Sunday, 19 December 2010

Ocean Sunfish - World's heaviest bony fish

ocean sunfish image
Ocean Sunfish
Kingdom: Animalia
Phylum: Chordata
Class: Actinopterygii
Order: Tetraodontiformes
Family: Molidae
Genus: Mola
Species: Mola Mola
Conservation Status: Not Evaluated
Common Name(s): Ocean Sunfish, Mola

The ocean sunfish (also known as Mola mola or just Mola) holds the world record for heaviest bony fish. The species inhabits tropical and temperate waters and occurs in all major oceans.

Ocean sunfish Description
The ocean sunfish is the heaviest bony fish of the world, with adults having an average weight of 1.000 kg (about 2,200 lbs). Adults have an average length of 1.8 m (6 ft) and an average fin to fin length of 2.5 m (8.2 ft). However, there have been reports of individuals measuring up to 3.3 meters (10.8 ft) in length and 4.3 meters (14 ft) from fin to fin. The heaviest specimen ever recorded had a weight of 2.300 kg (5,100 lbs). Females tend to be a bit larger than males.

The body is laterally flattened and as you can see on the images, M. Mola looks like a huge head with a tail. The caudal fin is replaced by a rounded clavus, giving the body its distinct truncated shape. The species has small, fan-like pectoral fins with both the dorsal fin and the anal fin being lengthened. When all fins are extended, the ocean fish has the same length and "height". The species lacks a swim bladder, an internal gas-filled organ that helps with the control of buoyancy. They have small eyes and mouth.

Color varies from brown to silvery-gray or white, with a great range of mottled skin patterns, some of which appear to be region-specific. These colorations are often darker on the dorsal surface, fading to a lighter shade ventrally as a means of counter-shading camouflage. Individuals can vary their skin color from light to dark, when they are under attack by predators. The skin contains large amounts of reticulated collagen and can be up to 7.5 cm (3 inches) thick on the ventral surface. It isn't covered by scales but by denticles (small tooth-like structures) along with a layer of mucus. When touched, it has a sandpaper-like feeling, except in the area of the clavus where the it is smooth.

Growth rate is yet to be determined, although a juvenile specimen at the Monterey Bay Aquarium increased its weight from 26 to 400 kg (58 to 880 pounds) and reached an overall height of about 1.8 m (6 ft) in just 15 months. They are known to live for up to 10 years in captivity, however, average lifespan in the wild is still unknown. Due to the large body size and reproductive output it is likely that they are long-lived. For instance, the sharptail mola (Masturus lanceolatus) -a member of the same family- is estimated to have a maximum lifespan of 82 to 105 years

Certain sources indicate that some of its internal organs contain a concentrated neurotoxin called tetrodotoxin, like the organs of other poisonous tetraodontiformes, however other sources dispute this claim.

Mola mola at Lisbon Oceanarium, Portugal

They are native to every ocean that has temperate and tropical waters. They are pelagic animals, swimming at depths of up to 600 m (2,000 ft).

Molas are mostly found in waters that are warmer than 15 °C (50 °F), since prolonged periods of staying in waters at temperatures of 12 °C (54 °F) or lower lead first to disorientation and eventually death. Sightings of the fish in colder waters that should normally be out of its normal habitat (e.g. southwest of England) are believed by some to be the result of global warming.

They are typically found in open waters although sometimes they venture near kelp beds. They do so to take advantage of various small fish species which remove the ectoparasites from their skin by eating them.

Mola mola skeleton
Mola Mola skeleton showing
the structure of the fins

Molas enjoy swimming near the surface, many times presenting their whole side to the sun. Scientists believe that they do so in order to get warm after making a deep dive (since the deeper they dive the colder the water is). Many times, they are mistaken for sharks -due to their protruding dorsal fins- however, it is easy to distinguish between the two. A shark’s fin stays motionless when above the water while an ocean sunfish's fin swings

They prefer to stay alone for the majority of their lives, although occasionally they do form pairs or small groups.

Image showing an Ocean sunfish basking
Ocean sunfish basking 

Mola mola mainly feed on jellyfish. They are also reported to consume:
  • Crustaceans
  • Eel grass
  • Fish larvae
  • Salps
  • Squids
  • Numerous small fish 

Since jellyfish are nutritionally poor, they have to consume them in very large amounts to maintain their gigantic size. The range of their diet indicates that they probably feed on varied sea levels, from the surface to the sea bottom.

They can suck and then spit water, and by doing so they tear apart any soft-bodied prey. Their teeth are fused inside a beak-like structure which allows the Mola mola to also break and consume harder-made animals. They use their pharyngeal teeth to grind their prey into small pieces before swallowing it completely.

Since they consume large amounts of prey, their presence indicates that the waters are rich in nutrients.

Mola Mola swimming near the surface

Most sea predators avoid hunting adult Molas due to their bulk size. However, younger Mola are known to be hunted by Bluefin tuna and mahi mahi (Coryphaena hippurus). The main predators of adult individuals are:
  • Sea lions
  • Sharks
  • Orcas

It's worth mentioning that sea lions commonly hunt Mola mola just for the heck of it! They simply tear of their fins, toss the body around for a while and then leave them to die in the sea bottom.

Their mating behavior isn't well studied and little is known about it. The ocean sunfish holds the record for most eggs produced by a vertebrate. Females lay up to 300 million eggs per season, which are then fertilized externally by the sperm released by males. The eggs are very small, with an average diameter of 0.13 cm.

Hatchlings are in the form of larvae and are very small, having an average length of only 2.5 mm (0.01 inches). They grow to become fry. Ocean sunfish fry bear little resemblance to their parents when compared to adults, looking more like pupperfish, a closely related species.

Mola Mola Fry

Off the coast of Japan spawning is thought to occur between August and October. Some of the suggested spawning areas include:
  • Indian Oceans
  • North Atlantic
  • North Pacific
  • South Atlantic
  • South Pacific

Interaction with humans
Despite their immense size, they are docile animals posing little to no threat to human divers. Reports of injuries are very rare and are usually non life-threatening. In one case, a Mola mola knocked a boy out of his boat when it jumped onto it. The boy was shocked but unharmed. According to certain reports, the species can get accustomed and familiarize with divers.

In some places, the Mola mola meat is considered a delicacy, including Taiwan, Japan and the Korean peninsula.

Conservation Status and Threats
The species conservation status has yet to be evaluated, however overall populations are believed to have declined in the past decades.

The most significant threat to the species' survival comes from fisheries, with Mola mola comprising a huge proportion of bycatch in the majority of fisheries operating in the Pacific ocean, Atlantic ocean and the Mediterranean Sea. In certain areas, it can make up as much as 90 percent of the total catch, greatly outnumbering the target species caught in many hauls. For instance, in the Mediterranean between 1992 and 1994, the species had a bycatch rate of 70 to 93%.

Another threat to these strange animals is that in certain places they are "finned" by fishermen, who regard them as worthless bait thieves. The finned fish lose their ability to swim and eventually die.

Finally, pollution is another problem. The species is threatened by floating litter like plastic bags which resemble jellyfish, its main source of food. Bags can choke and suffocate an individual or fill its stomach to the extent that it starves.

Mola mola and diver
Mola mola and a diver

In Captivity
Due to their demanding needs, individuals are rarely kept in aquariums. Some aquariums that have them in display today include:
  • Kaiyukan Aquarium in Osaka, Japan
  • Lisbon Oceanarium in Portugal
  • Valencia Oceanogr√†fic and the Aquarium Barcelona (both in Spain)
  • Monterey Bay Aquarium, California, U.S.

Interesting facts about the Ocean Sunfish
- More than 45 parasite species have been reported to reside on their skin. They have innovated quite a few ways to get relief from them. One is that in temperate areas they visit near kelp beds where cleaner wrasses (and other fish) remove the parasites on their skin by eating them. In tropic waters, reef fishes provide the same service. They also bask their side on the water surface, so that seabirds can come and feed on the parasites. They have been reported to breach, clearing the surface by more than three body lengths, this may be another method to dislodge parasites.
- They can swim at a top speed of only 3.2 km/h, covering distances of up to 26 km per day.
- The mola genus contains only one other species that looks a lot like the ocean sunfish, Mola ramsayi.
- Mola in Latin means "millstone", to which the species bears great "resemblance" due to its grey color, rough texture, and rounded body.

Mola ramsayi
Mola ramsayi

References & Further Reading
- Pope, E., Hays, G., Thys, T., Doyle, T., Sims, D., Queiroz, N., Hobson, V., Kubicek, L., & Houghton, J. (2010). The biology and ecology of the ocean sunfish Mola mola: a review of current knowledge and future research perspectives Reviews in Fish Biology and Fisheries, 20 (4), 471-487 DOI: 10.1007/s11160-009-9155-9
- Thys, Tierney. "Molidae Descriptions and Life History". 
- Thys, Tierney. "Molidae information and research (Evolution)"
- Konow, N., Fitzpatrick, R., & Barnett, A. (2006). Adult Emperor angelfish (Pomacanthus imperator) clean Giant sunfishes (Mola mola) at Nusa Lembongan, Indonesia Coral Reefs, 25 (2), 208-208 DOI: 10.1007/s00338-006-0086-9

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