Biodiversity 3mn

What is the largest bony fish in the world?

With a round body that can exceed three metres in length and weigh up to two tonnes, the sunfish is the largest of the bony fish.

The Mola mola sunfish is the fish of records! The largest and heaviest bony fish, it also has the particularity of continuous growth. It never stops growing, and the more it eats, the bigger it gets.

Another record: the sunfish can produce up to 100 million eggs in a single clutch. This makes it the bony fish that lays the most eggs. The whale shark is also the largest fish in the world, but in the cartilaginous fish category.

Where can I find the animal?

The sunfish is an open-sea fish that lives in tropical, subtropical and temperate waters around the world, both on the surface of the ocean and in the depths.

At the surface, they are easily spotted by their dorsal fin, which sticks out of the water and can be mistaken for a shark fin.

When it drifts on the surface and catches the sun's rays, the mole takes advantage of the opportunity to be dewormed by certain birds. In fact, it is generally infested with around forty different species of parasite, which it tries to get rid of by various means.

How do you recognise it? 

The sunfish has an unusual, easily recognisable shape. Its oval body, compressed laterally, varies in colour from silvery grey to white, with a protuberance on its head, tiny pectoral fins, and dorsal and anal fins that can be almost twice as long as the fish is tall.

During its evolution, the caudal fin disappeared and became an appendage called a clavus, which connects the dorsal fin to the anal fin but does not allow it to propel itself. To move forward, it uses its dorsal and anal fins.

In fact, the sunfish either swims upright with its dorsal fin erect, giving it a sculling motion, or lies on its side, drifting on the surface with the current. In the latter case, this is probably to replenish its thermal reserves. Its English name, sunfish, probably comes from this. Its French name comes from its round shape: at night, its silhouette resembles the reflection of the moon in the sea.

Did you know?

What does it eat?

Sunfish feed on jellyfish, salps, ctenophores, squid and small crustaceans. It is estimated that a 120 kg sunfish in the Mediterranean would eat around 71 kg of jellyfish a day.

What makes it special?

The sunfish has no scales and no swim bladder. It is a harmless animal with few predators. The decline in shark populations, one of the few predators of this fish, could explain the increase in the number of sunfish. The proliferation of jellyfish linked to climate change and overfishing of jellyfish predator species is also thought to explain the growing population of sunfish.

Did you know? When they are born, juveniles are only a few millimetres long and are covered in spines that disappear as they grow.

Threats and protection measures

A species classified as vulnerable on the IUCN red list, the sunfish is a victim of dragnets and plastic bags, which it mistakes for jellyfish.