Phyla Cnidaria and Ctenophora – Lecture 5.2 – Marine Biology – Biol 321
The purpose of this video is to introduce the viewer to Cnidarians and Ctenophores.
Hello marine biology students.
In this video we're going to talk about Cnidarians
So phylum Cnidaria.
This is one of those strange Latin words with
silent c, but we'll see that also will be
true for our next phylum, phylum Ctenophore.
So, phylum Cnidaria, some of the basic characteristics
of cnidarians is that they have radial symmetry.
Most species of cnidarians are marine, but
there are a few freshwater hydroids.
There are two basic body plans when it comes
to Cnidaria, those of the free swimming medusae
or of the attached polyps.
They have the same basic body plan, they're
just oriented slightly differently.
Regardless of whether we're talking about
a medusa or a polyp, there's a single opening
that serves both as the mouth and the anus
and there are two layers of tissue, the epidermis
and the gastrodermis.
Separated between them can be a gelatinous
layer known as the mesoglea.
So when we look at these two tissue layers,
covers the body's surfaces whereas the gastrodermis
lines the internal cavity.
We see a distinction here from the sponges,
which lacked any sort of tissue structure
The mesoglea is an acellular gelatinous matrix
between these two types of tissues, and this
makes up the jelly part of a jellyfish.
One key feature that cnidarians have are these
harpoon-like structures known as nematocysts
and they're found within a particular type
of cell known as a cnidocyte.
are these coiled harpoon-like structures that
usually have a trigger and when that trigger
is pressed, it will discharge the harpoon.
The cells that hold these nematocysts are
Instead of having a complete digestive system
or a digestive tube, cnidarians have a gastrovascular
cavity, and this gastrovascular cavity ends
up having a single opening that functions
both as a mouth and an anus.
Now these cnidarians don't quite have taste
buds like we, do so it's not as disturbing
a concept for them as it would be for us.
There's no centralized nervous system, but
there is a nerve net throughout the body of
the cnidarian to coordinate its movements.
Some jellyfish also have additional sensory
cells and contractile cells which allow it
to receive information from its environments
and respond accordingly.
When we look at reproduction in these cnidarians,
they can reproduce both sexually and asexually.
With sexual reproduction there can be different
patterns and in some cnidarians the medusae
is the sexual stage, releasing eggs and sperm.
Fertilized eggs result in a zygote which then
develop into a swimming larva which is known
as the planula.
This planula then settles on the bottom to
form a colony of polyps.
New medusae will then be formed by this mature
So here we can see a model of this lifestyle
with the medusae forming the gametes, the
gamete then forming an embryo, then to a planula
That planula larva settles on the bottom and
undergoes metamorphosis into a colony and
that colony would reproduce through asexual
means until eventually they form reproductive
This is the life cycle of one type of cnidarian
known as a hydrozoan.
Cnidarians are also capable of asexual reproduction.
This could end up being through the polyps
themselves that can reproduce by budding,
and we can also see that some polyps can reproduce
And again, the difference between fission
and budding, budding is when a smaller individual
starts growing off the body of a larger one
but fission is when a large adult simply breaks
or divides into two equally sized individuals.
So let's talk about the different types of
So the first class of cnidarians we want to
talk about are the hydrozoans.
They generally consist of a colony of polyps
with small reproductive medusa.
So these include some of the freshwater cnidarians
along as many of the marine cnidarians.
There's a specific group within the hydrozoans
known as the siphonophores, and siphonophores
are interesting in that they're a drifting
colony of polyps.
Now, these colonies are atypical in that some
of them are specialized for different tasks.
One example of a siphonophore is the Portuguese
Man o’ war from the genus Physalia.
In this case, the siphonophore colony has
a gas-filled float, so this is a surface floating
Many siphonophores are going to be found in
deep water, but not the Portuguese man-o-war.
With this colony, some of the polyps have
long tentacles filled with stinging nematocysts
and here when we look at this these diagrams
of these siphonophores or hydrozoans, we see
this feathery-like colony of polyps and then
we also see the Portuguese man o’ war with
its entangling nematocyst-filled tentacles
that allow it to prey on fish and other organisms
in the water.
Both of these types of cnidarians could have
nematocysts that could puncture human skin
and so you might notice a burning or stinging
sensation if you're in contact with either
The next class of cnidarians are the scyphozoans.
This is the class with the large marine jellyfish.
They're a very large medusa and the polyp
stage of a scyphozoans’ life is usually
very limited and the polyps themselves are
The medusa stage is definitely the dominant
life cycle for the scyphozoans.
They move by rhythmic muscular contraction
of the bell, yet even though they generate
the swimming motion, they are carried by water
currents so they are considered to be macroplankton.
The next group of cnidarians are the cubomedusae.
They look like scyphozoans, but they're typically
significantly smaller and it turns out they're
usually also far more dangerous to humans.
The cubomedusa have some of the most powerful
stings and could potentially be fatal either
from the toxin that they release or by causing
a swimmer to faint or pass out or be unable
to swim and therefore drown.
So these are usually small medusa with tentacles
armed with very powerful nematocysts.
These are often so small that they can be
difficult to see in the sea water, which makes
them all the more dangerous.
The last type of cnidarian that I want to
talk about are the anthozoans from class Anthozoa.
These are going to include corals, sea anemones,
and gorgonians or sea fans, which we see pictured
These can be single, such as some of the sea
anemones we saw earlier, or colonial, as is
the case with corals and gorgonians.
They have colonial polyps and typically anthozoans
do not have a medusa stage.
Corals can secrete calcium carbonate skeletons
and build complex coral reefs.
Now, not all corals are reef building corals.
In fact, there are soft corals as well and
a coral will not build a reef in every environment,
but in environments that promote the growth
of these corals over many generations and
long periods of time, they can become a dominant
structure in the marine environment.
Most corals have symbiotic zooxanthellae and
these symbiotic dinoflagellates are able to
provide the corals with the benefits of photosynthesis
while the coral polyps provide protection
to the zooxanthellae.
This completes our discussion of the cnidarians.
The next phylum I'd like to talk about are
So, phylum Ctenophora, these are the comb
Now, there are ways that they look similar
to jellyfish, they are primarily translucent
and gelatinous, but there are differences
Comb jellies have eight rows of ciliary combs
that beat continuously, and the way that these
cilia refract light ends up causing very beautiful
rainbow-like patterns to be seen along the
edges of these ctenophores.
Like the cnidarians, the ctenophores have
radial symmetry, however unlike the cnidarians,
ctenophores do not have nematocysts or stinging
cells, instead these ctenophores have adhesive
sticky cells called colloblasts.
These colloblasts on their tentacles allow
them to ensnare things like fish larvae and
zooplankton, which they would then feed on.
There are about 100 species, all of which
are marine and most of them are planktonic.
So that takes us to the end of our discussion
of ctenophores and cnidarians.
Now, before the next video, I would like you
to think about how many different types of
worms are you familiar with.
We'll probably introduce you to a few new
ones in the next video.
See you then.