Monday, August 27, 2012

The Ocean SunFish

            Not only is the sunfish (Mola mola) the world’s heaviest bony fish, with some individuals weighing in at a staggering 2.3 tonnes, but it also possesses a truly bizarre body shape, likened to a gigantic ‘swimming head’. The sunfish has no tail, with the caudal fin reduced to a rudder-like structure, called the clavus. The dorsal and anal fins are placed far back on the body, and are used as ‘oars’ in swimming. The sunfish does not have scales, and instead has a tough, elastic skin which is covered in mucus. Also unlike other fish, it has fewer vertebrae, lacks bony tissue in the skeleton, and does not possess ribs, pelvic fins or a swim bladder. The mouth of the sunfish is small, and its teeth are fused together to form a ‘beak’. The sunfish tends to be brownish-blue to silver in colour, often with a slightly iridescent sheen, and it may also exhibit various patterns on the skin.

Scientific Classification
Mola mola

Other Names
Sun Fish
Poisson Lune
Schwimmender Kopf
Pez Luna

           The sunfish has yet to be classified by the IUCN.

           Mola mola can produce more than 300 million eggs, each about 2 to 3mm large. More than any other known vertebrate. They are thought to live for over 10 years. However just like many marine creatures they are probably in danger of dwindling population from the increase pressure of fishing by human and other environmental factors. Scientists have not yet understood the biology of Mola mola and their distribution, hence they do not know yet for certain whether the population is dwindling, however a recent tagging by satellite to monitor the population of Mola mola in Californian waters show that there are consistent early indications that the population size has recently been reduced – possibly due to fishing pressure.

           Ocean sunfish are native to the temperate and tropical waters of every ocean in the world. Sunfish are pelagic and swim at depths of up to 600 m. 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 m, occupying both the epipelagic and mesopelagic zones.

            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 that sunfish moved around mainly by drifting with ocean currents, individuals have been recorded swimming 26 km in a day, at a top speed of 3.2 km/h. Sunfish are most often found in water warmer than 10 °C, prolonged periods spent in water at temperatures of 12 °C or lower can lead to disorientation and eventual death. Researchers theorize that surface basking behaviour, 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.  

           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.

         The reproductive biology and behaviour of the sunfish are poorly understood. However, female sunfish are known to carry an extraordinary number of eggs, with an individual female capable of producing up to 300 million eggs at one time, the largest number of eggs ever recorded in a vertebrate. Where and when the sunfish spawns is not well known, although five possible areas have been identified in the North and South Atlantic, the North and South Pacific, and in the Indian Ocean, where there are central rotating oceanic currents, called gyres. The newly hatched sunfish measure just 0.25 centimetres in length, and will increase in mass by over 60 million times in order to reach the size of a 3 metre adult.

            The sunfish is thought to feed mainly on jellyfish, and its diet may also include a variety of alternative prey species including crustaceans, molluscs, squid, small fish and deep water eel larvae. The frequently observed ‘basking’ behaviour, where the sunfish swims on its side at the ocean surface, is thought to be linked to the deep dives which it makes periodically throughout the day, and which are most likely made in search of prey. Basking may be a form of thermoregulation, allowing the sunfish to warm up after making forays into cooler waters; however, other explanations for this peculiar behaviour include illness, or possibly the solicitation of cleaner fish or birds, for the removal of parasites. Known to harbour many parasites, at least 40 different genera have been recorded using the sunfish as a host, and crustaceans have also been found attached to the skin and gills of many individuals.

           The most significant threat to the sunfish is from fisheries, with the species comprising a huge proportion of bycatch in most fisheries that operate in the Pacific and Atlantic Oceans and in the Mediterranean Sea. The sunfish is not a commercially important fish, but in some areas of the world, it can make up as much as 90 % of the total catch, and will often greatly outnumber the target species caught in many hauls. This is especially concerning if the sunfish exists in discrete populations, rather than one large global population, as small, localised populations are at much higher risk of depletion or extinction.

Conservation Measures
           There are currently no conservation measures in place to protect this species, and perhaps more worryingly, there is very little information on the basic biology of the sunfish, including its foraging and diving behaviour, population structure, and its distribution and seasonal movements throughout the world’s oceans. There is growing recognition among the scientific community that further research on the sunfish is essential, especially in relation to how fishing-induced mortality is affecting the global population.