An ocean sunfish looks like a giant head, sporting a goofy expression, that’s been flattened by a steamroller. Its appearance isn’t the fish’s only unusual feature, though; its anatomy is odd, too.
It doesn’t have a tail, so it swims by flicking its long dorsal and anal fins from side to side. On its blunt rear end, instead of a tail, is a rudder-like appendage – a clavus – for steering. Then there’s its size.
Sunfish can reach lengths of over 3m (9.8ft) from mouth to clavus, and 4m (13ft) from top to bottom. They can also weigh more than 2,000kg (4,400lbs) and are by far the world’s heaviest teleosts, a huge group of aquatic animals also known as bony fish on account of the composition of their skeletons.
The only fish that are heftier than sunfish aren’t bony fish, but belong to a separate group, the chondrichthyans, which have bendy skeletons made of cartilage. This includes the huge whale sharks and basking sharks.
Mola mola, the scientific name for the ocean sunfish, stems from the Latin word for millstone, because they look like huge grey discs of stone. To reach this enormous size and weight, sunfish put on a tremendous growth spurt.
They hatch as tiny larvae, but they don’t stay small for long and can gain 1kg (2.2lbs) a day. Juvenile sunfish have a spiky appearance, a dead giveaway that they‘re closely related to pufferfish.
They’re members of the same family, the tetraodontidae, but sunfish absorb their spikes as they mature, and you won’t ever catch one inflating its body into a balloon (they don’t share the pufferfish’s habit of gulping down seawater when they’re alarmed and expanding their bodies to make themselves difficult to attack).
Adult sunfish rely instead on their huge size for defence. Their common name derives from the sunfish’s habit of basking on its sides at the surface of the sea, soaking up the rays. This, combined with its ungainly way of swimming, gave the species a reputation for being rather slow and sedate animals.
The truth, however, came to light in 2015, when a team of Japanese scientists tagged some sunfish with temperature probes, cameras and accelerometers. These devices revealed that sunfish plunge into the deep ocean on long active dives to chase their favourite prey, jellyfish, which they smash between two fused plates of teeth in their mouths.
After their chilly sojourn in the deep, sunfish return to the surface and bask to warm up. Ocean sunfish aren’t alone in their odd looks. There’s the similar-looking bump-head sunfish (Mola alexandrini), and a sunfish that washed up on a beach in New Zealand eventually became the Mola tecta, the hoodwinker sunfish.
Its name arose because scientists figured the species had been there all along, cruising through the open ocean among the other sunfish, but nobody had spotted it.
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