Giant sunfish

Appearance

"Mola alexandrini" has a relatively small mouth, and its teeth are fused into a parrot-like beak. It can reach up to 3.3 m in length and 2,700 kilograms in mass, making it the heaviest bony fish. Their bodies are flat and round, with large fins that they swish back and forth to propel themselves with as they swim horizontally. Their skin has rough denticles and a leathery texture, with brown and gray coloring with pale blotches; at death, their scales turn white. The body has a thick white subcutaneous gelatinous layer that is smooth to the touch, with a laterally compressed body covered in small rectangular scales. All "mola" species have no caudal bones, ribs, or pelvic fins, and have fused vertebrae, leaving only their median fins to propel themselves. "Mola alexandrini" can be distinguished from "Mola mola" by its smaller number of ossicles and lack of a vertical band of denticles at its base. In "Mola", the lower jaws are intact while the upper jaws are slightly forked. On the head, the lateral lines have small, white, rounded otoconia. On the sides are small gill openings covered by a soft gill membrane and gill rakers, which are covered under a subcutaneous gelatinous layer. All fins are spineless and triangular. Pectoral fins are small and rounded, located midlaterally fitting into shallow grooves on sides of the body, and dorsal fins are located on both sides of the anal fin.

"M. alexandrini" can be distinguished by its unique characteristics of head bump, a chin bump, rectangular body scales, and rounded clavus. Although adult sunfish look generally similar, they are distinguishable using seven characteristics: number of clavus fin rays, number of clavus ossicles, shape of clavus margin, presence of head bump, proportion of body length compared to body height, shape of body scales, and the presence of a chin bump.

Alongside these species, there are no external differences between sexes; internally, however, the gonads differ in shape, with females having a single spherical ovary and males having a pair of elongated, rod-like testes.

Overall, the maximum recorded weight of "M. alexandrini" is 2,744 kg from a 325-cm-length specimen caught off the coast of Faial Island, Azores, Portugal in late 2021. With these records, this specimen is currently regarded as the world's heaviest bony fish specimen. It exceeds by nearly half a ton the largest previously known specimen, caught off Kamogawa, Japan in 1996.

Distribution

"Mola alexandrini" has been found all over the globe and is widely distributed throughout the world's oceans, except for the polar regions. These species have been collected from waters off Japan, Taiwan, the Galapagos Islands, New Zealand, Australia, Turkey, Oman, and Spain. It can be found in the southwest Pacific, especially around Australia and New Zealand, and the southeast Pacific around Chile. Its range also extends to the southeast Atlantic near South Africa. During seasonal changes in climate on the Pacific side of Japan, "M. alexandrini" moves northwards in the summer and southwards in the winter. Seasonal migration is driven by temperature differences and productive frontal areas.

Status

The conservation status of "M. alexandrini" has not been evaluated, unlike "M. mola". Fisheries around the world catch ocean sunfish as bycatch. "M. mola" was listed by the International Union for the Conservation of Nature as "vulnerable" due to the high level of estimated bycatch in South African longline fishery with an annual estimated 340,000 annual catches. Both "M. mola" and "M. alexandrini" were listed as "high risk" bycatch species in the longline fishery off eastern Australia. Threat levels are lower than what is stated on the IUCN listing in Australian, New Zealand, and South African fisheries. Currently, the Indonesian government's Ministry of Marine Affairs and Fisheries has placed sunfish on a plan for protection.

Behavior

Sunfish swim by moving their dorsal and anal fins back and forth, both fins moving in the same direction at the same time. Adults are reported to travel mainly alone or in pairs, and sometimes in groups. Migrating from one place to another requires high tolerance and it is found that sunfish have high thermal tolerance undergoing quick and large temperature changes diving down the ocean several hundred meters. Sometimes, sunfish come up to the shallow water to recover from hypoxia from feeding below the thermocline. Like many other fish, sunfish adapt in response to the environment. During the day, sunfish tend to dive deeper than they do at night. At night sunfish preferred temperatures between 18–24 C, and moved to different areas during the day; vertical movements were correlated with temperature shifts.  Sunfish may dive deeper into cooler waters to cool the muscles or to repay oxygen debts.

Habitat

Although members of the genus "Mola" are found in many oceans throughout the world, this species thrives best in the open ocean of tropical and temperate seas, preferring warmer temperatures ranging from 16.8 C to 25.6 C, averaging 19.9 C. Many encounters with this species are linked to the influence of ocean currents. Based on their immediate environment, sunfish have different movement patterns. During the nighttime, these species stay in the same areas, but during the daytime, they stay below the thermocline. Vertical movement patterns correlated with thermocline depth, and they differed from December to May.

Reproduction

Sunfish spawn in the outer circulation of the temperate Atlantic and Pacific Oceans, as well as in the Mediterranean Sea. The optimal time for sunfish to spawn in the wild is not known, but research has shown that spawning in fall or winter, especially during the month of September, results in bigger fish. Fertilization occurs when sperm and eggs are shed in the water.  Being that sunfish are so large, a single adult female can produce 300 million eggs. Unfertilized eggs were measured at 0.42-0.45 mm in diameter.

Food

They mainly consume jellyfish, which are of low nutritional content but abundant, and they will also eat brittle stars, small fish, plankton, algae, salps, and mollusks. Sunfish also feed on ctenophores, hydrozoans, and small crustaceans. Juvenile sunfish feed in coastal areas in the coastal food web while larger sunfish dive deeper. These species are active predators hunting in dynamic frontal systems.