Ocean sunfish

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

Adult sunfish range from brown to silvery-gray or white, with a variety of mottled skin patterns; some of these patterns may be region-specific. Colouration 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 colouration 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 harbour 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 behaviour

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.

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 behaviour 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.

Feeding

The diet of the ocean sunfish consists primarily of various jellyfish (similar to the diet of a leatherback turtle). Additionally, it consumes salps, comb jellies, zooplankton, squid, crustaceans, small fishes, fish larvae, and eel grass. This diet is nutritionally poor, forcing the sunfish to consume large amount of food to maintain its size. The range of food items found inside sunfish indicates that the sunfish feeds at many levels, from the surface to deep water, and occasionally down to the seafloor in some areas.

The sunfish can spit out and pull in water through its small mouth to tear apart soft-bodied prey. Its teeth are fused into a beak-like structure, allowing it to break up harder organisms. In addition, pharyngeal teeth located in the throat grind food into smaller pieces before passing them to the stomach.

Life cycle

Ocean sunfish may live up to ten years in captivity, but their lifespan in a natural habitat has not yet been determined. Their growth rate is also indeterminate. However, it is known that a young specimen at the Monterey Bay Aquarium increased in weight from 26 kg (57 lb) to 399 kg (880 lb) and reached a height of nearly 1.8 m (6 ft) in fifteen months.

The sheer size and thick skin of an adult of the species deters many smaller predators, but younger individuals are vulnerable to predation by bluefin tuna and mahi mahi. Adults are consumed by sea lions, orcas and sharks. Sea lions appear to hunt sunfish for sport, tearing the fins off, tossing the body around, and then simply abandoning the still-living but helpless fish to die on the seafloor.

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 can produce as many as 300 million eggs at a time, more than any other known vertebrate. Sunfish eggs are released into the water and externally fertilized by sperm.

Newly-hatched sunfish larvae are only 2.5 mm (one-tenth of an inch) long. They grow to become fry, and those which survive grow many millions of times their original size before reaching adult proportions. Sunfish fry, with large pectoral fins, a tail fin and body spines uncharacteristic of adult sunfish, resemble miniature pufferfish, their close relatives. Young sunfish school for protection, but this behaviour is abandoned as they grow.

Human interaction

Despite their size, ocean sunfish are docile, and pose no threat to human divers. Injuries from sunfish are rare, although there is a slight danger from large sunfish leaping out of the water onto boats. According to Cliff Benson of Sea Trust, the only known case of a sunfish killing a person was when it landed on someone and crushed him. Areas where they are commonly found are popular destinations for sport dives, and sunfish at some locations have reportedly become familiar with divers. In fact, the fish is more threatening to boaters than swimmers, as its immense size and weight can cause significant damage when struck by watercraft. Collisions with sunfish are very common in some parts of the world and may cause damage to the hull of a boat, and their bodies can become lodged in the propellers of larger ships..
A dish made with the meat of the ocean sunfish
A dish made with the meat of the ocean sunfish

The meat of the ocean sunfish is considered a delicacy in some regions, the largest markets being Taiwan and Japan. All parts of the sunfish are used in cuisine, from the fins to the internal organs. Some parts of the fish are used in some areas of traditional medicine.

Sunfish are accidentally but frequently caught in drift gillnet fisheries, making up nearly 30% of the total catch of the swordfish fishery employing drift gillnet in California. The by-catch rate is even higher for the Mediterranean swordfish industry, with 71% to 90% of the total catch being sunfish.

The fishery by-catch and destruction of ocean sunfish are unregulated worldwide. In some areas, the fish are "finned" by fishermen who regard them as worthless bait thieves. This process, in which the fins are cut off, results in the eventual death of the fish, because it can no longer propel itself without its dorsal and anal fins. The species is also threatened by floating litter such as plastic bags which resemble jellyfish, its main diet. Bags can choke and suffocate an individual or fill its stomach to the extent that it starves.

Many areas of sunfish biology remain poorly understood, and various research efforts are underway, including aerial surveys of mola populations, satellite surveillance using pop-off satellite tags, genetic analysis of tissue samples, and collection of amateur sighting data. Recent studies indicate a decrease in sunfish populations that may be caused by more frequent bycatch and the increasing popularity of sunfish in human diet.

Sunfish in captivity

Sunfish are not widely held in aquarium exhibits, due to the unique and demanding requirements of their care. Some Asian aquariums display them, particularly in Japan. The Kaiyukan Aquarium in Osaka, Japan, is one of few aquariums with mola on display, where it is reportedly as popular an attraction as the larger whale sharks. The Lisbon Oceanarium in Portugal is another aquarium where sunfish are showcased in the main tank.

The first ocean sunfish to be held in an aquarium in the United States arrived at the Monterey Bay Aquarium in August of 1986. Because sunfish had not been kept in captivity on a large scale before, the staff at Monterey Bay were forced to innovate and create their own methods for capture, feeding, and parasite control. By 1998, these issues were overcome, and the aquarium was able to hold a specimen for more than a year, later releasing it after its weight increased by more than fourteen times. Mola mola have since become a permanent feature of the Outer Bay exhibit. Monterey Bay Aquarium's largest sunfish specimen was euthanized on February 14, 2008 after an extended period of poor health.

As the ocean sunfish is not highly maneuverable, preventing specimens from injuring themselves by rubbing against the walls of a tank is of major concern to aquarists. In a smaller tank, hanging a vinyl curtain has been used as a stopgap measure to convert a cuboid tank to a rounded shape and prevent the fish from scraping against the sides. A more effective solution is simply to provide enough room for the sunfish to swim in wide circles. The tank must also be sufficiently deep to accommodate the vertical height of the sunfish, which can be nearly as tall as it is long, and may reach a height of 3.2 meters (10.5 ft).

Feeding captive sunfish in a tank with other faster-moving, more aggressive fish can also present a challenge. Eventually, the fish can be taught to feed from the end of a pole or from human hands.

artikel asli dapat dibaca penuh di http://en.wikipedia.org/wiki/Ocean_sunfish