You might be forgiven for thinking that sea pens, anemones and corals are all types of plant, but in fact they are an ancient branch of the animal kingdom known as Cnidaria. The Phylum Cnidaria¹ contains roughly 9,000 species, including the aforementioned animals, jellyfish, box jellies, and weird freshwater animals.

Cnidaria evolved a long time ago, the oldest known fossils of Cnidarians dating from around 680 million years ago. Before then little is known of their evolutionary history. This is because the chance of finding fossils in older rocks is slim, as they are much more likely to have been destroyed by geological processes such as erosion. Also, most Cnidarians except corals lack hard parts, like bones, which are easily preserved in rocks.

Stingers

As anyone who has encountered a jellyfish can tell you, Cnidarians can be painful little creatures to deal with. In fact, the phylum name 'Cnidaria' literally means 'stinging creature'. All Cnidarians have stinging cells known as nematocysts, or cnidae, at the end of tentacles surrounding the creature's mouth. These allow the carnivorous Cnidarians to capture and stun prey, and to protect themselves from predators, although some species have adapted the basic design to fill their ecological niche.

The microscopic nematocysts look like a harpoon on a coiled thread. When something touches the tentacle, or in more complex Cnidarians when a nerve impulse from the animal tells the nematocyst to 'fire', the thread uncoils like a spring under tension, and the harpoon goes into the prey, would-be predator, or unfortunate swimmer. Most Cnidarians also have a toxin in their stinger which helps to disable the prey. Fortunately, most Cnidarians' nematocysts are not strong enough to penetrate human skin deeply enough to cause harm. However, jellyfish, the Portuguese Man O' War, fire coral, and box jellies pack a punch, and can deliver painful and even fatal stings to humans.

Physiology

Body Plans

Cnidarians exhibit radial symmetry, meaning that their body parts extend out from the centre like spokes on a wheel. Subsequently, they also lack a true 'head' or 'tail' end, and what we would know as a front or back. Still, this does not stop them from taking on bizarre and beautiful forms.

Cnidarians can have two basic body plans: polypoid or medusoid. Jellyfish are a great example of the medusoid shape, with their mouths and tentacles pointed downwards. Cnidarians with a medusoid body plan are usually free swimming and make a living of floating around in the ocean, snagging whatever food drifts along with them. Polypoids are upside-down medusoids, attaching themselves to rocks with the end opposite to their mouths and tentacles. Polypoids are generally sessile, meaning they anchor themselves to one spot for their whole lives. Examples of polyps include anemones, corals and sea pens. In some species of colonial Cnidarians, individual polyps will specialise for roles such as food gathering, defence or sexual reproduction. This is known as polymorphism.

Basic Biology

Cnidarians have a simple muscle and neural network which allows them to respond to their environment, and in the case of medusoids to move around. They consist of two cell layers, the ectoderm or epidermis on the outside, and the endoderm or gastrodermis on the inside of the body cavity. These layers are separated by an internal fluid called the mesoglea. The mesoglea can either be just a thin layer to glue the animal together as in the hydra, or it can make up the bulk of the creature as in jellyfish.

Most Cnidarians ensnare prey with their tentacles and sting it to death. However, some species feed by trapping micro-organisms with mucus on their bodies. Some corals have a symbiotic relationship with algae known as zooxanthellae in their tissues, and gain energy from the algae's photosynthesis.

Unlike higher animals, which have a one-way digestive tract, there is only one opening to the digestive cavity or enteron of Cnidarians. This means the waste from everything they eat must go out the way it came in. In the digestive cavity, food is broken down into small particles and taken into food vacuole cells to be processed into energy. The enteron also serves as the 'lungs', allowing for gas exchange between the animal and surrounding waters.

Reproduction and Life Cycle

Cnidarians reproduce both asexually through budding² and sexually. Most Cnidarians have separate sexes, with specialized cells in each producing the gametes, which are excreted in hopes that they might meet up with gametes produced by the opposite sex. When this happens, a free-swimming, microscopic, pear-shaped larva known as a planula forms. It moves through the water by moving tiny hairs called cilia that cover its body. If it evades all predators, it eventually settles down on the sea floor to develop into a polyp. After the polyp becomes established, it will reproduce by budding. These will break off and form medusoid floaters, which reproduce sexually, and begin the cycle again. This alternation of generations is virtually unique to Cnidarians, although not all follow this pattern.

Types of Cnidarians (Phylogenetics)

There are five taxonomic classes of the Phylum Cnidaria.

Class Anthozoa - True Corals

Anthozoa means 'flower-animals', an apt name for the class of corals, sea fans, sea whips, and sea anemones. This class is found exclusively in marine environments, and most are colonial, sessile forms with elaborate polypoid stages. In reproduction, they usually forgo the medusa stage entirely, and are able to reproduce sexually in the polypoid form. Except for tube anemones, animals in this class produce a hard skeleton. This is the class of Cnidarians that is most often fossilized.

The class includes two major Subclasses, Octocorallia and Zoantharia. Octocorals such as the sea fans, sea whips, sea pens, and soft corals have eightfold radial symmetry, where each polyp has eight branching tentacles. This differs from the true corals and sea anemones of Zoantharia, which have sixfold radial symmetry. Both can be broken down into orders.

Octocorallia

• Telestacea: Snowflake-like soft corals, inhabiting the Pacific Ocean.
• Alcyonacea: Other soft corals which often inhabit low tidal zones of rocky shores.
• Stolonifera: 'Organ-pipe corals'.
• Gorgonacea: Sea fans and sea whips.
• Pennatulacea: Includes the sea pens and sea pansies.
• Helioporacea: This order includes only one genus, Heliopora, the so-called 'blue coral' of the Indian and Pacific Oceans.

Zoantharia

• Tabulata: This extinct colonial coral is distinguished by its structure: small, closely packed calcite tubes in which the organism probably lived, with small horizontal dividers called tabulae along the length of the tube. About 280 genera strong, this order first appeared in the Early Ordovician (480 million years ago), built giant reefs during the Silurian and Devonian periods (440-360 million years ago), and went extinct at the end of the Permian (235 million years ago).
• Rugosa: This Order of 800 genera were mostly solitary corals that lived from the Middle Ordovician to the Permian, when they succumbed to the same mysterious event as their Tabulata cousins. They are commonly known as horn corals, due to their unique horn-shaped chamber whose wall is wrinkled, or rugose.
• Scleractinia: This coral which builds the spectacular reefs we see today is the only surviving Order of Zoantharia. They appeared in the middle Triassic (220 million years ago) after the other coral's extinction. It is not known if they are descended from the extinct corals, or if the plan of a hard exterior evolved again from a soft-bodied ancestor. They share key morphological features, but their skeletons are made of aragonite, compared to the calcite of their extinct cousins. Whatever the case, they diversified to well over 800 genera, and developed massive, branching forms that continue to awe divers to this day.

Class Scyphozoa - Sea Jellies

Class Scyphozoa, while only about 90 living genera strong, is an unpleasant one to run afoul of, as it is primarily made up of jellyfish. Like the Anthozoa, they are all marine species and spend their lives drifting in the ocean currents, catching food with their tentacles, which can be up to 40 metres in length. It is unknown exactly how old this class really is because they rarely fossilize, and when they do, it is usually as an easily overlooked blob. However, some of these blobs have been found in the exceptional Burgess Shale, 505 million years old.

Class Hydrozoa - Fire Corals, Portuguese Man O'War, and Freshwater Hydras

This class contains the only freshwater Cnidarians, but most members are marine. Most spend the better part of their lives as polyps, though some have a free-floating medusa stage. Most are soft-bodied, and those that have hard parts make them out of chitin, which rarely fossilizes. Many members of this class are solitary but others form colonies, some pelagic³ like the Portuguese Man O' War, and others sessile, some of which form reefs. Fire Corals and freshwater hydras are also part of this class.

Class Cubozoa - Box Jellies

This aptly named class contains the box jellies, which look like cubes when viewed from above. Cubozoans are similar in form to scyphozoans, but they have only four evenly spaced tentacles or bunches of tentacles, and well developed eyes. Like their other soft-bodied cousins, they do not have a good fossil record. There are about 20 known species living in tropical and subtropical waters, with the Australian stinger Chironex fleckeri counted among the deadliest creatures in the world.

Class Staurozoa - Stalked Jellyfish

Until recently, the stalked jellyfish were regarded as members of the Scyphozoa class. They have recently been placed in their own class, as their development differs from that of other members of the order - staurozoans remain as polyps throughout their life, never entering a medusa stage.

What about Comb Jellies?

Ctenophores, or comb jellies, superficially resemble jellyfish in that they are 'wibbly' and have long trailing tentacles. In the past Ctenophora and Cnidaria were considered closely related because they have similar body plans. However, the resemblance between Cnidarians and Ctenophores is likely to be due to convergent evolution. This occurs when two evolutionarily distant or physically distant species or populations evolve the same behaviour or physically adapt to their environment in the same way. From two different origins, species can converge to appear similar or identical to each other even though they may be unrelated.

Conservation

The Phylum Cnidaria hosts a diverse and important group of species. Coral reefs provide food and habitat for many species and are often referred to as the 'rainforests' of the sea. However, due to coral's symbiotic relationship with photosynthetic organisms which give them their energy, they are particularly susceptible to problems like coral bleaching and pollution.

Coral bleaching occurs when the symbiotic zooxanthellae leave their coral hosts due to adverse conditions. It can occur for many reasons such as a change in sea temperature, increased pollution or reduced light levels. If the water temperature changes by sometimes as little as a few degrees or if there are too many pollutants in the water, it can kill them off. If their departure is extended for more than a month or so the coral will start to bleach and eventually die off. Coral bleaching is unfortunately on the rise and poses a serious threat to the biodiversity of the seas.

There is a wealth of information about this ancient phylum but, as with all species that live in the sea, there still remains much to be learnt about their way of life and their ecology.