Category: Sea Life

For an animal that lacks the basic structure of either an endo- or exoskeleton, jellies are capable of wreaking a great deal of havoc. Blooms of jellies, where millions of them congregate in one area, can devastate fisheries and halt tourism. Since jellies are plankton and are at the mercy of the currents, these blooms tend to occur where two currents meet. There are four thousand species of jellies, and they come in a wide variety of sizes, colors, and shapes.



Scientific & Common Names

Kingdom - Animalia

Phylum – Cnidaria

Subphylum – Medusozoa

Class – Cubozoa, Hydrozoa, Polypodiozoa, Scyphozoa, and Staurozoa

Common Names – Jellyfish, Jellies (considered more proper), Sea Jellies, Medusa


Jellyfish can be the size of a pinhead or a small car. They can be lethal, or completely harmless. They can be brightly colored, or nearly invisible. They have no brain, no musculature, no lungs or stomach, but they thrive in an often hostile world. Jellies either have a bell shape with tentacles hanging below, called medusas, or tentacles above, called a polyp. Lion’s Mane Jellies are the largest known species, with the bell measuring 6 feet across and the tentacles measuring nearly fifty feet. Some jellies have the ability to detect light and cubozoan jellies have rudimentary eyes, although it is not known how they interpret the signals from the eyes since they have no brains, only a neural net. The class scyphozoa are often considered to be ‘true’ jellies due to the fact that they spend most of their life in the familiar medusa shape. Hydroza spend very little time in the medusa stage and settle quickly on the bottom. Staurozoa are stalked jellies and they attach to rocks or plants. Cubozaoa jellies are the most dangerous and one of their number, the sea wasp, has some of the most potent venom on Earth. Polpodiozoa are often not included in lists of jellies and they are still being studied and evaluated, but they are a parasitic life form.


Due to the extreme differences in the species of jellies, there are many different ways they breed. The most common breeding technique seems to be mass spawning at either dawn or dusk on a daily basis, providing the environment is favorable. The eggs are released and fertilized in the water, where the developing jellies join the flow of plankton. Some very few species will brood the young, where they develop in their mother’s body.


Not all jellies sting, but many have tiny stinging cells in their tentacles called nematocysts they use to subdue prey or defend themselves. They eat phytoplankton and zooplankton, as well as eggs and larvae of fish and other animals. While some can undulate their bells, none can swim with any strength and their destinations are decided by the currents and tides. Comb jellies are slightly better swimmers, but they are not closely related to jellies and are found in a different phyla – Ctenophora.


Jellies are one of the oldest animals on Earth, with a history that dates back between 500 and 700 million years. They are the first known animal to have a nervous system, making them an important step in the evolution of life.

Present status

While it could appear that the numbers of jellies is increasing worldwide, there is too little historical knowledge of their numbers to know with any certainty. There are no known species of jelly that could be considered threatened.


  1. Stung!: On Jellyfish Blooms and the Future of the Ocean by Lisa-Ann Gershwin, Foreword by Sylvia Earle

  2. Amazing Jellies: Jewels of the Sea by Elizabeth Gowell

  3. Jellyfish Blooms Edited by Kylie A. Pitt, Cathy H. Lucas

  4. Zooplankton of the Atlantic and Gulf Coasts: A Guide to Their Identification and Ecology by William S. Johnson, Dennis M. Allen