Within the invertebrate animals we find the edge of the cnidarians, an interesting group that contains an important diversity of species. The name cnidarian is taken from the term "cnidocytes", specialized cells unique to the group. Some members of this phylum are deadly animals to humans, while others can cause minor nuisance and a few are almost imperceptible.
Do you want to know what are cnidarians? We invite you to continue reading this article on our site to learn about the types of cnidarians, specific examples, their main characteristics and much more.
What are cnidarians?
Cnidarians are invertebrate animals of aquatic life and with a more complex constitution than sponges, but smaller than bilateral ones. Despite this last aspect, they can be good predators of animals much more complex than themselves.
They have an ancient fossil record, which dates back about 700 million years. Therefore, within the cnidarians we find an important diversity, represented by a variety of beautiful and strange forms, which include plant-shaped hydroids, flower-like anemones, jellyfish and the peculiar horny corals and stony corals, which form the fascinating coral reefs, which are associated with a biodiversity of great ecological importance.
Types of Cnidarians
The classification of cnidarians, according to the Integrated Taxonomic Classification System [1], corresponds as follows:
- Animal Kingdom
- Phylum: Cnidaria
In turn, we distinguish three subphyla, in which the different types of cnidarians are grouped. The first of these is the anthozoa Here the class Anthozoa is included, which groups the corals, flower animals and sea anemones
The second subphylum is Medusozoa, which groups the medusasand is classified into the following classes:
- Cubozoa (sea wasps, box jellyfish)
- Hydrozoa (hydroid animals)
- Polypodiozoa (polypodiozoa)
- Scyphozoa (jellyfish)
- Staurozoa (stauromedusae)
Finally, we have the subphylum Myxozoa, which groups the myxozoa (which are microscopic parasites) and is divided into two classes:
- Malacosporea
- Myxosporea
As for the number of species within the cnidarians, it is estimated around 10,000 approximately.
Characteristics of cnidarian animals
Now that we know the different types of cnidarians, we can guess that it is difficult to establish the same characteristics for all. However, in general, we will mention the main characteristics of cnidarian animals:
- All members of the phylum are exclusively aquatic animals.
- They have radial symmetry.
- They have no defined head, so the ends of the body are called oral, near the mouth, and aboral (away from the mouth).
- Cnidarians have two basic forms: polyps or jellyfish. Some species present both, depending on the stage in which they are found, as in the case of some hydrozoans and the class Scyphozoa.
- In some species, such as corals and anemones, polyp forms generally have a tubular body, with tentacles around the mouth, and are usually fixed to the substrate at the aboral end, where there are a structure that is sometimes disc-shaped. This type of cnidarian can live alone or in an organization called a colony.
- Jellyfish-like cnidarians typically have bell- or umbrella-like bodies, with a quartered body arrangement. Commonly, in these cases the mouth is located in the center, on the concave side of the animal, with the tentacles extended around the umbrella.
- All members of this phylum have an endoskeleton or exoskeleton, which may be chitinous, calcareous or protein in composition.
- Some have a substance called mesoglea, which functions as a hydroskeleton and, although it contains mainly water, it is also made up of proteins, collagen and other substances. This substance is found, for example, in corals and jellyfish.
- The body of cnidarians has a single opening, which functions as both a mouth and anus. This is known as gastrovascular cavity.
- These animals generally have tentacles around their mouths.
- As a unique feature of the phylum, cnidarians have some specialized cells called cnidocytes, very abundant in the tentacles, with which they inject substances chemicals that immobilize their prey; they also use them for defense. Some species possess lethal substances for human beings, as is the case of certain species of the class Cubozoa.
- Cnidarians also have other specialized cells, including muscle and sensory cells. The latter are capable of synapsing.
- Another characteristic of cnidarians is that they don't have defined organs.
- Polyp-shaped cnidarians are sessile, while jellyfish-shaped ones can be carried away by currents. However, it has recently been proven that they are capable of swimming, for which they rely on contraction movements of the body.
- They have a nervous network that controls vital processes.
- They have sensory structures, which allow them to respond to stimuli such as light, temperature or pressure changes, among others.
- Undigested food remains are excreted through the mouth.
- Respiration occurs through the body's cells, which take in oxygen and release carbon dioxide.
Cnidarian habitat
Cnidarians, as we have mentioned, are exclusively aquatic animals, well adapted to live in this environment. The vast majority of species inhabit in s altwater ecosystems, however, some other species live in freshwater bodies.
Depending on the species, cnidarians can be cosmopolitan or restricted to particular habitats. On the other hand, depending on the animal they also develop in deep or shallow waters, as well as with different temperature conditions. Additionally, some are located in open waters, while others towards the coasts. Cnidarians that swim move from one place to another depending on the conditions of the environment.
Examples of cnidarian habitats
For example, we find the moon jellyfish (Aurelia aurita), which has a wide distribution throughout the oceans, except in the Arctic, so it inhabits the seas of America, Asia, Europe, Australia and some areas of Africa. The freshwater jellyfish (Craspedacusta sowerby), on the other hand, is native to freshwater bodies in Asia, as are members of the genus Hydra, which live in freshwater. And for its part, the fearsome sea wasp (Chironex fleckeri) is mainly restricted to Australian maritime watersand to Southeast Asia.
On the other hand, the coral reefs, which can be made up of various types of cnidarians, have a more defined habitat, the which is usually associated with tropical-type seas, at depths that do not usually exceed 50 meters, with temperature ranges from 20 to about 28 ºC and certain conditions of salinity. These form part of highly productive marine ecosystems with an ecological relevance for the planet, and are also associated with other coastal marine ecosystems, with which they provide mutual benefits.
Cnidarian feeding
Depending on the type of cnidarian, there is a feeding mode in these animals. Thus, some are active predators, such as jellyfish, which hunt their prey using their tentacles, oral arms and poisonous cnidocytes to paralyze them and then devour them. On the other hand, sessile life cnidarians, such as corals, feed by filtering dissolved substances in the water, absorbing the particles. Some species obtain nutrients through algae with which they establish symbiotic relationships, which live inside the animal, which is why they are known as endosymbionts.
Thus, depending on the species, cnidarians feed on:
- Phytoplankton
- Zooplankton
- Dissolved Particles
- Fishes
- Crustaceans
- Mollusks
- Eggs
- Other cnidarians
Cnidarian reproduction
Cnidarians can have a type of asexual or sexual reproduction, with the particularity that various species have both reproductive forms. In asexual reproduction, buds can sprout from outgrowths of the body that are later shed. In other cases, individuals may divide in half and even some of the class Anthozoa reproduce by dividing above the body base. In the article on Asexual Reproduction in Animals we talk in more detail.
As for sexual reproduction, it is related to species that have a life phase in the form of a polyp (which divides asexually) and another of a jellyfish, which corresponds to sexual reproduction itself. In this way, there are dioecious cnidarians, that is, some males and other females, which produce sperm and ovules, respectively. If you are interested in delving deeper into this topic, do not miss this post on Jellyfish Reproduction.
Examples of cnidarian animals
We have already mentioned that cnidarian animals are quite a diverse group, but let's meet some particular examples of species:
Class Anthozoa
In this class we find animals that have flower-shaped polyps, it also includes various types of corals,Anemones, Feathers and Seafans . Some examples are:
- Staghorn Coral (Acropora cervicornis)
- Pale anemone (Aiptasia pallida)
- Caribbean Giant Anemone (Condylactis gigantea)
- Slotted Brain Coral (Diploria labyrinthiformis)
- Common Sea Fan (Gorgonia ventalina)
Class Cubozoa
Here we have commonly called " box jellies" because of the square shaped cross section of the bells. They are active predators and good swimmers. Some examples are:
- Irukandji Jellyfish (Carukia barnesi)
- Sea wasp (Chironex fleckeri)
- Warted sea wasp (Tamoya gargantua)
- Hikurage or Fire Jellyfish (Virulent Morbakka)
- Mangkaprun Klong or sarong (Chironex indrasaksajiae)
Class Hydrozoa
It is quite a diverse group, with both polyp and medusa stages, with a complex reproductive process. Among its members we find:
- Portuguese warship (Physalia physalis)
- Freshwater polyp (Hydra viridissima)
- Flower Cap Jellyfish (Olindias formosa)
- Genus fire coral (Milleporidae dichotoma)
- Wind sailors (Velella velella)
Class Polypodiozoa
It is a genus of parasitic cnidarians and only has one species: Polypodium hydriform. Its taxonomic location is under review. Interestingly, it lives inside the cells of other animals:
Class Scyphozoa
Here we find the majority of jellyfishCnidarians of this class are well diversified in habitat, from arctic to tropical waters. They are very common cnidarians in coastal waters, although some live at certain depths. Examples include:
- Moon jellyfish (Aurelia aurita)
- Sea nettle (Chrysaora quinquecirrha)
- Lion's mane (Cyanea capillata)
- Golden jellyfish (Mastigias papua)
- Cannonball jelly (Stomolophus meleagris)
Class Staurozoa
The ones in this group are usually small animals, associated with boreal waters and are usually adhered to some substrate. Its members include:
- Stemmed jellyfish (Haliclystus antarcticus)
- Spotted kaleidoscope jellyfish (Haliclystus octoradiatus)
- Stalked jellyfish (Manania handi)
- Stalked Trumpet Jellyfish (Depastromorpha africana)
- Trumpet bell jellyfish (Lipkea stephensoni)
