Phylum Cnidaria

For the next few posts, I plan to go over the main phyla of the kingdom Animalia. Today’s post is about the Cnidarians (nahy-dair-ee-uh  ns). Jellyfish, anemones, and coral all belong to this phylum. Check out the Cnidarians we have in the Puget Sound at this site: http://www.bily.com/pnwsc/web-content/Cnidarian%20Identification.html

Aggregate Anemone on Sound View’s beach

Nematocysts Photo:http://www.mesa.edu.au/Cnidaria/default.asp

Cnidarians are most known for the fact that they have stinging cells, or nematocysts. They use these stinging cells to catch their food and protect themselves. Nematocysts work a lot like a harpoon. The cells have “trigger hairs.” When something touches them, they release the microscopic “harpoon” that goes into whatever touches it and releases a venom. The strength of the stinging effect varies by species. The sting of a Box Jellyfish can potentially be fatal to humans where the sting of the Plumose Anemone, a local species, is barely noticeable. Of the Cnidarians that are most common in Washington, the Lion’s Mane Jelly is the only one that has a notably painful sting. It is not generally life threatening. The sting of the anemones you can find at the beach is so mild they their tentacles just feel sticky.

Cnidarians are invertebrates, meaning they have no bones. They have very simple bodies with no organs. Their gastrovascular cavity, where they digest their food, only has one opening to it, their mouth. They get water and oxygen by simply absorbing into their bodies and they get rid of bodily waste by releasing it out of their cells or out of their mouth. They also have radial symmetry. You could “cut” a Cnidarian in half in anyway through the center and both sides would be identical.

Photo:http://www.mesa.edu.au/Cnidaria/default.asp

They have two types of body plans: the medusa and the polyp. The medusa is the typical jellyfish shape with a round bell on top and tentacles hanging down. They are free floating, though most cannot control their body movement against the water current. Their mouth points down. The polyp is the typical anemone shape. The body is generally stuck to something and the tentacles reach up. Some anemones can move where they are attached to but for most, it is a slow process to do so. Their mouth points up.

Jellies go through both the polyp and medusa shapes in their life. After they hatch from an egg, they are a free floating larva that settles down on something as a polyp. Once the polyp is big enough, it divides into many individuals that grow into medusa forms. Anemones and coral are polyps for their whole lives. Coral is a made up of a colony of polyps that, in many species, creates a calcium carbonate skeleton to live on. Coral also has a symbiotic relationship with an algae known as zooxanthellae. The algae uses sunlight to make sugar that feeds the coral polyps and the coral offers protection and carbon dioxide. Some anemones develop this relationship as well. Zooxanthellae is often what causes the bright colors of corals and some anemones.

Photo:https://ucmp.berkeley.edu/cnidaria/ctenophora.html

BONUS PHYLUM: There are a few sea jellies that belong to another phylum. Comb Jellies of the phylum Ctenophora (ti-nof-er-uh). While the comb jellies share a lot in common with the Cnidarian jellies, they do not have stinging cells. They do have specialized hairs, or cilia, that help them move and catch food. They also have sticky cells that help them catch food. Many Ctenophores are bioluminencent, producing their own light.