The bell-shaped jellies—distantly related to corals and anemones—launched their lifestyle long, long ago. Exquisite jellyfish fossils found recently in Utah display reproductive organs, muscle structure and intact tentacles; the jelly fossils, the oldest discovered, date back more than 500 million years, when Utah was a shallow sea. By contrast, fish evolved only about 370 million years ago.
The descendants of those ancient jellies haven’t changed much. They are boneless and bloodless. In their domelike bells, guts are squished beside gonads. The mouth doubles as an anus. (Jellies are also brainless, “so they don’t have to contemplate that,” one jelly specialist says.) Jellies drift at the mercy of the currents, though many also propel themselves by contracting their bells, pushing water out, while others—such as the upside-down jellyfish and the flower hat, with its psychedelic lures—can recline on the seafloor. They absorb oxygen and store it in their jelly. They can sense light and certain chemicals. They can grow quickly when there’s food around and shrink when there isn’t. Their tentacles, which reach up to 100 feet long in some species, are covered with cells called nematocysts that fire tiny poison harpoons, enabling the animals to immobilize krill, larval fish and other prey without risking their mushy bodies in a struggle. Yet if a sea turtle bites off a hunk, the flesh regenerates.
A breeding jellyfish can spit out unfertilized eggs at a prodigious rate: one female sea nettle may spew as many as 45,000 per day. To maximize the chances of sperm meeting egg, millions of moon jellies of both sexes assemble in one place for a gamete-swapping orgy.