Polyps, medusae, and jellyfish

Within Biology, there is a lot of terminology, to say the least. There are common names, Latin names, names for tiny structures that seem self-explanatory, two or more names for the same structure, and names for every context in which an organism can be considered.

Example: the terms medusae and polyp.

Now, most people know what a jellyfish is. But most likely, medusae and polyp are fairly foreign terms. That's because these words describe the forms which the jellyfish's body can take on at different stages of its life. The terms are used for other creatures in Phylum Cnidaria as well, which means that more than one specific animal could be under consideration when the words "medusae" or "poylp" are mentioned. It is important to realize that a medusae is not a species in and of itself, nor is a polyp. Think of it this way: a human can come in one of two forms, male or female. But a "male" is not some different sort of creature than a human. A dog can be a male; a lizard can be a male; but a "male" in and of itself is not a sort of separate creature. Likewise, a human can be considered an "adolescent" or an "adult," but these are stages of life that the human goes through, not particular creatures with their own DNA and unique physical features/life habits. "Polyp" and "medusae" are the two main life stages of a jellyfish, but they are not their own creatures.

A polyp is a form which certain Cnidarians take on for either their whole life, or just a stage of their life cycle This form is characterized by its cylinder shape. The bottom of the polyp attaches, sometimes permanently, sometimes not, to something solid. At the top of the polyp is a mouth opening surrounded by tentacles, which capture small food particles out of the water (all Cnidarians are aquatic). The following picture illustrates a coral polyp.

The medusae body form is usually saucer or bowl shaped, the space inside the "bowl" acting as the "stomach" and "lung" of the Cnidarian. Food and oxygen will be taken into this opening, digested in the case of food, and  absorbed into the body of the Cnidarian. Tentacles either cover the surface of the medusae, or hang down around the body opening, officially known as the gastrovascular cavity. The medusae form is motile, meaning it is able to float or purposefully propel itself from place to place. Examples can be seen in the following pictures (all photos are courtesy of Wikimedia Commons).

What I find particularly interesting about jellyfish is that they actually go through both a polyp and a medusae stage in their life. A jellyfish larvae looks very different from its parents--somewhat shoe-shaped and covered in tiny hairs. Eventually it finds a hard surface to cement itself to, and here it will enter its polyp stage, growing tentacles and feeding on tiny organisms. The polyp continues to grow, and will develop segments that begin separating from the top of the polyp. Each of these segments is a mini medusae. When sperm from a male medusae fertilizes an egg inside a female medusae, another little larvae will come into existence and start the process over again.

From one polyp comes many medusae--is it any wonder then that jellyfish numbers increase at exponential rates, and can easily become an infestation if not controlled by climate and predators?

In conclusion--polyp and medusae are terms that refer to the two main body forms of Cnidarians. They are not stand-alone creatures; rather, they describe the physical appearance of specific creatures.