The ocean sunfish, Mola mola, or common mola, is the heaviest known bony fish in the world. It has an average adult weight of 1,000 kg (2,200 lb). The species is native to tropical and temperate waters around the globe. 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.
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 3.2 m (10.5 ft) in height have been recorded.
The mature ocean sunfish has an average length of 1.8 m (5.9 ft), a fin-to-fin length of 2.5 m (8.2 ft) and an average weight of 1,000 kg (2,200 lb), although individuals up to 3.3 m (10.8 ft) in length 4.2 m (14 ft) across the fins and weighing up to 2,300 kg (5,100 lb) have been observed.
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, and pharyngeal teeth located in the throat.
The sunfish lacks a swim bladder. Some sources indicate that the internal organs contain a concentrated neurotoxin, tetrodotoxin, like the organs of other poisonous tetraodontiformes, while others dispute this claim.
In the course of its evolution, the caudal fin (tail) of the sunfish disappeared, to be replaced by a lumpy pseudo-tail, the clavus. This structure is formed by the convergence of the dorsal and anal fins. The smooth-denticled clavus retains twelve fin rays, and terminates in a number of rounded ossicles. Without a true tail to provide thrust for forward motion and equipped with only small pectoral fins, Mola mola relies on its long, thin dorsal and anal fins for propulsion, driving itself forward by moving these fins from side to side.
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. Sharks, like most fish, swim by moving the tail sideways while keeping the dorsal fin stationary. The sunfish, on the other hand, swings its dorsal fin and anal fin in a characteristic sculling motion which can be used to identify it.
Adult sunfish range from brown to silvery-gray or white, with a variety of mottled skin patterns; some of these patterns may be region-specific. Colouration is often darker on the dorsal surface, fading to a lighter shade ventrally as a form of counter-shading camouflage. Mola mola also exhibits the ability to vary skin colouration from light to dark, especially when under attack. The skin, which contains large amounts of reticulated collagen, can be up to 3 in (7.6 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.
More than 40 species of parasites may reside on the skin and internally, motivating the fish to seek relief in a number of ways. In temperate regions, drifting kelp fields harbour cleaner wrasses and other fish which remove parasites from the skin of visiting sunfish. In the tropics, the 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 more than three body lengths, possibly as another effort to dislodge parasites.