Weird fish of the week: Ocean sunfish


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This week's weird fish is another species that can be found swimming off UK shores. The Ocean sunfish, Mola mola, is weird in so many ways...

On first inspection, the fish looks more like half a fish, as it appears to be little more than a head with stiff, pointed dorsal and ventral fins attached.

Instead of a true tail fin, the sunfish has a structure known as a 'clavus' formed from elements of the dorsal and anal fins. This 'pseudo-tail' is of little use for propulsion, so the sunfish relies on moving its enlarged dorsal and ventral fins in a characteristic 'sculling' motion to get around, in a way similar to triggerfish to which they are related, both being from the order Tetradontiformes.

Sunfish are also the world's heaviest bony fish, with specimens weighing up to 2300kg (5100lb), measuring 3.3m (10.8ft) long and 4.2m(14ft) from dorsal to ventral fin tip recorded.

The fishes' huge size is achievable by a skeleton made largely from cartilaginous tissues which are lighter than bone, despite its bony fish ancestry.

Its scientific name comes from the Latin for 'millstone', alluding to its rounded, flattened shape and grey coloration, while its common name comes from its habit of appearing to sunbathe on the sea's surface. It is speculated that this behaviour serves the dual purpose of allowing the fish to warm itself in cooler waters, as well as attracting sea birds that remove parasites from its skin.

Little is known of their reproduction, but females can produce over 300 million eggs at a time - more than any other known vertebrate. They feed largely on jellyfish, but will also eat squid, crustaceans, small fish, eel grass and salps (a type of free swimming tunicate).