5 Amazing Facts about the Ocean Sunfish

Learn 5 cool facts about this massive fish!

Aug 27, 2023By Michael C., BA Fisheries and Wildlife
facts ocean sunfish

It may look like something out of a sci-fi movie, but the Ocean sunfish (also known as the mola) is an actual creature! These massive beasts can be found in both temperate and tropical oceans worldwide. Read on to learn more about this strange fish!

1. It is the Heaviest Bony Fish

mola sunbathing on water surface
Image credit: National Marine Sanctuaries

The ocean sunfish, also known as the mola (which translates to “millstone” in Latin, possibly due to their round shape), is a record-breaking animal, being the heaviest fish in all the deep blue sea. One species, the Giant sunfish, is considered the largest living bony fish species in the world. One individual found dead in the Azores weighed around 6,050 pounds, around as heavy as a rhinoceros!

The ocean sunfish is named after its “sunbathing” behavior, which is believed to allow the molas to “charge up” and thermoregulate. As the sun warms the molas right up, it allows them to pursue prey in the deeper, colder depths of the ocean. Molas are not meant to be confused with unrelated freshwater sunfishes, such as bluegills and pumpkinseeds!

Due to their large size, many ocean sunfishes are riddled with parasites. Around 40-50 species are known to parasitize the molas, both on the skin and internally. These parasites range from flatworms to nematodes.

Molas have a few certain traits that distinguish them from other fishes. Their “tails” aren’t actually true tails; the actual tail is nonexistent. Instead, their dorsal and anal fins combine, forming a “clavus”. A lot of the mola’s skeleton has been replaced with cartilage, similar in fashion to a shark’s. This is believed to have assisted the mola to reach larger sizes.

The skin of a mola, which is coated in mucus, is thick and rubbery with a rough surface whose texture is likened to sandpaper.

2. Females Lay Millions of Eggs

mola swimming in aquarium
Image credit: Nol Aders/Wikimedia Commons

The female ocean sunfish is a massive animal, and this allows her to produce and lay around 300 million eggs, possibly more than any other living vertebrate known! As she expels the eggs, the males will externally fertilize them, and they’ll then drift out into the sea.

Newborn mola fry are extremely tiny, only around 2 millimeters when freshly hatched. Mola fry look almost nothing like their adult counterparts, sporting large spines and large fins. As they grow, the sunfish can grow up to around 60 million times their original weight, which is believed to be the most extreme size change of any vertebrate as well. As juveniles, they school in small groups for protection but become solitary once they've reached adulthood.

3. There are Multiple Species

mola and diver
Image credit: Daniel Botelho/Barcroft Media/Getty Images

Until recently, only three species of mola were recognized: the Ocean sunfish (the group highlighted in this article), the Sharptail mola, and the Slender mola. However, recent studies have shown that within the Ocean sunfish, an additional two species existed this entire time. These two species are the Hoodwinker sunfish and the Giant, or Southern sunfish, and each species differs slightly in shape and size.

The Common mola is best distinguished from its relatives by its scalloped clavus or tail fin. Meanwhile, as mentioned earlier, the Giant sunfish is the largest mola species. Along with its size, it can be distinguished by its round clavus and rather bulbous head (though this can also be seen in some individuals of the common mola). Within the genus, the Hoodwinker sunfish is perhaps the smallest of the three species, and its body is smoother in shape. The clavus is also rounded, similar to the Giant sunfish.

The elusive Sharptail mola can be distinguished from the rest by the projection of its clavus, which forms a pointed area towards the center. The Slender mola, meanwhile is the smallest of the molas, reaching just up to a meter in length, and can be found in schools. Its body is very streamlined in shape, and it sports a rather flat clavus. Slender molas are also very agile, being adapted to pursue squid as their main prey.

4. They are Predators

mola and pilotfish
Image credit: Unknown

The ocean sunfish was once believed to specialize in eating jellyfish and salps. However, recent studies have shown that molas will eat small fishes, crustaceans, and cephalopods. Jellyfish, salps, and other similar prey items only make up around 15% of its overall diet. Younger individuals tend to hunt prey around coastal and benthic areas, while older fish tend to prey out in the open ocean. Algae and eelgrass have also been reported in the mola's diet. They’ll suck their prey up like a vacuum, which will then be broken down with their sharp pharyngeal teeth located in their throats.

Molas are commonly believed to be slow, lazy fish, but recent studies have dispelled this idea. Molas are active hunters during the day, taking deep dives up to around 2,000 underwater to pursue their prey. Though they congregated back towards the water’s surface at night, the molas can swim for miles a day in search of food.

5. Ocean Sunfish Need our Help

mola in aquarium
Image credit: Fred Hsu/Wikimedia Commons

Out of all the mola species living today, only the Common mola, Sharptail mola, and Slender mola are listed on the IUCN Red List. While both the Sharptail mola and the Slender mola are currently listed as Least Concern as of the 2011 assessment, the Common mola is meanwhile listed as a Vulnerable species. The other mola species have not been evaluated yet, but many are listed as threatened in various parts of their range.

The most prominent threat to molas is bycatch. They’re often accidentally caught while other fish species are being targeted. In one swordfish fishery in California, molas ended up making up around 30% of fish being caught! The mola is also sought after for its meat in some parts of the world, being considered a delicacy, and boat collisions also pose a risk for both boaters and molas. Molas have also been reported to consume plastics that pollute the oceans.

You can help the mola by raising awareness among your friends and family. You can also support conservation organizations that are working to conserve molas and their ocean environments.

Michael C.
By Michael C.BA Fisheries and Wildlife

Michael holds a BS degree in Fisheries and Wildlife from Michigan State University. He formerly worked at a pet store as an animal care associate and is the former president of the MSU Herpetological Society. Michael currently owns three snakes (a corn snake, a Kenyan sand boa, and a checkered garter snake) and a leopard gecko. Interests include almost anything animal-related. Michael enjoys drawing, gaming, and having fun in his free time.