BELOW THE SURFACE OF PUGET SOUND — Mysterious, luminous jellyfish, living lava lamps of Puget Sound, are some of the simplest and oldest animals on earth.

Jellies ride the chill, jade swells of the Sound all summer, their languid lolling shaming our central nervous system-charged hustle. Around since before the days of the dinosaurs, jellies are living proof that less is more.

How much less? How about no blood, no bones, no brain, no eyes, no ears, no heart. No lungs, no gills, no fins, no head, no tail, no teeth. No needs, other than to drift, feed and reproduce.

Elegantly simple, jellies operate with less infrastructure than nearly every other animal on earth.

In some parts of the world, such as Chesapeake Bay, robust populations of jellies are suspected to be linked to decreased water quality, with jellies taking over oxygen-starved, degraded waters that are no longer hospitable to fish.

Blooms of jellies from the Bering to the Black seas are being studied by scientists to determine if their presence is a sign of ecosystems thrown out of balance by overfishing, invader species and other human-caused disturbances.

But in Puget Sound, jellies are just part of life, not a trouble light that indicates problems under the ecological hood.

They are an important piece of the food web and useful to humans. One species abundant here, Aequorea victoria, was harvested for its bioluminescent and fluorescent proteins, useful in biomedical research. Cloning the proteins has eliminated the need for harvest.

The animal is colorless and entirely invisible at night, save for a dotted green, bioluminescence around the margin of its bell.

Which takes us directly to the jellies' undeniable mystique.

Their otherworldly form fascinates divers, snorkelers and anyone who has ever looked over the rail of a boat into the summer waters.

Asked what fueled her more than 20 years of research on jellies, jellyfish expert Claudia Mills of the University of Washington's Friday Harbor Labs doesn't hesitate. "They are beautiful," she said. "And very cool."

Like underwater UFOs

More than 95 percent water, jellyfish swim, feed, breathe, regenerate damaged tissue and even clone themselves. They accomplish all this with just two layers of skin, a gelatinous bit of filler, a radial network of simple nerves, a mouth and a drift of tentacles.

Seen underwater, they are like UFOs, pulsing in a green firmament starred with plankton. There is no hurry in their world, not even a right angle in their hypnotic movement.

There are about 1,000 species of jellyfish worldwide, about 60 in Puget Sound alone. Among the best-known in our waters are the hefty red Lion's Manes, big as a basketball. Denizens of cold water across the northern half of the globe, they are often seen washed up on Puget Sound beaches.

Egg Yolk jellyfish look like their namesake. Translucent tentacles swirl and trail the glowing yellow orb of their gut and gonads as they cruise the currents of the Northern Hemisphere.

Moon jellies — found worldwide — float throughout Puget Sound, their spectral white, lunar shapes as familiar a part of our seascape as the cry of gulls.

A strange life cycle

Jellies are mysterious, with a complex life history that includes two distinct phases: bottom dwelling and free-swimming. They take many forms along the way as they reproduce and metamorphose. Jellyfish inhabit all the waters of the world, even freshwater lakes.

Most jellies have a benthic, or bottom-dwelling stage, in which, as a tiny polyp, they attach to a surface — wood or stone are favorites — feeding with tiny tentacles and cloning new jellies.

A single colony of jellyfish can be traced back to a polyp that may have first cloned itself hundreds of years before. Any dock in Puget Sound clothed with what looks like white may be in fact paved with the polyps of Moon jellies.

In April, as daylight lengthens and intensifies and water temperatures warm, bottom-dwelling jelly polyps will also bud off tiny, baby jellyfish. By July, the juveniles have grown to the adult gobs we know so well. Some Lion's Manes can grow to nearly a yard wide.

Adult males release sperm into the water, where it is gathered by females that brood the eggs internally, then release them as ciliated beings no bigger than the head of a pin, called planula larvae. These drift to the bottom, where they attach, and begin the life cycle anew.

Most adults die by October, victims of bacterial infection, depleted plankton supplies and the appetites of fish and other predators.

Their ephemeral beauty and delicate tentacles belie their effectiveness as predators.

Jellies stun their prey with stinging cells called cnidocytes in their tentacles that harpoon a complex stew of neurotoxins into the victim's tissue. The Australian Box jellyfish packs a sting powerful enough to kill a human.

Most Puget Sound jellies have stinging cells too puny and ineffective to hurt people, Mills said. The exception, Mills said, is the Lion's Mane, which can sting like a bee.

The pulsing movement of jellies is governed by a pacemaker-type nervous system that contracts its tissues in a wavelike motion. The jelly can move up and down the water column, while its tentacles drift net for food.

Jellies eat plankton, fish eggs and larvae, and even other jellyfish.

The tentacles can retract to gather their prey to mouthlike feeding surfaces. These fragile oral lobes can hang like ruffled curtains as long as 6 feet from the middle of the jellyfish. The tentacles, too, are impressive, trailing up to 20 feet from a lion's mane.

The Lion's Mane, Egg Yolk and Moon jellies are all native species to Washington waters, found principally in the top 40 feet of the water, Mills said.

They will also bob right at the surface before pulsing below to feed. They are perhaps taking a siesta, just surfing the swell.

Lynda V. Mapes: 206-464-2736 or lmapes@seattletimes.com