Cnidarians are one of the simplest forms of multicellular organism. They include the coral animals that build reefs, jellyfish and sea anemones as well as some small fresh water forms. 95% of the mass of a jellyfish is water, combined they make up 40% of the biomass of the oceans. They are as elegant in their movement as they are alien in appearance

Cnidaria - Cnidarians Coelenterates - Coelenterata, now obsolete terms

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
A typical free swimming jellyfish, tentacles down.
picture used permission of stefani.drew - CC Attribution ShareAlike 2.0 License

A sea anemone, a typical polypoid Cnidarian
Attached to a rock with tentacles up, they catch small prey and then bend over to bring them to the central mouth. After digestion waste materials are voided from the mouth.
copyright Tomas Castelazo / Wikimedia Commons / CC BY-SA 4.0

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, they float at the surface and the sail sticks out of the water so the animal can sail in the wind. There are two kinds with the sail running in either of the possible diagonal directions, one kind will drift to the left of the wind direction, the other kind to the right.

Ctenophore - Bolinopsis infundibulum
Many Ctenophores 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 for viewing.
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.

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 Nick Hobgood - Creative CC Attribution ShareAlike 3.0 License

Kingdom - Animalia
    Phylum - Cnidaria

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 (limestone) 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)

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. Coral reef bleaching is a very significant form of environmental degredation and associated with global warming.

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 digestive tract can become blocked by them.

Life cycle of a medusa of the class Scyphozoa

Many but not all cnidarians have both hydroid (fastened to a substrate with tentacles up) and medusoid (free swimming, tentacles down) parts of their life cycle. Medusa are an excellent way of distributing themselves through the seas before settling down as they get larger.

In the Scyphozoa, the free swimming adult, no.14 produces small larvae (called planulae) by sexual reproduction which attach to a substrate and develop into a polypoid form - no.1. As the polyp grows and matures it forms a stage called a scyphistoma no.10 which then produces medusoid forms again 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", say it out loud, you'll like it.
Illustration - "Die Entwicklung der Meduse". In: "Das Meer" by M. J. Schleiden (1804-1881)

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


Lions mane jellyfish, Cyanea capillata - a cold water species that grows up to 2.5m (8ft) across
the tentacles can be over 30m (100ft) long

Ctenophores - comb jellies, the colours are caused by interference from
the effect of light on the rows of cilia

Banner picture at top of page - Semperina rubra soft coral with polyps extended, courtesy of Nick Hobgood under Creative Commons Attribution-Share Alike 3.0 Unported licence.