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Friday, Sept. 27, 2002


Moon jellyfish

* Japanese name: Mizukurage
* Scientific name: Aurelia aurita
* Description: Moon jellyfish are soft-bodied, transparent animals that swim by gently undulating their bell-shaped bodies. They range in size from 5 cm to 40 cm in diameter. Unlike what you may have expected, size is not an indication of age, rather it reflects how well the jellyfish has been eating recently: A dinner plate-size jellyfish will shrink to the size of a 10 yen coin without food. The body is divided into eight lobes, containing sense organs for balance. There are four horseshoe-shaped gonads, yellow in females and purple in males. There are also four "oral arms" surrounding the central mouth, and a fringe of about 250 stinging tentacles. The sting is very mild and has almost no effect on humans.
* Where to find them: In the seas all around Japan, mostly near the coast. They prefer warm seas, but can tolerate very cold water.
* Food: Jellyfish are carnivorous. They feed on plankton and on the developing larvae of other animals, such as mollusks, crustaceans, copepods and rotifers. They will also eat eggs and diatoms. Food collects on the surface of the jellyfish and gets stuck in the mucus, from where it is passed to the mouth opening and the stomach. The jellyfish propels itself upward in an undulating motion, simply to keep near the surface of the sea where the food is.
* Special features: Reproduction in jellyfish is as bizarre as might be expected from such alienlike organisms, but it is still done by the transfer of sperm to the female. Instead of a penis, the male releases sperm via special filaments, and the female draws it in and fertilizes her eggs. Her embryos develop for a few days on her oral arms, before falling off, after which they drift to the bottom of the ocean and become polyps. In a process called strobilization, each polyp buds off into several tiny jellyfish. Over the next four months, the babies grow into adult jellyfish called medusa. Most of them only live for six months.

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