News / Industry News & Events / This 5,000-pound behemoth is the world's heaviest bony fish (12/14/17)

This 5,000-pound behemoth is the world's heaviest bony fish

12/14/17

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The heaviest bony fish ever caught weighs in at a staggering 5,070 lbs. (2,300 kilograms). Now, scientists know its name.

The fish is aMola alexandriniocean sunfish, researchers reported Dec. 5in the journal Ichthyological Research. Originally, the fish, which was caught in 1996, was misidentified as aMola mola, a better-known species of sunfish. But recent research has upended the wholeMolagenus and led to the reidentification of some species.M. alexandriniis recognizable by its prominent head shape, lending it the common name the "bump-headed sunfish."

"For the same reason, we adopt the already proposed Japanese common name Ushi-manbo," said study leader Etsuro Sawai, a sunfish expert at Hiroshima University. "'Ushi' means 'cow,' and refers to the head profile of the fish."

Sunfish are the largest bony fish in the sea. Unlike sharks and rays, they have skeletons made of bone rather than cartilage. They're also very strange-looking. Their bodies are huge and round, shaped like wagon wheels or pancakes. They can grow up to about 10 feet (3 meters) long.

TheMolagenus wasn't well understood until in recent years, because studying such enormous specimens isn't easy. They're difficult to collect and even harder to transport for thorough anatomical examinations. Genetic studies cracked open the case, revealing that fish once classified asMola molawere in fact very different from one another, and that some gene sequences didn't fit nicely into pre-existing species categories. In July 2017, researchersnamed a new species of sunfishwith those sequences,Mola tecta, after finding a handful of specimens washed ashore on a New Zealand beach.

M. tectahas a round snout and a distinctive stripe that divides its body from the rudder-type fin on the fish, which is known as the clavus.

Redefining the sunfish

In the new study, researchers studied 30 specimens ofMolathat did not belong to theM. tectaspecies. They also hunted through historical photographs, looking for anatomical features that would help to distinguish existing species from one another. Ultimately, they used this information to redescribeM. alexandriniand to differentiate it fromM. tectaandM. mola.

The realization thatM. alexandriniwins the heavyweight prize for bony fish grew out of this newly clarified classification. Guinness World Records listsM. molaas theworld's heaviest bony fish, but Sawai and his team found that the largest catch on record was actually aM. alexandrinicaught in 1996 off Kamogawa, Japan. That fish was 8.9 feet (2.72 m) long, raising the question of whether some individuals of this species are even heavier. In 2004, fishers recorded the catch of a 10.9-foot-long (3.32 m)M. alexandrininear Aji Island, Japan, but they didn't weigh that behemoth.