Jellyfish aren't fish, and they aren't made of jelly. They look like a blob of jelly when they are out of the ocean, and most species can give you a nasty sting. But in their natural habitat, they are a work of nature's art.
The moon jellyfish is common to shorelines around the world, and often seen washed up on the beach, where it resembles nothing more than a transparent disc. It has short nonstinging tentacles and relies on ocean currents for movement. Here are some recipes, in case you have some fresh moon jellyfish.
More jellies, after the jump.
The crystal jelly is a bioluminescent jellyfish. The transparent jellyfish produces green flashes of light chemically due to the green fluorescent protein that scientists use to study gene splicing. These experiments have led to the development of other glowing animals.
The Portuguese Man O'War may be the jellyfish you are most familiar with, but it's not a jellyfish at all! It is actually a colony of specialized invertebrates that cling together to form one eating and reproductive being. The Man O'War floats on the surface of the ocean by filling its bladder with air. The tentacles hang down an average of three feet (but can reach 33 feet), and its sting is very painful and sometimes deadly to humans.
The fried egg jellyfish is native to the Mediteranian Sea and can grow up to 35cm wide. Most jellies depend on ocean currents for movement, but this one can swim on its own by flapping its dome. You can see it swim in this video.
There are many species of box jellyfish native to Australia, the Philippines, and other tropical areas. Some are not dangerous to humans, but one species, Chironex fleckeri, is the world's most venomous animal. The sting from this particular species can cause death in minutes! If stung by a box jellyfish, apply vinegar before removing the tentacles, or it will inject more venom. That's why Australians take vinegar along when swimming at some beaches during jellyfish season.
Jellyfish Lake on the island of Palau is a popular tourist destination. The lake was once connected to the sea, but became isolated by a reef. The jellies that were trapped in the lake thrived due to few predators. They have stinging tentacles, but they are so small that swimmers rarely feel any effects. Watch a video of the lake.
Jellyfish in motion, from National Geographic.