Mola mola: The swimming head

Ocean sunfish are sometimes found on the beaches of the Bras d’Or estuary in autumn and early winter, generating a lot of interest.

You may think that Ocean sunfish (Mi’kmaq: E’pma’jit) earned that name because of their disc-like appearance. Actually, the moniker is based on the fish’s habit of basking on the ocean’s surface on its side either to absorb the sun’s rays or to offer seabirds a feast of its skin parasites.

In the 1700s, the famous Swedish naturalist Carl Linnaeus compared the Ocean sunfish to a giant, grey millstone. He gave the species the scientific name of Mola mola, meaning ‘millstone’ in Latin. Ocean sunfish have round, knobby tails, and long dorsal (top) and anal (bottom, rear) fins. They are gray, bluish or white and the skin is gritty, covered with a thick layer of slimy mucus. The specimens that have met their demise in the Bras d’Or estuary are relatively small, whereas specimens from elsewhere qualify as the heaviest bony fish in the world, with the heaviest recorded individual measuring 2.7 metres and weighing 2.3 tonnes. That is a lot of slime and muscle for a gelativore (diet composed mostly of jellyfish).

Ocean sunfish have been reported from countries around the world and there are many reports from the Atlantic Coast of Canada as far north as Labrador. Despite its regular appearance in Nova Scotia, the Ocean sunfish does not really thrive in our colder ocean waters. The optimum temperatures for this species are in the range of 10 to 19 degress Celsius, so they will be quite comfortable in the warmer Gulf Stream off our shores. They have been tracked in colder waters so are capable of swimming into the colder Bras d’Or estuary from the Gulf Stream.

Pictured is a typical Mola mola, which can be found around the Bras d’Or Lake.

During the summer months, they probably hang about for awhile and then leave. In the autumn and early winter, the water is too chilly for them to stay for long periods and there is not enough food to sustain them so they end up dead on the shore if they can’t find their way back to the ocean. If you have found one during your beach walks you might wonder how this plate-shaped fish makes its way in the open ocean.

Recent tagging studies have demonstrated that the Ocean sunfish is capable of strong directional movement and deep dives. It actually does a good job of swimming using a technique with its fins. The spine of the Ocean sunfish is very short so the lateral muscles are unable to flex the body. However, the dorsal and anal fins are powerful and serve as the main means of locomotion. These fins are stroked synchronously to generate a lift-based thrust the same way that a penguin uses its wings.

My favourite name for these oddly shaped fish comes from the German, “schwimmender kopf” which means “swimming head.” They do look like swimming heads and this odd appearance may be the reason that so much interest is generated when one is found washed up on a Bras d’Or beach. We have records of eight beachings in the Bras d’Or estuary since 2005 and we know that many occurred before that.

Ocean sunfish have been around for a very long time. An eager young marine biologist from California named Tierney Thys has made the study of Ocean sunfish her life’s work. She states that Molas emerged in the fossil record between 45 million and 35 million years ago. This is a time when whales had legs and the dinosaurs had come and gone. The fossil evidence indicates that a group of puffer fishes—“built like little tanks,” (says Thys) left their coral reef homes for the open ocean. So, the oddly shaped body of the Ocean sunfish is a throwback to their previous habitat where they evolved to fit those surroundings. Dr. Thys maintains an excellent Web site (https://oceansunfish.org/) where you can learn about the species and log any sightings (even dead ones on the beach). “Nearly every day I have people reporting,” says Thys. “Molas have been seen north of the Arctic Circle and as far south as Chile and Australia.”

The beachings in the Bras d’Or since 2005 have occurred in East Bay and around Groves Point in the St. Andrew’s Channel. If you find a sunfish, we would appreciate it if you could take a picture and log it on I Naturalist (https://www.inaturalist.org/), a web-based platform which is very easy to use.

Dr. Annamarie Hatcher is a consulting ecologist and a board member of the Bras d’Or Lake Biosphere Reserve Association (BLBRA).