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The ocean sunfish ( Mola mola), also known as the common mola, is one of the largest bony fish in the world. It is the type species of the genus Mola, and one of five extant species in the family Molidae. It was once misidentified as the heaviest bony fish, which was actually a different and closely related species of sunfish, Mola alexandrini. Adults typically weigh between . It is native to tropical fish and temperate waters around the world. It resembles a fish head without a tail, and its main body is flattened laterally. Sunfish can be as tall as they are long when their Dorsal fin and ventral Pelvic fins are extended.
Many areas of sunfish biology remain poorly understood, and various research efforts are underway, including aerial surveys of populations, satellite surveillance using pop-off satellite tags, genetic analysis of tissue samples, and collection of amateur sighting data.
Adult sunfish are Vulnerability to few natural predators, but , , and will consume them. Sunfish are considered a delicacy in some parts of the world, including Japan, Korea, and Taiwan. In the European Union, regulations ban the sale of fish and fishery products derived from the family Molidae. Sunfish are frequently caught in Gillnetting.
The common name "sunfish" without qualifier is used to describe the seawater family Molidae and the freshwater sunfish in the family Centrarchidae, which is unrelated to Molidae. On the other hand, the name "ocean sunfish" and "mola" refer only to the family Molidae.
The caudal fin of the ocean sunfish is replaced by a rounded clavus, creating the body's distinct truncated shape. The body is flattened laterally, giving it a long oval shape when seen head-on. The pectoral fins are small and fan-shaped, while the dorsal fin and the anal fin are lengthened, often making the fish as tall as it is long. Specimens up to in height have been recorded.
The mature ocean sunfish has an average length of and a fin-to-fin length of . The weight of mature specimens can range from , but even larger individuals are not unheard of. The maximum size recorded, a specimen washed ashore in New Zealand in 2006, was in length, weighing .
The spinal column of M. mola contains fewer and is shorter in relation to the body than that of any other fish. Although the sunfish descended from Osteichthyes, its skeleton contains largely cartilage tissues, which are lighter than bone, allowing it to grow to sizes impractical for other bony fishes. Its teeth are fused into a beak-like structure, which prevents them from being able to fully close their mouths, while also having pharyngeal teeth located in the throat.
The axial musculature is completely lost during development. In addition they are missing a swim bladder. Instead they get their buoyancy from a stiff and gelatinous layer under the skin, which consist of about 90% water and a meshwork of collagen and elastin, acting like an exoskeleton. Due to its greasy texture, it may also contain lipids. The layer, which is horizontally separated by a septum, makes up a larger part of the animal's total mass the bigger the individual is. Some sources indicate the internal organs contain a concentrated neurotoxin, tetrodotoxin, like the organs of other poisonous tetraodontiformes, while others dispute this claim.
Ocean sunfish often swim near the surface, and their protruding dorsal fins are sometimes mistaken for those of . However, the two can be distinguished by the motion of the fin. Unlike most fish, the sunfish swings its dorsal fin and anal fin in a characteristic sculling motion.
More than 40 species of may reside on the skin and internally, motivating the fish to seek relief in a number of ways. One of the most frequent ocean sunfish parasites is the flatworm Accacoelium contortum.
In temperate regions, drifting kelp fields harbor cleaner fish and other fish which remove parasites from the skin of visiting sunfish. In the tropics, M. mola solicits cleaning help from reef fishes. By basking on its side at the surface, the sunfish also allows seabirds to feed on parasites from its skin. Sunfish have been reported to breach, clearing the surface by approximately , in an apparent effort to dislodge embedded parasites.
Although early research suggested sunfish moved around mainly by drifting with ocean currents (which has resulted in the sunfish sometimes being characterized as a megaplankton), individuals have been recorded swimming in a day at a cruising speed of . They are also capable of moving rapidly when feeding or avoiding predators, to the extent that they can vertically leap out of water. Contrary to the perception that the fish spend much of their time basking at the surface, M. mola adults actually spend a large portion of their lives actively hunting at depths greater than , occupying both the epipelagic and mesopelagic zones. Sunfish are most often found in water warmer than ; prolonged periods spent in water at temperatures of or lower can lead to disorientation and eventual death. Surface basking behavior, in which a sunfish swims on its side, presenting its largest profile to the sun, may be a method of "thermally recharging" following dives into deeper, colder water in order to feed. Sightings of the fish in colder waters outside of its usual habitat, such as those southwest of England, may be evidence of increasing marine temperatures.
Sunfish are typically observed in solitary environments, though infrequently they may be encountered in pairs.
The sheer size and thick skin of an adult of the species deters many smaller predators, but younger fish are vulnerable to predation by bluefin tuna and mahi mahi. Adults are consumed by orca, and .David C. Powell (2025). 9780520223660, University of California Press, Monterey Bay Aquarium. . ISBN 9780520223660
The mating practices of the ocean sunfish are poorly understood, but spawning areas have been suggested in the North Atlantic, South Atlantic, North Pacific, South Pacific, and Indian oceans. Females of the species can produce more eggs than any other known vertebrate, up to 300 million at a time. Sunfish eggs are released into the water and externally fertilized by sperm.
Newly hatched sunfish larvae are only long and weigh less than one gram. They develop into fry that resemble miniature pufferfish, their close relatives. Sunfish fry do not have the large pectoral fins and tail fin of their adult forms, but they have body spines uncharacteristic of adult sunfish, that disappear as they grow. Young sunfish school for protection, but this behavior is abandoned as they grow. The fry that survive can grow up to 60 million times their original weight before reaching adult proportions, arguably the most extreme size growth of any vertebrate animal.Wood, The Guinness Book of Animal Facts and Feats. Sterling Pub Co Inc. (1983), Kooijman, S. A. L. M., & Lika, K. (2013). Resource allocation to reproduction in animals. Am. Nat. subm, 2(06).
The flesh of the ocean sunfish is considered a delicacy in some regions, the largest markets being Taiwan and Japan. All parts of the sunfish are used in cuisine, from the fins to the internal organs. Some parts are used in some areas of traditional medicine. Fishery products derived from sunfish are forbidden in the European Union according to Regulation (EC) No 853/2004 of the European Parliament and of the Council, as they contain toxins that are harmful to human health.
Sunfish are accidentally but frequently caught in drift gillnet fisheries, making up nearly 30% of the total catch of the swordfish fishery employing drift gillnets in California. The bycatch rate is even higher for the Mediterranean swordfish industry, with 71% to 90% of the total catch being sunfish.
A decrease in sunfish populations may be caused by more frequent bycatch and the increasing popularity of sunfish in human diet. The fishery bycatch and destruction of ocean sunfish are unregulated worldwide. In some areas, the fish are "finned" by fishermen who regard them as worthless bait thieves; this process, in which the fins are cut off, results in the eventual death of the fish, because it can no longer propel itself without its dorsal and anal fins. The species is also threatened by floating litter such as plastic bags which resemble jellyfish, a common prey item. Bags can choke and suffocate a fish or fill its stomach to the extent that it starves.
In Kamogawa Sea World the ocean sunfish named Kukey, who started captivity in 1982, set a world record for captivity for 2,993 days, living for eight years. Kukey was at the time of delivery, but was in size at the time of death. While the first ocean sunfish to be held in an aquarium in the United States is said to have arrived at the Monterey Bay Aquarium in August 1986, other specimens have previously been held at other locations. Marineland of the Pacific, closed since 1987 and located on the Palos Verdes Peninsula in Los Angeles County, California, held at least one ocean sunfish by 1961, and in 1964 held a specimen, said to be the largest ever captured at that time. Los Angeles Times. June 15, 1964. p. 3 However, another specimen was brought alive to Marineland Studios Aquarium, near St. Augustine, Florida, in 1941.The Miami News, March 16, 1941, p. 5-C
Because sunfish had not been kept in captivity on a large scale before, the staff at Monterey Bay was forced to innovate and create their own methods for capture, feeding, and parasite control. By 1998, these issues were overcome, and the aquarium was able to hold a specimen for more than a year, later releasing it after its weight increased by more than 14 times. Mola mola has since become a permanent feature of the Open Sea exhibit. Monterey Bay Aquarium's largest sunfish specimen was euthanized on February 14, 2008, after an extended period of poor health.
A major concern to curators is preventive measures taken to keep specimens in captivity from injuring themselves by rubbing against the walls of a tank, since ocean sunfish cannot easily maneuver their bodies. In a smaller tank, hanging a vinyl curtain has been used as a stopgap measure to convert a cuboid tank to a rounded shape and prevent the fish from scraping against the sides. A more effective solution is simply to provide enough room for the sunfish to swim in wide circles. The tank must also be sufficiently deep to accommodate the vertical height of the sunfish, which may reach .
Feeding captive sunfish in a tank with faster-moving, more aggressive fish can also present a challenge. Eventually, the fish can be taught to respond to a floating target to be fed, and to take food from the end of a pole or from human hands.
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