Ocean sunfish are gigantic fish that can be found in many of the world’s tropical and temperate waters, from the length of the Atlantic from Iceland to Chile.
They are the heaviest known bony fish in the world that can weigh anywhere between an impressive 250 – 5000 pounds.
In this post, we’re going to go through everything you need to know about the sunfish size and why they are so big.
These fascinating fish have many unique traits and have captivated scientists all over the world for many years.
Let’s take a closer look at the sunfish’s size and how it got so big…
How Big Is A Fully Grown Sunfish?
Full-grown adult sunfish are the heaviest bony fish in the world. They measure up to 10-feet (3 meters) long and weigh more than 4,000lbs (2,000kg).
Despite adult sunfish being gigantic, they start their lives extremely small. An adult female can produce up to 3 million eggs, however, most of which will be eaten by predators.
Of those fertilized, most of which will hatch into only millimeter long larvae, and again most of the larvae will be gobbled up by hungry predators.
Those sunfish that make it to adulthood are truly special. To even witness an adult sunfish is incredibly rare, although they have few natural predators they are pretty much defenseless.
They much prefer to live on the open ocean where they can sunbathe and relax, which makes encountering one in the wild super rare.
How Big Is The Largest Sunfish?
The largest ever recorded sunfish was a whopping 14 feet (4.2 meters) fin-to-fin and weighed over 5,000 pounds (2.26 tonnes).
Sunfish this large are incredibly rare and are not often seen by humans. They generally stay between 60 – 125 miles off the coastline and they migrate within their ranges.
They prefer to spend summer at higher latitudes and their winters relatively closer to the equator. Their ranges are typically along roughly 300 miles of coastline.
But one sunfish off the coast of California was mapped as traveling over 400 miles.
Check out this amazing footage of a gigantic sunfish off the coast of Portugal to get a true understanding of its size.
Why Are Sunfish So Big?
Only recently researchers published the first genome sequence of the Ocean sunfish, and the results rendered insight into the fish’s fascinating growth rate.
“Several sunfish genes involved in the growth hormone and insulin-like growth factor 1 axis signaling pathway were found to be under positive selection or accelerated evolution, which might explain its fast growth rate and large body size,”Gigascience.biomedcentral.com
Insulin-like growth factor 1 (IGF-1) is also one of the primary growth hormones in humans. People that are insensitive or deficient in the hormone experience dwarfism or stunted growth.
However, the Ocean sunfish seems to have tonnes of the stuff! This means that combined with plenty of food the fish can grow and grow to reach its monumental size.
But that’s not the only reason why sunfish are so big. A large body helps these fish to adapt to their hunting environment because they lose heat slowly.
They are the ultimate sunbathers that can regularly be found near the surface catching some rays. They do this to regulate their body temperature.
After diving deep into the icy waters, they swim to the surface and catch some rays, warming up by spending some time near the surface.
The large surface area of their bodies allows them to soak up the sun, helping them warm up after their deep dives.
How Big Are Baby Sunfish?
Strangely enough, newly hatched sunfish as incredibly small. Sunfish larvae are only 2.5mm in length and weigh less than one gram.
From there they develop into fry, and the survivors can grow up to an impressive 600 times their original size before reaching adult proportions.
This amount of growth is arguably the most extreme in any vertebrate animal. These fish are so small as babies that you could easily fit a dozen of them on your fingertip.
Ocean sunfish can lay up to 300 million eggs at once, more than any vertebrate. However, most of these are eaten by predators before they reach anywhere close to adulthood.
The chances of any of these eggs surviving is small. But once they are fertilized they grow at a rapid rate of around 1 kilogram per day until they are fully grown.
Are Sunfish Friendly?
Despite the Ocean sunfish’s massive size, they are friendly toward humans and other creatures. They rely heavily on other fish to rid them of parasites which is why they are naturally curious.
These giant fish are completely harmless to humans and pose no threat at all. They move incredibly slowly and are likely to be more frightened of us than we should be of them.
Ocean sunfish are one of the most helpless creatures on the planet, with no way to defend themselves against predators.
Most sunfish are too large to be threatened by small predators. But Orca, Sea Lions, and Large sharks do sometimes eat these fish.
California sea lions have even been known to bite the fins off Ocean sunfish and use them as frisbees.
Hopefully, this post has been helpful and you’ve learned more about the sunfish size today and why they grow to be so huge.
These fascinating fish are still very much a mystery to scientists but we are discovering more and more about them every year.
Ocean sunfish have one of the most interesting lives from birth to adulthood, with an incredibly slim survival rate, it’s a spectacle to even see an Ocean sunfish in the wild.
These fish are gigantic when fully grown, and can be found all across the Atlantic, even in the UK during the summer months.
However, it’s unlikely to find one of these fish swimming close to the shoreline, as they much prefer to be between 60 – 125 miles off the shore.
Thanks for taking the time to read this post and feel free to stick around to learn more about the Ocean sunfish and many other types of marine wildlife.
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Hi, I’m George – the founder of MarinePatch. I created this blog as marine wildlife has been my passion for many years and I’ve spent decades learning and dedicating myself to documenting all I can about the topic.