Cnidaria is a phylum of the animal kingdom. It is a wide-ranging group of aquatic animals on planet earth. While professional and amateur biologists might be able to call certain animals to mind by phylum name alone, the rest of us will recognize this group by more common names: jellyfish, sea anemone, corals, and more. This phylum partially consists of the strange salt-water creatures in shallow rifts and semi-tropical streams. They wash up along the shore and sometimes inspire stinging terror in the waves where we swim. 

Today we are going to explore the world of cnidarians. We will learn about their basic anatomy, life cycles, and evolutionary changes. Additionally, we'll look at how these creatures are impacted by climate change. 

The Basics of Phylum Cnidaria 

Pronounced nid-AIR-ee-ah, this phylum consists of over 10,000 species of aquatic animals. Another name that people occasionally use is Coelenterata. "Cnidaria" derives from the Greek word "cnidos" which means  "stinging nettle". These stinging cells surround the mouths of cnidaria species. It is an integral part of their predatory feeding habits.  

Cnidarians are incredibly diverse in form. There are huge corals, box jellies with complex eyes, and feathery strands of hydroids. But again, what these diverse forms all share are cnidocysts that contain stinging cells. As a result, scientists presume that these organelles called nematocysts, stem back to a common ancestor. And, nearly all of the species are predators. 

Cnidarian species live in tropical waters and semi-tropical waters. Sometimes we find them in freshwater. Overall, it depends on the species and the various habitats in which they have evolved to inhabit. 

Predators of cnidaria include starfish, sea slugs, parrotfish. Marine turtles eat jellyfish and often mistake plastic bags floating in the water for a food source. Consequently, this can block the gut of the sea turtle and result in death. 

Cnidarian Body Forms

These creatures contain two tissue layers (which is known as diploblastic) but no organs. While these organisms contain no heart, brain or lungs, they do use their internal cavity as a means for respiration. The ectoderm, or epidermis outer layer and body wall, maintains cells that aid in capturing food and other cells that secrete mucus. Contrarily, the endoderm, or inner layer, produces cells that excrete enzymes to break down food particles. The gastrodermis forms around the stomach of the organism. 

Important body functions are conducted through a process called diffusion. This is a process in which molecules move from an area of higher concentration (think of crowded zones) into a less concentrated area. This process allows an organism to get the oxygen it needs to survive. 

And within this phylum, there are two basic body types. These are swimming medusae and sessile polyps. Both categories exhibit radial symmetry. In both cases, the invertebrates have a mouth that is surrounded by cnidocytes, also known as tentacles. 

Furthermore, cnidarian body forms contain mesoglea. This is a jelly-like substance. It lies between the two tissue layers of the epithelium. In cnidarians, this tissue is usually one cell thick. 

Sessile Polyps 

Firstly, the polyp form is shaped like a hollow cylinder or a tube. It's like a bag that opens at the top and bottom of the entity’s tissue. The top, or the mouth, leads to the polyp’s central body cavity which is called the gastrovascular cavity. Once attached to hard surfaces, these creatures are only able to capture prey that wanders into their tentacles. The layer of mesoglea between this specific body form's tissue is very thin. 

Swimming Medusae

Next are the medusae. The medusa (singular) form is free-swimming. Compared to the open-tubed polyps, these radially shaped cnidarians look similar to an umbrella. Medusoid bodies have a mouth and tentacles that hang downwards in the water. Different from the polyps, the downward-facing mouth of medusae leads up to the gastrovascular cavity. Also, their mesoglea jelly is thick and makes up most of their body mass. 

In the medusa form, there is an inner nerve ring and an outer nerve ring, cell layers in the body that contain nerve cells. This decentralized nerve system is called a nerve net. 

In particular species, polyps will transition to a medusa stage and back again to a polyp stage, and vice versa. Usually, one stage or the other is dominant within a species's lifecycle. 

In many ways, these two body forms are simply the inverse of one another, though their parts function in similar ways. 

Life Cycles of Cnidarians

All animals undergo a life cycle - which is a series of changes that occur in the lifetime of an organism including reproduction. 

In the case of cnidarians, reproduction happens sexually and asexually. Depending on the species, it is possible for the same organism to produce both eggs and fertilizing sperm cells. Scientists refer to this phenomenon as simultaneous hermaphrodites. In these instances, gametes, little bundles of egg-and-sperm mixes, are released into the oceans.

Fertilization happens internally and, in the case of many corals, externally. Spawning can happen en mass in relationship to changing temperatures or the full moon. Alternatively, other species of cnidarians utilize a “brooding” technique of sexual reproduction. Both brooding and spawning sexual reproduction result in larvae. 

We identify asexual reproduction in these organisms as budding and fragmentation. If the organism produces many buds, we identify it as a colony. These sorts of reef-building colonies are famous for their size and alter the ocean floor. 

Similarly, these organisms are able to reproduce lost or damaged parts of their body tissue - tentacles can grow back and chunks of floating tissue can become entirely new organisms. 

A Jelly Life Cycle Example

Let's look at the example of a jellyfish: the adult medusa forms produce fertilized eggs or sperm, which then develop into a planula, or larva. Then, the larva eventually attaches to a hard surface and forms a polyp. Once the polyp developed into a strobila, a term used to describe a polyp ready to "bud". When the strobila buds out, it releases an ephyra. The ephyra is a clone of the polyp that then grows into an adult medusa. Eventually, the adult medusa has a jellied body. Equally important, these adult jellyfish have gonads in the gastroderm - which release sperm and ova into the water depending on the breeding season. This is considered a complex life cycle.

Cnidarians vs. Ctenophora

In learning about cnidaria, it is easy to confuse this phylum with a different one: Ctenophora. However, the main difference and the easiest way to remember is that cnidarians are radially symmetrical whereas ctenophora exhibit biradial symmetry. Both groups of animals are diploblastic animals with tentacles around their mouth, so they do share many traits in common.  

The Four Cnidarian Classes

Out of the 10,000 different species of cnidarians, there are four distinct groups or classes to know. Within those groups, there are some true show stoppers and cold killers worth remembering. 

Anthozoa

Anthozoans is a group that includes true corals, anemones, and sea pens. Class Anthozoa is arguably the most famous of the four groups because of the renowned corals that form in tropical waters and play a critical role in ecosystems in addition to human religions, mythology, and identity. 

Scientists have been able to track a long and variable fossil record of anthozoans, as well. Extending back at least 550 million years, anthozoans were present in the late Precambrian period, though true coral did not develop until later on in the middle Triassic period when the first dinosaurs were evolving on our planet. 

In regards to morphology, Anthozoans complete lack the medusa stage and are polyps throughout their life cycle. Each individual has a sack-like body divided into radio pieces called septa. Septa are visible with the human eye and are key in categorizing various species. 

Hydrozoa 

The class hydrozoa is the most diverse class of cnidarians and includes siphonophores, hydroids, fire corals, and many free-swimming medusae species. Many introductory biology students will recognize Hydra from a class - and while this is one of the more popularly known species within the class, it is actually a-typical, since it never goes through a medusoid stage. 

This class is often colonial in that hydrozoans will form branching colonies that often resemble true corals. 

Other species of hydrozoans develop floating colonies. You might easily confuse them with jellyfish. However, these colonies are made up of many individuals with specialized functions. “Portuguese-Man-O-War” that occasionally wash up upon the shore to the amazement of visiting tourists is an example of such a hydrozoan colony. 

Scyphozoa

Scyphozoans are the group for true jellyfishes, and to any regular beach-goers, these species might be familiar. These graceful, dangerous animals use their stinging tentacles and if contacted, can cause skin rashes, muscle cramps, and death. 

The largest species we know of in class scyphozoa is Cyanea arctica which wields tentacles over 40 meters long. Their life cycle alternates between polyp and medusa stages and in some parts of the world, these creatures are a food source of communities of people. The broad, plate-like shape of the jellyfish’s tissue is fringed with tentacles. 

Cubozoa

The cubozoan is a grouping that features the Box Jellyfish, with its powerful toxins and complex eye structure. 

This class is very similar to the true jellyfish of the class Scyphozoa. You’ll be able to quickly spot the difference when viewing from above as box jellyfish are more square in shape. Hence their common name!

There are about 20 species and subspecies of cubozoan box jellies found in tropical and semitropical waters. These creatures can swim fast, maneuver around objects, and see well without the advantages of a brain.

For instance, in Australia - the land of poisonous animals big and small - the sting of Chironex fleckeri is considered one of the deadliest creatures in the world and causes human fatalities in 3 minutes. It grows to about the size of a human head and its tentacles grow up to 3 meters long. 

Cnidarians and Climate Change

Like all creatures and ecosystems on planet earth, species of cnidaria are vulnerable to changes in temperature, light, and chemical shifts in the earth’s atmosphere and water bodies. 

Ecologists have been sounding the alarm on coral reefs for years, and many people watch in horror as “mass bleaching” events take place in what was once a thriving nursery for a wide variety of marine animals - housing 25% of all life in the ocean. This is due to the rising temperatures of the ocean water - which shows no indication of slowing down unless we make some radical changes to the ways in which our societies function. 

Climate change continues to intensify. We struggle to gather the political will to curb human behavior. Less heat-tolerant species will struggle to survive, while other cnidaria species will be able to potentially travel and spread to new areas. The added stress of extreme conditions is a critical area of study in ecology and zoology. It is imperative that we all care enough to reimagine our societies for a livable planet. You might be relieved to swim in waters with fewer stinging creatures. However, our ecosystems are delicate, intricate webs of life that depend on certain species' continued survival. 

Conclusion 

Cnidaria is a phylum of ancient water inhabitants. As a result, these creatures mystify and sometimes painfully electrify our imaginations and bodies. 

Some people fixate on the vast mysteries of the sky above. But, we know from the wild world of the cnidaria phylum that the ocean's critters can be just as alien to us as creatures from a far-away galaxy. In studying these creatures, we come closer to glimpsing the intricacies of a delicate planet. Cnidarians are a gorgeous array of jelly-filled, colorful, and sometimes deadly organisms. They are just one example of strange, brainless creatures of intelligence. Above all, they are beings we need to simultaneously protect and continue to investigate.