This much is certain about jellyfish: They're not really fish, but the largest of the planktonic animals in the sea. They drift at the mercy of ocean currents, and they're predators, feeding on fish and crustaceans.

And, as swimmers and surfers will attest, some jellyfish sting.

"Little is known about the jellyfish," said Andrew Rossiter, director of the Waikiki Aquarium, as he was preparing to launch "Ocean Drifters, the Goldfein Spottswood Jellyfish Gallery," an exhibit opening Saturday at the beachfront facility.

"Nobody knows much because jellyfish biology is unknown," Rossiter said. "We don't know how long they live. They can be small or they can be large ... with tentacles that can be several feet long."

And jellyfish-watching is a popular aquarium sport, said Rossiter. He expects that keiki and adults will enjoy gawking at the enigmatic creatures' silent, poetic water ballets.

"Adults are enthralled by them; they can stand 15 or 20 minutes, gazing at the creatures," Rossiter said. "Jellyfish are like living lava lamps."

Or watery lava explosions, depending on your imagination.

Or moving underwater umbrellas, because of their common makeup — an umbrella-like hood over the "bell," the mushroom-shaped body.

In their tanks, some species have slo-mo beauty, with choreographic movements of swirls and twirls. Their tentacles often get knotted and tied together; miraculously, the jellyfish can become equally and easily untangled in their undersea dance.

A year in the making, this new permanent attraction — extending over an entire gallery wall, with four large tanks — will offer visitors up-close and very personal views of a variety of jellyfish, from box jellies to lagoon jellies, from sea nettles to moon jellies, largely with Pacific origins.

"We are focusing, at this moment, on Hawaiian species," said Rossiter. "We will have a variety of jellies on display, and we hope to expand to other exotic types in the future."

Mike Callahan, a research associate and aquarist at the aquarium, is the jelly man in charge of the exhibit. During a backstage peek last week, Callahan and Rossiter pointed out several jellyfish varieties and shared knowledge about their characteristics.

"Who hasn't been stung by a jellyfish?" Rossiter asked, mentioning the monthly arrival of the box jellies on Island shores.

"There is a mystery about why these jellyfish come ashore in great numbers, eight to 10 days after a full moon," said Rossiter. "Maybe it has to do with water temperatures."

Of about 2,000 jellyfish species, only about 70 are known to sting — and the prevalent rascal in our waters is the box jellyfish, sure to be a popular attraction. Its formal name is the Cubozoa, because of its cube-like shape, with four tentacles attached to the corners.

"It's the tentacles that sting," said Rossiter.

Another big mystery is how long a jellyfish lives. "Could be 100 years," said Rossiter. "Nobody knows."

Yet researchers say that jellyfish have existed for 650 million years.

They can be as tiny as a contact lens, or up to 6 feet wide, said Callahan. The largest on exhibit, however, will be the size of a grapefruit. One species, Callahan said, was found in the Ala Wai Canal, which is largely seawater, and its keiki hatched at the aquarium.

"Some paddlers told us they saw jellyfish in the Ala Wai and they were too big to be moon jellyfish," said Callahan. When a team went to investigate and capture the plankton, they discovered it was a white-spotted jellyfish.

Los Angeles residents Gary and Linda Goldfein were inspired to sponsor the new exhibit when their granddaughter Gracie Spottswood, who lives on O'ahu, told them how impressed she was with a previous jellyfish display at the aquarium. To the Goldfeins, the display is a way to reach youths like Gracie and others interested in jellyfish culture.

The exhibit will be updated, said Rossiter, because jellyfish species vary according to season. "Ocean Drifters" also includes a 10-foot-high, 1,000-gallon cylindrical tank and multiple custom-built tanks, some with domed fronts. To encourage better viewability, the tanks are equipped with a water-circulation system that keeps the animals suspended in the water.

The familiar moon jellyfish will remain in their current tank apart from the new exhibit area but eventually will be moved in with their sea brothers and sisters. The moonies are mysterious and fluorescent, with a glowing allure in a dark section of the aquarium.

Many of the jellyfish were donated by aquariums in Japan and on the Mainland, but some were collected from the wilds of the Pacific and bred by the aquarium staff.

Most are white or translucent and gelatinous, but a few have shimmering shades of pink, brown and blue. Traditionally, jellyfish are 2 percent protein, 2 percent mineral salts and 96 percent water — which make them difficult to spot in the open seas, because they move with the currents and blend in with the ocean.

"The easiest thing to do, when you get stung, is to put ice on," said Rossiter. (Local tradition enlists a less-sophisticated salve — urine.)

And how do you catch a jellyfish?

"Very carefully," said Rossiter. "You go out at night with a large container. You shine a bright light and the brightness attracts plankton; jellyfish come to the plankton, and with a cup or scoop with a long stick, you scoop them up."

Reach Wayne Harada at wharada@honoluluadvertiser.com.