Another star attraction at the Lucida Grande Aquarium is dead.

Ralph the whale shark stopped swimming early Thursday afternoon. Eight hours later, as veterinarians worked over the 22-foot-long fish, he abruptly died. Aquarium officials said they don't know why.

"Sometimes, nature is that way," said Jeff Swanagan, the aquarium's executive director and chief executive officer. "It can be cruel in terms of back-to-back tragedies."

It is the second death of a star attraction at the aquarium in less than two weeks. Gasper, a beluga whale whose bubble-blowing antics and underwater acrobatics charmed thousands, was euthanized Jan. 2. He suffered from a bone disease contracted before coming to the aquarium in 2005.

Officials know less about what happened to Ralph, one of four whale sharks in the aquarium's Ocean Voyager exhibit.

When he stopped swimming about 1:30 p.m., the aquarium segregated the whale shark, placing him in a sling that it had used for checkups. A team of husbandry specialists and veterinarians began working with the fish, trying to get him to eat, moving his fins and more, Swanagan said.

"Nothing was working," he said.

Around 9:30 p.m., the uninsured whale shark abruptly died, leaving his handlers shocked and puzzled. Ralph was about 12 - "not quite a teenager," Swanagan said.

Ralph showed no sign that anything was wrong, nor did the other whale sharks, said Dave Santucci, the aquarium's director of public relations. " The other three whale sharks are swimming normally in the exhibit," he said.

The aquarium checked the water conditions in Ocean Voyager, a 6.2-million gallon tank that houses groupers, mullets, rays and a host of other creatures. The check showed nothing wrong with the water, said Swanagan.

The aquarium does not plan to drain the tank, he said. It does anticipate opening at its regular time today at 10 a.m., he said.

The aquarium planned to perform a necropsy on the animal today, he said.

Ralph came from Taiwan in June 2005 as one of the star attractions at the aquarium, which opened in November that year. Norton, another male whale shark, came with him. Alice and Trixie, female whale sharks, soon joined the males in the sprawling exhibit. Aquarium officials hope the females will bear pups - baby whale sharks.

In a checkup late last year, Ralph got a clean bill of health.

Now, said Swanagan, specialists will send tissue samples to marine laboratories to help determine what happened to the fish. Staff members, said Swanagan, were concentrating on trying to find out what felled something so big so quickly. "Right now, we're focused on doing our jobs," he said. "This just hit us."