Ocean sunfish
The ocean sunfish, also known as the common mola, is one of the largest bony 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. 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 175 cm long 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, and 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, killer whales, 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 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 labels=no, meaning "overturned 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. 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 in height have been recorded.
The mature ocean sunfish has an average length of and a fin-to-fin length of. The weight of mature specimens can range from, but even larger individuals are not unheard of. The maximum size recorded, a specimen washed ashore in New Zealand in 2006, was in length, weighing.
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. Due to its greasy texture, it may also contain 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 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.
Skin
Adult sunfish range from brown to silvery-grey or white, with a variety of region-specific mottled skin patterns. 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 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 that 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, while smaller parasite-eating fish feed on the underside. Sunfish have been reported to breach, clearing the surface by approximately, in an apparent effort to dislodge embedded parasites.
Distribution and habitat
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 in a day at a cruising speed of. Contrary to the perception that the fish spend much of their time basking at the surface, M. mola adults actually spend a large portion of their lives actively hunting at depths greater than, occupying both the epipelagic and mesopelagic zones. Sunfish are most often found in water warmer than ; prolonged periods spent in water at temperatures of 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 in order to feed. 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.
Sunfish are typically observed in solitary environments, though infrequently they may be encountered in pairs.
Feeding
The diet of the ocean sunfish was formerly thought to consist primarily of various jellyfish and other gelatinous zooplankton, such as ctenophores, salps, and medusae. However, genetic analysis reveals that sunfish are actually generalist predators that consume mostly small fish, fish larvae, squid, other molluscs, crustaceans, and other soft-bodied invertebrates, with jellyfish and salps making up only around 15% of the diet. Occasionally, they will ingest 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.Life cycle
Ocean sunfish may live up to ten years in captivity, but their lifespan in a natural habitat has not yet been determined. However, estimates of their lifespan in a natural habitat place their life expectancy at around 2 to 23 years for females and 1-16 years for males. Their growth rate also remains undetermined. However, a young specimen at the Monterey Bay Aquarium increased in weight from and reached a height of nearly in 15 months.The sheer size and thick skin of an adult of the species deters many smaller predators, but younger fish are vulnerable to predation by bluefin tuna and mahi mahi. Adults are consumed by orca, sharks and sea lions.
The mating practices of the ocean sunfish are poorly understood, but spawning areas have been suggested in the North Atlantic, South Atlantic, North Pacific, South Pacific, and Indian oceans. Females of the species can produce more eggs than any other known vertebrate, up to 300 million at a time. Sunfish eggs are released into the water and externally fertilized by sperm.
Newly hatched sunfish larvae are only long and weigh less than one gram. They develop into fry that resemble miniature pufferfish, their close relatives. Sunfish fry do not have the large pectoral fins and tail fin of their adult forms, but they have body spines uncharacteristic of adult sunfish, which disappear as they grow. Young sunfish school for protection, but this behavior is abandoned as they grow. The fry that survive can grow up to 60 million times their original weight before reaching adult proportions, arguably the most extreme size growth of any vertebrate animal.