The fertilized moon-jelly eggs cling to pits on the female's oral arms until they turn into swimming larvae. Whatever degree of parenting they receive, moon-jelly quality or not, fertilized eggs become larvae about .5 millimeter long and settle on rocks to become polyps, barely visible, like small and colorless sea anemones. (The unlucky ones sink to the ocean floor where they may be covered with sediment and have a harder time.) Eventually the polyps clone into layers, resembling a miniature stack of Frisbees. The one at the top begins to pulse and then flies off, followed over a week or two by 30 or so of his or her siblings. The polyp stage can last for years, cloning and sending out more medusas at irregular intervals. The medusas don't live much more than a year.
Jellyfish may meet a violent death in the beak of a sea turtle, the mouth of a fish, or the bill of a seagull. The tuna and swordfish that eat them have been overfished, but in the Gulf of Mexico, a Volkswagen-sized beast, the Ocean sunfish, specializes in feeding on jellies. So does the leatherback turtle. One of the many mysteries of ocean life is how the sunfish and sea turtle can grow as big as 2,000 pounds on a diet of creatures whose bodies are 95 percent water.
If they escape predators, jellyfish die not long after sex. Their bells turn raggedy, the tissues that once were clear become opaque, and they drop to the ocean floor.
People who watch the sea—lifeguards, surfers, and fishermen—believe that there are more jellyfish than ever. Though this has definitely been the summer of beach closings, marine biologists find it hard to pin down the numbers. It could be, the more cautious say, that we're seeing more near beaches this year because of changing currents or shifting winds, or we could be at some point in a climatic cycle that causes jelly populations to increase before they wane again. It's impossible to map the long-term ebbs and flows because we don't have good data on jellyfish blooms in past decades or centuries. Census and tracking are difficult because it's very, very hard to put a tag on a jellyfish, they have a short life span, and no one cares to come back and recapture them. (It's even hard to net them without having them fall to pieces.)
It's easy to assume that global warming must be involved in the apparent jellyfish explosion. After all, we bump into these scary floaters in the summer. But they're around in numbers in the summer because there's plenty to eat then. (There are also jellyfish floating under Arctic and Antarctic ice.) It's true that jellyfish might make more medusas at higher temperatures, but recent experiments showed that each juvenile medusa has a lower chance of survival in warmed water.
We do know for certain that changes in food supply can cause a population increase. Jellyfish consume tiny fish, shrimp larvae, and plankton. (Some jellyfish specialize in eating other jellies; these cannibals include the extremely useful Aequorea, the source of a luminescent substance—green fluorescent protein—used as a marker by virtually every molecular-biology lab in the world.) Fertilizer and sewage runoff from our coastal cities and towns puts nutrients in the water that fertilize the tiny plants—phytoplankton—that are eaten by the small marine animals—zooplankton—that are the primary jellyfish food. Jellyfish have always hovered near the coast; we may be inadvertently beckoning more of them closer with our organic waste. And overfishing and pollution have greatly decreased the jellyfish's main competitors for food—the little 4- to 8-inchers like anchovies, sardines, and menhaden.
We don't know enough now to predict the future of jellyfish populations; we do know we don't want them at our beach. Still, there's something captivating about the creatures, even outside an aquarium. E.O. Wilson, an ant expert and the greatest environmental scientist of our time, begins his autobiography with a boyhood reminiscence of seeing jellyfish in the waters of Northwest Florida. That jellies are not deep thinkers doesn't diminish their appeal. Despite the appearance of passivity, there's some precise and efficient hard wiring at work that has kept them around for millions of years. In polluted parts of the ocean, they may be the last ones standing—or, rather, floating.