Given that it is 3 metres long, weighs 2300 kilograms and looks like a severed head, you would think there could be no mistaking the identity of the world’s heaviest bony fish. But in fact we have been misidentifying this ocean-going giant for years.
The record has stood since 2002, on the basis of a specimen caught off the Japanese coast in 1996. But now a detailed analysis of photographs and other information about the specimen has revealed that it is not an ocean sunfish, but a relative named Mola alexandrini.
Sunfish look extremely peculiar to our eyes. They appear almost disc-shaped from the side and extremely narrow when viewed head-on. They have a curved rudder-like lobe at the rear, where most fish have a tail fin. Many exceed 2000kg and reach 3m long.
Etsuro Sawai at Hiroshima University in Japan and his colleagues reviewed the three known Mola species: M. mola, M. ramsayi and the rare M. tecta – which was only discovered in July. They examined 30 specimens of M. mola and M. ramsayi, and trawled through accounts of sunfish sightings and captures going back 500 years.
On viewing pictures of the monster catch, Sawai realised that it was actually M. alexandrini. He also argues that this species is the same as M. ramsayi, which should now be called M. alexandrini because that name was used first.
Unlike the other two species, M. alexandrini/ramsayi has a large swelling on its forehead, hence its common name: the bump-head sunfish. It also has a giveaway bump on its chin and a differently shaped “tail”. These features allowed Sawai to distinguish it from M. mola.
Sawai also thinks the record will be beaten. In 2004, a female sunfish 3.32m long was caught off Japan’s Aji Island, but it was not weighed.
Journal reference: Ichthyological Research, DOI: 10.1007/s10228-017-0603-6
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