According to one BostInno reader there was a possible shark sighting off Castle Island on Tuesday afternoon. The above photo of a sleek looking fin was taken around 6pm ET on Castle Island.

The Boston Globe ran a similar story today confirming that a large fin was spotted in Pleasure Bay. According to them it’s either a big sun fish or a 5-6 foot blue shark… however it’s most likely a blue shark.

UPDATE: According to an update from the Globe late Tuesday night, it does appear to be a sun fish. Here’s the update they issued. “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.”

While sharks have made Cape Cod a home over the past few summers, it’s also not uncommon for them to make an appearance in Boston at the end of the summer. Don’t worry though blue sharks are not man eaters, however, they could take a bite out of an ankle if provoked. Be careful out there.

Did you see a fin today off Castle Island? Send us your photos at greg at bostinno dot com.