Jellyfish

Jellyfish
Sea nettle, Chrysaora quinquecirrha
Scientific classification
Kingdom:	Animalia
Phylum:	Cnidaria
Class:	Scyphozoa Goette, 1887

White-spotted jellyfish

Jellyfish are animals of the phylum Cnidaria. Most of them live in the oceans, in salt water, where they eat small sea animals like plankton and little fish, and float in the sea. Only a few jellyfish live in fresh water.

They have soft bodies and long, stinging, poisonous tentacles that they use to catch fish. Venom is injected by stinging cells called nematocysts. A jellyfish is 98% water.

Most jellyfish have a bell-shaped body and long tentacles at the underside of the body. Tentacles are long "arms" with special stinging cells called nematocysts. They move by contracting their bodies, but they do not have much control over where they go: most of the time, they drift with the water current. The largest type of jellyfish is the Lion's mane jellyfish, which has tentacles that can be as long as 60 meters, but most jellyfish are much smaller.

Jellyfish occur in four of the Cnidarian classes:

• Scyphozoa: the true jellyfish
• Cubozoa: the box jellyfish
• Staurozoa: the stalked jellyfish
• Hydrozoa: the hydroids

These four classes are sometimes linked as the sub-phylum Medusozoa.

There are many types of jellyfish. The smallest jellyfish are just a few inches across. The largest jellyfish is the Lion's mane (Cyanea capillata), whose body can be over 3 feet (1 m) across, with much longer tentacles. Some jellyfish glow in the dark (this is called phosphorescence). Some of the most dangerous jelly fish include the box jelly (Genuses Chironex, Chiropsalmus and Carybdea) and the tiny, two-cm-across Irukandji jelly (Carukia barnesi); the venomous sting of these jellyfish can kill a person.

Many animals eat jellyfish, including sea turtles and some fish (including the sun fish). Humans eat jellyfish too; especially in Asia jellyfish are considered a delicacy. Jellyfish spoil very quickly after they are caught. Sometimes they are dried to preserve them. There is a different process in which they are cleaned, which can take up to 40 days. They are often eaten in a kind of salad, with soy sauce or vinegar.

Life cycle

Most jellyfish undergo two distinct life history stages (body forms) during their life cycle. The first is the polypoid stage, when the animal takes the form of a small stalk with feeding tentacles; this polyp may be unable to move around, living on the bottom of the ocean or it may be free-floating. Polyps generally have a mouth surrounded by upward-facing tentacles. Polyps may be on their own or in groups, and some bud asexually by various means, making more polyps. Most are very small, measured in millimeters.

In the second stage, the tiny polyps asexually produce jellyfish, each of which is also known as a medusa. Tiny jellyfish swim away from the polyp and then grow and feed in the plankton. Medusae have a radially symmetric, umbrella-shaped body called a bell, which is usually supplied with marginal tentacles that capture prey. A few species of jellyfish do not have the polyp portion of the life cycle, but go from jellyfish to the next generation of jellyfish through direct development of fertilized eggs.

Jellyfish eat plankton and little fish, which are caught by their poisonous tentacles. Jellyfish may live in symbiosis with algae. The jellyfish transports them into sunlight and get nutrients from the algae's photosynthesis. Both forms of jelly fish have small tentacles with nematocysts (stinging cells) that sting and can hurt people on contact.