Cnidaria (hydroids, sea fans, jellyfish, corals, sea anemones)


Life > Eukaryotes > Opisthokonta > Metazoa (animals)



Cnidarians are bell-shaped or tubular animals. The term cnidaria is derived from stinging cells, called cnidocytes, present in this group. They have a, two layered, sac body plan; the outer tissue layer is a protective epidermis and the inner endoderm layer secretes digestive juices into the internal cavity called the gastrovascular cavity. Two basic body forms are exhibited by cnidarians during their lifecycle; a polyp has a mouth that is directed upward whereas that of a jellyfish or medusa is directed downward. In some cnidarians, one stage is dominant and the other is reduced or in other species, one form may be absent altogether.

Having both muscles fibres and nerve fibres, cnidarians are capable of directional movement by either contracting or extending their bodies. Polyps usually stay in one place (e.g. sea anemones) and medusae float or swim around in the water column (e.g. jellyfish). Tentacles, forming a ring around the mouth, can reach out and grasp prey that may have been stung by the cnidocytes. Each cnidocyte houses a nematocyst that contains a long, sometimes barbed, and spirally coiled hollow thread. When a potential prey item touches the trigger of a cnidocyte, the nematocyst is discharged and the thread either traps it or penetrates and paralyses it with toxins. The gastrovascular cavity has a dual purpose in that food items get digested there and it provides the animal with a hydrostatic skeleton (using water pressure).

There are over 10,000 species of cnidarians known globally which are placed within their classes based on the lifecycle stages that are dominant within each. The Anthozoa are the most primitive of the cnidarians and completely lack a medusoid "jellyfish" stage in the life cycle, with a polyp stage persisting throughout. In the class Hydrozoa the polyp stage is dominant and they may be solitary or colonial. The Scyphozoa have a primary medusa stage and the polyp remains small and insignificant. The Cubozoa are similiar to the Scyphozoa with respect the their lifecycles. However, they are boxed shaped, possess four evenly spaced out tentacles or bunches of tentacles and well-developed eyes, and do not strobilate like true jellyfish do. At present ~842 Cnidarian species are known from South Africa.



Some cnidarians use their cnidocytes to trap prey items. Jellyfish use their long cnidocyte laced tentacles to capture and then feed on a variety of planktonic invertebrates and are food themselves for other marine organisms like sea turtles and sunfishes. The ability to ward off would be predators, using stinging cells may also benefit other organisms, as is demonstrated by the association of sting resistant clownfish with anemones. Other Anthozoans depend on zooxanthellae, symbiotic dinoflagellates within the body walls, to live. Corals are photosynthetic animals in a sense, in that the dinoflagellates undergo photosynthesis and pass on the carbon compounds that they produce to their hosts. Cnidarians may either be completely dependant on zooxanthellae or use them to supplement their diets of trapped food prey items. Corals living deeper than the photic zone (the layer of water penetrated by sunlight) may not be dependant on symbionts but colonial, reef-forming corals depend on them, thus, reefs can only exist in shallow water.


Cnidaria and humans

Besides being fatally stung by a box jellyfish or having our commercial fishing nets clogged by jellyfish, cnidarians, especially Anthozoans, have considerable economic importance. Many Cnidarians are the source of novel biomedical compounds that have anti-carcinogenic (anti-cancer) properties. Corals provide habitats for commercially fished organisms (eg., invertebrates, algae and fish) while tempting tourists to spend their money in foreign countries, which possess reefs for diving, snorkelling, boating and fishing. Corals are traded in the aquarium and jewellery markets, while their aggregate may even be mixed with concrete to be used in the building industry in countries like the Maldives, Sri Lanka, India, Indonesia and Malaysia.

Many of these positives for humans have, however, been to the detriment of these organisms. Human population growth and urban development have contributed largely to habitat (reef) destruction, over harvesting, increased pollution and increased temperature. Coral bleaching occurs when masses of corals die due to stress induce mortalities of their colourful photosynthetic symbionts, which in turn is caused by the effects of global warming and the destruction of the ozone layer. Destructive fishing using poison, like cyanide, or dynamite is still practiced in some countries today and kill off masses of reef as a consequence.




Scyphozoa (jellyfish)



Anthozoa (sea fans, anemones and corals)



Further reading

  • Branch, G. and Branch, M. 1981. The Living Shores of Southern Africa. C. Struik Publishers, Cape Town.

Text by Wayne K. Florence

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