These alien-looking creatures are named for their translucent, moonlike circular bells. Instead of long, trailing tentacles, moon jellies have a short, fine fringe (cilia) that sweeps food toward the mucous layer on the edges of the bells. Prey is stored in pouches until the oral arms pick it up and begin to digest it.
The coloration of a moon jelly often changes depending on its diet. If the jelly feeds extensively on crustaceans, it turns pink or lavender. An orange tint hints that a jelly's been feeding on brine shrimp.
Scientists have studied the life cycle of this jelly extensively. They know the adult male moon jelly releases strands of sperm, which are ingested by female moon jellies. After fertilization, larvae settle on or near the seafloor and grow into polyps. Polyps alternate between feeding and reproductive stages for up to 25 years. In the reproductive phase, polyps launch buds of cloned juveniles, known as ephyrae, which grow into adult medusae.
Found worldwide in temperate and tropical waters, moon jellies feed in quiet bays and harbors. Although moon jellies have a sting, they pose little threat to humans.
Moon jellies are very plentiful. However, plastic bags that end up in the ocean often look like jellies to animals that depend on these drifting creatures for food. Thousands of turtles and birds die each year after swallowing indigestible wads of plastic mistaken for jellies. You can help by picking up plastic on the beach and near storm drains.
Like other jelly populations, overabundant moon jellies indicate an unbalanced ecosystem. Scientists have discovered that jellies reproduce best when the water has too many nutrients—usually the result of run-off from land—and too little oxygen.
Although they didn't get to the moon, nearly 2,500 moon jelly polyps and ephyrae—two early stages in the jelly life cycle—went into orbit aboard the space shuttle Columbia in May 1991. They were part of a study on the effects of weightlessness on development of internal organs in juvenile jellies.