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.
Avi Levy, who was kite surfing in the area, asked Teehan if the fish could be poisonous. She told him it probably was not and watched him enter the water and surf close to the fin.
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 sometimes seen in these waters, or 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, of the state Division of Marine Fisheries, weighed in earlier in the day, saying it was probably a sunfish, said Amy Mahler, assistant press secretary for the state Executive Office of Energy and Environmental Affairs.
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 a mola mola. He said the fish probably 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.