Download Cnidarians - Westgate Mennonite Collegiate

Amy, Irene, Nicolien
• Radial symmetry – means one half of the animal mirrors the
other half.
• Body plan – Sac-like structure, has an internal cavity called the
Gastrovascular Cavity, which has one opening (used as both the
mouth and the anus)
• Diploblastic – body and tentacles have two cell layers. There is
the endoderm and ectoderm and the mesoglea layer is found in
between them. The mesoglea layer can just be there to hold the
other two layers together, or it can make up the majority of the
animal.
Polypoid
• Tentacles and mouth are
face up
• The animal is attached to
the ground (eg. Rock)
• Coral and anemones are
Polypoid in structure
Medusoid
• Mouth and tentacles are
face down
• Free swimmers
• Jellyfish are Medusoid in
structure
• Many Cnidarian species can reproduce using sexual and
asexual reproduction methods.
• Sexual reproduction is used by fertilizing gametes externally by
spawning them in to the water.
• Sexual reproduction is done by adults with usually opposite
sexes although some are found same sexes.
• Asexual reproduction is done by cloning with budding,
fragmentation, and fission.
• True Jellies (Cnidarian) go through two body forms during the
life stage. The asexual polyp and sexual medusa.
• The adult jellyfish in the medusa form reproduce as male releases
sperm in water and female collect the sperm to her mouth to hold her
eggs. Fertilized eggs grow in to larvae and detach from the mom
and drift through water, settling on to the sea bottom.
• Such organisms called polyps now reproduce asexually by diving or
budding.
• Ephyra, cloned buds, swim away from polyp base and eventually
grow in to adult medusa.
• After becoming the adult medusa, the two shape cycle repeats all
over again.
• Many polyps are found in
shallow water, but anemones
(also polyps) are found in
deep water too.
• Medusae drift in currents,
mostly in surface water of
bays and coasts, but
sometimes in the open ocean.
• Nematocyts are “coiled thread-like stingers”.
• The nematocysts are found in cells called cnidoblasts, which are
on tentacles, but may also be on other layers.
• The nematocysts can act as defense and offense.
• Nematocysts can paralyze and kill animals that are much
bigger than the animal itself.
• There are small sensory hairs close to the nematocysts, which
are sensitive to vibrations in the water. The nematocyst is
triggered when a prey swims by. The nematocyst becomes
straight and shoots out a “barb”, which injects the prey with
venom, then a tentacle brings the prey toward the mouth.
• Some of the chemicals that coral produce helps scientists study
cancer and anti-cancer
• Cnidarians can be several tonnes in weight
• Cnidarians are carnivorous
• A Cnidarian is the longest animal in the world, can reach up to
120 feet in length
• Includes sea anemones, coral
• Polypoid cnidarians
• Don’t have medusa stage, they release sperm and eggs that
form planula, which attaches to substrate, and cnidarian grows.
Some Anthozoans can reproduce asexually through budding
• Mostly feed on plankton, some fish
• Approx. 6,000 species
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Coral are threatened by acidification in the ocean
Coral are a rich source of food
Coral provide protection for shores
Most coral form colonies
Tubular body with mouth and ring of tentacles (which are
hollow)
• Long diverse fossil record extends back around 550 million
years
Anthozoan
• “True jellyfish”
• Drifting and of the medusoid body form
• Size can range from 12 mm to 2 m across (Cyanea arctica is
over 40 m long!)
• They have no head, skeleton, or special organs for breathing or
excreting
• Scyphozoa have a more developed nervous system than other
cnidarians; a nerve ring circling the edge of the bell-shaped
part of the body connects with nerves all over the body,
allowing for quick nerve signalling and creating movement
through bell contraction and release.
• The sting of a scyphozoan can cause rashes, cramps, and
sometimes, although not very often, human death.
• Divided into 4 orders:
• Stauromedusae are cup-shaped and are not free swimming
• Corontae are deep-dwelling
• Semaeostomae are disc-shaped as well as the largest and most
common
• Rhizostomae have no tentacles, but eight oral arms fused in the
middle
• Cubozoa, box-shaped jellyfish, used to be an order, but are not
a sister group to scyphozoa.
Scyphozoa
• Most hydrozoans are marine but some cnidarians have adapted
to fresh water.
• Most hydrozoans alternate between polyp and medusa stage.
They spend lives as what it looks like “jellyfish”
• Many hydrozoans are colonial. They form massive colonies
depending on specialized shapes and functions.
• Some examples or hydrozoan colonies
include Hydractinia, forming crusts on shells occupied by hermit
crabs and defend the crab from predators.
• Scyphozoa (the “true” jellyfish) and hydrozoans differ when the
polyp stage is left with the medusa absent. Usually the medusa
(also known as a sporosarc in this case) never breaks away from
the polyp, and the medusa stage is large when the polyp stage
is small.
• The hydrozoan medusae have a muscular shelf known as velum.
This is not found in “true jellies” or scyphozoans.
• Hydrozoans lack some cells in the mesoglea (jelly layer found
between the basic cell layers