Re: What do jellyfish do during their lifetime?

Hello Little (?),

You have no idea how excited I am to be assigned to answer your question! You see, I study jellyfish, and very much enjoy talking about them. So let's get started, shall we?

First of all, there are no animals living in the sea that should be called "jellyfish." There are many sea creatures whose bodies are made of jelly, but none of them are fish. So let's agree to call them "jellies." That way, the ichthyologists (scientists who study fish) won't think we're talking about their animals.

Okay, now to address your question. The animals that you're thinking of are probably medusae (singular form is "medusa"). Medusae typically have a roundish body, called a "bell," with lots of tentacles hanging down. The tentacles are loaded with stinging cells that the medusa uses both to catch its own prey and avoid becoming someone else's lunch. Medusae swim by using pulsations of the bell to propel themselves through the water. The following site has a lot of information and beautiful photos of some of the medusae that live in Monterey Bay, California: http://www.mbari.org/~kraskoff/medusae2.htm

And then there are the ctenophores (pronounced "teen-o-fors"), also known as comb jelles. The comb jellies are also roundish creatures, that use eight rows of tiny paddles to swim through the water. If a ctenophore has any tentacles at all, it has only two, so it probably won't look like a medusa. Comb jellies feed either by engulfing prey whole (similar to the way a snake eats) or catching small prey using sticky cells on their tentacles. These sites have excellent pictures of ctenophores:

http://www.imagequest3d.com/catalogue/ctenophores/index.htm
http://lifesci.ucsb.edu/~haddock/plankton/cteno1.html
http://lifesci.ucsb.edu/~haddock/plankton/cteno2.html

Another fascinating group of jelly animals in the sea are the siphonophores. Believe it or not, siphonophores are related to medusae; they both belong to the same phylum, or major group of animals that are all related to each other. Siphonophores are long colonies of interconnected individuals. Some of the individuals swim, some feed, some reproduce, and some protect the colony. Siphonophore colonies can be very long, extending to 30 meters (over 100 feet) in length! Here's a link to a picture of a siphonophore:
http://lifesci.ucsb.edu/~haddock/plankton/Bargmannia.jpg

There are also lots of other groups of jelly animals, but I think we've covered the major groups. As for what jellies do all day, …well …, they pretty much just hang out. Of course, they do all the things that other animals do - they eat, protect themselves from being eaten, and reproduce. Many medusae and ctenophores "fish" for prey by dangling their tentacles in the water. They catch whatever unfortunate creature happens to blunder into the tentacles.

Jellies are usually what marine biologists call "plankton" - the animals and plants that drift along with the ocean's currents. Some plankton can swim, but they are still at the mercy of the currents. So you can think of jellies as spending their days (and nights) literally hanging out, drifting along with the movement of the water. The following sites have excellent information and pictures of jellies, and will give you a good idea of what a jelly gets to do in its life:

http://www.discovery.com/area/nature/jellyfish/jellyfish2.html

Thanks for your question!

Allison J. Gong
Mad Scientist