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
A medusoid jellyfish
picture used permission
published under Creative Commons Attribution
ShareAlike 2.0 License
Two sea anemones, typical polypoid Cnidarians
catch small prey and then bend over to bring them to the central
mouth. After digestion waste materials are voided from the mouth.
Velella - "Jack-sail-by-the-wind"
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
published under GNU Free Documentation
License, Version 1.2
Ctenophore - Bolinopsis infundibulum
are are translucent but can appear multicoloured. Viewed from
one direction they are like a non-descript plastic bag, but
with the sun on them against a dark background, rainbow lines of
interference colours chase each other along the lines of cilia,
best seen while snorkeling or diving so you can get in the right
position to see.
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 snorkeling 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.
- 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
- Usually no skeletal material though hard coral polyps build
calcium carbonate structures around themselves that over time
become the massive 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
- 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
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
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
Illustration - "Die
Entwicklung der Meduse". In: "Das Meer" by M. J.
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 Attribution ShareAlike 3.0