An 18-foot whale shark was found dead Thursday at Port Everglades, attracting a group of marine biologists eager to take samples from the rare fish.

The badly decomposed shark, missing its head, was first misidentified as a pygmy sperm whale. But marine biologists from Nova Southeastern University arrived at the scene and quickly identified the distinctive spotted pattern of the world's largest fish.

Whale sharks commonly grow more than 40 feet long and have been known to reach 60 feet. They are harmless -- except to plankton, small fish and crustaceans, which they suck in as they swim slowly near the surface.

While they inhabit Florida waters, whale sharks are rarely seen.

"They're not terribly common here," said Demian Chapman, a Nova graduate student in marine biology who examined the shark. "Divers see them once in a while. Some were seen off West Palm Beach a couple of years ago."

Because their numbers are low, they are protected from fishing in U.S. waters. But active fishing, generally by harpoon, takes place in the Philippines to feed the demand for shark-fin soup, said Margo Schulze-Hausen, a fishery biologist with the National Marine Fisheries Service.

It's likely that the shark died at sea and floated into the port or was pushed in by a ship, Chapman said.

When the dead shark was discovered at Port Everglades, the U.S. Coast Guard sent a boat to the scene and threw a line onto it.

The cause of death will probably remain a mystery, said Debra Abercrombie, a research biologist at Nova's genetics lab. While strips of blue paint on the belly indicated a boat had probably hit it, the collision probably took place after it died, she said.

The Nova group took samples from the fins and planned to analyze the DNA. The results could tell scientists which sub-population this shark came from.

After the biologists finished their work, a 41-foot Coast Guard utility boat towed the carcass five miles offshore and released it.