessile medusa
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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, which is complex in
  structure with 4 gastric pouches that connect
  with a series of radial canals that join the ring
  canal.

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Moon Jellyfish Aurelia aurita
Gastric pouches
Radial canals
Ring canal
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Scyphozoa

• In Scyphozoa the sexes are separate and
  fertilization is internal.
• 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.

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Aurelia (moon jellies Scyphozoa)
Lions Mane jellyfish (scyphozoa)
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Cubozoa

• The cubozoans are a small 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 the name cubozoans) and there is a
  flattened structure at the end of each tentacle
  called a pedalium, which facilitates
  identification

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Box jellyfishes (Cubozoa)
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Cubozoa

• Cubozoans are very strong swimmers and are
  extremely effective predators mainly on fish.
• They produce highly toxic venom and often have
  very long tentacles.

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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.
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Anthozoa

• Anthozoa are polyps with a flowerlike appearance.
  There is no medusa stage.
• The Anthozoa includes three groups
• Zoantharia sea anemones, hard corals
• Octocorallia sea fans, sea pansies, sea pens
• Ceriantipatharia tube anemones and thorny
  corals. A small group with few species.

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Orange sea pen (Octocorallina)
Sea anemones (Zoantharia)
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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.

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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.

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Fig 7.19
Sea anemones
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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.

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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 and the
  coral can retreat into it when threatened.

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Hard coral polyp
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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.

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Coral polyps
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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.

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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.

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Brain coral (Anthozoa)
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Anthozoa Octocorallia

• Octocorals include soft corals, sea pens, sea
  fans, and gorgonians .
• They all have 8 pinnate tentacles. Almost all
  are colonial and 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.

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Polyps of an octocorallian coral
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Anthozoa Octocorallia

• Octocorals have an endoskeleton in which
  stiffening elements are secreted into the
  mesoglea.
• Soft corals have fleshy bodies that contain
  calcareous spicules in the mesoglea.

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7.27
Soft coral (Octocorallia)
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Anthozoa Octocorallia

• Other octocorals couple spicules (sometimes
  fused) with a stiff, but flexible protein
  gorgonin (similar to keratin). This combination
  provides structural support that allows large
  fan-like or branched colonies to develop.

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Red gorgonian
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Sea fan (gorgonian)
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Anthozoa Octocorallia

• Octocoral colonies may be in the form of mats or
  ribbons, feather-like, quill-like or clusters of
  vertical branches.
• Often they are brightly colored red, orange,
  yellow or purple.

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Orange sea pen (Octocorallia)
Fig 7.20
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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.

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Coral Reefs

• Reef-building corals contain symbiotic algae
  (zooxanthellae) that supply a significant part of
  the corals 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).

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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.

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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.

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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).

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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 gt1000 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.

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Shumann Island, Papua New Guinea (fringing reef).
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Pohnpei Atoll Micronesia
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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.

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Coral Reefs

• In late Pleistocene sea levels were about 120m
  below current levels when lots of water locked up
  in ice sheets.
• As 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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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nudibranch
Puffer fish
Blenny
Green Sea turtle
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Coral reef, Indonesia
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Schooling jack fish
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• Nature Conservancy Video clips
• http//www.nature.org/joinanddonate/rescuereef/exp
  lore/video.html
• Kimbe Bay
• Belize fish spawning

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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.

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Threats to coral reefs

• Global warming from increased levels of carbon
  dioxide also threaten reefs because when water
  becomes too warm corals expel their zooxanthellae
  (coral bleaching) and die.
• In addition, the higher carbon dioxide levels in
  the atmosphere are lowering marine marine pH
  levels making the water more acidic, which makes
  it harder for corals to produce calcium carbonate
  and may also dissolve corals.

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• Nature Conservancy Video clips
• http//www.nature.org/joinanddonate/rescuereef/exp
  lore/video.html
• Palau

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Youtube corals reefs

• Truk Lagoon 230
• http//www.youtube.com/watch?vju-6QxK6FJMmodere
  latedsearch

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Coral reef, Komodo Island National
Park, Indonesia
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Phylum Ctenophora

• The phylum Ctenophora is a small phylum of fewer
  than 100 species of comb jellies all of which are
  marine.
• They are named for the 8 rows of comb-like plates
  they beat in order to move.
• Like the cnidarians they are radially
  symmetrical, diploblastic, have a nerve net and
  an oral cavity, but no anus.

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Phylum Ctenophora

• Most ctenophores are free swimming.
• On the surface they have 8 rows of transverse
  plates that bear long fused cilia.
• Beating of the cilia in each row begins at the
  aboral end and all plates beat in synchrony to
  move the ctenophore.

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Ctenophore video clips

• http//www.oceanfootage.com/stockfootage/Comb_Jell
  y_Footage/owner3Dekovacs//?DVfSESSCKIE8cdc73b166
  081cdf4f033fe0c520befaa7ddb171

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Phylum Ctenophora

• Ctenophores make adhesive cells called
  colloblasts, but not nematocysts. However, one
  ctenophore does carry nematocysts, which it
  appears to obtain from cnidarians it eats.
• Like cnidarians ctenophores are predators. They
  feed on zooplankton including small crustaceans
  and fish larave.