LA JOLLA ... They have no spine, no brain, no eyes and no heart.

No, this isn't a story about politicians. We're talking jellyfish, the mysterious, often beautiful ... and sometimes deadly ... drifting aquatic ghosts once considered more plant than animal.

"They are animals. And while they don't have what we think of as a brain, heart and eyes, most have organs that crudely work in some of those roles,"said Leslee Matsushige, assistant curator at the Birch Aquarium at Scripps Institution of Oceanography.

Jellies are in the news this summer as thousands of huge, and extremely rare, black jellyfish suddenly appeared off San Diego beaches last month,causing an upsurge in reports of swimmers getting stung.

The mysterious black jelly, Chrysaora achlyos, is making only its fifth known visit to Southern California shallows this summer.

Also known as the black sea nettle, the gigantic jelly was previously reported (and photographed) in San Diego in 1926, 1965, 1989 and 1999. Named for the dark Burgundy color of an adult's bell, the black jelly is not expected to be seen here much longer this year.

Black jellies grow quickly, feeding on plankton blooms in the spring, Matsushige explained. They continue growing and reach sexual maturity in late summer, releasing their gonads before withering and dying after a life cycle lasting just a few months.

The gelatinous beast ... whose bell can exceed 3 feet in diameter and whose tentacles can stretch 25 feet ... was not officially described in scientific literature until 1997.

It was the largest invertebrate (animals without spines) named in the 20th century, said Mike Schaadt, exhibit director at the Cabrillo Marine Aquarium in San Pedro, who has studied the animals for several years.

"This is a plankton, even though it's so huge," Schaadt said. "Plankton is a designation for plants and animals which are at the mercy of ocean currents."

Despite its size, the black jelly's sting is considered moderate, no more potent than that of more common cousins, such as the purple-striped jelly.

Scientists believe the black jelly probably thrives in deep water south of here and shows up in San Diego through some capricious shifting of currents and tides.

"It's such a rare visitor, we have little knowledge about its diet, habitat, lifestyle or physiology," said Vincent Levesque, a Birch aquarist.

Like all jellies, the black jellyfish is about 95 percent water, Levesque said. It feeds constantly on such things as plankton, smaller jellies, brine shrimp and fish not old enough to swim against ocean currents, using the venom in its tentacles to sting and incapacitate prey.

Though jellies typically lack eyes and a brain, he said, nerve centers direct the animal's pulsating movement and sensors detect light and objects, working as rudimentary eyes.

Jelly nursery
Jellyfish are not really fish. They belong to phylum cnidaria, included in the class scyphozoa (from the Greek skyphos, for cup, and zoon, for animal).

The animals, which have been around for about 650 million years, pulse along in every ocean. There are also a few fresh-water varieties among the 200 or so known species of jellyfish.

Though most jellies are basically bell-shaped and transparent with trailing tentacles, they come in a vast array of sizes, shapes and colorations.

They range from the size of your fingernail (the Caribbean's thimble jellyfish is less than an inch across) to a bell reaching 8 feet in diameter (the Arctic's lion's mane jelly, whose venom-laden tentacles trail 100 feet).

Some jellies thrive near the surface or in shallow water, often washing up on beaches. Others prefer the ocean depths. Levesque said there are undoubtedly countless species of jellyfish yet to be discovered.

Developing from fertilized eggs, jellyfish have what's euphemistically known as an "incomplete digestive system," meaning the same orifice is used for both food intake and waste expulsion.

Along with colleagues here and at the Cabrillo Aquarium and the Long Beach and Monterey Bay aquariums, Levesque has worked to perfect husbandry techniques to reproduce black jellies for public display. (They are currently on exhibit at Birch Aquarium, along with moon, bell and fried-egg jellyfish.)

In a behind-the-scenes tank, Levesque showed how sperm and eggs taken from the gonads of black jellies (in both 1999 and from 30 collected this summer) are combined in petri dishes filled with 59-degree seawater.

Within a few days, polyps develop, attaching themselves to Plexiglas plates.

"We can keep the polyps for years," Levesque said. Using certain triggers (usually involving temperature change of the seawater), the scientists induce "strobilation," in which the polyps begin their transformation into juvenile jellyfish.

Mike Schaadt said the cooperation in the black jelly "husbandry program" among the four California aquariums is a "remarkable and exciting learning experience."

"And we're sharing this with visitors as it's happening," Schaadt said. "Here (in San Pedro) we work closely with young people who are learning science and they were able to see the emerging baby stage of these animals as we first saw it.

"These youngsters had that wonderful feeling of discovery right along with us. It was magical."

Floating death
In this summer of shark attacks in the Gulf of Mexico, a lurid fascination with dangers of the deep has once again captured the public's imagination.

Spend a little time near the black-jelly exhibit at Birch Aquarium and you'll hear wondrous comments (usually from teenagers) about the kind of sting a 25-foot-long tentacle might pack.

But when it comes to stinging, the black jelly is 20,000 leagues removed from the terrifying Chironex fleckeri, a species of the "box jellyfish," so named for their cubed-shaped head. (The Chironex is also known as the sea wasp).

It plys the waters off North Queensland, Australia and its incredibly potent venom can kill a man in minutes.

It is the most venomous marine creature on the planet and has killed at least 70 people in the last 120 years, according to researchers at Australia's CRC Reef Research Center.

Box jellies differ from most of their brethren in that they have functioning eyes – 24 of them, in fact, like some monster out of science fiction. They also have four nerve centers and, unlike other jellies, they can swim as they hunt prawns, fish, crabs and other prey.

The head of an adult Chironex is the size of a basketball and its deadly tentacles – it has up to 60, 15 dangling from each corner of its head – usually strike a bather's lower legs in shallow, calm water.

The venom of Chironex, the Australian researchers say, directly attacks both a victim's nerves and heart. Human victims can die within minutes of being stung; survivors often are left with dark-red-striped scars on their skin.

"There is an antivenin for Chironex fleckeri stings, and medical assistance should be sought as soon as possible so that it can be administered," according to the CRC Reef Research Center.

The box jelly family, distinct enough to have its own aptly named class, Cubozoa, includes relatives that live off our Pacific coast.

But the local box jelly variety's sting is nowhere near as powerful as the Chironex's, Vincent Levesque said.

"They have a decent sting," he said of the relatively rare and transparent box jelly, which is usually seen from August to November near the ocean bottom, inside the kelp beds, from Santa Barbara to La Jolla.

"But it's certainly not deadly; nothing like the sea wasp in Australia."

• San Diego Union article