The phylum Cnidaria contains around 13,000 living species. Surprisingly simple, yet beautiful organisms. They are classified based on a number of distinct features. For example, they tend to be radially symmetrical, meaning that their body structures extend outward from the centre. They are multicellular, and generally consist of simply an internal cavity and a mouth. But cnidarians come in two basic body forms - polyp and medusa - which often occur within the life cycle of a single cnidarian.

Generally speaking, Polyps are tube shaped and sessile with a ring of tentacles around the mouth. Anemones are a good example of this. Medusae, such as adult jellyfish, are umbrella or bell shaped, and can move freely with the currents.

The life cycle of a moon jellyfish sees its larva swim down to the seabed and attach onto rocks. There, it develops into a small polyp called a scyphistoma. Over a long period of time, the polyp undergoes a process called strobilation, rearranging itself into a tube-like structure that resembles a stack of coins. Each coin is in fact a tiny medusa, which are released in turn to drift off and develop into sexually mature adults. Some species are able to revert to an earlier stage of their life cycle, allowing them to regenerate themselves continuously.

Commonly called jellyfish, medusas possess tentacles that hang down below the umbrella. Their diversity is staggering. The largest, the lion’s mane jellyfish, has tentacles that grow longer than 27 metres - longer even than the blue whale. Many are slow-moving planktonic animals - meaning they aren’t efficient at actively swimming and must rely on ocean currents to help them move. But don’t be fooled by their apparent placid nature. Jellyfish have long been described as being the most important predators in the seas. Long tentacles are used for a range of functions, including catching food, and attacking using stinging cells called nematocysts.

It is these nematocysts which unite nearly all members of the phylum Cnidaria, from corals and anemones to jellyfish. They are a unique feature, a capsule containing a coiled threat which can be shot outwards to catch prey, or repel predators. A modified flagellum acts as a sensory trigger, rapidly ejecting the nematocyst thread when it’s touched.

This adaptation is an advantage to organisms that cannot actively hunt their prey. Instead, they adopt ‘sit and trap’, or ‘float and trap’ strategies, using their nematocysts to stun or kill prey that collide with their transparent tentacles. One of the deadliest examples of this is the Portuguese Man of War, a colony organism consisting of many small animals called zooids working together. It is made up of four separate polyps. The uppermost, a gas-filled bladder called a pneumatophore, sits atop the water like a ship at full sail.

The tentacles are the man-of-war's second polyp. Long, thin tendrils that extend 50 metres (165 feet) below the surface, covered in powerful nematocysts used to kill small creatures. Once a prey animal is captured, muscles in the tentacles draw prey up to a polyp containing the digestive organisms called gastrozooids. The fourth polyp contains the reproductive organisms.

Many cnidarians produce colonies like the Man of War, composed of tiny creatures. Siphonophores are abundant in the open ocean, consisting of medusa or polyp-like zooids. On the sea floor, sedentary colonies of polyps secrete a calcium carbonate skeleton to provide a rigid structure. These are the corals. Perhaps the most important cnidarians in the marine ecosystem, building expansive reefs that support a huge diversity of creatures. Their rocky structures provide shelter for reef fish and invertebrates, and protect coastlines from the damaging effects of waves and tropical storms.

They also play a part in the carbon cycle as a carbon sink, sequestering carbon in their skeletons. In doing so, they keep this carbon from returning to the atmosphere. Even after they die, the stony structures can store it for thousands of years.

Coral polyps form a number of important associations with other organisms. Notably, stony corals’ survival depends upon a symbiotic relationships with tiny algae that live in their tissues, called zooxanthellae. The algae use sunlight to produce sugars via the process of photosynthesis, and provide the corals with energy. In return, the corals provide protection for the algae within their tissues. It is this mutualistic relationship that upholds reefs, and consequently supports 25% of all known marine species.

Another relationship forms between crabs, and sea anemones that live on their shells. This association benefits the anemone by providing it with transport, and giving it better chances of finding food. The sea anemone protects its host from predators in return, of course by making use of its stinging nematocysts. Similarly, anemone fish like clown fish rely on symbiosis with cnidarians like anemones. The fish have developed immunity to the stings by secreting a thin layer of mucus over their bodies. Without it, the clownfish would be stung to death and consumed by the anemone.

Other fish have a similar association with larger drifting medusae. Those that are immune can take shelter among the jellyfish’s tentacles, baiting the trap for other fish. In all these examples, we see creatures from vastly different groups depending on cnidarians for survival. This goes to show just how important jellyfish and their relatives are in increasing biodiversity in the oceans.

Overall, there are so many different kinds of cnidarians that they have many points of importance. Coral reefs are formed of many millions of cnidarians and are home to staggering biodiversity. Jellyfish are important predators in the open ocean food web, but they serve as food for many other species as well. Sea anemones fulfil a similar role but in the shallows, also serving as hiding places for clownfish. Thus, this unique and abundant group of creatures is integral to the function of many marine communities, from the open ocean to coasts and the benthic zone of the deep sea.