Swimmers and sunbathers at Midland Beach on Staten Island reported seeing about 30 sharks surfacing several yards from shore around 10:30 a.m. yesterday, and the Parks Department barred beachgoers from entering the waters for more than four hours amid widespread – albeit misplaced – fears of flesh-eating fish.

Several beachgoers likened the incident to the 1975 Steven Spielberg film “Jaws.” Far from a blockbuster, however, yesterday’s supposed shark sighting turned out to be a short subject.

Law enforcement officials and a New York Aquarium shark expert quickly concluded that the awe-inspiring oceanic creatures were probably harmless cow-nosed rays. As a result, the city agency reopened the beach to swimmers at 3 p.m., according to a Parks spokesman, Ashe Reardon.

Not that the creatures in question are guppies. Cow-nosed rays do carry venom in their serrated spines, and “when the spine pokes into something, the gland injects poison,” the director of the Florida Program for Shark Research, George Burgess, said.

He added that the species is docile and he has never heard of its inflicting a serious injury. Fisherman magazine reported in 2003, though, that a beach goer off Monmouth County, N.J. was treated for puncture wounds after being hit by a cow-nosed ray’s tail.

As the ray swims, the tips of its two fins emerge from the water, “so if 30 fins were seen, that would be 15 animals,” Mr. Burgess said. Because the rays, which travel in groups, feed on clams and shrimp-like organisms, the presence of a large mass of rays near the shore of an Atlantic beach is “quite common,” he said. According to a 2001 bulletin from the U.S. Fish and Wildlife Service, the species – which can be as long as 7 feet – has been sighted as far south as Rio de Janeiro and as far north as Cape Cod.

But for four hours yesterday, beachgoers thought they were observing an extraordinary natural phenomenon.

“Everywhere you looked, you saw fins popping up,” a sunbather, Cathie Felitti, said.

The fearsome fish “seem to be frolicking,” Ms. Felitti, 52, a receptionist from Rosebank, Staten Island, observed.

One regular beachgoer, Steven Veksler, 20, of Duncan Hills, Staten Island, was ankle-deep in the ocean when lifeguards ordered swimmers to exit the water, and he quickly hopped ashore.

“It’s all fun and games until somebody loses a leg,” Mr. Veksler, a New York University student, said.

His cousin, Oleg Vugman, 20, a Baruch College student, was taking off his flippers near the water’s edge when he first saw the fins.

“People thought it was a big fish. No one could believe their eyes,” Mr. Vugman said.

That incredulity turned out to be wise.

“You wouldn’t find 30 sharks together unless they were feeding on something such as a dead whale,” Mr. Burgess said. But he noted that three species – including the most fearsome type, white sharks – live in the waters off New York year-round. And two other species, sandbar sharks and dusky sharks, migrate here from Florida during the warm-weather months, he said.

According to data compiled by Mr. Burgess’s program, five unprovoked shark attacks have been confirmed in New York since 1670, with no fatalities ever recorded. The last recorded shark attack death in the Northeast occurred in Massachusetts in 1936.