Ocean sunfish
(Mola mola)

The ocean sunfish or common mola (Mola mola) is one of the two heaviest known bony fish in the world, the other being the southern sunfish (Mola alexandrini) of the same genus. Adults typically weigh between 247 and 2,000 kg (545 and 4,409 lb).

The species is native to tropical and temperate waters around the world. It resembles a fish head with a tail, and its main body is flattened laterally. Sunfish can be as tall as they are long when their dorsal and ventral fins are extended.

Sunfish are generalist predators that consume largely small fishes, fish larvae, squid, and crustaceans. Sea jellies and salps, once thought to be the primary prey of sunfish, make up only 15% of a sunfish\'s diet. Females of the species can produce more eggs than any other known vertebrate, up to 300,000,000 at a time. Sunfish fry resemble miniature pufferfish, with large pectoral fins, a tail fin, and body spines uncharacteristic of adult sunfish.

Adult sunfish are vulnerable to few natural predators, but sea lions, killer whales, and sharks will consume them. Sunfish are considered a delicacy in some parts of the world, including Japan, Korea, and Taiwan. In the EU, regulations ban the sale of fish and fishery products derived from the family Molidae. Sunfish are frequently caught in gillnets.

Description

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 3.3 m (10 ft 10 in) in height have been recorded.

The mature ocean sunfish has an average length of 1.8 m (5 ft 11 in) and a fin-to-fin length of 2.5 m (8 ft 2 in). The weight of mature specimens can range from 247 to 1,000 kg (545 to 2,205 lb), but even larger individuals are not unheard of. The maximum size is up to 3.3 m (10 ft 10 in) in length, 4.2 m (13 ft 9 in) across the fins, and up to 2,300 kg (5,070 lb) in mass.

The spinal column of M. mola contains fewer vertebrae and is shorter in relation to the body than that of any other fish. Although the sunfish descended from bony ancestors, its skeleton contains largely cartilaginous 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 sunfish lacks a swim bladder. Some sources indicate the internal organs contain a concentrated neurotoxin, tetrodotoxin, like the organs of other poisonous tetraodontiformes, while others dispute this claim.

Fins

In the course of its evolution, the caudal fin (tail) of the sunfish disappeared, to be replaced by a lumpy pseudotail, the clavus. This structure is formed by the convergence of the dorsal and anal fins, and is used by the fish as a rudder. The smooth-denticled clavus retains 12 fin rays and terminates in a number of rounded ossicles.

Ocean sunfish often swim near the surface, and their protruding dorsal fins are sometimes mistaken for those of sharks. 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.

Skin

Adult sunfish range from brown to silvery-grey or white, with a variety of region-specific mottled skin patterns. Coloration is often darker on the dorsal surface, fading to a lighter shade ventrally as a form of countershading camouflage. M. mola also exhibits the ability to vary skin coloration from light to dark, especially when under attack. The skin, which contains large amounts of reticulated collagen, can be up to 7.3 cm thick on the ventral surface, and is covered by denticles and a layer of mucus instead of scales. The skin on the clavus is smoother than that on the body, where it can be as rough as sandpaper.

Range and behavior

Ocean sunfish are native to the temperate and tropical waters of every ocean in the world. Mola genotypes appear to vary widely between the Atlantic and Pacific, but genetic differences between individuals in the Northern and Southern hemispheres are minimal.

Although early research suggested sunfish moved around mainly by drifting with ocean currents, individuals have been recorded swimming 26 km (16 mi) in a day at a cruising speed of 3.2 km/h (1.7 kn). While this might be the case most of the time, 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 sunfish 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 200 m (660 ft), occupying both the epipelagic and mesopelagic zones.

Sunfish are most often found in water warmer than 10 °C (50 °F); prolonged periods spent in water at temperatures of 12 °C (54 °F) 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, although the proximity of England\'s southwestern coast to the Gulf Stream means that many of these sightings may also be the result of the fish being carried to Europe by the current.

Feeding

The diet of the ocean sunfish was formerly thought to consist primarily of various jellyfish. However, genetic analysis reveals that sunfish are actually generalist predators that consume largely small fish, fish larvae, squid, and crustaceans, with jellyfish and salps making up only around 15% of the diet. Occasionally they will ingest eel grass. This range of food items indicates that the sunfish feeds at many levels, from the surface to deep water, and occasionally down to the seafloor in some areas.