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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Fig. 7.2
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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 to form a circular muscle layer.
• There are also gland cells that secrete digestive
  enzymes.

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Figure 7.3
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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.
  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

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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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7.9
Life cycle of Obelia, a marine hydroid.
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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). 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.

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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.

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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.

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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 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.

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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)

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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.
• 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.

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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 (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.

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

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Box jellyfishes (Cubozoa)
Pedalium
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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.

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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 (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.

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Orange sea pen (Octocorallia)
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 (CaCO3) 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 all have 8 pinnate 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.

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Polyps of an octocorallian coral
7.26
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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.)

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Anthozoa Octocorallia

• The coupling of spicules (sometimes fused) with
  the stiff, but flexible protein gorgonin (similar
  to keratin) provides enough structural support
  for large fan-like or branched colonies of
  octocorals to develop.

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Sea fan (gorgonian)
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Red gorgonian
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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.

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7.27
Soft coral (Octocorallia)
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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 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.

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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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13.34
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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 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.

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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.
• 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.

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7.28 7.30
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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.

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Phylum Ctenophora

• 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.

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http//www.marlin.ac.uk/images/taxonomy_descriptio
ns/Ctenophora.jpg
111
Phylum Ctenophora

• 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

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Ctenophore video clips

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