I am a cnidarian. That means I have a sac-like body with a mouth and tentacles. Anemones and corals are also cnidarians. I can be as small as 2 inches (5 cm) or grow over 1 foot wide (30 cm). Sometimes I hang out and just float by myself but you can also find me in groups of hundreds of my closest moon jelly friends. I am one of the most common jellyfish; you can find me in reefs all over the world.
I am one of the strangest animals on the reef. I am like a floating sac with a mouth and arms. I can swim, but not to get from one place to another. I am happy to float along, but I need to make sure that I stay near the surface of the water. That way my thin tentacles have lots of room to hang and catch food. Other jellyfish can kill humans with their stinging cells, but mine don't bother people much at all.
I spend my time floating near the surface of the water, waiting for tiny animals called zooplankton to float by. When the zooplankton brushes up against my tentacles I fire thousands of stinging cells, or nematocysts, to poison them! My tentacles bring the food to my mouth. Because I only eat other animals, I am a carnivore.
There are no dangers to me, but I can be dangerous to the reef if there are too many of me. I keep the zooplankton population under control and they keep the phytoplankton numbers in check. If I eat all the zooplankton, though, there is nothing to eat the tiny plants and they grow out of control! They multiply so greatly that they can start to block out the sunlight which is the source of energy (or food) in the reef!! If there is no sunlight, coral and the plants that live in them will die.