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Ocean Sunfish

mola mola

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 tonne (2,200 lbs). The species is native to tropical and temperate waters around the globe. 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 and ventral fins are extended.

Sunfish live on a diet that consists mainly of jellyfish. Because this diet is nutritionally poor, they consume large amounts in order to develop and maintain their great bulk. Females of the species can produce more eggs than any other known vertebrate. 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, orcas and sharks will consume them. Among humans, sunfish are considered a delicacy in some parts of the world, including Japan, the Korean peninsula and Taiwan, but sale of their flesh is banned in the European Union. Sunfish are frequently, though accidentally, caught in gillnets, and are also vulnerable to harm or death from encounters with floating rubbish, such as plastic bags.

A member of the order Tetraodontiformes, which also includes pufferfish, porcupinefish and filefish, the sunfish shares many traits common to members of this order. It was originally classified as Tetraodon mola under the pufferfish genus, but it has since been given its own genus, Mola, with two species under it. The ocean sunfish, Mola mola, is the type species of the genus.

Naming and taxonomyMany of the sunfish's various names allude to its unique flattened shape. Its specific name, mola, is Latin for "millstone", which the fish resembles because of its grey color, rough texture, and rounded body. Its common English name, sunfish, refers to the animal's habit of sunbathing at the surface of the water. The Portuguese-, French-, Spanish- and German-language names, respectively peixe lua, poisson lune, pez luna and Mondfisch, mean "moon fish", in reference to its rounded shape. In German, the fish is also known as Schwimmender Kopf, or "swimming head", because it has no true tail. In Taiwan's Hualien County, where sunfish are featured as the official mascot, they are known as the "mambo fish" for their swimming motions. The ocean sunfish has various obsolete binomial synonyms, and was originally classified in a pufferfish genus, as Tetraodon mola. It is now placed under its own genus, Mola, with two species under it: Mola mola and Mola ramsayi. The ocean sunfish, Mola mola, is the type species of the genus.

The Mola genus belongs to the Molidae family. This family comprise 3 genera: Masturus, Mola and Ranzania. The common name "sunfish" without qualifier is used to describe the Molidae marine family as well as the freshwater sunfishes in the family Centrarchidae which are unrelated to Molidae. On the other hand, the name "ocean sunfish" and "mola" refer only to the family Molidae.

The Molidae family belongs to the order Tetraodontiformes, which includes pufferfish, porcupinefish and filefish. It shares many traits common to members of this order, including the four fused teeth that form the characteristic beak and give the order its name (tetra=four, odous=tooth, and forma=shape). Indeed, sunfish larvae resemble spiky pufferfish more than they resemble adult molas.

 

Description

A sunfish caught in 1910, with an estimated weight of 3,500 pounds (1,600 kg)The ocean sunfish resembles a fish head without a tail. Its caudal fin is replaced by a rounded clavus, creating the body's distinct shape. The main body is flattened laterally, giving it a long oval shape when seen head-on. The pectoral fins are small and fan-shaped. However, the dorsal fin and the anal fin are lengthened, often making the fish as tall as it is long. Specimens up to 3.2 meters (10.5 ft) in height have been recorded.

The ocean sunfish has an average length of 1.8 meters (5.9 ft), and an average weight of 1,000 kilograms (2,200 lb), although individuals up to 3.3 meters (10.8 ft) in length 4.2 m (14 feet) across the fins[8] and weighing up to 2,300 kilograms (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. The spinal cord of a specimen measuring 2.1 meters (7 ft) in length is under 25 millimeters (1 in) long. Even though sunfish descended from bony ancestors, its skeleton actually contains largely cartilage tissues which is lighter than true bone and allows it to grow to sizes uneconomical for other bony fishes.

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.

 

Fins

The dorsal fin of a sunfish, sometimes mistaken for that of a sharkIn 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, it is possible to distinguish a shark from a sunfish, by observing the trajectory made by the dorsal fin on the surface, while the fish itself moves underwater and remains unseen. Sharks, like most fish, swim by waving the tail sideways while keeping the dorsal fin moving in a straight line. The sunfish, on the other hand, swings its dorsal fin and anal fin in its characteristic sculling motion. Thus, the sideways movement of the dorsal fin on the surface can be used to identify the sunfish.

 

Skin

M. mola in typical swimming positionAdult sunfish range from brown to silvery-gray 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 counter-shading camouflage. Mola 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 3 inches (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 harbor cleaner wrasses and other fish which remove parasites from the skin of visiting sunfish. In the tropics, the mola will solicit cleaner help from reef fishes. By basking on its side at the surface, the sunfish also allows seabirds to feed on parasites from their skin. Sunfish have been reported to breach more than ten feet above the surface, possibly as another effort to dislodge parasites on the body.

 

Range and behavior

M. mola exhibiting its characteristic horizontal basking behaviorOcean 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.

Sunfish are pelagic and swim at depths of up to 600 meters (2,000 ft). Contrary to the general perception that sunfish spend much of their time basking at the surface, research suggests that adult M. mola actually spend a large portion of their lives submerged at depths greater than 200 meters (700 ft), occupying both the epipelagic and mesopelagic zones.

They usually stay in water warmer than 10 °C (50 °F). In fact, prolonged periods spent in water at temperatures of 12 °C (53 °F) or lower can lead to disorientation and eventual death. Researchers theorize that the basking behavior at the surface, in which the 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. Others point to sightings of the fish in colder waters such as those southwest of England outside of its usual habitat as evidence of increasing marine temperatures.

Sunfish are usually found alone, but occasionally in pairs or in large groups while being cleaned. They swim primarily in open waters, but are sometimes seen near kelp beds taking advantage of resident populations of smaller fish which remove ectoparasites from their skin. Because sunfish must consume a large volume of prey, their presence in a given area may be used as an indicator of nutrient-rich waters where endangered species may be found.

Source: encyclopedia.thefreedictionary.com


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