Jellyfish (also known as jellies or sea jellies or Medusozoa) are free-swimming members of the phylum Cnidaria. Jellyfish have several different morphologies that represent several different cnidarian classes including the Scyphozoa (over 200 species), Staurozoa (about 50 species), Cubozoa (about 20 species), and Hydrozoa (about 1000–1500 species that make jellyfish and many more that do not). Medusa is another word for jellyfish, and refers to any free-swimming jellyfish stages in the phylum Cnidaria.
Jellyfish are found in every ocean, from the surface to the deep sea. Some hydrozoan jellyfish, or hydromedusae, are also found in fresh water; freshwater species are less than an inch (25 mm) in diameter, are colorless and do not sting. These are the large, often colorful, jellyfish that are common in coastal zones worldwide. Scientists have evidence of Jellyfish roaming the seas for about 500 million years.
In its broadest sense, the term jellyfish also generally refers to members of the phylum Ctenophora. Although not closely related to cnidarian jellyfish, ctenophores are also free-swimming planktonic carnivores, are generally transparent or translucent, and exist in shallow to deep portions of all the world’s oceans.
The word jellyfish (which has been in common usage for more than a century) is used to denote several different kinds of cnidarians, all of which have a basic body structure that resembles an umbrella, including scyphozoans, staurozoans (stalked jellyfish), hydrozoans, and cubozoans (box jellyfish). Some textbooks and websites refer to scyphozoans as “true jellyfish”.
Since jellyfish are not even vertebrates, let alone true fish, the usual word jellyfish is considered by some to be a misnomer, and American public aquariums have popularized use of the terms jellies or sea jellies instead.
In its broadest usage, some scientists occasionally include members of the phylum Ctenophora (comb jellies) when they are referring to jellyfish. Other scientists prefer to use the more all-encompassing term “gelatinous zooplankton”, when referring to these, together with other soft-bodied animals in the water column.
A group of jellyfish is sometimes called a bloom or a swarm. “Bloom” is usually used for a large group of jellyfish that gather in a small area, but may also have a time component, referring to seasonal increases, or numbers beyond what was expected. Another collective name for a group of jellyfish is a smack.
Jellyfish are “bloomy” by nature of their life cycles, being produced by their benthic polyps usually in the spring when sunshine and plankton increase, so they appear rather suddenly and often in large numbers, even when an ecosystem is in balance. Using “swarm” usually implies some kind of active ability to stay together, which a few species like Aurelia, the moon jelly, demonstrate.
Most jellyfish have a second part of their life cycle, which is called the polyp phase. When single polyps, arising from a single fertilized egg, develop into a multiple-polyp cluster, connected to each other by strands of tissue called stolons, they are said to be “colonial.” A few polyps never proliferate and are referred to as “solitary” species.
All jellyfish sting their prey using nematocysts, also called cnidocysts, stinging structures located in specialized cells called cnidocytes, which are characteristic of all Cnidaria. Contact with a jellyfish tentacle can trigger millions of nematocysts to pierce the skin and inject venom, yet the sting of only some jellyfish species causes an adverse reaction in humans. When a nematocyst is triggered by contact by predator or prey, pressure builds up rapidly inside it up to 2,000 lbs/sq. inch until it bursts open. A lance inside the nematocyst pierces the victim’s skin, and poison flows through into the victim. Touching or being touched by a jellyfish can be very uncomfortable, sometimes requiring medical assistance; sting effects range from no effect to extreme pain to death. Because of the wide variation in response to jellyfish stings, it is wisest not to contact any jellyfish with bare skin. Even beached and dying jellyfish can still sting when touched.
Scyphozoan jellyfish stings range from a twinge to tingling to savage agony. Most jellyfish stings are not deadly, but stings of some species of the class Cubozoa and the Box jellyfish, such as the famous and especially toxic Irukandji jellyfish, can be deadly. Stings may cause anaphylaxis, which can be fatal. Medical care may include administration of an antivenom.
In 2010 at a New Hampshire beach, pieces of a single dead lion’s mane jellyfish stung between 125 and 150 people. Jellyfish kill 20 to 40 people a year in the Philippines alone. In 2006 the Spanish Red Cross treated 19,000 stung swimmers along the Costa Brava.