Torben Lonne, Dive Instructor
Explore Marine Life
Meet The World’s Largest Bony Fish:
Mola mola, commonly known as ocean sunfish, is the largest bony fish on Earth and also… one of the weirdest. It can be found in tropical and temperate waters all around the world.
However, its populations are decreasing due to fisheries, bycatch and water pollution.
Mola mola taxonomy
These bony fish belongs to one of the youngest fish families in the ocean. Some of the Mola mola fossil records are dated from 50 million years ago. Young Mola mola are very similar to pufferfish, with whom they are actually closely related.
Where does the name come from?
The name Mola in latin means millstone which resembles the silvery color of ocean sunfish as well as their rough superficial texture. In some European countries, ocean sunfish are commonly called “moon fish” or “swimming head”. In China, they are known as “toppled wheel fish”.
What do they look like?
An average adult ocean sunfish can weigh 1000kg, but the largest recorded weight is 2300kg. It measures up to 3 meters horizontally and 4 meters vertically. To have a better idea of how large they are, here is a curious story that happened in 1998. A Mola mola weighing approximately 1400 kg collided with a cement carrier ship and decreased the ship’s speed from 14 to 11 knots. The ship arrived at the Sydney harbor with the animal still impaled on its bow.
Ocean sunfish have a disc-like shape which enables them to adopt a vertical and horizontal position. Despite their large size, they have a brain that weighs only 6 grams. They have a small mouth that is permanently open, and their teeth are fused, forming a beak in the upper and lower jaws. In addition, they have hook-shaped pharyngeal teeth.
Mola mola have 17 vertebrae. Even though they are born with a caudal fin, it folds into itself to create a wavy clavus, or rudder. Ocean sunfish use their dorsal and anal fins for propulsion, and they can reach a velocity of 3 km/h.
Unlike most fish, Mola mola don’t have a swim bladder. Instead, they have a thick layer of gelatinous tissue under their skin that provides them neutral buoyancy.
Ocean sunfish like to bask in the sun near the surface, especially when they get cold. It is suggested that they depend on the sun for thermoregulation. So, they hunt during the day, while at night they rest in superficial waters. Sometimes Mola mola are mistaken for sharks when their dorsal fins emerge. At first, they were seen as lazy animals, but now we know they are actually very active.
Reproduction and life spawn
Ocean sunfish are considered the most fecund organism among vertebrates. In fact, a female Mola mola can produce up to 300 million eggs at a time which are released into the water and fertilized. When they hatch, larvae are only 1 mm long and are protected by a star-shaped, transparent covering that gradually disappears as Mola mola grow. They can grow to 60 million times their size since they hatch. For example, an ocean sunfish from Monterey Bay Aquarium grew 373kg in only 15 months.
Ocean sunfish in captivity can live up to 10 years, but nobody really knows how long they live in the wild.
What do Mola mola eat?
Ocean sunfish can dive to a depth that varies between 90 and 170 meters to find prey. The average dive lasts approximately 10 minutes. These strange bony fish usually feed on large quantities of jellyfish. In reality, it is though that the beak is an adaptation linked to sunfish dependence on jellyfish. Besides, a thick layer of mucus protects the animal against jellyfish stings. Recent research has shown that they also feed on crustaceans, small fishes, fish larvae, squid, and eelgrass. Some Mola mola spend a lot of time in coastal areas where they exploit alternative food.
Furthermore, Mola mola have big eyes and a great visual acuity that helps them to hunt prey that are hard to see under water, as is the case of jellyfish.
Who eats them?
The main predators of ocean sunfish are sea lions, orcas and sharks.
Interestingly, sea lions hunt Mola mola apparently just for fun. They tear off their fins and, when they have eaten enough or gotten bored, they let them sink to the bottom and die. Some researchers suggest that Mola mola have few pain receptors, because when they are being attacked, they don’t respond at all.
Ocean sunfish parasites and interspecies relationships
The ocean sunfish skin is one of the thickest in the world with approximately 7 cm. It stabilizes the animals body, but doesn’t protect it against parasites. In fact, about 40 kinds of parasites can be found on ocean sunfish. Interestingly, one of the most common parasitic species is the larval stage of shark tapeworm which grows inside Mola mola. If someday a shark eats the sunfish, the tapeworm will be able to reproduce and continue its life cycle inside shark’s body.
In order to tackle this problem, Mola mola often go to the surface and seek the help of small cleaner fish and birds to feast on those parasites infesting the skin. It is also hypothesized that ocean sunfish floats horizontally at the water surface so the UV radiation from the sun light can kill the parasites. Mola mola also jump out of the water and splash in it to try to remove them.
Where do Mola mola live?
Ocean sunfish abundance isn’t completely understood. However, it is known that they are typically found in tropical and temperate waters all around the world. Also, there are some reports of its presence in Icelandic waters and along Norwegian Coast. They are epi- (0-200m) and mesopelagic (200-1000m) animals, and the deepest dive observed was 644 meters.
Globally, summer is considered the best season for spotting ocean sunfish. This species are commonly found in northwestern Atlantic during spring and summer months. Besides, large aggregations of small Mola mola are reported in coastal waters.
There are some places where it is possible to dive with Mola mola, such as Indonesia, where they are seen from July to November. They are harmless to humans and they often approach divers.
What are the main threats for ocean sunfish?
Water pollution is a great concern for the survival of Mola mola because it can suffocate on plastic bags which are often mistaken for jellyfish.
They are considered a delicacy in many countries and can be found in markets around Japan and Taiwan. They are also targeted by fishers in western Pacific and south Atlantic. Some scientists believe that their guts are poisonous.
Because of their epipelagic habits, it is estimated that 36450 ocean sunfish are bycatch annually. Even though the majority of these are returned to the ocean alive, they can show traumas and survival data are lacking. In some places, like California, more Mola mola are caught that the target species. In 2007, Moroccan government approved a law to ban the use of driftnets in which ocean sunfish are often catch inadvertently.
Nowadays, this outstanding fish is classified as vulnerable by the International Union for Conservation of Nature (IUCN) due to fishing and harvesting of aquatic resources. Mola mola population is severely fragmented and experts suspect that it is declining by 30% over 3 generations (24-30 years).
Mola mola in captivity
Due to their large size and swimming patterns, ocean sunfish are rarely kept in captivity. However, there are few places where they are displayed, such as the Two Oceans Aquarium (South Africa), Nordsøen Oceanarium (Denmark), Monterey Bay Aquarium (USA), Lisbon Oceanarium (Portugal) and Kaiyukan Aquarium (Japan). The majority of sunfish exhibited there were bycatched, or became trapped and injured in ports, and then, they were rescued.
Interestingly, they become rapidly adapted to life in captivity, likely due to the fact that they don’t display great migratory instincts or mating behaviors, and also because they are friendly animals. For instance, a Mola mola taken to the Two Oceans Aquarium was eating from the hand of scuba divers within one day.