CTENOPHORA

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CTENOPHORA
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Common Names

• Ctenophores are often called comb jellies, sea
  gooseberries, sea walnuts, or Venuss
  girdles.

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Examples

• There are two classes in the phylum Ctenophora
• Tentaculata, of which the most characteristic
  species is Hormimorpha plumosa
• Nuda, of which the most characteristic species is
  Beroe punctatus.
• The most commonly seen ctenophores are of the
  genus Pleurobrachia. There are about one hundred
  species of Ctenophora that have been discovered.

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Characteristics

• All organisms in Ctenophora have comb rows that
  are actually large cilia used for transportation.
  These are the largest cilia any organism uses.
  Ctenophores lack the ability to sting and instead
  use tentacles covered by colloblasts . All
  ctenophores are bioluminescent, meaning they are
  capable of producing light. Some have the
  appearance of rainbow coloration. This is caused
  by the beating of their combs, causing the
  diffraction of light to form a rainbow.
  Classification is generally done through
  comparative morphology.

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Characteristics (continued)

• Ctenophores have two digestive openings, but
  rarely use the anus for defecation
• Biradial symmetry and three germ layers
  (epidermis, gastrodermis, mesoglea if included)
• Mesoglea holds muscle cells and amoebacytes
• Digestive system consists of mouth, pharynx, and
  stomach with a canal system
• Have no CNS or brain, but a nerve net
• Use an aboral sense organ with a statocyst to
  maintain balance
• Balance is maintained by mood (turn toward prey
  if in danger)
• All Ctenophores have a distinct larval form
  before adulthood

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Morphology

• Most ctenophores are round or spherical in body
  shape. They generally have two large tentacles
  and eight rows of comb plates. Their tentacles
  have the adhesive colloblasts to capture prey and
  they have a stomach, mouth, pharynx, and anal
  pores. They have a nerve network that is rather
  complex and an apical sense organ. All
  ctentophores are hermaphrodites and most
  reproduce sexually. They generally range in size
  from .6 cm to 30.5 cm.

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Colloblasts (Morphology)
Colloblasts are microscopic, sticky structures
used to adhere to the prey of a ctenophore.
Usually the classes Cydippida and Lobata have
colloblasts on their tentacles. After a prey is
caught by a colloblast, the tentacle is
contracted and the prey is brought by the
predators mouth. There the prey is released and
ingested.
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Cydippids (Morphology)

• Egg-shaped bodies
• Order includes pleurobrachia
• Tentacles on opposite sides of body
• Body can be flattened to increase range of
  tentacles
• Tentacles have tentilla (small tentacles) with
  colloblasts
• Generally coil around prey to kill
• Combs are spaced evenly around the body
• From each statocyst balancer, two combs are moved
  at once
• Movement is dependent on water disturbances

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Lobata (Morphology)

• Order also called Lobates
• Have pair of muscular lobes extending from mouth
• Tentacles in grooves on lobes, have auricles
  between lobes and mouth
• Auricles have cilia to create current and flush
  prey into the mouth
• Two comb rows on each lobe and two on each side
  between lobes
• Comb movement dependent on nerves rather than
  water movement
• Some can clap combs to push water rapidly and
  move opposite
• Generally prey on plankton

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Beroids (Morphology)

• Order also known as Nuda
• No feeding appendages, but pharynxes have
  macrocilia
• Marcocilia are large bundles of cilia that work
  to bite off pieces of whatever is being consumed
• Prey on other ctenophores
• When not eating, a ridge shuts the mouth by
  connecting with the other part of the ridge

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Embryology

• When the egg is fertilized, development of the
  embryo begins. Cleavage occurs completely but
  unequally, first producing two cells, then four,
  then eight, and so on until the embryo is full
  developed. The embryo forms within the egg cover.
  It develops double rows of cilia, a pair of
  lateral tentacles, and a large, apical
  sense-organ. The ectodermal layer of the
  gastrovascular system undergoes Epiauxesis.
  Epiauxesis involves the flattening and extension
  of the ectoderm germ layer in the gastrovascular
  system. Soon, the cilia begin to function and
  the developed larva breaks the egg shell and
  enters the water. The following pages are
  detailed steps to the process of initial cleavage
  and development.

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Development (Embryology)
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Development (Embryology)
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Biochemical Evidence

• Little biochemical evidence exists regarding the
  phylogeny of Ctenophora. However, by using 18S
  ribosomal RNA for comparison, it has been
  determined that the ctenophores are a distinct
  monophyletic group that is closest related to the
  cnidarians. The ancestral ctenophore seems to
  have been like Cydippida. Thus, it was
  determined that Cydippida is a polyphyletic
  group, all other orders having been secondarily
  derived from the ancestor. The relatively close
  distance between the 18S ribosomal RNA sequences
  leads to the conclusion that ctenophores are
  derived from a recent common ancestor.

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Life History

• Mnemiopsis leidyi is a ctenophore of the order
  Lobata. These have a distinctive level of
  bioluminescence that makes them appear to glow
  even more than others. Locomotive flappers hide
  the phosphorescent cells within the meridional
  canal, perpendicular to the plane of the comb
  plate. It exists in various morphological forms,
  chiefly where transparence is inversely
  proportional to body size. They can be up to 5-7
  cm long and always live in a marine environment.

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Life History (continued)
M. leidyi is a hermaphroditic and are capable of
self-fertilization. They are considered
developed when capable of producing offspring,
even though they have not ceased growing yet.
When reproduction begins, the gonads in the
meridional canals produce one spermatophore and
one to four eggs. The auricalular gonads produce
only one egg. The egg is covered in a thick
membrane one minute after seawater contact.
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Life History (continued)
When embryogenesis begins, the eggs are spherical
and soft. After the larvae is formed, the egg is
covered with a thick membranous cuticle. The
embryo experiences epiboly, gastrulation, ctenes
appear, apical organs grow, tentacles grow, and
the embryo enters the tentacular stage. Here,
the larval form hatches and continues
development. Complete embryonical development
takes 20-24 hours.
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Life History (continued)
After hatching, the M. leidyi enters the lobate
stage. Next, the meridional canal meets and the
auricles begin to grow out. At this point, the
M. leidyi is capable of reproduction and
considered an adult.
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Life History (continued)
M. leidyi are capable of regeneration of damaged
body parts. After damage is done, the cells
begin dividing to reform the organ by doubling.
M. leidyi feeds on fish eggs and larvae,
phytoplankton, and holoplankton. It ingests any
organism it can grasp with its oral lobes. They
are generally considered carnivores but have been
known to consume phytoplankton when needed.
Their feeding on phytoplankton and zooplankton
has reduced their populations as well as those of
the kilka in the Caspian ecosystem. Thus, the
economy in Caspian countries has suffered due to
the loss of kilka.
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Life History (Digestion)

• Enzymes and contractions of pharynx liquidize
  prey
• Cilia move the resultant mush into the stomach
  canal system
• Nutritive enzymes break down the mush in the
  stomach
• Ciliary rosettes in canal transfer nutrients to
  muscles in mesoglea
• Most waste is ejected through the mouth, rarely
  through the anal pores.
• Exact mode of excretion is unknown

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Summary

• Are ctenophores carnivorous, herbivorous, or
  omnivorous?
• How do ctenophores capture prey?
• How do ctenophores reproduce and what organs do
  they have for such?
• To what phylum are ctenophores most closely
  related?
• What is the function of a colloblast?
• When are ctenophores considered fully developed?