(Cnidarian Facts) – 11 Amazing Facts About Cnidarians In Detail

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Cnidarians are all invertebrates that fall under the Phylum Cnidaria of the animal kingdom. Phylum Cnidaria is also known as the Phylum Coelenterata.

They are all radially symmetrical simple organisms that are found only in aquatic habitats. Cnidarians include over 11,000 known species.

They are called Cnidarians because of the presence of distinguishing cell types called cnidocytes, which are the specialized cells that they use mainly for capturing prey.

Famous examples of Cnidarians are jellyfish,  sea anemones, stony corals, soft corals, medusozoa, gorgonian, sea pens, and hydrozoans.

One interesting fact to note is that the Cnidarians were formerly grouped with Ctenophores under the phylum Coelenterata of the Animal Kingdom. But later as researchers have found out more clues, differences, and findings of their existence, they got placed under separate phyla.

Find Out More About These Amazing Creatures That Includes Some Completely Mobile Species While Others Being Completely Sessile With Various Life Cycle Stages.

Here Are The 11 Amazing Facts About Cnidarians You Must Know About. Let’s Know

1. Cnidarian is a phylum of aquatic, freshwater, and most marine animals

Yes, the phylum Cnidarian includes aquatic, some freshwater, mostly marine, sessile, and free-living, radially symmetrical animals. They can be solitary or colonial.


Cnidarians can be typically seen in almost all ocean habitats in shallow to deep water that can be warm or cold.

In shallow water in the coastal regions, you will often find them as Coral Reefs forming colonies and having a mutualistic relationship with the algae living inside them.

Some cnidarians like many species of Jellyfish can even migrate from one place to another within the ocean in search of food, for reproduction, etc.

Many species of cnidarians are known to live for many years, as they can regenerate their body parts, thus making them effectively immortal.

Hydrozoans like Hydra live in freshwater, Obelia live in coastal ocean water.

Anthozoans like scleractinian corals, sea pens, and sea fans live in the deep cold sea waters of the polar regions.

Scyphozoans are the True jellyfish species like Aurelia, Crown jellyfish species, etc. that are seen in mid to deep ocean water. They can also be seen in a variety of water depths and closeness to shore depending on the species and life cycle stage.

2. Depending on the species, Cnidarians can live for a few days to more than 4,000 years

Cnidarians can have a life span of about a few months to more than 4,000 years, while some are even immortal. Many can regenerate their lost body parts and that’s an advantage to them.

Most cnidarians that include jellyfish are short-lived, like for example, the Moon Jellyfish (Aurelia) live approximately for 8 to 12 months only.

Some species of Jellyfish can live for several years, or even decades. Immortal jellyfish (Turritopsis dohrnii) is the one species of Jellyfish that is almost immortal.

Sea anemones are known to live more than Jellyfishes. They can live for as long as 60 to 80 years of age because they are able to clone themselves and as such, they don’t get old so easily.

Other cnidarian species like Hydra are known to reach maturity in about 5 to 10 days, with having a lifespan of at least 4 to 6 years.

Corals are notable for having the most life span in the whole ocean ecosystem on planet earth. Some species of corals can live for over 4,000 years like the deep-water black coral (Leiopathes glaberrima).

3. There are two forms of Cnidarians, called the Polyp and Medusa

Polyp and Medusa are the two different body forms of Cnidarians. The polyp body form is non-motile while the Medusa body form is free-swimming.

Polyp is the hydroid form that is sessile with the mouth of the organism faced upwards and is being surrounded by extensible tentacles bearing the nematocysts.

In polyp, the upper-end bearing the mouth is also called the free end of the body which is actually hollow and cylindrical in nature. While the lower end of the polyp is typically adapted for attachment to the surface substratum.

Examples of solitary polyp include sea anemones, while those of the colonial polyp include corals.

Medusa is the umbrella or bell-shaped body with the mouth facing downwards. They are all free-swimming in nature. It is the typical form of jellyfish.

Some cnidarians exist only in the polyp form, while others exist in the medusa form throughout their life. Some species of Cnidaria have both stages, that is the polyp and medusa stage during their life cycle, for example in the species of Class Hydrozoa.

4. They can reproduce both sexually and asexually, and their life cycle varies within the classes

Depending on the species type they can reproduce both sexually and asexually.

The polyp form is known to reproduce both sexually as well asexually, while the medusa form is known to reproduce sexually only.

So, in a more clear-cut way, it can be said that the polyps primarily reproduce asexually by budding. Some polyps can also produce male and female gametes to reproduce sexually.

Medusae on the other hand, must reproduce sexually, using both eggs and sperm cells.

Now for the sexually reproducing cnidarian species, the organism can be either monoecious or dioecious.

Monoecious have both the male and female reproductive organs in the same individual, while dioecious species have separate sexes for gamete production.

5. They are all stinging creatures

All Cnidarians have tentacles with stinging apparatus called cnidae (singular: cnida) which are used to capture and subdue prey.

There are three main types of cnidae: nematocysts, ptychocysts, and spirocysts, with many variations. Depending on the type of species, one or more types of cnidae may be present on the organism that’s unique to that type of species.

The nematocysts are equipped with barb-like tips that help to inject venom into their prey. These barb-like tips help in embedding the nematocysts into the victim’s body. Most cnidarian species have nematocysts.

The ptychocysts are those cnidae that put out a sticky substance and are used to hold on to the prey. These are found exclusively in the tube-dwelling anemones (Ceriantharia).

The spirocyst is a lasso-like string that is fired at the prey which later wraps around the prey.

The cnidocyte, cnidoblast, or nematocyte cells have the cnida with a hollow, coiled, thread-like structure.

These cells can be triggered by touch to get the stimuli to attack the prey within a few microseconds after the touch.

6. They have tissue level of body organization

Cnidarians are classified as the simplest organism of the animal kingdom that has the tissue level of body organization. They are the true tissue having organisms.

They lack the organ level of body organization, as they don’t have any organs like hearts or lungs. So the various types of tissues help them in the proper functioning of the body.

Their tissue level of body organization can be well seen as their body surface is made up of three layers viz. the outer ectoderm, the middle mesoglea (a jelly-like substance containing scattered cells and collagen fibers), and the inner endoderm.

They do also have distinctive tissue for digestion, movement, catching prey, and also for attachment to the substratum.

All of the tissues work together and have specialized jobs.

7. They do have a mouth, but don’t have an anus

Yes, they do have an internal cavity that acts both as respiration cavity and stomach (gastrovascular cavity) with the mouth opening through which the food is ingested.

They do not have a separate anus, and so the mouth opening acts as the anus.

In the case of polyps, the mouth is located on the upper body side, while in the case of medusae, the mouth is located on the lower body side.

When the food is ingested through the mouth it enters the gastrovascular cavity, where the enzymes are secreted that break down the ingested food particles, and the cells lining the cavity absorb the nutrients from that food.

Excretion occurs by means of waste products removal either through the mouth or through the surface cells by water circulation.

8. Their body is transparent due to the presence of jelly-like mesoglea

Mesoglea acts as a transparent hydrostatic skeleton in the body of the cnidarian. It acts as an internal skeleton for the cnidarians.

It is non-cellular and is situated between the two-cell layer (outer ectoderm and inner endoderm) of the body surface.

The mesoglea contains a jelly-like substance with freely floating cells, collagen, heparan sulphate proteoglycans along with muscle bundles and nerve fibres.

The mesoglea also contains amoebocytes that aid in the process of phagocytosis and so in removing debris out of their body. Amoebocytes cell can also fight various kinds of infections by producing many antibacterial chemicals.

It is due to the presence of the mesoglea that the cnidarian body remains elastic in nature and such properties help restore the shape of the body after it is deformed by the contraction of the muscle tissues.

One notable fact about the mesoglea is that it is not rigid enough and is jelly-like to touch and feel. So, due to this reason, the mesoglea can’t bear the weight of their body when they are taken out of the water and so the body gets deformed.

So, without the buoyancy property of water to support the organism when it is inside the water, the mesoglea won’t be able to bear the weight of the whole body.

9. The existence of Cnidarians have been estimated to be 740 million years old

Phylogenetic studies and Molecular Clock analysis show that Cnidarians have evolved on earth around 740 million years ago.

Paleontological studies show that fossil cnidarians have been found in rocks formed about 580 million years ago.

The study of fossils also shows that the existence of corals can be dated before 490 million years ago.

Various theories were also suggested based on the evolution of cnidarians. Among these, almost the majority of the theories state that the radial symmetry and tissue level of body organization is the proof that indicates that the cnidarians have evolved before the bilaterally symmetrical organisms.

Many of the theories also indicate that the medusa body form being the sexually reproductive phase of their life cycle is the original body form of the cnidarians that had evolved from the ancestral organisms.

A few of the theories also bring light to the concept that the original cnidarian was a planula-like organism that had preceded both polyp and medusa body forms.

10. They don’t have any digestive or circulatory system, but simple nervous tissue is present

Yes, as already said that the cnidarians have a tissue-level of body organization. They do lack the organ system level of body organization.

They don’t have any distinctive digestive or circulatory system, but simple nervous tissue is present as nerve net.

There are also no respiratory organs, and the both cell layers (outer ectoderm and inner endoderm) absorb oxygen and expel carbon dioxide into the surrounding water.

It is also to be noted that the Cnidarians don’t have any organs like hearts or lungs. They have an internal cavity used for respiration and a gastrovascular cavity.

Cnidarians of the class Scyphozoado (True jellyfish) and class Cubozoa (Box jellyfish) have sensory organs called the rhopalium that remain borne on the margin of the bell in the medusa. These organs contain the statocyst and eyes with lenses.

Rhopalium are actually those special sensory organs with specialized structures that help the cnidarians to sense light (ocelli) and perceive gravity (statoliths).

Rhopalium also controls the pace of swimming-muscle contraction and in enabling the balance and orientation of the body while moving.

11. There are only four recognized classes of Cnidaria viz. Hydrozoa, Cubozoa, Scyphozoa, and Anthozoa

According to the classification scheme followed by Ruppert and Barnes in 1994, the Phylum Cnidarian is classified as Class Hydrozoa, Cubozoa, Scyphozoa, and Anthozoa.

Class Hydrozoa includes species like Hydra, Obelia, Millepora, Physalia, Valella, Porpita, etc. in which the individuals are either exclusively poly­ploid or medusoid or constituted by both forms in the life cycle.

Class Cubozoa includes species like Carybdea, Chiropsalmus, Chironex, Tripedalia, etc. that are Medusoid with a bell-like distinctly flattened body with four sides having four tentacles or tentacle clus­ters at the four corners.

Class Anthozoa includes species like Gorgonia (sea fan), Pennatula (Sea-pen), Alcyonium (Deadmen’s finger), Helipora, Telesto, Tubipora (organ-pipe coral), Corallium (red coral), etc. that exists only in the polypoid form with having a cellular mesoglea. The polyps are with eight tentacles and eight septa.

Class Scyphozoa includes species like Aurelia, Pilema, Lucernaria, Pericolpa, etc. that significantly exist in the Medusoid form which is dominant in the life cycle of the organism, while the polypoid form is very insignifi­cant.

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