The mystery of Castle Island was solved Tuesday night.
The tale unfolded early in the day when Pam Teehan was taking a run around Pleasure Bay and saw a mysterious dorsal fin carving a line through the water.
Other people saw it, too, she said, and she heard them exclaiming, “There’s a shark in the water!’’
[Cue the “Jaws’’ theme.]
“This was a friendly, clumsy fin,’’ Teehan said. “The more I looked at it, I thought it was a shark,’’ said the Dorchester resident.
She said the fish was smooth and shiny and that schools of small silver fish were jumping up from the water around it at Castle Island in South Boston.
After reports of the sighting came in, staff from the New England Aquarium thought the fish might have been a shark, too. Images of the fish were examined by experts and nonexperts, and its identity left many scratching their heads — was it a mola mola, a type of ocean sunfish that is sometimes seen in these waters, or was it a blue shark?
“It’s problematic to get an animal in there,’’ said Tony LaCasse, a spokesman for the Aquarium.
The bay is sealed off by a gate, which means the fish could have been dragged in by the tide and could be injured.
Although some experts believed it was a shark, the thick rounded edge of the fin and the swirls it made in the water convinced others it was a mola mola, LaCasse said.
Greg Skomal, a shark expert from the state Division of Marine Fisheries, weighed in earlier in the day, saying it was probably a sunfish, according to Amy Mahler, assistant press secretary for the state Executive Office of Energy and Environmental Affairscq.
And after viewing late video turned over to the Aquarium by fishermen, LaCasse said they have solved the mystery — and that the fish that captivated onlookers in the morning was indeed a mola mola.
LaCasse said that this summer he has seen as many reports of mola molas as he has in six or seven years combined. He said the fish likely squirmed its way into Pleasure Bay in order to feast on the abundant sea jellies.
“They look like a marine hybrid experiment gone wrong,’’ he said.
“I felt honored and fortunate to see it,’’ said Teehan.