Look out for that medusa! Coming up!
Jonathan investigates the world of stingy, squishy animals—the cnidarians! Welcome
to Jonathan Bird’s Blue World!
Jellyfish—technically not fish at all.
Biologists prefer to call them jellies. Everyone has heard of them, but what are they?
Jellies are squishy invertebrates, members of a phylum of animals called Cnidarians.
The
phylum Cnidaria which means “stinging animal” contains around 11,000 species worldwide. They are
among the simplest of the "higher" organisms—more complex than sponges, but not as complex as
something like sea stars.
The creatures in this phylum are radially symmetrical, just like
sea stars. This means that the parts of the body extend outward from the center
like the spokes on a bicycle wheel.
The Cnidarians include the hydroids, jellies,
anemones, and corals.
All Cnidarians have tentacles with stinging cells in their tips,
which are used to capture and subdue prey.
The stinging cells are called cnidocytes and
contain a structure called a nematocyst. The nematocyst is a coiled thread-like stinger.
When the nematocyst fires, the thread uncoils, and springs straight. The harpoon-like thread
punctures through the cell wall and into the prey.
Most Cnidarians also have a toxin in
their stinger that helps disable the prey.
Many Cnidarians are not harmful to humans, as
the nematocyst cannot penetrate sufficiently into human skin to inflict any harm.
There are
some Cnidarians however, like certain jellies and hydroids, that can deliver extremely painful, and
in a few cases, even fatal, stings to humans.
The Cnidarian can have one of two
basic body types. The polypoid is the configuration of corals and anemones, with
the tentacles and mouth generally facing upward, and the other side affixed to a substrate. In the
medusoid form, the organism is a free-swimmer, with the mouth and tentacles
generally pointed down.
The hydroids, anemones, and corals are mostly
all benthic animals, in a polypoid shape. This means that they live attached to the bottom
with their tentacles and mouth pointing up.
The Tealia Anemone, found in the north Atlantic,
is a good example of a polypoid cnidarian. Anemones are solitary, with each individual living
alone. It sits patiently and waits for prey.
And once something wanders in to the tentacles, the prey goes into a central mouth,
then down into what is called a blind gut. That means there is only one way in
or out. Once the food has been digested, it gets spit back out through the same mouth
it went in. All Cnidarians have a blind gut.
Hydroids tend to be quite small and flower-like.
They often appear as a tiny anemone on a stalk. Both don’t let their fragile appearance fool
you. They can sting their prey quite well.
There is a hydroid in the tropics that looks
like peach fuzz on the reef. But it has an appropriate name: Fire coral. It’s not actually
coral, but it definitely stings like fire. Contact with this hydroid is quite unpleasant
for divers.
Unlike the Tealia anemone, coral polyps are colonial, meaning that they live
in groups, connected together with living tissue, and they share food resources. Some corals
create limestone skeletons that build up to form boulder-like structures called coral
heads. Many coral heads together form reefs, creating habitat for fish. It’s hard to believe
that something so small can create structures that can be seen from space.
Cnidarians don’t
have much in the way of specialized organs. For example they have no gills. They absorb oxygen
from the water directly through their skin. Having a lot of tentacles gives them plenty of surface
area exposed to the water for metabolism.
The most well-known Cnidarians are of course
the members of the medusoid body form—the jellies. Jellies drift along in the water
generally with the mouth and tentacles pointing down--a net to capture small prey.
Some jellies are extremely venomous to humans. Some, like the box jellies, can even kill people
with enough contact of tentacle on bare skin, but most are not that dangerous, producing
only a mild rash and some short term pain.
All Cnidarians can reproduce asexually at some
form of their development by budding off tiny clones of themselves, or dividing themselves.
But some cnidarians also reproduce sexually, releasing eggs and sperm into the water to
fertilize and disperse with the currents, seeking new habitats. Corals in
particular, which can’t move, spread to other areas using this
form of broadcast spawning.
For such simple animals, the Cnidarians
make a huge impact on the oceans. They are predators, reef-builders, and
drifters. It’s hard to imagine what the Blue World would be like without
these squishy, stingy Cnidarians.
Hey Everyone! Have you subscribed to
our extras Channel BlueWorld_plus? It’s full of great behind-the-scenes and
additional fun content! Check it out now!