Cnidarians
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Morphology & Behavior Cnidarians are invertebrates that have cnidocytes (sting cells). A local example is the moon jelly (Aurelia Labiata). They use these as a defensive mechanism and also as a method for catching their prey. Each cnidocytes has a hairlike projection called a trigger, if one comes into contact with protein all of the cnidocytes will fire their nematocyst which have poison in their sharp tips. They have two different morphologies: a polyp stage and a medusa stage (not all classes will have both through out their life time). Both stages have radial symmetry. The polyp stage tends to be sessile and the medusa stage is mobile. Some species of Cnidarians have bioluminescence.
There are four classes of Cnidarians and over 10,000 species that have been identified (Castro ) |
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To see video footage of nematocysts firing follow the link below
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Anthozoans: Consists of solitary and colonial polyps. Polyps of this class have a more complex structure that have septa. These septa allow for extra support since this class only has the polyp phase and will never enter the free swimming medusa stage.
Just a few of the local species found on the pacific coast are Metridium senile (plumose anemone) and Ptilosarcus gurneyi (orange sea pen). Their habitat tends to be rock substrate, but the Metridium senile can be seen hanging off the bottom of the docks. Sea anemones will sting their neighbors when engaging in intraspecific competition often seen in a small band of empty space between two genetically different colonies |
Hydrozoans: This class can share both the polyp stage and the medusa stage. The polyp stage can divide asexually by budding to form colonies. The colonies can then release small medusas that will reproduce sexually by releasing gametes. This is often seen in Obelia sp. The polyp stage tends to be more dominant and some organisms may lack a medusa stage. This can be seen in the Portuguese man-of-war which is a colony of Hydrozoans.
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Scyphozoans:
Scyphozoans are the more commonly thought of Cnidarians. They have a true medusa stage and can be very large. This are often referred to as Jellyfish, the largest one known can have a bell over 10ft in diameter. Scyphozoans spend most of their life in the medusa stage and some species may often lack a polyp stage. The Cyanea capillata (lion's mane) viewed above tends to wash up on the shores of the Puget Sound, its bell will look gelatinous and red-brown. |
Cubozoans: Cubozoans tend to be the most deadly of all the Cnidarians. The toxins in their nematocysts have been known to cause excruciating pain and death with in minutes of being stung. This class is often distinguished by its box shaped dome and the four tentacle clusters found at the corners.
Recent studies show that Chironex fleckeri (box jellyfish) have eyes, although studies are still being performed to identify whether they actually see. |
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