WHAT IS KILLING THE GREAT SEA creatures of southern Florida? Nobody knows. But during the past three weeks, more than 50 manatees—endangered sea mammals that can weigh more than 1,000 pounds—have been found floating along a 25-mile stretch of Gulf of Mexico coastline north of Ft. Myers and Sanibel Island. Most have been brought by truck or boat trailer to the J. N. "Ding" Darling National Wildlife Preserve on Sanibel. There, in a makeshift lab, Scott Wright, director of the pathobiology lab at the Florida Marine Research Institute in St. Petersburg, heads a team of seven scientists and volunteers autopsying the corpses. "They're dying of pneumonia," says Wright. "We just don't know what's causing the pneumonia."

When first reports came in, Wright wasn't unduly alarmed; manatees, like human beings, often die of pneumonia. But when the count reached 9 or 10 animals a day—out of an estimated population of just over 2,600—it became clear that something was seriously wrong.

The dead manatees, all adults, including some pregnant females, have been found to be symptom-free apart from the pneumonia. There has been no known chemical spill in the area and pollution levels appear normal. So far, public speculation has focused on two theories for the epidemic. One is that red tide, a toxic algae infestation that is heavier than usual this year, may play a role. The other theory—also without any scientific basis so far—is that the unusually cold winter might have been a contributing factor. "What we're doing," says Wright, "is collecting samples. It will take a month or more to get things ferreted out."

Sweating under the Florida sun, covered with the stench of his labors, Wright looks at the Dumpster where the autopsied manatees, covered with quicklime, have been piled. "In an event like this you have to stay focused," he says. "But there are manatees we've worked with for years. I really hope we don't see them here."