WESTERLY, R.I. (WPRI) — After a fin was spotted in the water at Misquamicut State Beach last week, experts are hoping to educate beachgoers about the difference between a shark’s fin and an ocean sunfish’s fin.

The Rhode Island Department of Environmental Management (DEM) initially thought the fin belonged to a mako shark but changed course after seeing footage of the fin in question.

“In the video, it appears that, actually, the dorsal fin does move whereas a shark’s dorsal fin is stationary,” the DEM’s Mike Healey told Eyewitness News.

Mystic Aquarium’s Chief Clinical Veterinarian Dr. Jen Flower seconds the DEM’s findings.

“I actually thought sunfish for a minute after seeing the video, just because of the erratic movements at the surface,” she explained. “This swimming pattern would be a bit unusual for a shark as they don’t often make such quick changes in direction at the surface.”

An ocean sunfish, also known as mola mola, is considered the heaviest fish in the world because most of its body is made up of bone, according to the New England Basking Shark and Ocean Sunfish Project.

Ocean sunfish are frequent flyers in New England waters, mostly during the summer and fall months. They can be 4 to 8 feet long and can weigh between 300 and 1,000 pounds.

The most telling difference between a shark and a sunfish is their swimming pattern. While both have dorsal fins, ocean sunfish tend to swim on their sides with their fin bobbing in and out of the water, as opposed to a shark whose fin cuts smoothly through the water.

Ocean sunfish pose no threat to humans.

The shark scare prompted lifeguards to restrict swimming for about an hour after the sighting. Once it was determined the fin belonged to an ocean sunfish, beachgoers were allowed back in the water.

“It’s important to be knowledgeable about marine species native to our ocean waters and respect their environment, especially if you plan on swimming in their habitat,” Flower said. “Of course, when a fin is spotted close to shore, it’s always best to play it safe and evacuate the water. While most finned animals – including sharks – do not pose a threat to humans, there is no predicting their behavior or response to something new.”

Whether the fin belongs to an ocean sunfish or shark, the DEM highly encourages anyone who spots one to get out of the water immediately and alert a lifeguard.