What are Cnidarians?

Cnidarians are animals that have a variety of colours and shapes. They range from transparent jellyfishes in the ocean to the vibrant colours of a sea anemone on a intertidal zone. A Cnidarian’s body is soft while their mouths are lined with stinging structures called nematocysts.

Many life cycles have two different and distinct states, a sessile flowerlike polyp, and a motile bell-shaped medusa. Both of these stages have a body wall that surrounds the gastrovascular cavity. A gastrovascular cavity is an internal space inside of the organism where digestion takes place. The body wall has three layers: epidermis, gastroderm, and mesoglea. Epidermis is the layer of cells that is on the outer surface of the body. Gastroderm is the layer of cells that covers the inner surface, lining the gastrovascular cavity. Mesoglea is the layer between the other two, and it ranges from a think noncellular membrane to a thick jellylike material that may contain wandering amebocytes. The mesoglea is thin in polyps and thick in medusae. Most Cnidarians are only a few layers thick because they have not had the need to evolve into more complicated body systems for basic survival.

Cnidarians lack a brain and a centralized nervous system. What they have is a simple nervous system that is called a nerve net. Nerve nets are throughout the bodies of Cnidarians. The nerve nets are concentrated where the mouth is to inform the nematocysts when to sting.

Cnidarians lack three organized systems: the circulatory (internal transport) system, the respiratory system, and the excretory system. Instead, they respire, and excrete wastes through their body walls by diffusion.

All Cnidarians have radial symmetry, two germ layers, and specialized cells and tissues.

Most Cnidarians live in marine water, but some live in freshwater. Their habitats range from the middle of the ocean to intertidal zones to lakes. One imporant adaption for Cnidarians is that they live in symbiosis with tiny photosynthetic protists that live inside of them. (more on this later).

Examples of Cnidarians are hydroids, jellyfish, sea anemones (image above), portuguese man of war, sea pansies, sea wasps and corals.

Important vocabulary is in bold.

Feeding + Digestion

Cnidarians are primarily predators: they catch and eat other organisms. Cnidarians capture food via stinging tentacles, harpoons and even tangling nets, but poisonous stingers called nematocysts are the most common (see right). Animals in the phylum Cnidaria eat a variety of food. Some of these animals include large protists, crabs, worms, fish and even other cnidarians! As said in class, some groups of coral (cnidaria) have a symbiotic relationship with algae. The algae feed on carbon dioxide from the coral, photosynthesizes and releases oxygen. In turn, the coral is given energy filled carbohydrates and both organisms prosper. Digestion is very simple in Cnidarians. The cell membrane is very thin so most waste is able to be diffused through the cells. Also, any undigested food is expelled from the gastrovascular cavity, out the mouth and into the water.

Reproduction

In the Phylum Cnidaria, most organisms can reproduce sexually and asexually. In asexual reproduction, polyps reproduce new polyps by budding. Budding starts with a swelling in the adult organism before breaking off and becoming a new polyp. It is important to note that polyps that spring from budding can also be from tiny medusae.

This diagram below shows the life cycle of a Jellyfish undergoing sexual reproduction. Sexual reproduction in Cnidarians happen when mature medusae produce gametes (male and female) and the male gametes are released into the water for fertilization. Depending on the organism, fertilization can happen in a medusa or in the water. Female gametes are also released into the water if the fertilization of the organism happens in the water. The zygote (fertilized egg) grows into a ciliated larva before it becomes a polyp on a surface in which reproduction will take place again.

Movement + Response to Environment

Most cnidarians use their fluid filled body cavity as a hydrostatic skeleton. Muscle surrounding the cavity contracts and relaxes, causing the incompressible liquid to be pushed back and forth. As water moves in one direction, the cnidarian moves the opposite way. Because most cnidarians do not have a central nervous system, they rely on sense organs to determine whether they are near a predator, moving up or down, and towards or away from light. The oceans current plays a huge role on determining where the organism will move. Although some species appear to be sessile, all cnidarians are motile to some degree (such as sea anemones, see left). There are both advantages and disadvantages of how these organisms move. The lack of inner structures (bones etc.) makes it very difficult for predators to grab a hold of. Also, it is very light and agile, and water current allows for greater genetic diversity when reproducing. However, the structure is very fragile. In a storm, various cnidarians are quite vulnerable to being washed up on shore, or forced out of their habitat/niche.

Comparisons to Other Phyla

There are a few similarities between phylum Porifera and phylum Cnidaria.

• Both corals (Cnidaria) and sponges create chemicals so they do not become infected and other organisms cannot grow on them.
• Cnidarians and sponges have symbiotic relationships with organisms that are not animals.

A difference between phylum Porifera and phylum Cnidaria.

• Sponges manufacture a toxic chemical to fend off the enemy, while Cnidarians sting enemies/prey with nematocysts which are connected to a simple nervous system.

A similarity between phylum Cnidaria and phylum Platyhelminthes.

• They both use a gastrovascular cavity as their digestive system.

A difference between phylum Cnidaria and all the other animal phyla.

• Only Cnidarians use nerve nets as their nervous system to capture prey and to protect itself.

Interesting Facts

Here are some interesting facts about Cnidarians.

• Jellyfish is a Chinese delicacy and is very common as an appetizer.
• Some of the chemcials produced by corals help us learn about anti-cancer drugs, and the disease itself.
• Some Cnidarians are the longest animals in the world, one of them that can stretch as long as 40m is the Praya dubia (see image right).
• Cnidarians can weigh up to several tonnes
• The Cnidaria are the oldest of the true metazoan phyla. A fossil (Hydrozoan) from South Australia called Ediacara is 700 million years old!
• There are four main classes of cnidarians:
  - Anthrozoa (corals, sea anemones)
  - Scyphozoa (jellyfish, siphonophores)
  - Cubozoa (box jellyfish)
  - Hydrozoa (various spectrums)

WORKS CITED

Stevens, Allan. "Cnidaria." Snow College. 2001. 15 Feb. 2007 <http://www.snow.edu/~allans/biology1320/cnidaria.html>.

Bird, Jonathan. "Cnidarians: Simple But Deadly Animals!" Oceanic Research Group. 2001. 11 Feb. 2007 . <http://www.oceanicresearch.org/cnidarian.html>.

Collins, Allen, Ben M. Waggoner, and David Smith. "Introduction to Cnidaria." Univeristy of California Museum of Paleontology. 9 May 2006. Berkeley Natural History Museums. 8 Feb. 2007 . <http://www.ucmp.berkeley.edu/cnidaria/cnidaria.html>.

"Corals and Jellyfish." ReefED. 8 Feb. 2007 . <http://www.reefed.edu.au/explorer/animals/marine_invertebrates/corals/index.html>.

Haddock. The Bioluminescence Website. 18 Jan. 2007. University of California Santa Barbara. 16 Feb. 2007 . <http://www.lifesci.ucsb.edu/~biolum/organism/pictures/praya.html>.

Hotash. "Sea Anemone." Flickr. 31 Dec. 2006. 8 Feb. 2007 .<http://www.flickr.com/photos/hotash/342630417/>.

McDarby, Michael. "Fmcc Website." Biology of Plants and Animals. Jan. 2001.<http://faculty.fmcc.suny.edu/mcdarby/Animals&PlantsBook/Animals/02-Sponges&Cnidaria.htm>.

Miller, Kenneth R., and Joseph Levine. Biology. Fourth ed. Upper Saddle River, New Jersey: Prentice Hall, 1998. 563-569.
<http://www.snow.edu/~allans/biology1320/cnidaria.html>.

Sjostrom, Timothy. "Cnidarians." Bellarmine. 2004. Bellarmine College. 14 Feb. 2007 .<http://cas.bellarmine.edu/tietjen/images/cnidarians.htm>.