The ocean sunfish (Mola mola), also known as the common mola, is one of the largest ray-finned fish in the world. It is the type species of the genus Mola, and one of five extant species in the family Molidae. It was formerly misidentified as the heaviest bony fish, which is actually a different and closely related species of sunfish, Mola alexandrini. Adults typically weigh between 247 and 1,000 kg (545 and 2,205 lb). It is native to tropical and temperate waters around the world. It resembles a fish head without a tail, and its main body is flattened laterally. Sunfish can be as tall as they are long, between 175 and 180 centimetres (5.74 and 5.91 ft), when their dorsal and ventral fins are extended. Many areas of sunfish biology remain poorly understood, including mating practices and spawning locations, early life stages, movement and migration patterns, population structure and status, diet and trophic ecology, and post-release survival rates. Various research efforts are underway, including aerial surveys of populations, satellite surveillance using pop-off satellite tags, genetic analysis of tissue samples, and collection of amateur sighting data. Adult sunfish are vulnerable to few natural predators, but sea lions, orcas, and sharks will consume them. Sunfish are considered a delicacy in some parts of the world, including Japan, Korea, and Taiwan. In the European Union, regulations ban the sale of fish and fishery products derived from the family Molidae. Sunfish are frequently caught in gillnets.
Naming
Its common English name, sunfish, refers to the animal's habit of sunbathing at the surface of the sea. Its common names in Catalan, Dutch, French, German, Greek, Hungarian, Italian, Norwegian, Portuguese, Russian, Spanish, and Ukrainian (peix lluna, maanvis, Poisson lune, Mondfisch, φεγγαρόψαρο, holdhal, Pesce luna, månefisk, peixe lua, рыба-луна, pez luna, and риба-місяць, respectively) mean "moon fish", in reference to its rounded shape. In German, the fish is also known as Schwimmender Kopf, or "swimming head". In Polish, it is named samogłów, meaning "head alone" or "only head", because it has no true tail. In Swedish and Danish it is known as klumpfisk, in Dutch klompvis, in Finnish möhkäkala, all of which mean "lump fish". The Chinese translation of its academic name is 翻車魚; fān chē yú, meaning "overturned car fish". Many of the sunfish's various names allude to its flattened shape.
Taxonomy French polymath Guillaume Rondelet wrote about the ocean sunfish in his 1554 work de Piscibus, using the term Orthagoriscus, "sucking pig" for the likeness of its body and mouth. It was originally classified in the pufferfish family as Tetraodon mola, its epithet mola is Latin for "millstone", which the fish resembles because of its gray color, rough texture, and rounded body. It is now placed in its own genus Mola and family name Molidae as the type species with two other species: Mola tecta and M. alexandrini (previously known as Mola ramsayi). Extinct relatives of Mola mola lived in the Oligocene and Miocene epochs. However, the earliest known fossil remains of Mola mola itself were found in archaeological middens dating to the Holocene epoch. The common name "sunfish" without qualifier is used to describe the marine family Molidae and the freshwater sunfish in the family Centrarchidae, which is unrelated to Molidae. On the other hand, the name "ocean sunfish" and "mola" refer only to the family Molidae.
Description
The ocean sunfish shares many traits common to members in the order Tetraodontiformes, including pufferfish, porcupinefish, and filefish, like having a beak formed from four fused teeth; sunfish fry resemble spiky pufferfish more than they resemble adult molas. 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 recorded, a specimen washed ashore in New Zealand in 2006, was 3.3 m (10 ft 10 in) in length, weighing 2,300 kg (5,100 lb). 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 axial musculature is completely lost during development. In addition, they are missing a swim bladder. Instead, they get their buoyancy from a stiff and gelatinous layer under the skin, which consists of about 90% water and a meshwork of collagen and elastin, acting like an exoskeleton, with a greasy texture that may indicate it also contains lipids. 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. 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 11–14 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.
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