Brief Knowledge on Cnidaria

Organisms are classified by their phylum. Cnidaria is divided into four classes: Hydrozoa (hydrozoans), Scyphozoa (scyphozoans), Anthozoa (anthozoans), and Cubozoa (cubozoans) (cubozoans). All cnidarians share a number of traits, which supports the idea of a common ancestor. Cnidarians fascinate both laypeople and scientists due to their symmetrical body forms, varied colours, and sometimes complicated life histories. These species can be found in both marine and freshwater environments, but tropical seas are the most frequent and diverse. In most tropical areas, including Australia’s Great Barrier Reef, which stretches over 2,000 kilometres along the country’s northeastern coast, their calcareous skeletons provide the framework for reefs and atolls.

Cnidaria:

Only cnidarians develop nematocysts, or cnidae, the phylum’s name for microscopic internal stinging capsules. Because of their basic anatomy, which revolves around a central body cavity, they are also known as coelenterates (the coelenteron). When coelenterates were first defined, they contained not only the animals now recognised as cnidarians, but also sponges (phylum Porifera) and comb jellies (phylum Ctenophora). The name “coelenterate” now solely applies to cnidarians, but the latter term is used to avoid ambiguity.

Cnidarians have a body plan that is radially symmetrical (i.e., similar parts are arranged symmetrically around a central axis). They lack cephalization (concentration of sensory organs in the head), have two cell layers instead of three in so-called higher animals, and the saclike coelenteron only has one entrance (the mouth). They are the world’s simplest organisms, with cells organised into different tissues but no organs. Polyp and medusa are two cnidarian body forms that occur frequently during a single cnidarian’s life cycle.

A medusa, often known as a jellyfish, has a bell-shaped or umbrella-shaped body with tentacles dangling from the edge. The manubrium is a tube-like structure that hangs from the bell’s centre, connecting the lower end of the mouth to the coelenteron within the bell. Medusae are mostly planktonic, slow-swimming animals. Polyps, on the other hand, have an upward-facing mouth with encircling tentacles, and their cylindrical bodies are frequently attached to a hard substratum by their opposite end. A mouth is found at the end of a manubrium in many hydrozoan polyps. In anthozoan polyps, an internal pharynx, or stomodaeum, connects the mouth to the coelenteron.

Most cubozoans, hydrozoans, and scyphozoans pass through the medusoid and polypoid body forms, with medusae producing larvae that sexually change into polyps and polyps producing medusae asexually. The polyp is thus a juvenile form, whereas the medusa is an adult form. Polypoid cnidarians lacking a medusa stage are known as anthozoans. Asexual reproduction is possible for polyps and, to a lesser extent, medusae.

It’s possible that one physical type stands out more than the other. Because the medusa form is larger and more well-known than the polyp form, scyphozoans are commonly referred to as true jellyfishes. In hydrozoans, the polyp phase is more noticeable than the medusa phase in families like hydroids and hydrocorals. Hydromedusae are smaller and more delicate than scyphomedusae or cubomedusae, and they may be completely absent from the life cycle of some hydrozoan species. Although medusae are found in other species, they never separate from the polyps. Cubozoans have medusae, which are known as box jellyfish because of their form. Human deaths are caused by some species, such as the so-called sea wasps, which are predominantly found in tropical Australia and Southeast Asia. The polyp is unnoticed since it is small.

Cnidarian polyps can be as small as a millimetre in diameter. Most hydroids, hydrocorals, and soft and hard coral polyps, on the other hand, reproduce asexually to form colonies that can grow to be considerably larger and endure much longer than the individual polyps. Some tropical sea anemones (class Anthozoa) can reach a diameter of a metre, while some temperate sea anemones can reach roughly the same height. Anthozoans live a long time, both individually and collectively; some sea anemones are hundreds of years old. Medusae and sea anemones can only be found alone. Scyphomedusae can be over a tonne in weight, while hydromedusae are only a few centimetres across. Medusae, on the other hand, may have numerous and extensible tentacles that allow them to manage a considerably broader region than their body size would suggest. Large colonies of hydroids can occur on docks, boats, and rocks. Similarly, some medusae can attain incredible densities—up to tens of thousands per litre of water—but only for a brief period of time.

Many of the world’s benthic (bottom-dwelling) environments are dominated by the species Anthozoans due to their quantity and range. Although soft and hard corals coexist in virtually all tropical areas suitable for either, in the tropical Indo-Pacific, members of the anthozoan order Scleractinia (hard corals) dominate coral reefs, whereas in the Caribbean, members of the anthozoan subclass Alcyonaria (soft corals) dominate coral reefs. Corals, in addition to being the most abundant and spanning the largest area of any reef species, also help to shape the environment after they die. Soft corals are important in reef construction because their skeletal debris (spicules) fills in gaps between hard coral skeletons, acting as a cement.

Soft-bodied anthozoans are also common in other seas. Sea anemones can be found in temperate rocky intertidal zones all over the world. They take up space, making it inaccessible to other organisms, and hence have a major impact on community organisation. Liponema, a bizarre hemispheric anemone, is the most abundant benthic invertebrate in the Gulf of Alaska in terms of both numbers and biomass. Anemones can be found around deep-sea hot vents and cover portions of the Antarctic seafloor.

The most prevalent species that foul water-borne vessels are sedentary cnidarians, particularly hydroids. The muscles that give scyphomedusae their strength as swimmers are dried and consumed in Asia. Sea anemones are eaten in numerous places of Asia and North America.

Conclusion:

As the tropics, when reefs are accessible, coral skeletons are used as a building material, either in blocks or slaked to produce cement. The skeletons of cnidarians are frequently worn as jewellery. The skeleton of a hydrocoral species has a pink colour known as “coral.” Other hydrocorals have purplish bones. Skeletons are available in a variety of colours, with the more attractive ones commanding a premium. The cores of some sea fans, sea whips, and black corals are used to make beads, bracelets, and cameos.