Cnidarians: characteristics and examples

They are aquatic organisms , most of them marine, and make up the phylum Cnidaria . More than 10,000 living species of cnidarians are known and there are also fossil species dating from the Ordovician. These are the main characteristics of cnidarians :

• They are diblastic organisms, that is, their body is made up of 2 layers of cells: the ectoderm or epidermis (outer layer) and the endoderm or gastrodermis (interior), as well as a gelatinous substance between the two called mesoglea, which is especially visible in jellyfish.
• Most cnidarians have radial symmetry, which means that the parts of the body are arranged around a central point. Many cnidarians present a second bilateral axis of symmetry and others only present bilateral symmetry, which implies a single cut plane through the center.
• They are carnivorous organisms that feed mainly on crustaceans (for example, plankton), although they can also be omnivores.
• They have a gastrovascular cavity connected to the outside by means of a mouth that is surrounded by tentacles that serve, in part, to capture prey. Some cnidarians absorb dissolved organic matter directly from water. Others, such as certain types of corals, have algae that photosynthesize and provide them with carbon.
• Most cnidarians have complex reproductive cycles that include an asexual and a sexual stage. The sexual reproduction is effected by spawning (both male and female gametes release their water column, combined generating larvae) and asexual reproduction occurs by budding (an individual grows from another).
• They have a very variable life expectancy. There are types of polyps that live less than 10 days, while some corals in reefs can live in the same layer for more than 4,000 years. This is because they are able to regenerate their bodies .
• They lack specialized respiratory and excretory organs, but they do have a nervous system.
• They have stinging cellscalled cnidocytes, which serve both to feed and to defend themselves. Inside the cnidocyte there is a capsule called the cnid, which may have a lid or operculum. Inside the cnid is the characteristic filament of this cell, which is coiled and folded, called the nematocyst. When the cnidocyte receives a series of stimuli (chemical or mechanical), these are processed on its surface and the cnido’s operculum opens and the filament fires. Cnidocytes are present in the tentacles and also in other parts of the body (such as the epidermis and gastrodermis). Nematocysts contain toxic substances and serve as a defense mechanism against predators, while in other cases they have adherent substances that help them capture prey or adhere to the substrate.
• They have a digestive cavity called the cellar, which contains the stomach, esophagus, and intestines. This cavity has an opening that serves as the mouth and anus.
• Some cnidarians are mobile and others completely sessile, but most have medusa and sessile polyp life stages. The hydroid or sessile polyp has a more or less cylindrical shape and is attached to the substrate on the aboral side, so that the mouth and tentacles are on the opposite side, freely exposed. Hydroid colonies are made up of different types of individuals, some dedicated to food, others specialized in defense and others in reproduction. The jellyfish shape, mobile, is flattened, generally with the tentacles arranged on the margins of the body. The aboral surface of the jellyfish faces upwards, while the oral face faces downwards.

There are four types or classes of cnidarians : hydrozoans, cubozoans, scyphozoans and anthozoans.

Hydrozoa (class Hydrozoa)

It includes small predatory animals that live in fresh water (hydras) or marine environments. Most generate calcite shells. Some live in isolation and others in colonies. They have a non-cellular mesoglea, lack tentacles in the gastric cavity and do not have an esophagus. In their life cycles the hydroid phase predominates although, in some species, the medusoid phase is more important.

Cubozoa (class Cubozoa)

It groups the box jellyfish, which are related to the siphomedusae but differ from them in that they are smaller, have a primitive nervous system and eyes, as well as a characteristic cube-shaped morphology. Box jellyfish stings can be fatal to humans. Opinions diverge as to whether they are to be considered as an order or a class. When they reproduce, they give rise to a relatively long-lived polypoid state called scyphistoma, which can reproduce asexually giving rise to other scyphistomas, through the formation of lateral buds. At the end of this larval stage of scyphistoma, the organism passes into the cubomedusa form, characterized by having four edges on the edges of the body where one or more tentacles with a flattened base are placed, that make up the so-called pedalio. In addition to the pedaliums, they also present ropalios, which are conglomerates of sensory organs, photoreceptors, and neurons that also appear in sciphomedusae.

Scifozoans (class Scyphozoa)

It is the class of “true jellyfish”. All its specimens live in the ocean. They have a short phase in the form of a polyp (scifopolyp or scyphistoma), while most of their lives are spent in the form of a jellyfish (sciphomedusa). They are larger than the hydrozoan jellyfish, and can reach up to two meters in length, although it is normal for them to be between 2 and 40 centimeters in diameter. They are characterized by presenting a cellular mesoglea, as well as tentacles in their gastric cavity. There are jellyfish in which fertilization takes place in the manubrium, where the embryos develop to give rise to the “planula larva”, which remains free and, at the end of its swimming life, goes to the bottom and gives rise to scyphistoma. Scyphistoma is like a polyp that can live for a few months and, through budding or strobilation processes, gives rise to new individuals. By strobilation, scyphistoma generates ephrae, small discs that remain free in the water and develop into the shape of a jellyfish. The time each phase occupies is highly variable: there are longer-lived scyphistomas, while, in other species, it is practically non-existent and the planula larva gives rise directly to an ephira …

If you want to know more about the so-called “real jellyfish”, here you can learn how jellyfish reproduce .

Antozoos (clase Anthozoa)

They include anemones, corals, and sea feathers. It is the largest class of cnidarians, with more than 6,000 known species (there are many fossil specimens) in all seas, even at great depths. They only present a polyp form, not a medusoid, and live in marine environments. They can live alone or in colonies. Individuals are column-shaped with an aboral end where they are attached to the substrate and an oral end where the mouth is surrounded by tentacles. In anthozoans, the mouth continues through a pharynx that occupies a good part of the gastrovascular cavity. In the area where the pharynx is located, anthozoans present complete or incomplete septa or mesenteries that increase the absorption and digestion surface. The most important group of anthozoans is the subclass Hexacorallia which, mainly,

Once we have explained what cnidarians are and their characteristics, we now present some examples of cnidarians indicating each class.