Ocean Sunfish

Mola mola

Ocean Sunfish: The Giant Fish That Sunbathes After Dives

The ocean sunfish (Mola mola) is one of the heaviest bony fish in the world, a tall, flat disc with no true tail that lives in tropical and temperate oceans everywhere. It dives into cold water to hunt jelly-like prey and then rewarms at the surface, often lying on its side. It is listed as Vulnerable by the IUCN.

Habitat
Open ocean, from the surface down to about 1,500 m
Size
Up to 3.3 m long; heaviest published weight 1.3 tonnes
Diet
Mainly siphonophores and other gelatinous zooplankton
Range
Tropical and temperate waters of all oceans
Lifespan
Unknown in the wild; over 8 years in aquariums
Conservation
Vulnerable (IUCN, assessed 2011)

Habitat & range

The ocean sunfish lives in tropical and temperate waters of every ocean. It is an open-ocean fish that has been recorded from near the surface down to about 1,500 m, and it prefers water of roughly 12 to 25 °C (FishBase). Adults also come in to slopes next to deep water, where small cleaner fish pick parasites off their skin (FishBase; Pope et al. 2010). Sightings near the coast are usually in summer. In November 2026 one was found stranded on the Isle of Man, the first stranding of its kind recorded there, after an unusually warm year kept sunfish in local waters late into the autumn (BBC News, Manx Wildlife Trust).

Diet & foraging

Sunfish were long thought to be lazy jellyfish drifters. Tagging with cameras and accelerometers off Japan showed they are active hunters by day: they dive again and again to 100 to 200 m, into water colder than 12 °C, and the prey they caught most often were siphonophores, long chains of jelly-like animals (Nakamura, Goto and Sato 2015). One sunfish filmed approaching a jellyfish ate only its gonads and oral arms, the most nutritious parts. Older records also mention crustaceans, small fish, squid and eel larvae in their stomachs, so the full diet is still being worked out (Pope et al. 2010).

Behavior & adaptations

The ocean sunfish has no true tail. The back of its body ends in a stiff, rudder-like fringe called the clavus, and it swims by flapping its very tall dorsal and anal fins from side to side. It has no scales, but its skin is extremely thick and elastic, its small mouth has teeth fused into a beak, and it has no swim bladder (FishBase). The famous surface basking is mostly about heat: body temperature drops during deep dives and recovers at the surface, and the fish gains heat 3 to 7 times faster than it loses it, which lets it spend more time hunting in cold water (Nakamura, Goto and Sato 2015). Sunfish may also bask to let seabirds and cleaner fish remove parasites; 54 parasite species have been recorded on them (Pope et al. 2010). Ocean Sunfish appears as a Pisces creature card in Wilder, a free browser collectible card game.

Conservation

The IUCN lists the ocean sunfish as Vulnerable (assessed 2011, published 2015), mainly because so many are caught by accident in fisheries for other species (IUCN Red List). Sunfish made up 70 to 93 percent of the fish catch in Spanish Mediterranean drift gillnets in 1992 to 1994, 29 percent of all bycatch in the Californian swordfish fishery, and 51 percent of the bycatch in a South African midwater trawl fishery (Pope et al. 2010). Most are thrown back alive, but how many survive is not known. Floating plastic bags may also be mistaken for jelly-like prey (Pope et al. 2010).

Fun facts about Ocean Sunfish

  • A 137 cm female ocean sunfish was estimated to hold 300 million eggs, the most of any known vertebrate (Pope et al. 2010).
  • Growing from a 2.5 mm larva to a 3 m adult means increasing body mass about 60 million times (Pope et al. 2010).
  • One sunfish at Monterey Bay Aquarium gained 373 kg in 15 months (Pope et al. 2010).
  • The record for heaviest bony fish, 2,744 kg, belongs to a close relative, the giant sunfish Mola alexandrini, found dead off the Azores in 2021 (Gomes-Pereira et al. 2023).
  • Sunfish warm up at the surface 3 to 7 times faster than they cool down during dives (Nakamura, Goto and Sato 2015).

Ocean Sunfish vs Orca

The orca wins. A tough hide and a huge body are not enough against a fast, intelligent pack hunter.

Orcas are among the few animals recorded killing and eating ocean sunfish, along with sea lions and large sharks (Pope et al. 2010). A sunfish basking on its side at the surface is slow to get away, and its best defence is its size and its very thick, tough skin. An orca is faster, smarter and hunts in groups, so it can work around the sunfish and attack the fins, the way California sea lions have been seen biting fins off sunfish in Monterey Bay (Pope et al. 2010).

Read more about Orca.

Common questions about Ocean Sunfish

Why do ocean sunfish lie on their side at the surface?

Mostly to warm up. Tagged sunfish off Japan dove again and again to 100 to 200 m, into water colder than 12 °C, to hunt. Their body temperature dropped on each dive and recovered at the surface, and they gained heat 3 to 7 times faster than they lost it (Nakamura et al. 2015). Basking may also let seabirds and cleaner fish pick off parasites (Pope et al. 2010).

What do ocean sunfish eat?

Mainly siphonophores and other jelly-like animals. Camera tags showed siphonophores were the prey sunfish caught most often, at 50 to 200 m deep, and one sunfish ate only a jellyfish's gonads and oral arms, the most nutritious parts (Nakamura et al. 2015). Older stomach records also list crustaceans, small fish, squid and eel larvae (Pope et al. 2010).

Is the ocean sunfish the heaviest bony fish in the world?

Almost. The record belongs to its close relative, the giant sunfish Mola alexandrini: a 2,744 kg fish found dead off the Azores in 2021 (Gomes-Pereira et al. 2023). The heaviest published ocean sunfish, Mola mola, weighed 1.3 tonnes (FishBase). The biggest fish of all is the whale shark, but sharks have skeletons of cartilage, not bone.

How many eggs does an ocean sunfish lay?

Possibly hundreds of millions. One 137 cm female was estimated to hold 300 million eggs, the most known for any vertebrate. The eggs are tiny, and growing from a 2.5 mm larva to a 3 m adult means increasing body mass about 60 million times (Pope et al. 2010).

Are ocean sunfish endangered?

They are listed as Vulnerable, one step below Endangered (IUCN Red List). The main threat is bycatch: sunfish were 70 to 93% of the fish caught in Spanish Mediterranean drift gillnets in 1992 to 1994 and 29% of all bycatch in the Californian swordfish fishery (Pope et al. 2010).

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