Cnidaria is is a Phylum of Invertebrate Animals consisting of Sea Anemones, Corals, Sea Pens, Jellyfish, Box Jelly, Portuguese Man o' War etc. They are found exclusively in aquatic and mostly marine environments. There are over 9,000 Species of Cnidaria. Cnidarians were for a long time grouped with Ctenophores in the phylum Coelenterata, but increasing awareness of their differences caused them to be placed in separate Phyla. The word 'Cnidaria' is pronounced as "Nidaria" and not "Sneedaria" as the 'C' in 'Cnidaria' is silent.

• Cnidaria Examples

• Cnidaria Characteristics

• Cnidaria Distribution

• Cnidaria Evolution

• Cnidaria Reproduction

Fossil of Cnidarians have been found in rocks formed about 580 million years ago and other fossils show that Corals may have been present shortly before 490 million years ago and diversified a few million years later. In the form of Jellyfish and Box Jellies, Cnidarians are dangerous as their stings have killed several hundred people, at the same time some Cnidarians like some large Jellyfish are considered a delicacy  in eastern and southern Asia. Coral Reefs are important part of the marine environment as they provide home and food to various Species of Animals.

Scientific Classification

Domain - Eukaryota.
Kingdom - Animalia.
Phylum - Cnidaria.

Classes, Orders, Subclasses & Suborders of Cnidarians (with examples)

1.  Anthozoa - Example Sea Anemones, Corals, Sea Pens.

ORDERS

• Stolonifera
• Telestacea
• Gorgonacea
• Alcyonacea
• Helioporacea (Coenothecalia).
• Pennatulacea
• Subclass Ceriantipatharia
• Antipatharia
• Ceriantharia
• Subclass Zoantharia
• Actiniaria
• Corallimorpharia
• Scleractinia (Madreporaria)
• Zoanthinaria (Zoanthidea)

2.  Scyphozoa - Example Jellyfish.

ORDERS

• Coronatae
• Rhizostomae
• Semaeostomeae
• Stauromedusae

3.  Cubozoa - Example Box Jellies.

4.  Hydrozoa - It includes all the freshwater Cnidarians as well as many marine forms, and has both sessile members such as Hydra and colonial swimmers such as the Portuguese Man o' War.

ORDERS

• Actinulida
• Chondrophora
• Hydroida
• Suborder Anthomedusae
• Suborder Leptomedusae
• Suborder Limnomedusae
• Milleporina
• Siphonophora
• Stylasterina
• Trachylina
• Suborder Laingiomedusae
• Suborder Narcomedusae
• Suborder Trachymedusae

Phylum Cnidaria Characteristics

• Diploblastic - Cnidaria are Diploblastic Animals, i.e. they have two main cell layers, while more complex Animals are Triploblasts, having three main layers. The two main cell layers of Cnidarians form Epithelia that are mostly one cell thick, and are attached to a fibrous basement membrane, which they secrete. They also secrete the jelly-like mesoglea that separates the layers. The layer that faces outwards is known as the Ectoderm. This outside skin generally contains Epitheliomuscular cells,  harpoon-like 'Nettle Cells' known as 'Cnidocytes', Sensory cells, Interstitial cells etc.

• Radially Symmetrical - Cnidarians are Radially Symmetrical. This means that their gastrovascular cavity, tentacles, and mouth are aligned such that if an imaginary line was to be drawn through the centre of their body, from the top of their tentacles through the base of their body, one can then turn the Animal about that axis and it would look roughly the same at each angle in the turn. Another way to look at this is that Cnidarians are Cylindrical and have a top and bottom but no left or right side.

• Two Basic Forms - Cnidarians come in two basic forms.

i)  Medusa - The Medusa form is a free-swimming structure which consists of an umbrella-shaped body called a Bell, a fringe of tentacles that hang from the edge of the bell, a mouth opening located on the underside of the bell, and a gastrovascular cavity. The mesoglea layer of the medusa body wall is thick and jelly-like. Some Cnidarians only exhibit the Medusa form throughout their life while others first pass through other phases before maturing into the Medusa form. The Medusa form is most commonly associated with adult Jellyfish.

ii)  Polyp - The Polyp is a Sessile i.e. it is attached to the sea floor and often forms large colonies. The Polyp structure consists of a basal disc that attaches to a substrate, a cylindrical body stalk, inside of which is the gastrovascular cavity, a mouth opening located on the top of the Polyp, and numerous tentacles which radiate out from around the edge of the mouth opening. Some Cnidarians remain a Polyp for their entire life, while others pass through the Medusa body form. The more familiar Polyp Cnidarians include Corals, Hydras, and Sea Anemones.

• Presence of Cnidocytes - Cnidocytes are specialized cells located in the epidermis of all Cnidarians. These cells are unique to Cnidarians as no other organism possesses them. Cnidocytes are most concentrated within the epidermis of the tentacles. Cnidocytes contain organelles called 'Cnidae'.

There are several types of Cnidae which include:

Nematocysts - Nematocysts consist of a capsule containing a coiled thread and barbs known as 'Stylets'. Nematocysts, when discharged, deliver a stinging venom that serves to paralyse prey and enable the Cnidarian to ingest its victim.

Spirocysts - Spirocysts are Cnidae found in some Corals and Sea Anemones that consist of sticky threads and help the animal capture prey and adhere to surfaces. Ptychocysts are found in members of a group of Cnidarians known as the 'Ceriantharia'. It includes Sea Anemones and Corals.

Ptychocysts - Ptychocysts are not used for prey capture, instead the threads of discharged Ptychocysts are used for building protective tubes in which their owners live. Ptychocysts are found only in the order 'Ceriantharia', Tube Anemones.

In Hydras and Jellyfish, the Cnidocytes cells have a stiff bristle that projects out from the surface of the epidermis. This bristle is called a Cnidocyl (it is not present in corals and sea anemones, which instead possess a similar structure called a ciliary cone). The Cnidocyl serves as a trigger to release the Nematocyst. Cnidocytes can only fire once, and about 25% of a Hydra's Nematocysts are lost from its tentacles when capturing a Brine Shrimp. Used Cnidocytes have to be replaced, which takes about 48 hours. To minimise wasteful firing, two types of stimulus are generally required to trigger Cnidocytes: their cilia detect contact, and nearby sensory cells "smell" chemicals in the water. This combination prevents them from firing at distant or non-living objects. Groups of Cnidocytes are usually connected by nerves and, if one fires, the rest of the group requires a weaker minimum stimulus than the cells that fire first.

• Absence of Excretory, Circulatory & Respiratory Systems - Cnidaria have no brains or even central nervous systems. Instead they have decentralized nerve nets consisting of  Sensory Neurons that generate signals in response to various types of stimulus, such as smells; motor neurons that tell muscles to contract; all connected by cobwebs of intermediate neurons. As well as forming the 'Signal Cables', intermediate neurons also form ganglia that act as local coordination centres. The cilia of the Cnidocytes detect physical contact, and nerves inform Cnidocytes when prey or attackers are smelt and when neighbouring Cnidocytes fire. Most of the communications between nerve cells are via 'Chemical Synapses', small gaps across which chemicals flow. As this process is too slow to ensure that the muscles round the rim of a 'Medusa's Bell' contract simultaneously in swimming the neurons which control this communicate by much faster electrical signals across gap junctions. Gas exchange takes place directly across the surface of their body and waste is released either through their gastrovascular cavity or by diffusion through their skin.

• Regeneration - All Cnidarians can regenerate, allowing them to recover from injury and to reproduce asexually. Medusa have limited ability to regenerate, but Polyps can do so from small pieces or even collections of separated cells. This enables Corals to recover even after apparently being destroyed by predators.

Distribution (geographical range & habitat)

Coral Reefs are a dominant feature of shallow tropical saline waters (at least 23.5 degrees centigrade). The northern and southern extend of present Coral Reefs define the extreme boundaries of the tropics. Cold water is not suitable for the formation of larger Reefs, although some solitary Corals can live in colder water. The Indo-Pacific region has the most diverse Coral population that include over 700 Species. In the tropical Atlantic there are only 35 Species and are concentrated in the Caribbean area.

Diet - Most Cnidarians prey on organisms ranging in size from plankton to animals several times larger than themselves, but many obtain much of their nutrition from endosymbiotic algae, and a few are parasites. Some Cnidarians are parasites, mainly on Jellyfish but a few are major pests of Fish.

Predators - Their Predators include Animals like Starfish, Sea Slugs, Fish like Butterfly Fish and Parrot Fish, Turtles etc.

History and Evolution of Cnidarians

The origin of the Phylum Cnidaria is not very clear. The oldest fossilized Cnidarians were soft-bodied. Representatives of all four modern classes have been identified in Ediacaran fauna of the Precambrian Period (that is, those appearing between about 635 million and 542 million years ago) known from more than 20 sites worldwide. However, there are few fossil Cnidarians of the Cambrian Period (542 million to 488.3 million years ago). Presumed fossil sea anemones are found in the lower Cambrian System. Colonies of 'Stromatoporoidea', considered to be an order of the class Hydrozoa that extended from the mid-Cambrian Period to the Cretaceous Period (about 145.5 million to 65.5 million years ago), produced massive skeletons. Although there were two groups of Paleozoic Corals, neither of which has modern descendants, they were not great reef-builders during that era. Scleractinians arose in the mid-Triassic Period; Blue Corals, Gorgonians, Millepores, and Hydrocorals have records from the Jurassic Period (199.6 million to 146 million years ago) or the Cretaceous Period to the present. Most other Cnidarians are known only from the Holocene Epoch (within the last 11,700 years).

Cnidarian Reproduction

All Cnidarian Species are capable of sexual reproduction, which occurs in only one phase of the life cycle, usually the Medusa. Many Cnidarians also reproduce asexually, which may occur in both phases. In asexual reproduction, new individuals arise from bits of tissue that are budded off from a parent, or by a parent dividing lengthwise or crosswise into two smaller individuals. Polyps that remain physically attached to one another or embedded in a common mass of tissue constitute a colony. In some colonies, Polyps share a common Coelenteron (sac like cavity) through which food captured by any member is distributed to others. Hydrozoan Polyp colonies, called Hydroids, are prostrate, bushy, or feathery in form. Examples of other colonies are anthozoan soft corals and most reef-forming hard corals. Polyps that are produced asexually and then physically separate are called Clones, or Ramets. In this way, a single genotype can be represented by many separate “individuals.”

Cnidaria sexual reproduction often involves a complex life cycle with both Polyp and Medusa stages. For example in Scyphozoa  (jellyfish) and Cubozoa (box jellies) a Larva swims until it finds a good site, and then becomes a Polyp. This grows normally but then absorbs its tentacles and splits horizontally into a series of disks that become juvenile Medusa, a process called Strobilation. The juveniles swim off and slowly grow to maturity, while the Polyp re-grows and may continue strobilating periodically. The adults have gonads in the gastroderm, and these release ova and sperm into the water in the breeding season. Spawning (laying of eggs) is generally driven by environmental factors such as changes in the water temperature, and their release is triggered by lighting conditions such as sunrise, sunset or the phase of the moon. Many Species of Cnidaria may spawn simultaneously in the same location, so that there are too many ova and sperm for predators to eat more than a tiny percentage. Such an event occurs in the Great Barrier Reef, where at least 110 Corals and a few Non-Cnidarian Invertebrates produce enough to turn the water cloudy.