Oh man, where to even begin? The sheer size of ocean sunfish is perhaps a good starting point. I’m sure you’ve seen photos of these hauntingly beautiful leviathans, and visualised how large they are. That said, however big you think a sunfish is, it’s probably bigger.
The most common species of ocean sunfish, the famous Mola mola, is found in warm coastal waters the world over. Its two towering fins can give it an overall height nearly twice my own. Specimens have been caught that weigh up to 2,300 kg! What’s more, despite all this, it’s not even the biggest species…
Pictured below is the giant sunfish, also known as the southern sunfish. As its latter name suggests, it cruises the oceans found below the equator for the most part. It somehow manages to look even weirder than its more well-known cousin, its head covered in thick lumps. A few years ago, a southern sunfish was caught that weighed an absolutely gargantuan 2,744 kilograms. It is by far the largest bony fish on our planet, about the size of an African forest elephant.
The sleek hoodwinker sunfish was only discovered in 2017, having previously been misidentified as Mola mola. The second part of its scientific name (Mola tecta) means “hidden”, a reference to this aspect of its history. It doesn’t seem to grow as large as the two aforementioned species, though it’s still a massive creature. Some recorded individuals achieve 2,000 kilograms.
The most mysterious sunfish species of all is likely the sharptail mola. Its back end is not just a rounded stump like that of its brethren, instead having a sharp spike protruding from the centre. This, of course, is how it got its name. To this day, very little is known about the species, as it’s exceptionally rare. We do know it’s big though, regularly reaching two ton weights.
The final species of the five alive today is a little different to those we’ve met so far. This slender sunfish is “only” a metre long, by far the smallest in the family. With its much leaner and more hydrodynamic shape, it’s no surprise that it’s also a lot more agile than its relatives. To hunt down its prey (squid) it has to able to swim at great speed and turn on a dime.
The larger, more cumbersome species aren’t quite cut out for chasing squid, though they are possessed of surprising power in short bursts. Instead, they famously feed on jellyfish, along with other gelatinous organisms like comb jellies and salps. Though these animals are comprised almost entirely of water, their abundance and ease of capture more than makes up for their lack of nutritional value.
Interestingly enough, ocean sunfish have beaks. These seemingly evolved from four of the teeth, which fused together to form hardened mouthparts. Such crushing dentition is usually seen in animals which prey on hard shellfish, so it’s surprising that the jelly-eating sunfish have it. The beaks prevent the animals from ever closing their mouths, so they appear to be perpetually surprised.
Like many other fish, sunfish have a second set of teeth deep within the throat - the pharyngeal teeth. These are long and thin, shaped like claws or fangs - ideal for ensuring any slippery jellyfish don’t manage to float their way back out of the mouth. Fascinatingly, the pharyngeal teeth also seem to have a secondary function: making sound! When ground together, they produce an eerie raking noise that can travel through the water for long distances.
The front teeth aren’t the only feature that ocean sunfish have lost to evolution. The swim bladder - that crucial, buoyancy-controlling organ found in almost all fish - is also nowhere to be seen. Instead, sunfish seemingly regulate their buoyancy using a weird gel-like substance. Just beneath the skin is a layer of gelatinous tissue, a good two or three inches thick. When you consider how flattened these animals are, it’s amazing how much of their cross-section this layer must make up.
They only have sixteen vertebrae, whereas many fish have thirty or more. The tail fin has entirely disappeared, along with all the bones it would have contained. Indeed, what bones are left in the skeleton are mostly made of cartilage! Instead of a caudal fin, it has that rounded rump which we call the clavus, a kind of pseudo-tail.
This makes steering complicated, as all other fish use their caudal fins to change direction. Ocean sunfish, on the other hand, will squirt powerful jets of water from their mouths to steer! I can’t quite fathom how much force it would take to pivot a 2,700 kilogram fish by blowing a raspberry. The two immense fins then handle propulsion. They swing left and right in synchrony, almost like oars on a boat, a form of locomotion that no other aquatic animal has.
Sunfish skin is bizarre, too. They don’t have regular scales, but rather denticles - yes, teeth. All anatomical evidence seems to point to these dermal structures having the same evolutionary origin as their teeth! The same is true of shark scales. Because of this, ocean sunfish are extremely rough to the touch, with skin like sandpaper.
Unfortunately, it also makes them awfully susceptible to parasitism, as do their vast surface area and sluggish nature. These gentle giants suffer more from skin parasites than perhaps any other fish, with an infestation to be found on most individuals. There are more than forty different parasite species that are known to dwell on sunfish skin and gnaw at their denticles.
As such, ocean sunfish have developed an arsenal of strategies to rid themselves of their tormentors. The most spectacular of these is breaching. Somehow, even fully-grown molas can generate enough power to launch themselves up to three metres into the air before crashing back down. Isn’t that astonishing? Upon impact with the surface, many embedded parasites get dislodged.
Breaching sunfish have been known to accidentally land on boats, causing severe injuries to the people crushed under their massive weight. There’s even a report of a boy once being killed in this manner in Southeast Asian waters, though I can’t find any verified record of this event.
Molas also love to seek the services of smaller fish to free themselves of parasites. Floating bundles of kelp in the open ocean serve as hotspots of life, which the ocean giants flock to in search of wrasses and other tropical fishies. These helpers scrub them clean, while getting a free meal in the process - it’s a mutually beneficial relationship.
Surprisingly, fish aren’t the only animals that assist them with cleaning. Ocean sunfish are often seen floating just below the water’s surface, lying horizontally. This sunbathing habit is actually what gives them their English name. In most languages, the word for these fish ironically translates to “moonfish”, in reference to their circular shape.
In any case, the behaviour has two functions - first, as a form of thermal recharging. Basking in the sun’s warmth allows them to boost their body temperature before they dive again in search of more jellyfish. Secondly, to attract seabirds! Gulls and the like will readily land on their exposed flanks, and slurp up any external parasites they can find.
Sadly, there are some creatures which bite off more than just parasites. Fully-grown molas are safer from predation than you’d think - though they can’t exactly escape in a hurry or fight back, their enormous size and incredibly thick skin render most attackers powerless to hurt them. It would also appear that they just don’t taste that good. Tiger sharks are well-equipped to chomp through gelatinous armour, but only very rarely go for sunfish.
The two most common predators of ocean sunfish are actually the orca and the California sea lion! You may have seen images like this of sea lions just chowing down on sunfish who can do nothing about their unfortunate situation.
Orcas, being freakishly intelligent as usual, have figured out that there are tasty organs hiding beneath the unpalatable layer of gel that puts off sharks. They open the poor fish like pitta bread, before pulling out the intestines. I regret to say that all this happens while the prey is still alive.
The cetaceans have also been observed balancing and spinning killed sunfish on their noses for fun, and throwing them around like frisbees! How miserable it must be to be any creature in the sea that isn’t an orca.
Now let’s talk a little about reproduction. Ocean sunfish spawning is an incredible sight - each female can release 300 million eggs at a time! This is by far the highest reproductive capacity of any vertebrate in the world. Infant mortality is sky-high, of course - otherwise the seas would be more sunfish than water! The larvae, or fry, stick together in large schools for safety, but nonetheless few of them make it to adulthood.
They are absolutely tiny at birth, weighing a fraction of a gram. If they survive the perils of youth, they can grow to as much as sixty million times their original size! This is absolutely mind-bending growth, easily the most extreme among Earth’s vertebrates. Fascinatingly, sunfish larvae are covered in transparent spikes. In this regard they resemble pufferfish, which are quite closely related. At this stage of life I find they actually look like little suns! When the larvae begin their transformation to adulthood, they lose their spikes, along with a lot of their more “normal” fish characteristics.
So, that’s pretty much everything I know about ocean sunfish. Deemed ugly by many, but possessed of stunning beauty in my eyes! I hope you’ve enjoyed this answer - I know I enjoyed writing it. Thank you very much, everyone, for reading, and have a great day!