Ocean Sunfish Facts

Ocean Sunfish Profile

Considering how much damage we’ve done to it, it’s remarkable how little we understand the ocean. Some of the biggest animals in the world are also the most elusive and it’s commonly touted that we know more about the moon than we do about our oceans. 

This isn’t true, at least, not anymore, but the fact that the heaviest bony fishes can be so common yet so poorly understood does lend it some thought. 

Once thought to be the largest planktonic organism, the ocean sunfish has lost that title, but still carries many records.

Ocean Sunfish Facts Overview

Sunfish are the largest fish nobody understands. This group of enormous, flattened bony fish are the heaviest in the world, and we still know almost nothing about where it’s going and what it’s eating. 

What we do know is that it has one of the most amazing growth cycles of any animal and perhaps the most eggs of any known vertebrate. 

Sadly, it’s most commonly seen brought up as bycatch for the industrial fishing industry, and while it benefits from the jellyfish blooms that come from overfishing, its destiny is a bit of a mystery. 

Interesting Ocean Sunfish Facts

1. They’re one of the largest bony fish in the world

Sunfish make up a genus known as Mola, as well as two other genera in the family Molidae. Every species in Mola grows to at least 2.5 meters across, the smallest of which weighs 1800kg! 

Mola mola gets significantly larger and heavier, at well over three meters long, and comes with a corresponding increase of weight that has it topping 2.3 tonnes. This is an immense fish and one of the heaviest bony fish in the world. 

The only bony fish recorded with more mass is its close cousin, the Southern Sunfish, the largest of which showed up dead around the Azores Islands in the Atlantic in 2001. It weighed a formidable 2744kg, making it the heaviest bony fish ever recorded. 

And this is a fair accolade, but considering how big cartilaginous fishes can get, it’s worth keeping some perspective. Even the largest rays can top 3000kg, and the whale shark, the heaviest fish, has been weighed at 21,000kg.

Still, the Mola mola is a the very high end of the size spectrum, and this is particularly impressive not only for its final size but for the growth it took to get there. 

2. Yet they come from tiny star fry

Perhaps the most incredible thing about sunfish is how many eggs they lay. A single female can produce over 300,000,000 eggs, all minuscule and floaty. This is more eggs than any known vertebrate can carry at once and gives them yet another record to their names. 

They’re fertilised out in the open and hatch into incredibly small fry whose fins give them the shape of little ninja stars. They’re around 2.5mm across at this stage and will increase in weight roughly sixty million times if they reach full size. 

This is possibly the most extreme growth of any animal on Earth. ¹

3. They rise and set

When very young, ocean sunfish will school together for support, but as they age, they’ll become less and less sociable and will spend most of their time drifting about alone. 

They exhibit a daytime cycle of up and down migrations that researchers are still trying to figure out, but they can often be found basking at the upper surface layer of the ocean during the day, hence their name, Sunfish. 

They then sink down for a bit, and return to the surface again, suggesting to some that this pattern might have something to do with thermoregulation. 

On the other hand, it might reflect a hunting strategy, as the sunfish follows the diurnal vertical migrations of its prey. ²

4. They can change colour

The sunfish has gritty, rough skin, similar to sharks. It’s usually grey and slightly spotted, and covered in mucous. But one strange characteristic about it is its ability to change colour.

While in a relaxed state, this grey skin appears silvery, or even slightly opalescent, they are said to be able to darken and lighten these effects, changing colour in response to stress, and exhibiting different patterns for different contexts. 

Not much is known about it yet because, despite the enormous size of this animal, it’s woefully misunderstood. It wasn’t until recently that people even realised it could swim at all. ³

5. They’re not plankton

This might seem intuitive, considering their gargantuan proportions and the fact that most plankton are tiny.

But the true definition of plankton is something which is more or less entirely at the mercy of the ocean currents. This means it can be as big as you like, as long as it doesn’t control its own movement, and it was only recently that the sunfish was accepted as a deliberate swimmer. 

This goes to show what little we understand about the largest bony fishes, but there does appear to be an increase in interest in recent years. ⁴

6. They’re active predators

One thing we now know is that not only can they swim, they actively hunt. This might be what they’re doing when they descend for their vertical migrations, but nobody quite knows how they do it, yet. Still, stomach contents surveys show that they’re able to catch their own food. 

Their characteristic basking behaviour represents only a small portion of their behavioural repertoire, and it’s now a race against time to figure out what else it gets up to before populations are decimated by the fishing industry. 

7. They’re heavily threatened by the fishing industry 

Bycatch is the biggest threat to the species, including that from longlines. 29% of bycatch from the swordfish fishery in California was made up of this species, more than the amount of swordfish that’s caught. 

The IUCN has them listed as Vulnerable, but their condition could get worse. 

8. Future prospects are uncertain

90% of large predatory fish have already disappeared from the ocean since industrialised fishing took off and ocean sunfish are heavily affected by the same forces that drove this. 

On the other hand, shark decline from industrial fishing may have led to an increase in sunfish numbers, relative to where they’d be without it. And overfishing in general has created an abundance of jellyfish, which is a bad sign for the ecosystem but a great help to sunfish numbers. 

While this would be a good thing for the sunfish, it’s hard to call it good news, and regardless, it’s looking like the human destruction of this species outweighs any benefit they might have been getting from the sudden loss of sharks. 

This is a fish that’s very poorly understood, even down to its feeding preferences, and a lot remains to be figured out about the direction of its future. 

Ocean Sunfish Fact-File Summary

Scientific Classification

1. Kerryn Parkinson (2023), “Ocean Sunfish, Mola mola”, Australian Museum.
2. “Ocean Sunfish”, IUCN Red List..
3. “Mola mola (Common mola)”, Ocean Sun Fish.
4. Edward C. Pope (2009), “The biology and ecology of the ocean sunfish Mola mola: a review of current knowledge and future research perspectives”, Sci Hub.