The ocean sunfish is one of the strangest animals in the sea. With a huge, flat, almost round body, it looks like the front half of a fish that forgot to grow a tail. Known scientifically as Mola mola, the sunfish is among the heaviest bony fish in the world and still holds many mysteries for scientists.
Advertisement
What does a sunfish look like?
The sunfish has a tall, flattened body with a tall fin on top (the dorsal fin) and another below (the anal fin). Instead of a normal tail, it has a rounded structure called a clavus, formed by parts of these fins. Its mouth is small, and its skin is thick and rough, covered by a layer of mucus.
Adults can reach more than 3 meters from fin tip to fin tip and weigh well over a ton. In 2021, researchers reported a giant sunfish of a closely related species found near the Azores that weighed more than 2,700 kilograms, considered the heaviest bony fish ever recorded.
Where sunfish live
Sunfish live in tropical and temperate oceans around the world. They spend much of their time in the open ocean, often diving to depths of hundreds of meters to feed. They are sometimes seen near the coast, including off California and in parts of the Atlantic and Mediterranean.
Why it is called a sunfish
The name comes from a curious behavior: sunfish are often seen lying on their side at the surface, as if sunbathing. Scientists believe this behavior may help them warm up after diving into cold, deep waters. Lying at the surface may also allow seabirds to pick parasites off their skin. Smaller fish at cleaning stations also help remove parasites.
Advertisement
What sunfish eat
For a long time, sunfish were thought to eat only jellyfish. Jellyfish and other gelatinous animals are indeed an important part of their diet, but studies show that sunfish also eat small fish, squid, crustaceans and other prey, especially when they are young. Because jellyfish have low nutritional value, sunfish need to eat large quantities.
Remarkable growth
Sunfish have one of the most impressive growth rates among vertebrates. They hatch from tiny eggs and start life as minuscule larvae, but they can grow to enormous sizes, increasing their body weight many millions of times between larval stage and adulthood.
Female sunfish are also known for producing enormous numbers of eggs, more than almost any other vertebrate studied. Most of these eggs and larvae do not survive, which balances the population.
New discoveries
Even today, scientists are still learning about sunfish. In 2017, researchers described a new species, the hoodwinker sunfish (Mola tecta), which had gone unnoticed for centuries because it looks similar to other species. The name reflects how it “hid” from science.
Researchers also use tags and satellite tracking to study how sunfish move, dive and migrate, revealing that they are much more active swimmers than once believed.
Are sunfish dangerous?
Sunfish are harmless to people. They do not attack swimmers or divers. The main risk comes from their size: in rare cases, sunfish have collided with boats or jumped out of the water.
Threats to sunfish
- Bycatch: many sunfish are accidentally caught in fishing nets and longlines.
- Plastic pollution: floating plastic bags can be mistaken for jellyfish and swallowed.
- Changes in the ocean that affect their prey.
The ocean sunfish is listed as vulnerable on the IUCN Red List, which highlights the need to reduce bycatch and protect the oceans.
How to see a sunfish
Some places are known for sunfish sightings, such as Bali in Indonesia, where divers can observe them at cleaning stations during certain seasons, and parts of the California coast. Responsible tourism means keeping a respectful distance and never touching the animals.
Sunfish species
Several species are known as sunfish. The ocean sunfish (Mola mola) is the best known, but scientists also recognize the southern sunfish (Mola alexandrini), which holds records for the heaviest bony fish ever measured, and the hoodwinker sunfish (Mola tecta), described as a new species in 2017 after researchers noticed differences in specimens that had been misidentified for a long time. Its name reflects how it "hid in plain sight." Other related species include the slender sunfish and the sharptail mola.
Anatomy of a strange fish
Sunfish have a flattened, almost circular body, tall dorsal and anal fins used for swimming and no true tail. Instead, they have a rudder-like structure called a clavus. Their skin is thick and covered with mucus, and they often carry many parasites. Cleaner fish and seabirds sometimes help remove these parasites, which is one explanation for why sunfish bask at the surface, where birds can pick parasites from their skin.
Diving behavior
Although often seen lying at the surface, sunfish are active divers. Tracking studies show they regularly dive to depths of hundreds of meters to feed on jellyfish and other gelatinous animals in cold, deep waters. Basking at the surface may help them warm up after these dives, which supports the idea behind their common name.
Reproduction
Female ocean sunfish are known for producing an enormous number of eggs, possibly hundreds of millions at a time, more than almost any other vertebrate. The larvae are tiny and spiky, very different from adults, and only a small fraction survive. Sunfish then grow at an impressive rate, increasing their weight many times over during their lives.
Sunfish and fisheries
Sunfish are frequently caught accidentally in fishing nets and longlines meant for other species, a problem known as bycatch. In some regions, they are also targeted for food, especially in parts of Asia. Because sunfish are difficult to study, scientists still lack precise data on population trends, which makes conservation decisions challenging.
Plastic pollution
Floating plastic bags can resemble jellyfish, a major part of the sunfish diet. Swallowing plastic can harm digestion and nutrition. Reducing plastic use and preventing waste from reaching the ocean helps sunfish and many other marine animals, such as sea turtles.
Responsible observation
Tourists who see sunfish while diving or on boats should keep a respectful distance, avoid touching them and follow local guidelines for marine wildlife watching.
Related reading:
- Penguins: Species, Adaptations, Behavior and Threats
- The Animal World: Diversity, Instincts and Our Bond With Animals
Conclusion
The ocean sunfish is a true giant of the sea, with an unusual body, extraordinary growth and behaviors that still intrigue scientists. Protecting this gentle species means reducing bycatch, fighting plastic pollution and caring for the health of the oceans where it lives.