The Great Barrier Reef Library

A comprehensive guide to The Great Barrier Reef

Cnidaria – Jellyfish and Anemones

Cnidaria -Anemone

By on December 27, 2017


Cnidaria (pronounced nai-daria) begin life as a transient, microscopic larvae. This is generally followed by settling into a benthic (bottom-dwelling), polyp phase. This phase is essentially the same body form as an adult, only upside down, with their tentacles in the air. Some species will never grow out of this polyp phase, such as corals, where as others metamorphose into free swimming, adult medusae.3

Cnidaria have no internal organs, such as hearts, or brains, but they have a two cavities.  One for respiration and the other for feeding. This means that they have no anus and so excretion usually occurs through the same opening as feeding (although not in all species).  They have a twinned body wall made of the endoderm / gastroderm (internal) and exoderm / epiderm (external). These are separated by a layer called the mesoglea. With no nerves cells or brain to control their functions, cnidaria have a decentralised network, or nerve net. There may be one or two nets present.1,3


Nearly all Cnidaria are predators and have stinging organelles within their cells, called Cnidae, or more commonly, Nematocysts. This is a distinguishing feature of this Phylum. These cells are capable of injecting a venom that can kill prey much larger than themselves. Some are even capable of killing humans, although in most species, the sting is harmless to us. Other nematocysts do not sting at all, rather cling to their victim.

Small hairs sense changes and vibrations in the surrounding water column, which signals that a prey item is near. If it strays too close, the lid (Operculum) opens and the nematocyst will fire. A coiled barb is released, which will inject the venom into the skin of the prey.  This is one of the fastest known cellular processes and takes only 1-3 milliseconds to complete. They can even fire after the Cnidarian has died!3

Some Cnidaria feed on microscopic plankton, whereas others feed on much larger items. Some get energy from symbiotic algae in their tissues and some are parasitic.  On the other side, a variety of animals prey upon Cnidaria, including Nudibranchs, Fish, including the Butterfly Fish, Parrot Fish and Sun Fish, Crown of Thorns Starfish and even Turtles.1,2

There are over 11,00- Cnaidaria worldwide, with only a select few surviving in fresh water. Many inhabit brackish water, as well as open oceans and shallow seas.3

The longest Cnidarian in the world is The Lion’s Mane Jellyfish, which has tentacles of over 35m in length. This makes it one of the longest known animals to ever live on Earth.

Class – Next level of classification, grouping animals with similar characteristics

  • Coral Macro Anthozoa – Sessile organisms, such as corals, sea pens and anemones

  • Cubozoa – Box Jellyfish

  • Hydrozoa – Bluebottle

  • Scyphozoa Scyphozoa – Jellyfish