Lifeguards spotted what they believed was a great white shark off Martha's Vineyard yesterday, forcing the closing of beaches and prompting the inevitable references to "Jaws," the movie thriller that was filmed on the island.

The dorsal fin of the shark, sticking some 2 1/2 feet out of the water, was spotted 75 yards offshore at South Beach in Edgartown. Authorities received reports of other sightings along State Beach, on the island's northeast and the site of the opening scene of "Jaws."

"It definitely creates some excitement in town," said Trish Lyman, a resident who works at The Boneyard surf shop. "People are tentative but still excited."

Lisa Capone, spokeswoman for the state Executive Office of Environmental Affairs, said the Coast Guard received several reports of the shark sighting. The state sent a plane to scan the waters, she said, but the pilot could not confirm the sighting.

Though unconfirmed, the sightings left residents wondering whether they would be able to see the massive creature. Arthur Smadbeck, chairman of the Board of Selectmen, said the police chief joked to him that he ought to head to the shores with a red plaid jacket, an allusion to a character in "Jaws."

A great white shark sighting is rare, but not unheard of in Massachusetts waters, said Greg Skomal, a shark specialist with the Division of Marine Fisheries. He said the species has a range spreading from the Gulf of Mexico into Canadian waters.

In 2004, a great white was entrapped for two weeks in a salt pond not far from the island, giving Skomal and other researchers a rare opportunity to study the animal. "That was a telling sign for us that the animals are here," Skomal said.

Last year, great white sharks were believed to be feeding on the local seals. Other local sightings have been reported over the years, which Skomal attributed to a greater awareness, perhaps a fascination, with the animal. But he stressed that the greater number of reports does not necessarily translate into an influx of sharks.

Yesterday, the state took the threat seriously enough to close beaches in South Beach State Park, along the southern coastline, where lifeguards reported spotting the great white. Edgartown Police Chief Paul Condlin said local officials were acting in the best interest of public safety.

The last believed great white attack in the Massachusetts area was in 1936, Skomal said, and there are believed to be only three in history.

Smadbeck said he did not think the sighting will have a negative impact on tourism, now in the island's busiest season.

"People will be so darn curious we'll probably be inundated with people wanting to see it," he said.

Lyman pointed out that the popular Monster Shark Tournament is planned for next weekend, giving participants a benchmark as they head into the waters.

"You can just surf cast and get a winning shark," she said.

Globe correspondent Casey D. Ramsell contributed to this report. Valencia can be reached at mvalencia@globe.com.

© Copyright 2008 Globe Newspaper Company.