Jellyfish are not actually fish, but rather invertebrates in the phylum Coelenterata, which also includes the hydras, corals, and sea anemones. As with the other coelenterates, jellyfish feature radial symmetry and specialized stinging cells known as nematocysts. Although almost completely marine, there are some freshwater species of jellyfish that occasionally bloom in reservoirs and lakes. Worldwide, there are about 200 different species of jellyfish that have been described. From the tiny spherical thimble jellyfish of the Caribbean Sea to the largest, the Arctic lion's mane with tentacles that stretch over 100 feet in length, the basic body plans for jellyfish (a bell, tentacles and a mouth) are similar. With over 95 percent of the typical jellyfish composed of water, they do not feature hearts, blood, brains, or gills.

Technically known as the medusa stage, jellyfish are free-swimming and reproduce sexually. This is in contrast to the polyp or hydroid stage that is usually asexual, benthic, and sessile. The jellyfish produce eggs and sperm that, once fertilized, develop into zygotes and hatch as ciliated larvae (planulae). In many species, the complete life cycle takes two generations.

The nematocysts, mounted on the ends of long tentacles, serve several functions including stunning, holding, and reeling in prey in a manner similar to other invertebrates and fishes. In addition to feeding, the stinging cells are used for defense. The sting of most jellyfish is comparable to a bee sting, unless the victim suffers from allergies that can lead to anaphylactic shock and possibly death. The most dangerous jellyfish, the Australian box jelly, synthesizes a toxin more potent than cobra venom. Despite stinging cells tinged with neurotoxins, the ocean sunfish and sea turtles, such as the giant leatherback, feed primarily on jellyfish. In fact, this diet places leatherback turtles at risk because of human carelessness, littering the world's oceans with small plastic bags that resemble the medusas, but which cannot be digested.

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