The 5 Classes Of Phylum Cnidaria

Phylum Cnidaria also known as Phylum Coelenterate is a group of around 11,000 soft and hard-bodied, stinging organisms commonly found in marine habitats, yet some can be found to live in freshwater environments.

Examples of these species include jellyfish, hard corals, sea anemones and hydras.

In order to better understand these organisms, we’re going to break them down into different categories as well as look at the differences in reproductive cycles, lifecycles and habitats.

The 5 Classes of Phylum Cnidaria

History

The phylum name is derived from the Greek word for stinging nettle. Previously known as Coelenterata, they are thought to be one of the oldest groups in evolutionary history with a distinct form.

Fossil records vary but they can be traced up to 600-700 million years ago which would make the group one of the oldest forms of metazoan.

Classification

The phylum Cnidaria can be classified into five categories:

  • Anthozoa (anthozoans)
  • Scyphozoa (scyphozoans)
  • Hydrozoa (hydrozoans)
  • Cubozoa (cubozoans)
  • Staurozoa (staurozoans)

Those classified as cnidarians share several characteristics, supporting the theory of a single origin.

For example, Cnidarians all display radially symmetry as they have a central point or axis through their body shape meaning they are nearly perfectly symmetrical.

They are diploblastic, which means their bodies are made up of two embryonic cellular layers called the ectoderm and the endoderm.

Ectoderm refers to the outmost layer or epidermis of the organism whereas endoderm refers to the inner portion of the organism making up the inner gastrodermis.

Each of these layers contains basic types of cells that perform different functions in place of organs more advanced animals would rely on.

For example, secretory and gastrovascular cells needed for the digestion of nutrients. Cnidarians are very basic biological organisms which lack sensory organs in the head.

Forms

Interestingly, there are two different life stages or body forms in these animals, dependent on the specific species.

Although the majority of polyps don’t transform into medusae, exhibiting one stage their whole life, the majority of medusae (jellyfish) have a polyp stage with some even alternating between the two such as hydrozoans.

The polyp stage is characterized by tubular-shaped animals with their mouths facing up towards the water.

These animals lack sense organs and can reproduce sexually and asexually making them known for their primitive attributes.

Medusa stages mean that an animal is mobile, such as a jellyfish and are further categorized by their bell-shaped bodies and downward hanging tentacles.

In contrast to polyp, medusa species have their mouths facing downwards, they possess sense organs such as photoreceptors and exclusively reproduce sexually.

Tentacles

Another shared characteristic between Cnidaria is their unique stinging structures usually found in tentacles surrounding the mouth.

Nematocysts cells are found on the tentacles, which when triggered wrap around the prey injecting poison into it causing a paralyzing or stunning effect in order to easily capture and consume it.

These stringers can also be used to defend themselves against predators. All Cnidarians are carnivorous.

Bodily Processes

Cnidarians can lack cephalization which basically means an organism has evolved without sensory or neural organs in an anterior head.

If you’ve ever seen a jellyfish up close you’ve probably noticed the lack of any discernible organs in their clear shapes. But they do have some bodily processes that help to aid in their survival, however simple.

Digestive System

The digestive system of Cnidarians is incomplete, meaning the gastrovascular cavity is both the mouth and anus.

When prey is captured and drawn into the mouth, enzymes are discharged in order to break down the food.

This is an example of extracellular digestion.

When the enzymes have finished breaking down the food particles, the cell lining of the gastrovascular cavity can absorb the nutrients.

In place of an anus or other excretory organs, the waste material is then expelled through the mouth opening or can diffuse into the water outside the animal from the cells themselves.

Nervous System

Their nervous systems are very simple and primitive but help the organisms respond to the environment.

They have something called a nerve net which is distributed across the body and largely consists of groups of cells, motor neurons and sensory neurons.

This system acts as a sensory locator allowing the animal to detect changes in temperature, capture food, expel waste and move in response to stimuli in the environment.

A more developed nerve net is associated with medusae due to their free-swimming nature. This swimming motion is coordinated by their nervous system.

Let’s look at these five in a little more detail:

Anthozoa

Anthozoa are marine invertebrates such as sea anemones, stony corals and soft corals. This is probably the largest and most well-known of the cnidarians.

Classes of Phylum Cnidaria

Most of us have seen coral either in documentaries, pictures, aquariums or in their natural habitat but what are their defining characteristics?

These organisms are characterized by their cylindrical column shape topped by a disc containing the mouth in the centre, also known as a polyp.

The mouth is surrounded by some form of tentacles. The aboral end or lower part of the body of adults attaches itself to surfaces or the seabed using a structure called a pedal disc.

If you’ve ever been stung by a jellyfish, you know why we quickly learn to fear animals with tentacles regardless of the size.

Despite their stationary nature Anthozoans tentacles can still sting those passing as they contain specialized cells called cnidocytes which house an organelle called a cnidocyst (or stinging cells).

Cniodcysts are important for prey capture and also help to act as an anti-predator defense.

You therefore won’t be surprised to learn that anthozoans are carnivores that use their tentacles to catch their prey.

Some have venom in their tentacles and can be used to harpoon and subdue their prey whereas others have longer tentacles which can be used as weapons.

Shallow dwelling corals are reef-building anthozoans and in spite of their predatory nature, they also have another source of food.

Single-celled algae called zooxanthellae live in the tissues of corals and photosynthesize in the warm, shallow waters passing on some of the food to their hosts.

Zooxanthellae can also provide corals with oxygen and remove some of their waste.

In exchange, the coral provides some of its own nutrients to the algae alongside providing shelter.

This symbiotic relationship grants enough energy for the corals to rapidly multiply and grow to build the vast reef structures we see in tropical waters.

Scyphozoa

Class Scyphozoa consists of about 200 different species of jellyfish making them solely marine animals.

This category includes the jellyfish most commonly seen at the beach or in the sea by the general public, generally characterized by their bell-shaped or helmet-shaped appearance.

However, this along with color and size varies between species.

Their bodies are diploblastic and have radial symmetry, exhibiting the main characteristics of cnidarians. In line with this, they feature venomous tentacles surrounding the mouth on the underside of the animal, which helps them to capture prey.

Their mouth is the only part of their structure that is connected to their digestive system.
If a jellyfish stings a human this can be extremely painful and in extreme cases can even cause death.

The typical size of these species varies from 2 to 40 cm in length however some, like the Lion’s mane jellyfish (Cyanea capillata) can grow up to 2 meters in diameter with tentacles reaching a staggering 30 – 40 meters.

The medusa form is the dominant form associated with Scyphozoa as they are mostly motile animals.

They lack velum meaning they have to rely on the contracting and relaxing of muscles in the bell-shaped part of their body which pushes the water out in order to move the jellyfish along.

Scyphozoans, both polyp and medusa, lack any sensory organs or a brain, but some species are known to have light-sensitive eyespots located on the margin of the medusa.

Hydrozoa

The subgroup Hydrozoa contains approximately 3,700 diverse species. These can live in their marine environments or freshwater.

Most have both polyp and medusa stages in their lifecycle making them polymorphs, although it should be noted that some have one or the other.

Freshwater species tend not to have a medusoid stage.

In terms of physical characteristics, they are mostly small animals measuring around 2 inches or less but there are still some larger species which can reach around 17.7 inches (40cm).

Typical hydrozoans like hydra can have a small tube or polyp-like body featuring tentacles arranged around the ‘head’ of the organism.

Examples of hydrozoans are freshwater jellyfish, freshwater polyps, the Portuguese man o’ war and the pink-hearted hydroids.

Hydrozoa can live in solitary conditions or in large colonies depending on the species.

Cubozoa

One of the smaller groups of phylum Cnidaria are the cubozoa, with only around 50 known species.

Members of this group are defined by their box-shaped medusa which is why you may know them referred to as box jellyfish.

5 Classes of Phylum Cnidaria

Physically they range from 15 to 25 centimetres in diameter. They predominantly exist in medusa form with a higher range of mobility.

Cubozoans have muscular pads known as pedalia in each corner of their box-shaped body from which one or more tentacles stem from.

The nemocytes are arranged in a spiral pattern along the tentacles of some Cubozpans. Similarly to other jellyfish, the mouth faces downwards to the water underneath the animal.

Other organs found on the curved side of the animal include the gastric pouch, gonad and Rhopalium.

In certain species, the connection between the mouth and digestive system has been seen to extend into the pedalia.

Staurozoa

Another smaller, yet relatively new class of phylum Cnidaria consists of around 50 species, known as Staurozoa. They can measure between 1 to 4 centimeters in diameter.

Staurozoans are polyp-like medusae otherwise known as ‘stalked jellyfish’ and they spend their lives attached to a base or substratum such as algae via a stalk-like feature called a peduncle.

Certain species can be quite colorful, whereas others mimic and camouflage themselves amongst the algae they live on. Others can have distinct patterns on their calyx.

Similarly to the common freely moving jellyfish, staurozoans have an umbrella-shaped body called the calyx, in contrast, they have four bifurcated or eight normal arms to which their tentacles are attached.

Many of the species have a trumpet or goblet-shaped body.

The tentacles which contain the stinging characteristics of the animal are often arranged in pairs. Sometimes the tentacles are on extended arms of the main body and often arranged in pairs.

In certain species, arms and tentacles are evenly distributed giving the appearance of them being evenly distributed.

The eight sets of tentacles are referred to as secondary tentacles in order to differentiate them from the eight sets of primary tentacles that occur in the development of juvenile animals.

Primary tentacles are often lost in adulthood with some species able to retain them and others they can develop to serve as anchos due to their great adhesive properties.

The mouth can be found in the main umbrella part of the body and has four distinct corners. Four funnel-like structures or depressions called infundibula are visible 45 degrees askew of these corners.

The funnels indicate the beginning of four longitudinal muscles that run down the length of the stalk (peduncle).

Location

Cnidarians can be found in almost all oceans as well as a few freshwater habitats.

They live in water regardless of how shallow or deep, or warm or cold it is. As mentioned some can live in colonies whilst others are solitary.

Corals only exist as polyps. Although some live in large colonies, others can live in solitary conditions in the depths of the ocean.

The majority of polyps need solid substrata to attach themselves to which commonly appear in shallower waters closer to the shoreline.

Reef-building corals inhabit shallow tropical waters. Here they are the most abundant and diverse.

The algae in the corals’ tissue need sunlight to photosynthesize, their symbiotic relationship means that the corals stay in shallower waters.

Tropical coral reefs are mainly inhabited by hard anthozoans and Caribbean reefs are dominated by soft anthozoans.

A well-known example of this is The Great Barrier Reef, Australia. Sea anemones however can live in extremely deep parts of the oceans.

Jellyfish live their lives as medusae and can live almost anywhere in the open ocean. Certain groups such as hydrozoans and scyphozoans live in surface waters which is why you commonly see them on the beach or near areas high in human activity.

Reproduction

As Cnidarians can reproduce using sexual or asexual methods there are many different variations between species in the reproductive cycle.

Polyp and medusa body forms exhibit different reproductive cycles. Polyps usually reproduce asexually and medusae sexually.

Certain species are known as hermaphrodites as they are able to produce both sperm and eggs (monoecious) whereas others are only capable of producing one gamete and as considered to be one sex (dioecious).

Although hermaphroditic species can produce both gametes they can’t always self-fertilize therefore sexual reproduction requires two animals.

Budding is a method of asexual reproduction in polyp species by which a small ‘bud’ develops as a growth from a parent organism, eventually breaking away.

This form of reproduction can produce another polyp or both a polyp and medusa organism.

A medusa bud is produced when the top section of the polyp separates into structures with horizontal lines, creating a medusa jellyfish.

This process is called strobilation. Note here that this is an asexual process, the jellyfish will all be the same sex.

It is common for polyps to produce more of their own kind asexually, for example in reef-building corals such large colonies can be formed that they can change the structure of the ocean floor.

Broadcast spawning is another external reproduction method in which all individuals in a colony release their gametes in a synchronous spawning event.

Once an egg is fertilized by a sperm released from a male organism in the colony a larva forms which is very important in the dispersion of sessile species such as coral as they have no other methods of creating new colonies.

The larva can settle nearby or be taken with the current and float hundreds of kilometres away.

Medusae have reproductive organs allowing for sexual reproduction.

Internal or external fertilization (dependent on the species) forms a zygote which develops into a larva called a planula.

This in turn develops into a polyp when settling on a structure such as a rock or a seabed substrate. The cycle then continues.

Lifespan

Scyphozoan jellyfish can live anywhere from a few days up to a year depending on the species.

A Lion’s Mane jellyfish for example lives up to a year and a moon jellyfish is known to live for up to eighteen months.

Other factors that can influence the lifespan of a jellyfish are food availability, predation and temperatures.

A more recent, fascinating discovery is the Turritopsis dohrnii or ‘immortal jellyfish’ which is found across the globe, mainly in tropical waters.

This hydrozoan has been found to regenerate or reverse their development from medusa back into a polyp form when they are injured, ill or even starving. Hence they have been named immortal.

When the jellyfish reverses back into polyp-form, buds are produced releasing medusae genetically identical to the original adult.

This occurs through a process called transdifferentiation; this is where an adult cell can change form into a different specialized polyp cell, therefore, allowing the jellyfish to regrow into the different polyp body form and mature again as part of their natural lifecycle.

Corals on the other hand can live much longer. Studies of elkhorn coral, an anthozoan found in the Caribbean can live up to 5,000 years, making them the oldest animals on the planet.

There is uncertainty between those in the scientific community on how to date corals, so lifespans can differ.

Individual polyps however have been found to have a median age of 4 years and a maximum lifespan of 12 years, when they die they are absorbed into the foundation for a new polyp.

Predators

All Cnidarians are predators, however, they are not immune from becoming prey themselves despite their stinging tentacles.

The most common predator that comes to mind when we think of animals such as jellyfish are turtles, however, they are also eaten by seabirds and sunfish.

Soft tissued corals and anemones are commonly eaten by sea slugs, sea stars, butterfly fish, carbs and parrot fish. Predators can be dependent on the location of the organism.

Threats

Anthozoans are listed as critically endangered species.

The greatest threat to these species is human activities. Pollution, over-fishing, trade, development of coastal zones and tourism are the most destructive human causes of coral reef degradation.

However, the greatest threat comes from climate change; ocean acidification and rising water temperatures occur as a result of increased carbon dioxide levels.

This causes corals to bleach, which can eventually kill them and their colonies.

Our oceans have lost half their coral reefs since the 1950s. The benefits coral reefs provide in terms of biodiversity, tourism and land defences are endless.

Cnidarians such as jellyfish are more tolerant of climate change, with some reports suggesting they are thriving and even taking over habitats in some areas.

Other studies have found warmer temperatures to influence metabolic activities as well as affect sexual reproduction impacting the growth rate and ability to settle and feed during their first few months of life.

Regardless of the opinion, it’s evident that warming temperatures negatively impact some Cnidarians even if it gives others the potential to thrive.

Final Thoughts

Cnidaria is a phylum containing over 11,000 marine and freshwater species of soft-bodied and hard-bodied stinging animals.

Contrary to what people may think, these animals are indeed alive despite their simple structure and lack of basic sensory organs and brains.

These creatures are categorized by their body form, they exist as either a swimming, mobile medusae or a sessile polyp. Both are known for their radial symmetry, mouths surrounded by tentacles and stinging powers.

Their life cycles can be complex and some can go through both a polyp and medusa stage. All Cnidarians can reproduce sexually and asexually and can even switch between the two.

Although many of us may be scared of being stung by a jellyfish on holiday, their ability to survive and reproduce is truly fascinating.

Those that make up tropical reef structures are under threat from human activity, despite the numerous benefits they provide to both humans and animals.

Therefore it is in our best interest to preserve these species for future generations.

Jennifer Dawkins

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