Ocean Sunfish (Mola mola): Facts, Size and Diet

What does an ocean sunfish look like?

An ocean sunfish is shaped like a tall, flattened oval, as if the back half of a normal fish had been cut off. A long dorsal fin rises from its back and an equally long anal fin hangs underneath, so the fish is often as tall as it is long. Where a tail should be, it has a stiff, rounded flap called the clavus, formed from parts of those two fins. The clavus works as a rudder, while the tall fins provide the power.

The mouth is small and round, and the teeth are fused into a beak-like plate, so the sunfish cannot fully close it. Extra teeth in its throat help shred prey. The pectoral fins are small and rounded, and the gill openings are tiny holes just in front of them.

Instead of normal scales, the skin is covered in tiny toothlike points and a layer of mucus, and it feels as rough as sandpaper. It is also very thick, up to about 7 cm (3 in) on the belly. Most sunfish are gray, brown or silvery white, often with paler blotches, and they can change from light to dark, for example when disturbed.

Inside, the sunfish is just as unusual. Its skeleton is largely made of light, flexible cartilage, it has fewer vertebrae than any other fish and it lacks a swim bladder. Buoyancy comes instead from a thick, jelly-like layer under the skin that is about 90% water. It belongs to the same order as pufferfish, porcupinefish and triggerfish, and its larvae even look like tiny spiny pufferfish. See Why do pufferfish puff up? for more about this family tree.

How big do ocean sunfish get?

Adult ocean sunfish average about 1.8 m (6 ft) in length and about 2.5 m (8 ft) from the tip of the dorsal fin to the tip of the anal fin. Most weigh 247–1,000 kg (545–2,205 lb). FishBase gives a maximum length of about 3.3 m (11 ft) and a heaviest published weight of about 1.3 tonnes for Mola mola.

The famous record for the heaviest bony fish belongs to a relative. In 2021, a giant sunfish (Mola alexandrini) found dead off the Azores weighed 2,744 kg (6,049 lb). The details, and why old sunfish records are confusing, are explained in How big do ocean sunfish get?

The hoodwinker sunfish was described only in 2017, after DNA studies revealed it had been hiding among other sunfish for centuries.

Where do ocean sunfish live?

Ocean sunfish live in the open ocean in temperate and tropical waters around the world, mostly in water warmer than about 10–12 °C (50–54 °F). In the northeast Atlantic they are seen from the Bay of Biscay and Portugal to the waters off Cornwall, Ireland and Wales, mainly in summer and early autumn, and they are regular in the Mediterranean. In North America they occur off California and, in summer and fall, in New England waters such as the Gulf of Maine.

Sunfish were once thought to be passive drifters that floated wherever currents took them. Satellite tags have shown otherwise. Tagged sunfish have been recorded covering about 26 km (16 mi) in a day at around 3 km/h (2 mph), and they make seasonal migrations, moving into cooler, higher-latitude waters in summer and back toward warmer water in winter. They spend much of their time in the top 200 m (660 ft) but regularly dive deeper, down to 800 m (2,600 ft) or more.

What do ocean sunfish eat?

Sunfish are famous jellyfish eaters, and gelatinous animals make up a large part of their diet: jellyfish, salps, comb jellies and siphonophores, long colonial relatives of jellyfish. These animals are mostly water and contain few calories, so a sunfish has to eat a lot of them. It shares this diet with the leatherback sea turtle, another giant that cruises temperate seas in search of jellyfish.

Studies of stomach contents and prey DNA show that sunfish are less fussy than once thought. They also eat small fish, fish larvae, squid, crustaceans, brittle stars and other soft-bodied animals, and smaller sunfish take more of this mixed prey, while big adults rely more heavily on jellies. Sunfish feed at many depths, and in some places they hunt near the seafloor.

Basking, diving and cleaning

Ocean sunfish often appear at the surface lying on their side, a habit that gave them their English name. Much of this is warming up: sunfish dive repeatedly into deep, cold water to hunt and then return to the warm surface to raise their body temperature. Basking also lets seabirds pick parasites off their skin. The full story is told in Why do ocean sunfish lie on their side?

Parasites are a constant problem. More than 40 species of parasites have been recorded on and inside ocean sunfish. Near floating kelp and reefs, sunfish visit cleaner fish that nibble parasites from their skin, and they are occasionally seen leaping clear of the water, perhaps to shake parasites off.

When swimming near the surface, a sunfish’s dorsal fin often sticks out of the water and is sometimes mistaken for a shark’s. The difference is in the movement: the sunfish flaps its dorsal and anal fins together from side to side, so its fin waves back and forth rather than cutting straight through the water.

Reproduction and growth

Little is known about how ocean sunfish mate. Spawning is thought to take place in several oceans, and eggs and sperm are released into open water, where the eggs are fertilized. Sunfish are extraordinarily productive: a female about 1.5 m (5 ft) long was found carrying an estimated 300 million eggs, more than any other known vertebrate.

Newly hatched larvae are only about 2.5 mm (0.1 in) long. They pass through a spiny, pufferfish-like stage and then grow at an astonishing rate, increasing their weight by up to about 60 million times by adulthood, one of the greatest size increases of any vertebrate. A 2010 review of sunfish biology reported that young sunfish in aquariums 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).

How long sunfish live in the wild is still unknown. In captivity, one sunfish at an aquarium in Japan lived for about eight years.

Predators and threats

Adult sunfish are protected by their size and thick skin, but they still have predators. Sea lions, orcas and large sharks attack them, and off California, California sea lions have been seen biting the fins off sunfish. Young sunfish are eaten by tuna and dorado.

The biggest threat comes from fisheries. Sunfish are caught in large numbers as bycatch, especially in drift gillnets and longlines set for swordfish and tuna. In some swordfish driftnet fisheries they made up a large share of the total catch, and in parts of the Mediterranean the majority. Many are thrown back injured or dead. Sunfish also swallow floating plastic that resembles jellyfish, and they are struck by boats when lying at the surface.

Conservation status

The IUCN Red List has classified the ocean sunfish as Vulnerable since 2015, with a decreasing population trend, mainly because of the high rates of bycatch. Population sizes are hard to measure for a wide-ranging fish of the open ocean, and the recent discovery that three similar Mola species overlap in some areas makes old data even harder to interpret.

In some places sunfish are protected or released when caught, and some fisheries now use methods that reduce bycatch. In the European Union, the sale of fish from the sunfish family is banned under food safety rules, because these fish may contain toxins similar to those of pufferfish.

Ocean sunfish and people

Ocean sunfish are popular with divers and wildlife watchers, and the sight of one lying at the surface is a highlight of many summer boat trips off Cornwall, Ireland and California. In many European languages the fish is named for the moon rather than the sun, because of its round shape: poisson-lune in French, Mondfisch in German and pez luna in Spanish. The scientific name Mola is Latin for millstone, after its round, gray, rough-skinned body.

Only a few aquariums keep ocean sunfish, because they need very large, rounded tanks and careful feeding. Monterey Bay Aquarium in California, the Lisbon Oceanarium in Portugal and the Osaka Aquarium Kaiyukan in Japan are among the best known. Sunfish are eaten in some parts of East Asia, including Japan and Taiwan.