How Big Do Ocean Sunfish Get? Mola Size and Records

How big is a typical ocean sunfish?

The ocean sunfish (Mola mola) is one of the largest bony fish in the sea, and its unusual shape makes it look even bigger. The tall dorsal and anal fins mean that an adult is often as tall as it is long, or taller.

The figures for the largest Mola mola come from FishBase, the global fish database. Animal Diversity Web and some older books give heavier maximums for Mola mola, of more than 2 tonnes, but at least some of those giant fish are now known to have been a different species.

The heaviest bony fish: a giant sunfish

In late 2021, a dead sunfish was found floating off Faial Island in the Azores, Portugal. It was brought ashore and weighed: 2,744 kg (6,049 lb), with a length of 3.25 m (10.7 ft). Scientists identified it as a giant or bump-head sunfish (Mola alexandrini), a close relative of the ocean sunfish, and described it in the Journal of Fish Biology in 2023 as the heaviest bony fish ever recorded.

It beat the previous record by about 444 kg (979 lb). That earlier fish, a 2,300 kg (5,070 lb) sunfish caught off Kamogawa, Japan, in 1996, was long listed as Mola mola, but researchers later concluded that it was also a giant sunfish.

Sharks and rays are heavier still, but they are cartilaginous fish rather than bony fish. The whale shark is the largest fish of all, as shown in our list of the largest fish in the world.

Why sunfish size records are confusing

For most of history, people assumed that any very large sunfish was Mola mola. In fact, three similar species of Mola exist: the ocean sunfish, the giant or bump-head sunfish, and the hoodwinker sunfish, which was described as a new species only in 2017. Adults can be told apart by details such as a bump on the head or chin, the shape of the clavus and the scales, or by DNA. Many old records include none of this information.

Measuring a giant sunfish is also difficult. Few people have the chance to weigh a fish that may weigh more than a car, and most record fish are measured after they have been caught in nets or found dead. Weights in popular books are sometimes estimates rather than readings from a scale. Length is usually taken from the snout to the end of the clavus, while height is measured from fin tip to fin tip, and not every report says which was used.

From tiny larvae to giants

Ocean sunfish begin life extremely small. Newly hatched larvae are only about 2.5 mm (0.1 in) long and weigh a tiny fraction of a gram. They pass through a spiny stage that looks like a miniature pufferfish, then lose their spines and develop the adult disc shape.

The survivors grow at an astonishing rate. A sunfish may increase its weight by up to about 60 million times between hatching and adulthood, one of the greatest size increases of any vertebrate. In aquariums, young sunfish have gained an average of 0.02–0.42 kg (0.04–0.9 lb) a day, and occasionally more than 0.8 kg (1.8 lb). One young sunfish at Monterey Bay Aquarium grew from 26 kg to 399 kg (57–880 lb) in just 15 months before it was released.

How long it takes a wild sunfish to reach full size, and how long it lives, is still unknown.

How do sunfish compare with other giant fish?

The whale shark is many times heavier than any sunfish, but among bony fish, sunfish and sturgeon are the heavyweights. Sunfish are also much deeper-bodied than other giant fish, which is why a sunfish of modest length can weigh as much as a fish twice as long.

Why do sunfish grow so big?

Several features make a giant body possible. Sunfish have a light skeleton made largely of cartilage instead of heavy bone, and they have no swim bladder. Much of their weight is thick skin and a firm, jelly-like layer under the skin that is about 90% water and helps them float.

A large body also helps with their way of life. Sunfish dive repeatedly into cold, deep water to hunt jellyfish and other prey, and a big body loses heat slowly, so they can stay down longer before returning to warm up at the surface, as explained in Why do ocean sunfish lie on their side? Finally, size and thick skin protect adults from most predators, although sea lions, orcas and large sharks still attack them.