something from another planet
If you attached a large fin to the top and bottom of a dinner plate, drew a face on the side and threw it into the ocean, you might have created a fairly decent likeness of an ocean sunfish.
From a distance, their large fins might look like that of a shark slicing through the water but get closer and you can’t mistake these fish for anything else. Their strange shape makes them look like floating heads bobbing gently at the surface.
What is a sunfish?
Sunfish are enormous, strange-looking fish found in tropical and temperate oceans around the world (although there is also a group of totally different freshwater fish that are also called sunfish). There are five recognised species, the most famous of which is the ocean sunfish or Mola mola.
How big are sunfish?
Ocean sunfish are huge – and hefty. In fact, the bump-head sunfish (Mola alexandrini) is the heaviest bony fish on the planet. One individual found dead off the Azores, Portugal, in 2021 was more than three metres long and tipped the scales at a whopping 2,744 kilos. That’s more than an adult walrus.
Their English common name – sunfish – comes from their tendency to loiter in the shallows soaking up the sun. Researchers think this helps the fish warm themselves up again after diving to deep, cold waters. It may also be a way of having a little spa treatment: by flapping their fins, the sunfish can attract seabirds to come and peck pesky parasites off their skin. They can also breach – like some sharks and whales – to try to shake off these annoying freeloaders.
What do they look like?
Massive weirdos. These fish can grow to several metres and have huge, round bodies that give them their scientific name Mola, which comes from the Latin word for millstone.
When you look at these fish, you might notice they seem to be missing something: a proper tail. Their rounded shape is partly because of their stunted tail fin, which looks more like a rudder than something that should be attached to a fish.
When sunfish are born, it’s hard to imagine they might grow into these gigantic swimming faces. Babies are less than 3mm long – smaller than a grain of rice – and have a teeny tiny tail fin. These fins never grow into a proper tail, leaving them with their rudder-like behind, known as a clavus, which helps them steer through the water.
One species – the sharp-tail sunfish (Masturus lanceolatus) – has a more prominent clavus than other sunfish. Researchers think this might by why orcas have been seen doing something very strange, and rather disturbing, to this fish. One orca grasps the sunfish’s clavus in its teeth while its pod-mate swims full pelt towards it, blasting it to pieces on impact. Exactly why the orcas are doing this hasn’t yet been confirmed but one leading theory is that they are playing. These social interactions can help young orcas learn.
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Where do they live?
Sunfish are found in tropical and temperate oceans around the world
What do sunfish eat?
More than anything, adult sunfish love to eat jellyfish. The abundance of jellies, along with warmer waters, bring the fish to UK waters in the summer.
They also like to snack on plankton, particularly colonial organisms called salps which form long chains that look like a line of matchsticks hovering in the water column.
How do they hunt?
Sunfish feed by sucking their jelly-like prey into their mouths. Their teeth are fused together, more like a parrot’s beak, making it impossible for them to close their mouths all the way. So, once they’ve slurped their foot, they spit it out, repeating the process over and over to break their meal into manageable chunks. Once they swallow, rows of spiky teeth in their throat help to stop their slippery food slithering back up their throat.
How do they reproduce?
Sunfish, like many other animals in the ocean, are broadcast spawners. Females may be able to carry 300 million eggs, which they release into the water column for males to fertilise.
The tiny larvae undergo an incredible transformation. Littluns look more like itty bitty pufferfish than sunfish. By the time the little hatchlings reach adulthood, they will become 60 million times heavier
Are they dangerous to humans?
No. Despite their impressive size, sunfish are slow-moving, gentle creatures. After all, how quickly would you swim if you were just a floating head?
These enormous creatures are much happier sunbathing at the surface than getting involved in any kind of skirmish. The only way they might cause humans harm is totally by accident: if a boat collided with one of these enormous, heavy fish, it may well be damaged.
Are they under threat?
Yes. Ocean sunfish (Mola mola) are a vulnerable species, according to the IUCN Red List. They are particularly vulnerable to fishing pressures and are often caught by accident as bycatch. Like turtles, they can be fooled into thinking floating plastic is a tasty jellyfish. If they try to eat it, they can choke and suffocate.
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