Typical Cnidarians
A diver with fire coral in the Red Sea.
The name is well deserved as it can give a very painful sting from
the nematocysts if it is accidentally brushed against. The "fronds"
look soft like seaweed but are actually stiff, which can cause unwary
divers to have an unpleasant and unexpected accident with it.
A medusoid jellyfish
picture used permission of stefani.drew
published under
Creative Commons
Attribution ShareAlike 2.0 License
Two sea anemones, typical polypoid Cnidarians
The tentacles catch small prey and then bend over to bring them
to the central mouth. After digestion waste materials are voided from
the mouth.
Cool Cnidarians
Velella - "Jack-sail-by-the-wind"
"By the wind sailor" or several other names by which this is known.
About 10cm long, widespread in the world's oceans, the sail sticks out
of the water and so the animal sails in the wind. There are two kinds
with the sail running in each of the possible diagonal directions, one
kind will drift
to the left of the wind direction, the other kind to the right.
picture used permission of
yakafaucon
published under
GNU Free Documentation License, Version 1.2
Ctenophore - Bolinopsis infundibulum
Many Ctenophores are are translucent and give off a bioluminescent
glow. Viewed from one direction when diving they are like a
non-descript plastic bag, with the sun on them against a dark
background, rainbow lines of interference colours chase each other
along the lines of cilia.
Picture courtesy NOAA
Portuguese Man o' War
A colonial Cnidarian that belongs to a group called Siphonophores
so not a true jellyfish. They can swarm in huge groups and give a very
painful sting. My wife will happily tell anyone the story of how she
surfaced directly beneath one while snorkelling and ended up in hospital
(no lasting effects fortunately).
The air bladder (pneumatophore or sail) protrudes above the sea and
enables the Portuguese Man o' War to sail. It must be kept moist and
so is regularly tipped over into the sea if it dries out. The sail can
be up to 30cm long and tentacles can stretch for up to 50m (yes metres!)
below the surface.
Wire coral
Showing the extended polyps in feeding mode. During the day,
many corals hide their polyps away for safety and so can appear hard
and dead, night-time brings many of them out and a whole new show of
colour on the coral reef.
picture used permission of Nick Hobgood
published under
Creative Commons
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Basic
Features:
- Aquatic organisms
- Radial symmetry, central mouth
- No central nervous system (CNS) or head - instead
have a "nerve net" of interconnected nerve cells
- No circulatory system, heart or blood
- Some have tentacles down - free floating medusa
- like jellyfish
Some are tentacles up - sedentary polyp - like sea anemones
Some have both stages in their life-cycle, some only have one stage
- Usually no skeletal material though hard
coral polyps build calcium carbonate structures around
themselves that over time become the rocky material of coral
reefs
- Stinging cells called nematocysts are arranged
on tentacles and used to capture food
- Can reproduce sexually or asexually (not all
species can do both though)
Kingdom - Animalia
Phylum - Cnidaria
The
group includes:
- All forms of jellyfish such as:
- Box jellyfish
- Portuguese man-o-war
- Hydra
- Ctenophores - comb jellies
- All forms of soft and hard coral such as:
- Brain coral
- Fire coral
- Elkhorn coral
- All forms of sea anemone such as:
- Beadlet anemones
- The various kinds that live on hermit crab
shells
- The various kinds of anemone that clown fish
live with
What do Cnidarians eat?
Many Cnidarians eat small planktonic animals
that they catch with their tentacles and stinging nematocysts. They
don't go "hunting" but have to wait for their prey to blunder into
the tentacles. Once this has happened other tentacles are brought to
the prey to secure capture and subdue it with more stinging cells.
The larger jellyfish can capture small fish this way too.
They do not eat a great deal individually, but can
be present in huge numbers so eat a lot between them. Jellyfish are
about 95% water, so a huge amount of animals doesn't represent much
dry biomass.
Some ecologists are predicting that as some
areas of the oceans become overfished, then jellyfish will take over
from the fish as more food becomes available to them. This has
already been seen in some areas such as the Black Sea, Caspian Sea
and off the coast of South Africa and Namibia. Jellyfish swarms can
be a serious threat to
fish farms.
Anthrozoid Cnidarians that make up hard corals
have symbiotic algae called zooxanthellae that live inside the
polyps. These zooxanthellae are photosynthetic and can provide
up to 90% of the energy requirements of the polyp.
Under conditions of stress, these coral polyps may
expel their algae for reasons that are unclear in a process called
coral bleaching. These events are known to be associated with
high sea temperatures. After ejecting the zooxanthellae, the coral
polyps then die leaving behind whitened coral skeletons and dead
reefs. There are regions of the sea around the Maldives, the
Seychelles and Sri Lanka amongst others where up to 90% of coral
cover has been lost due to this process.
What eats Cnidarians?
Coral polyps are eaten by some specialist
fish such as parrot fish and butterfly fish. Sea stars such as the
crown of thorns starfish also eat them and can be very damaging if
they build up to high levels on a coral reef.
Jellyfish are eaten by many other animals, it is
thought that fish such as sunfish are probably the most important
predator, though in some parts of the world they form the main
diet of sea turtles. This is a reason that plastic bags at sea
are so dangerous, turtles can mistake them for jellyfish and their
guts can become blocked by them.
Life cycle of a medusa of the class Scyphozoa
The free swimming adult, no.14 produces planula larvae by sexual reproduction
which attach to a substrate and develop into a polypoid form - no.1.
As the polyp matures it forms a stage called a scyphistoma no.10 which
then produces medusoid forms by asexual reproduction no.11, a process
called strobiliation. This means that no.11 has become one of my favourite
sounding phrases ever - a "Strobilating scyphistoma".
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