All About the Jellyfish

Jellyfish floating in the water
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Jellyfish are fascinating, beautiful, and for some, frightening. Here you can learn more about the ocean drifters known as jellyfish.

Jellyfish may also be called sea jellies, because they're not really fish! Jellyfish are marine invertebrates in the Phylum Cnidaria - which means that they are related to corals, sea anemones, sea pens and hydrozoans.

Even though jellyfish are often at the mercy of winds, currents, and waves that carry them around, they do have the ability to propel themselves by pulsing their bell. This mostly allows them to control vertical movement, rather than horizontal movement.

Characteristics and Classification of Jellyfish

  • Radially symmetrical - their body parts are arranged around a central point
  • Composed of two layers of cells - the epidermis, or outer layer, and the gastrodermis, which lines the gut. In between, there is the mesoglea, which is a jelly-like substance, which is how these animals get their name.
  • Jellyfish have a digestive cavity (the coelenteron) which is their stomach, gullet, and intestine. This has one opening, which serves and both the mouth and anus.
  • They don't all have tentacles. Tentacles may be longer or shorter depending on the species, or may not be present at all. All jellyfish have 4 to 8 oral arms which are used to pass food into the mouth. The oral arms generally look "chunkier" than the tentacles.
  • Kingdom: Animalia
  • Phylum: Cnidaria
  • Classes: Cubozoa (box jellyfish); Scyphozoa or Scyphomedusae (jellyfish); and the Staurozoa (stalked jellyfish).

Habitat, Distribution, and Feeding

Jellyfish are found in all the world's oceans, from shallow waters to the deep sea.

They are carnivores. Jellyfish eat zooplankton, comb jellies, crustaceans, and sometimes even other jellyfish. Some jellyfish have tentacles to use for defense and prey capture. These tentacles have a structure called a cnidoblast, which contains a coiled, thread-like stinging structure called a nematocyst. The nematocyst is lined with barbs that may embed into a jellyfish's prey and inject a toxin. Depending on the species of jellyfish, the toxin may be hazardous to humans.

Reproduction and Life Cycle

Jellyfish reproduce sexually. Males release sperm through their mouth into the water column. This is received into the female's mouth, where fertilization occurs. Development has to occur pretty quickly, as the lifespan of a jellyfish is only a few months. The eggs develop either inside the female, or in brood pouches located on the oral arms. Eventually, swimming larvae called planulae leave the mother and enter the water column. After several days, the larvae settle on the sea floor and develop into scyphistoma, polyps that use tentacles to feed on plankton. They then turn into a larva resembling a stack of saucers - this is called a strobila. Then each saucer turns into a free-swimming jellyfish. It grows into the adult stage (called a medusa) in a few weeks.

Cnidarians and Humans

Jellyfish may be beautiful and peaceful to watch, and they are often displayed in aquariums. They are also considered a delicacy and are eaten in some countries. But the thought that most likely comes to mind when you see a jellyfish is: will it sting me?

As mentioned above, not all jellyfish are harmful to humans. Some, such as the Irukandji jellyfish - a tiny jellyfish found off Australia - do have powerful stings. Jellyfish tentacles may also discharge toxins even when the jellyfish is dead on the beach, so you should exercise caution if you are unsure of the species. Click here for a guide to stinging and non-stinging jellyfish.

How to Avoid a Jellyfish Sting

  • Use caution when swimming - and if you see a jellyfish, stay far away. Tentacles in some species may be several feet long.
  • Swim carefully after a storm, as tentacle remnants may be present in the water.
  • Use a wetsuit or drysuit to offer additional protection to your skin. In lieu of a wetsuit, waterproof sun block may also help.
  • Don't touch a dead jellyfish on shore, as it could still sting.

How to Treat a Jellyfish Sting

Depending on the species, pain from a jellyfish sting may last from several minutes to several weeks. If you've been stung, there are some steps to take to minimize the pain of a jellyfish sting:

  • Get out of the water as soon as possible.
  • Remove tentacles using something other than your bare hands (towels, clothing, sand, seaweed, etc.)
  • Items such as meat tenderizer, sugar, vinegar, and sodium bicarbonate may take some of the pain out of the sting.
  • Seek medical attention if pain and swelling persist.

Examples of Jellyfish

Here are some examples of interesting jellyfish:


  • Coulombe, Deborah A. 1984. The Seaside Naturalist. Simon & Schuster.
  • National Geographic Society. Jellyfish. Accessed October 5, 2011.
  • Whitaker, J. D, King, R. and Knott, D. Sea Science: Jellyfish. South Carolina Department of Natural Resources. Accessed October 5, 2011.