Imagine a fish the size of a small car. Now remove its tail. Give it two enormous fins — one on top, one on the bottom — that it uses to row itself, slowly and with apparent indifference, through the open ocean. Make it silver, slightly iridescent, with small eyes that give it an expression of permanent mild surprise. Make it weigh 2,300 kilograms. Make it the heaviest bony fish on earth.

Now imagine encountering it while scuba diving — a disc of silver the height of a full-grown adult human, materialising out of the blue water in front of you, regarding you briefly with one small eye, and then rowing away with the unhurried dignity of something that has absolutely nothing to fear from anything in the ocean.

This is the ocean sunfish. Mola mola. And it is one of the most improbable animals in existence.

The Fish That Evolution Forgot to Finish

The sunfish’s shape is its most immediately striking feature — and also its most misleading one. It does not look like a fish that is missing its tail. It looks like a fish that never had one. But the sunfish is, in fact, a member of the order Tetraodontiformes — the same group that includes pufferfish and triggerfish — and its ancestors had a conventional fish body, complete with a tail fin.

Over millions of years of evolution, the sunfish’s tail fin was progressively reduced and eventually replaced by a structure called the clavus — a rudder-like appendage formed from the fusion of the dorsal and anal fins, which wraps around the back of the body and gives the fish its characteristic truncated appearance. The clavus is not a tail. It does not propel the fish forward. It steers.

Propulsion comes instead from the two enormous vertical fins — the dorsal fin on top and the anal fin below — which the sunfish moves in a rowing motion, sweeping them back and forth to push itself through the water. It is an inefficient system by the standards of most fish, and the sunfish is not a fast swimmer. It cruises at around 3 kilometres per hour, which is roughly walking pace for a human.

For an animal that can weigh more than 2,000 kilograms, this seems like a significant vulnerability. In practice, it is not — because almost nothing in the ocean is willing to attack something that large.

A Life Measured in Jellyfish

The sunfish eats jellyfish. Almost exclusively. This is, nutritionally speaking, a terrible diet — jellyfish are approximately 95% water, with minimal caloric content — and the sunfish must consume enormous quantities of them to sustain its bulk. A large sunfish may eat hundreds of jellyfish per day, spending most of its active hours cruising through the water column in search of them.

This dietary dependence on jellyfish has made the sunfish unexpectedly vulnerable to plastic pollution. Plastic bags floating in the ocean are visually indistinguishable from jellyfish when viewed from below, and sunfish ingest them regularly. Plastic accumulation in the digestive system causes blockages that are frequently fatal. In waters with high plastic pollution, sunfish mortality from plastic ingestion is a significant and growing problem.

The sunfish’s relationship with jellyfish also explains one of its most observed and least understood behaviours — basking. Sunfish are frequently seen lying on their sides at the ocean surface, apparently motionless, in what looks like sunbathing. For years, researchers assumed this was thermoregulation — the fish warming itself after deep dives into cold water.

More recent research suggests the reality is more complex, and considerably more unpleasant.

40 Species of Parasite

The sunfish is, by a significant margin, the most heavily parasitised vertebrate on earth. A single sunfish can carry up to 40 different species of external parasite simultaneously — copepods, isopods, flukes, and various other organisms that attach to its skin, burrow into its flesh, and feed on its tissues. The total number of individual parasites on a large sunfish can reach into the tens of thousands.

This parasite load is not incidental. It is a consequence of the sunfish’s size, its slow movement, and its inability to clean itself the way smaller, more agile fish can. And it appears to be the primary reason sunfish spend time at the ocean surface.

When a sunfish lies on its side at the surface, it is not sunbathing. It is presenting itself to seabirds — particularly gulls — which land on its exposed flank and pick parasites from its skin. The sunfish is, in effect, using the ocean surface as a cleaning station, offering its body to birds in exchange for parasite removal. The behaviour has been observed repeatedly and is now considered the most likely explanation for surface basking in this species.

The sunfish also visits cleaning stations on coral reefs, where small wrasse fish perform the same service. A large sunfish arriving at a reef cleaning station is a significant event — the fish may remain stationary for extended periods while dozens of wrasse work across its surface, removing parasites from areas the sunfish cannot reach on its own.

300 Million Eggs

The sunfish reproduces in a manner consistent with everything else about it — on a scale that is difficult to process.

A single female sunfish can produce up to 300 million eggs in a single spawning event — more than any other vertebrate on earth. The eggs are released into open water, fertilised externally, and then entirely abandoned. The larvae that hatch are tiny — a few millimetres long — and bear almost no resemblance to the adults they will eventually become. They have spines, a tail fin, and a body shape that looks like a conventional fish. As they grow, the tail is progressively reduced, the body becomes disc-shaped, and the animal transforms into something that looks like no other fish in the ocean.

The survival rate from egg to adult is vanishingly small. Of 300 million eggs, the vast majority will be eaten within hours of fertilisation. Of the larvae that hatch, most will be consumed before they reach any significant size. The sunfish’s reproductive strategy is pure quantity — produce enough offspring that a tiny fraction will survive to adulthood, and trust that adulthood, once reached, is sufficiently protected by sheer size to allow a long life.

It is a strategy that has worked for millions of years. The sunfish is not endangered. It is found in tropical and temperate oceans worldwide, and while plastic pollution and bycatch in fishing nets pose genuine threats, its populations remain large enough that it is currently classified as vulnerable rather than critically endangered.

The Fish That Shouldn’t Work

The ocean sunfish is, by almost any measure, an animal that should not exist. It is too large to be efficient, too slow to be safe, too dependent on a nutritionally poor food source to sustain its bulk, and too heavily parasitised to be comfortable. Its reproductive strategy requires it to produce hundreds of millions of offspring to replace itself. Its body plan looks like an evolutionary accident.

And yet here it is — 2,300 kilograms of silver disc, rowing through the open ocean at walking pace, eating jellyfish, collecting parasites, and basking at the surface while seagulls pick creatures off its skin.

It has been doing this, in essentially the same form, for millions of years. The ocean, apparently, has room for the improbable.

If you encountered a 2,300kg sunfish while diving, would you swim toward it or away from it?