Ocean sunfish

Appearance

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 in height have been recorded.

The mature ocean sunfish has an average length of 1.8 m and a fin-to-fin length of 2.5 m. The weight of mature specimens can range from 247 to 1,000 kg, but even larger individuals are not unheard of. The maximum size is up to 3.3 m in length
4.2 m across the fins and up to 2,300 kg 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, and it has 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. Sharks, like most fish, swim by moving the tail sideways while keeping the dorsal fin stationary. The sunfish, though, 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 mottled skin patterns; some of these patterns may be region-specific. 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 (2.9 in) 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. One of the most frequent ocean sunfish parasites is the flatworm, Accacoelium contortum.

In temperate regions, drifting kelp fields harbor cleaner wrasses 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 3 m (10 ft), in an effort to dislodge embedded parasites.

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 in a day, at a top speed of 3.2 km/h.
Sunfish are pelagic and swim at depths to 600 m. 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 submerged at depths greater than 200 m, occupying both the epipelagic and mesopelagic zones.

Sunfish are most often found in water warmer than 10 °C; prolonged periods spent in water at temperatures of 12 °C 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. 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.

Sunfish are usually found alone, but occasionally in pairs. They swim primarily in surface waters, and their presence in a given area may be an indicator of nutrient-rich waters where other endangered species may be found.

Food

The diet of the ocean sunfish consists primarily of various jellyfish. It also consumes salps, squid, crustaceans, small fish, fish larvae, and 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. The diet is nutritionally poor, forcing the sunfish to consume a large amount of food to maintain its size.