Ocean Sunfish Facts

Scientific Name: Mola mola

Close up of Mola Mola, a Ocean Sunfish

 Stephen Frink/Getty Images

The ocean sunfish (Mola mola) is certainly one of the more unusual-appearing fish in the oceans. This bony fish, also known as the common mola, is famous for its enormous bulk, striking appearance, high fertility, and free moving lifestyle.

Fast Facts: Ocean Sunfish

  • Scientific Name: Mola mola
  • Common Name(s): Ocean sunfish, common mola, common sunfish
  • Basic Animal Group: Fish
  • Size: 6–10 feet
  • Weight: 2,000 pounds
  • Lifespan: 22–23 years
  • Diet: Carnivore
  • Habitat: Pacific, Indian, Atlantic oceans, Mediterranean and North Seas
  • Population: Unknown
  • Conservation Status: Vulnerable

Description

The ocean sunfish is a bony fish—it has a skeleton of bone, which distinguishes it from cartilaginous fish, whose skeletons are made of cartilage. The fish doesn't have a normal-looking tail; instead, it has a lumpy appendage called a clavus, which evolved through the fusion of the fish's dorsal and anal fin rays. Despite its lack of a powerful tail, the ocean sunfish is an active and graceful swimmer, using its dorsal and anal fins to perform rapid changes in direction and horizontal movements independent of the prevailing current. It can also leap out of the water.

Ocean sunfish vary in color from brown to gray to white. Some even have spots. On average, ocean sunfish weighs about 2,000 pounds and range between 6 and 10 feet across, making them the largest bony fish species. Female sunfish are larger than the males—all sunfish larger than 8 feet long are females. The largest ocean sunfish ever measured was nearly 11 feet across and weighed over 5,000 pounds.

Underwater view of mola mola, ocean sunfish, Magadalena bay, Baja California, Mexico
 Rodrigo Friscione/Getty Images

Species

The word "mola" in its scientific name is Latin for millstone—a large round stone used to grind grain—and the fish's name is a reference to its disc-like shape. Ocean sunfish are often referred to as common molas or simply molas.

The ocean sunfish is also known as the common sunfish, as there are three other species of sunfish that live in the ocean—the slender mola (Ranzania laevis), the sharp-tailed mola (Masturus lanceolatus), and the southern ocean sunfish (Mola alexandrini). The sunfish group gets its name for the fish's characteristic behavior of lying on its side at the sea surface, seemingly basking in the sun.

Habitat and Range

Ocean sunfish live in tropical and temperate waters, and they can be found in the Atlantic, Pacific, and Indian Oceans as well as inlets such as the Mediterranean and North seas. They generally stay within 60–125 miles of the coastline, and they apparently migrate within their ranges. They spend the summers at higher latitudes and their winters relatively closer to the equator; their ranges typically are along about 300 miles of coastline, although one sunfish off the coast of California was mapped at traveling over 400 miles.

They move during the day horizontally at rate of about 16 miles a day. They also move vertically through the day, traveling between the surface and up to 2,600 feet below, moving up and down the water column during the day and night to chase food and regulate body heat.

To see an ocean sunfish, though, you'll likely have to find one in the wild, because they are difficult to keep in captivity. The Monterey Bay Aquarium is the only aquarium in the U.S. to have live ocean sunfish, and the fish are kept at only a few other aquaria, such as the Lisbon Oceanarium in Portugal and the Kaiyukan Aquarium in Japan.

Diet and Behavior

Ocean sunfish like to eat jellyfish and siphonophores (relatives of jellyfish); in fact, they are among the most abundant of the world's jellyfish eaters. They also eat salps, small fish, plankton, algae, mollusks, and brittle stars.

If you're lucky enough to see an ocean sunfish in the wild, it may look like it's dead. That's because ocean sunfish are often seen lying on their sides near the ocean surface, sometimes flapping their dorsal fins. There are a few theories about why sunfish do this; they often undertake long, deep dives in cold water in search of their favorite prey, and may use the warm sun at the surface to re-heat themselves and aid digestion. The fish may also use the warm, oxygen-rich surface water to recharge their oxygen stores. And they may visit the surface to attract seabirds from above or cleaner fish from below to clean their skin of parasites. Some sources suggest that the fish wave their fins to attract birds.

From 2005 to 2008, scientists tagged 31 ocean sunfish in the North Atlantic in the first study of its kind. The tagged sunfish spent more time near the ocean surface during the night than during the day, and they spent more time in the deep when they were in warmer waters such as the Gulf Stream and the Gulf of Mexico.

Sunfish, Mola mola, Molidae, Witless Bay Ecological Reserve, Newfoundland, Canada
Barrett&MacKay Barrett&MacKay/Getty Images 

Reproduction and Offspring

Ocean sunfish in Japanese waters spawn in late summer through October and likely multiple times. Age at sexual maturity is inferred at 5–7 years of age, and they spawn an enormous number of eggs. An ocean sunfish was once found with an estimated 300 million eggs in her ovary—more than scientists have ever found in any vertebrate species.

Although sunfish produce many eggs, the eggs are tiny and essentially scattered into the water, making their chances of survival relatively small. Once an egg is fertilized, the embryo grows into tiny spiked larvae with a tail. After hatching, the spikes and tail disappear and the baby sunfish resembles a small adult.

The lifespan of an ocean sunfish is up to 23 years.

Conservation Status

The International Union for Conservation of Nature (IUCN) has listed the ocean sunfish as "Vulnerable." Currently, sunfish are not targeted for human consumption, but they are endangered by bycatch. Reported estimates in California are that 14 percent to 61 percent of the fish caught by people seeking swordfish is sunfish; in South Africa, they make up 29 to 79 percent of the catch intended for horse mackerel, and in the Mediterranean, an astounding 70 to 95 percent of the total catch for swordfish is, in fact, ocean sunfish.

The global population of sunfish is difficult to determine, since they spend so much time in deep water, although tagging has become more common. Sunfish may be a crucial part of the planet's changing ecosystem under climate change: They are among the world's most abundant eaters of jellyfish, and global warming appears to be resulting in an upsurge of jellyfish numbers.

The biggest natural predators of ocean sunfish are orcas and sea lions.

Ocean Sunfish and Humans

Despite their enormous size, ocean sunfish are harmless to humans. They move slowly and are likely more frightened of us than we are of them. Because they are not considered a good food fish in most places, their biggest threats are likely being hit by boats and being caught as bycatch in fishing gear.

Ocean Sunfish and Diver, Mola Mola, Bali Island, Indo-Pazific, Indonesia
 Franco Banfi/Getty Images

Sources