A photo of a sunfish A photo of a sunfish

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10 things you might not know about sunfish

Known for its truncated, bullet-like shape, we explore what might be the world's weirdest fish.

During a weekend kayaking at Kimmeridge, Dorset, ABPmer Marine Consultant Alice Bowles spotted an ocean sunfish in the water, and took this photo:

Photo of a sunfish fin off Kimmeridge Bay, Dorset

The fin of a sunfish breaching the water's surface, captured off Kimmeridge Bay in Dorset. Click for full resolution.

The sunfish might be the world's weirdest fish; they develop their truncated, bullet-like shape because the back fin they are born with simply never grows.

Sunfish are found in tropical and temperate waters in all the oceans of the world (excluding the icy polar seas). Records of shore-caught sunfish from the UK date back to 1976, though increasing numbers are appearing around the south and west of the British Isles in summer months.

Here are 10 things you might not know about the ocean sunfish:

1. The sunfish’s scientific name is Mola mola, from the Latin word for "millstone", describing the sunfish’s grey, round body and rough texture.

2. Sunfish hold a Guinness World Record for being the world's heaviest bony fish. The average size of a mature sunfish is usually several hundred kilograms. The very largest was measured at over 4m vertically and 3m horizontally, weighing over 2,000kg.

3. Sunfish have a limited ability to hunt, and feed by sucking easy-to-catch jellyfish into their strange, beak-like mouth, which they are not capable of fully closing.

4. Sunfish have teeth, but they are located further back in the throat, fused together in two plates that look like a parrot’s beak. These teeth crunch up food before passing it to the stomach.

5. Sunfish are often observed basking on their sides at the surface of the sea, sometimes flapping their dorsal fins. They are often mistaken for sharks when their huge dorsal fins emerge above the water.

6. Ocean sunfish can become so infested with skin parasites they will often invite small fish or even birds to feast on their skin. In an attempt to shake the parasites, they will even breach the surface up to 3m in the air and land with a splash (as observed by Alice Bowles, but unfortunately not caught on camera).

7. Female sunfish can produce up to 300 million eggs at one time, more than any other vertebrate. Each egg is tiny and fertilised externally, so its chances of survival are very small.

8. Small sunfish have been observed moving in shoals, although it is thought that the species becomes solitary once mature.

9. Within many Asian countries (including Japan, China and Taiwan) sunfish are seen as a delicacy, though its trade is banned in the EU.

10. On a global basis, the IUCN classes sunfish as a ‘Vulnerable’ species, due to its exposure to fishing activity and propensity to be caught as bycatch.

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Sources and further information

British Sea Fishing
IUCN Red List
National Geographic

Image copyright Jeremy Koreski