A large roughtail stingray was seen swimming in shallow water about 50 yards off State Beach on Sunday morning.

Roughtail stingray seen Sunday around noon.

Louisa McCullough

Roughtail stingray seen Sunday around noon.

Louisa McCullough

The six-foot wide stingray with a long tail was identified by biologists at New England Aquarium, who viewed a photo of the animal. The species is not uncommon in Massachusetts waters, New England Aquarium spokesman Tony LaCasse said, though the Vineyard is at the northern edge of its range.

The stingray was spotted from aboard a boat in about eight feet of water. Mr. LaCasse said it is unusual to see them in waters that shallow; they prefer deep water and like sandy or muddy ocean floor, where they are camouflaged.

Mr. LaCasse said these stingrays are usually about five to seven feet across in size and usually weigh between 200 and 400 pounds, though they can grow to be 800 pounds. They are “very well armored,” he said, with venomous barbs on their tails and on their bodies. But they are also known among divers as very non-aggressive, gentle animals, he said, and most negative interactions with humans take place when stingrays are stepped on in shallow water.

Roughtail stingrays visit local waters to feed on crabs, squid, and marine worms. Local fishermen that trawl the ocean floor are probably familiar with the species, Mr. LaCasse said.

The rays are considered a species of least concern in the New England region. They were said to be abundant around Woods Hole in the 1950s but are nowhere near as prevalent now, Mr. LaCasse said.

They are commercially harvested in some areas of the world but not in the northwest Atlantic.

Video by Louisa McCullough.