The pair of 200-pound loggerhead sea turtles lay motionless in side-by-side plastic tanks, wet towels draped over their shells and eyelids stitched shut to help eye lesions heal.

A battery of volunteers helped veterinarians at the Clearwater Marine Aquarium tried to keep the suffering creatures wet and alive after their nervous systems were crippled -- likely by the particularly brutal strain of red tide lingering in the Gulf of Mexico this summer. One would die within days.

Most anybody who has spent time at southwest Florida beaches knows about red tide, a nasty but not uncommon offshore algae bloom that in humans irritates the throat and makes it hard to breathe, and leaves dead fish littering the sand after every high tide.

But the massive red tide bloom that has plagued coastal waters this year from Honeymoon Island north of Clearwater to south of Sarasota has been extra toxic and deadly to sea life, scientists say, the worst in more than 30 years.

Fish, sand dollars, sponges, crabs, coral and other undersea life suffocated as the red tide -- this strain is called Karenia brevis -- choked off the oxygen in the water. Bottlenose dolphins, sea turtles, manatees and birds that swim through it, inhale the nerve-impairing toxins in the surf spray or eat contaminated fish also have perished.

The casualties, according to the state Fish and Wildlife Research Institute, have included at least 94 endangered or threatened sea turtles and 57 endangered manatees.

Divers and fishermen have reported a 2,000-square-mile "dead zone" void of undersea life off Pinellas County, where the worst of the red tide is concentrated.

"We've essentially had a large, toxic bloom out there for several months, and it's working its way through the ecosystem, we believe," said Cindy Heil, senior research scientist with the Fish and Wildlife Research Institute.

Heil said that red tide is a nearly annual occurrence in southwest Florida but that it usually happens in the fall. The severity and longevity of this year's episode are unusual, she said, but not unprecedented in Florida. The bloom showed up in January, and its intensity and size has fluctuated through the spring and summer.

It is hard to predict where it will go or how much longer it will last.

"It's a dynamic system," said Richard H. Pierce, a senior scientist at Mote Marine Laboratory in Sarasota, Fla. "It moves with the currents, it concentrates in some areas and dissipates in others. It's hard to make a definitive statement about how much is there from one day to the next."

One of the elements this summer has been a strong "thermocline," a layer of water where the temperature changes quickly from cold down deep to warmer near the surface. The thermocline traps the toxic algae bloom near the bottom and kills fish, then the bacteria breaking down the dead fish saps the water of oxygen.

Oxygen is starting to return to some areas of the dead zone, but a full recovery could take years, scientists say. That is bad news for such people as Ben Dauterman, who runs Tanks-A-Lot Dive Charters in Clearwater Beach.

His business has taken a hit because he cannot take divers out to reefs and the other usual undersea destinations. It is not safe to dive in the red tide, and there are no fish to see. Other merchants who depend on lots of people visiting the beach are concerned, too.

Video taken recently by divers shows devastation off Pinellas, said Dauterman, 60, who has spent four decades on the water here.

"I've never seen anything close," he said. "This is like somebody dropping a bomb out here. It's like Hiroshima or something. There's nothing left alive."

A loggerhead turtle that swam through a red tide is cared for at the Clearwater (Fla.) Marine Aquarium. The algae crippled its nervous system.