Many areas of sunfish biology remain poorly understood,[needs update] and various research efforts are underway, including aerial surveys of populations,[9] satellite surveillance using pop-off satellite tags,[10][9] genetic analysis of tissue samples,[10] and collection of amateur sighting data.

[citation needed] Its common names in Dutch, Portuguese, French, Spanish, Catalan, Italian, Russian, Greek, Hungarian, Norwegian, and German (maanvis, peixe lua, Poisson lune, pez luna, peix lluna, Pesce luna, рыба-луна, φεγγαρόψαρο, holdhal, månefisk and Mondfisch, respectively) mean "moon fish", in reference to its rounded shape.

The Chinese translation of its academic name is 翻車魚; fān chē yú, meaning "toppled wheel fish".

[citation needed] French polymath Guillaume Rondelet wrote about the ocean sunfish in his 1554 work de Piscibus, using the term Orthagoriscus,[13] "sucking pig" for the likeness of its body and mouth.

[20] The caudal fin of the ocean sunfish is replaced by a rounded clavus, creating the body's distinct truncated shape.

Instead they gets its 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.

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.

[6] The skin, which contains large amounts of reticulated collagen, can be up to 7.3 cm (2+3⁄4 in) thick on the ventral surface, and is covered by denticles and a layer of mucus instead of scales.

[36] In temperate regions, drifting kelp fields harbor cleaner wrasses and other fish which remove parasites from the skin of visiting sunfish.

Sunfish have been reported to breach, clearing the surface by approximately 3 m (10 ft), in an apparent effort to dislodge embedded parasites.

[25] Mola genotypes appear to vary widely between the Atlantic and Pacific, but genetic differences between individuals in the Northern and Southern hemispheres are minimal.

[35] 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.

[38][42] 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.

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.

However, a young specimen at the Monterey Bay Aquarium increased in weight from 26 to 399 kg (57 to 880 lb) and reached a height of nearly 1.8 m (5 ft 11 in) in 15 months.

[48] The fry that survive can grow up to 60 million times their original weight before reaching adult proportions,[32] arguably the most extreme size growth of any vertebrate animal.

[49][50] In 2016, researchers from China National Genebank and A*STAR Singapore, including Nobel laureate Sydney Brenner, sequenced the genome of the ocean sunfish and discovered several genes which might explain its fast growth rate and large body size.

Analysis from this data suggests that sunfish and pufferfishes diverged approximately 68 million years ago, which corroborates the results of other recent studies based on smaller datasets.

[52] Areas where they are commonly found are popular destinations for sport dives, and sunfish at some locations have reportedly become familiar with divers.

Collisions with sunfish are common in some parts of the world and can cause damage to the hull of a boat,[53] or to the propellers of larger ships, as well as to the fish.

[54] The species is also threatened by floating litter such as plastic bags which resemble jellyfish, a common prey item.

[64] 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.

[32] Monterey Bay Aquarium's largest sunfish specimen was euthanized on February 14, 2008, after an extended period of poor health.

[65] 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.

Eventually, the fish can be taught to respond to a floating target to be fed,[66][better source needed] and to take food from the end of a pole or from human hands.