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PHYLUM CNIDARIANS
THE SCYPHOZOANS | THE HYDROZOANS | THE ANTHOZOANS
THE CHARACTERISTICS OF CNIDARIANS
Cnidarians can be found throughout California in a myriad of locations. From down in the sand, to perched on a rock, and all throughout the water column, cnidarians are well adapted to survival off of our coast. Though they vary in size, the general structures of cnidarians do not vary greatly.
THE BODY STRUCTURE OF CNIDARIANS
The body of cnidarians have radial symmetry in which any cut along any portion of the sponges axis will produce identical sides. The body of cnidarians are comprised of an outer cell layer exposed to the environment called the epidermis and the internal cell lining the gastrodermis. Between the gastrodermis and the epidermis is a jelly substance sandwiched in between called the mesoglea. Cnidarians have only one body cavity, named the coelenteron, which is responsible for digesting the cnidarians prey. This central body cavity serves as both the mouth and anus in the cnidarian phylum. Surrounding the mouth is a barrage of tentacles are with stinging cells named nematocysts. Nematocysts are structures with microscopic spring loaded harpoons which are triggered when another organism brushed against the cnidarians tentacles.
Two growth forms dominate the cnidarian family. These two growth forms are described as the medusa growth form and the polyp growth form. Both forms are classic cnidarian body forms with a central mouth surrounded by tentacles. In the case of the medusa form, the body is a free swimming form in which the mouth faces the ground as the cnidarian is swept by currents. A popular representative of the medusa growth form are the jelly fish which wash up onto the shore in the summer months. The polyp form is not always completely stationary in all forms of cnidarians. However all these polyp cnidarians will spend the vast majority of their lives on a rock substrate once they reach adult hood. The corals of more tropical waters and anemones are popular members of the polyp growth forms.
PREDATORS AND PREY
Cnidarians are predominantly carnivores, although many form symbiotic relationships with single cellular algae called zooxanthellae. Carnivorous cnidarians capture their food with the stinging nematocysts located all along their tentacles which are triggered by their prey as the prey swims by unassumingly. The first nematocyst sting triggers a whole series of events in which the stunned prey is pulled towards the mouth. As the prey is pulled to the mouth it is grabbed by many other tentacles and bombarded by their charged nematocysts. Here it should be noted that the nematocyst is attached to the tentacle and even though the nematocyst is released from the cell, it is still attached to the tentacle in a similar fashion that a shot spear is still attached to the spear gun. Once stung by the nematocyst the enzymes released serve to stun the prey as well as begin the digestive process. Although a very unsavory thought for us, the mouth of the cnidarian also doubles as an anus, capable of excreting waste material. Cnidarians lack organs capable of digesting their meal. Instead cnidarians have a digestive cavity called the coelenteron. The coelenteron itself is commonly subdivided into a central digestive cavity (gastro) which begins digestion and a peripheral distributive region. In the gastro cavity additional enzymes are released to further breakdown the prey. Next the monociliated cells in the peripheral distributive area take the partially digested prey and begin transferring it throughout the body. The digestive process is then completed individually by each cell in the body. Cnidarians will eagerly devour virtually anything it can fit into its mouth.
While cnidarians will eat just about anything, there are some creatures immune to their nematocysts who prey on the cnidarians themselves. Of these perhaps the nudibranchs are the most colorful. Many nudibranchs will bite off the tips of the tentacles of their prey and wear these nematocyst loaded tentacles for protection before making a meal of the rest of the cnidarian.
REPRODUCTION
Cnidarians such as the Aggregating Anemone can reproduce asexually via a process called "budding." During budding a small knob begins growing at the base of the anemone and over time this bud develops into a miniature version of the original anemone and separates.
THE CLASSES OF CNIDARIANS
THE SCYPHOZOANS
| MOON JELLIES
The members of the class scyphozoans are free swimming, spending all, or the vast majority of their life in what is called the medusa stage. In many ways these organisms can be likened to upside down anemones. The moon jelly to the right can be found in the summer months washing ashore from deeper waters in huge numbers.
HYDROIDS |
Corals are the most popular members of the hydrozoans.
THE ANTHOZOANS
SEA ANEMONES | SEA PENS | TRUE CORALS |
Off of our coast there are many anthozoans. From the intertidal zone to beyond the recreational dive limits, members of anthozoa can be found. Most anthozoans take the medusa growth form as represented by the anenome to your right. In this form the body of the anenome usually grips to a substrate while the stinging cell carrying tentacles line the mouth/anus of the anthozoan.
Copyrighted by Matthew J. Wahlrab 2002 - 20042